TitleRequirements for Internet Hosts - Application and Support
AuthorR. Braden, Ed.
DateOctober 1989
Format:TXT, HTML
UpdatesRFC0822, RFC0952
Updated byRFC1349, RFC2181, RFC5321, RFC5966, RFC7766, RFC9210

Network Working Group                    Internet Engineering Task Force
Request for Comments: 1123                             R. Braden, Editor
                                                            October 1989

       Requirements for Internet Hosts -- Application and Support

Status of This Memo

   This RFC is an official specification for the Internet community.  It
   incorporates by reference, amends, corrects, and supplements the
   primary protocol standards documents relating to hosts.  Distribution
   of this document is unlimited.


   This RFC is one of a pair that defines and discusses the requirements
   for Internet host software.  This RFC covers the application and
   support protocols; its companion RFC-1122 covers the communication
   protocol layers: link layer, IP layer, and transport layer.

                           Table of Contents

   1.  INTRODUCTION ...............................................    5
      1.1  The Internet Architecture ..............................    6
      1.2  General Considerations .................................    6
         1.2.1  Continuing Internet Evolution .....................    6
         1.2.2  Robustness Principle ..............................    7
         1.2.3  Error Logging .....................................    8
         1.2.4  Configuration .....................................    8
      1.3  Reading this Document ..................................   10
         1.3.1  Organization ......................................   10
         1.3.2  Requirements ......................................   10
         1.3.3  Terminology .......................................   11
      1.4  Acknowledgments ........................................   12

   2.  GENERAL ISSUES .............................................   13
      2.1  Host Names and Numbers .................................   13
      2.2  Using Domain Name Service ..............................   13
      2.3  Applications on Multihomed hosts .......................   14
      2.4  Type-of-Service ........................................   14

RFC1123                       INTRODUCTION                  October 1989

   3.  REMOTE LOGIN -- TELNET PROTOCOL ............................   16
      3.1  INTRODUCTION ...........................................   16
      3.2  PROTOCOL WALK-THROUGH ..................................   16
         3.2.1  Option Negotiation ................................   16
         3.2.2  Telnet Go-Ahead Function ..........................   16
         3.2.3  Control Functions .................................   17
         3.2.4  Telnet "Synch" Signal .............................   18
         3.2.5  NVT Printer and Keyboard ..........................   19
         3.2.6  Telnet Command Structure ..........................   20
         3.2.7  Telnet Binary Option ..............................   20
         3.2.8  Telnet Terminal-Type Option .......................   20
      3.3  SPECIFIC ISSUES ........................................   21
         3.3.1  Telnet End-of-Line Convention .....................   21
         3.3.2  Data Entry Terminals ..............................   23
         3.3.3  Option Requirements ...............................   24
         3.3.4  Option Initiation .................................   24
         3.3.5  Telnet Linemode Option ............................   25
      3.4  TELNET/USER INTERFACE ..................................   25
         3.4.1  Character Set Transparency ........................   25
         3.4.2  Telnet Commands ...................................   26
         3.4.3  TCP Connection Errors .............................   26
         3.4.4  Non-Default Telnet Contact Port ...................   26
         3.4.5  Flushing Output ...................................   26
      3.5.  TELNET REQUIREMENTS SUMMARY ...........................   27

   4.  FILE TRANSFER ..............................................   29
      4.1  FILE TRANSFER PROTOCOL -- FTP ..........................   29
         4.1.1  INTRODUCTION ......................................   29
         4.1.2.  PROTOCOL WALK-THROUGH ............................   29
    LOCAL Type ...................................   29
    Telnet Format Control ........................   30
    Page Structure ...............................   30
    Data Structure Transformations ...............   30
    Data Connection Management ...................   31
    PASV Command .................................   31
    LIST and NLST Commands .......................   31
    SITE Command .................................   32
    STOU Command .................................   32
    Telnet End-of-line Code .....................   32
    FTP Replies .................................   33
    Connections .................................   34
    Minimum Implementation; RFC-959 Section .....   34
         4.1.3  SPECIFIC ISSUES ...................................   35
    Non-standard Command Verbs ...................   35
    Idle Timeout .................................   36
    Concurrency of Data and Control ..............   36
    FTP Restart Mechanism ........................   36
         4.1.4  FTP/USER INTERFACE ................................   39

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    Pathname Specification .......................   39
    "QUOTE" Command ..............................   40
    Displaying Replies to User ...................   40
    Maintaining Synchronization ..................   40
         4.1.5   FTP REQUIREMENTS SUMMARY .........................   41
      4.2  TRIVIAL FILE TRANSFER PROTOCOL -- TFTP .................   44
         4.2.1  INTRODUCTION ......................................   44
         4.2.2  PROTOCOL WALK-THROUGH .............................   44
    Transfer Modes ...............................   44
    UDP Header ...................................   44
         4.2.3  SPECIFIC ISSUES ...................................   44
    Sorcerer's Apprentice Syndrome ...............   44
    Timeout Algorithms ...........................   46
    Extensions ...................................   46
    Access Control ...............................   46
    Broadcast Request ............................   46
         4.2.4  TFTP REQUIREMENTS SUMMARY .........................   47

   5.  ELECTRONIC MAIL -- SMTP and RFC-822 ........................   48
      5.1  INTRODUCTION ...........................................   48
      5.2  PROTOCOL WALK-THROUGH ..................................   48
         5.2.1  The SMTP Model ....................................   48
         5.2.2  Canonicalization ..................................   49
         5.2.3  VRFY and EXPN Commands ............................   50
         5.2.4  SEND, SOML, and SAML Commands .....................   50
         5.2.5  HELO Command ......................................   50
         5.2.6  Mail Relay ........................................   51
         5.2.7  RCPT Command ......................................   52
         5.2.8  DATA Command ......................................   53
         5.2.9  Command Syntax ....................................   54
         5.2.10  SMTP Replies .....................................   54
         5.2.11  Transparency .....................................   55
         5.2.12  WKS Use in MX Processing .........................   55
         5.2.13  RFC-822 Message Specification ....................   55
         5.2.14  RFC-822 Date and Time Specification ..............   55
         5.2.15  RFC-822 Syntax Change ............................   56
         5.2.16  RFC-822  Local-part ..............................   56
         5.2.17  Domain Literals ..................................   57
         5.2.18  Common Address Formatting Errors .................   58
         5.2.19  Explicit Source Routes ...........................   58
      5.3  SPECIFIC ISSUES ........................................   59
         5.3.1  SMTP Queueing Strategies ..........................   59
   Sending Strategy ..............................   59
    Receiving strategy ...........................   61
         5.3.2  Timeouts in SMTP ..................................   61
         5.3.3  Reliable Mail Receipt .............................   63
         5.3.4  Reliable Mail Transmission ........................   63
         5.3.5  Domain Name Support ...............................   65

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         5.3.6  Mailing Lists and Aliases .........................   65
         5.3.7  Mail Gatewaying ...................................   66
         5.3.8  Maximum Message Size ..............................   68
      5.4  SMTP REQUIREMENTS SUMMARY ..............................   69

   6. SUPPORT SERVICES ............................................   72
      6.1 DOMAIN NAME TRANSLATION .................................   72
         6.1.1 INTRODUCTION .......................................   72
         6.1.2  PROTOCOL WALK-THROUGH .............................   72
    Resource Records with Zero TTL ...............   73
    QCLASS Values ................................   73
    Unused Fields ................................   73
    Compression ..................................   73
    Misusing Configuration Info ..................   73
         6.1.3  SPECIFIC ISSUES ...................................   74
    Resolver Implementation ......................   74
    Transport Protocols ..........................   75
    Efficient Resource Usage .....................   77
    Multihomed Hosts .............................   78
    Extensibility ................................   79
    Status of RR Types ...........................   79
    Robustness ...................................   80
    Local Host Table .............................   80
         6.1.4  DNS USER INTERFACE ................................   81
    DNS Administration ...........................   81
    DNS User Interface ...........................   81
   Interface Abbreviation Facilities .............   82
         6.1.5  DOMAIN NAME SYSTEM REQUIREMENTS SUMMARY ...........   84
      6.2  HOST INITIALIZATION ....................................   87
         6.2.1  INTRODUCTION ......................................   87
         6.2.2  REQUIREMENTS ......................................   87
    Dynamic Configuration ........................   87
    Loading Phase ................................   89
      6.3  REMOTE MANAGEMENT ......................................   90
         6.3.1  INTRODUCTION ......................................   90
         6.3.2  PROTOCOL WALK-THROUGH .............................   90
         6.3.3  MANAGEMENT REQUIREMENTS SUMMARY ...................   92

   7.  REFERENCES .................................................   93

RFC1123                       INTRODUCTION                  October 1989


   This document is one of a pair that defines and discusses the
   requirements for host system implementations of the Internet protocol
   suite.  This RFC covers the applications layer and support protocols.
   Its companion RFC, "Requirements for Internet Hosts -- Communications
   Layers" [INTRO:1] covers the lower layer protocols: transport layer,
   IP layer, and link layer.

   These documents are intended to provide guidance for vendors,
   implementors, and users of Internet communication software.  They
   represent the consensus of a large body of technical experience and
   wisdom, contributed by members of the Internet research and vendor

   This RFC enumerates standard protocols that a host connected to the
   Internet must use, and it incorporates by reference the RFCs and
   other documents describing the current specifications for these
   protocols.  It corrects errors in the referenced documents and adds
   additional discussion and guidance for an implementor.

   For each protocol, this document also contains an explicit set of
   requirements, recommendations, and options.  The reader must
   understand that the list of requirements in this document is
   incomplete by itself; the complete set of requirements for an
   Internet host is primarily defined in the standard protocol
   specification documents, with the corrections, amendments, and
   supplements contained in this RFC.

   A good-faith implementation of the protocols that was produced after
   careful reading of the RFC's and with some interaction with the
   Internet technical community, and that followed good communications
   software engineering practices, should differ from the requirements
   of this document in only minor ways.  Thus, in many cases, the
   "requirements" in this RFC are already stated or implied in the
   standard protocol documents, so that their inclusion here is, in a
   sense, redundant.  However, they were included because some past
   implementation has made the wrong choice, causing problems of
   interoperability, performance, and/or robustness.

   This document includes discussion and explanation of many of the
   requirements and recommendations.  A simple list of requirements
   would be dangerous, because:

   o    Some required features are more important than others, and some
        features are optional.

   o    There may be valid reasons why particular vendor products that

RFC1123                       INTRODUCTION                  October 1989

        are designed for restricted contexts might choose to use
        different specifications.

   However, the specifications of this document must be followed to meet
   the general goal of arbitrary host interoperation across the
   diversity and complexity of the Internet system.  Although most
   current implementations fail to meet these requirements in various
   ways, some minor and some major, this specification is the ideal
   towards which we need to move.

   These requirements are based on the current level of Internet
   architecture.  This document will be updated as required to provide
   additional clarifications or to include additional information in
   those areas in which specifications are still evolving.

   This introductory section begins with general advice to host software
   vendors, and then gives some guidance on reading the rest of the
   document.  Section 2 contains general requirements that may be
   applicable to all application and support protocols.  Sections 3, 4,
   and 5 contain the requirements on protocols for the three major
   applications: Telnet, file transfer, and electronic mail,
   respectively. Section 6 covers the support applications: the domain
   name system, system initialization, and management.  Finally, all
   references will be found in Section 7.

   1.1  The Internet Architecture

      For a brief introduction to the Internet architecture from a host
      viewpoint, see Section 1.1 of [INTRO:1].  That section also
      contains recommended references for general background on the
      Internet architecture.

   1.2  General Considerations

      There are two important lessons that vendors of Internet host
      software have learned and which a new vendor should consider

      1.2.1  Continuing Internet Evolution

         The enormous growth of the Internet has revealed problems of
         management and scaling in a large datagram-based packet
         communication system.  These problems are being addressed, and
         as a result there will be continuing evolution of the
         specifications described in this document.  These changes will
         be carefully planned and controlled, since there is extensive
         participation in this planning by the vendors and by the
         organizations responsible for operations of the networks.

RFC1123                       INTRODUCTION                  October 1989

         Development, evolution, and revision are characteristic of
         computer network protocols today, and this situation will
         persist for some years.  A vendor who develops computer
         communication software for the Internet protocol suite (or any
         other protocol suite!) and then fails to maintain and update
         that software for changing specifications is going to leave a
         trail of unhappy customers.  The Internet is a large
         communication network, and the users are in constant contact
         through it.  Experience has shown that knowledge of
         deficiencies in vendor software propagates quickly through the
         Internet technical community.

      1.2.2  Robustness Principle

         At every layer of the protocols, there is a general rule whose
         application can lead to enormous benefits in robustness and

                "Be liberal in what you accept, and
                 conservative in what you send"

         Software should be written to deal with every conceivable
         error, no matter how unlikely; sooner or later a packet will
         come in with that particular combination of errors and
         attributes, and unless the software is prepared, chaos can
         ensue.  In general, it is best to assume that the network is
         filled with malevolent entities that will send in packets
         designed to have the worst possible effect.  This assumption
         will lead to suitable protective design, although the most
         serious problems in the Internet have been caused by
         unenvisaged mechanisms triggered by low-probability events;
         mere human malice would never have taken so devious a course!

         Adaptability to change must be designed into all levels of
         Internet host software.  As a simple example, consider a
         protocol specification that contains an enumeration of values
         for a particular header field -- e.g., a type field, a port
         number, or an error code; this enumeration must be assumed to
         be incomplete.  Thus, if a protocol specification defines four
         possible error codes, the software must not break when a fifth
         code shows up.  An undefined code might be logged (see below),
         but it must not cause a failure.

         The second part of the principle is almost as important:
         software on other hosts may contain deficiencies that make it
         unwise to exploit legal but obscure protocol features.  It is
         unwise to stray far from the obvious and simple, lest untoward
         effects result elsewhere.  A corollary of this is "watch out

RFC1123                       INTRODUCTION                  October 1989

         for misbehaving hosts"; host software should be prepared, not
         just to survive other misbehaving hosts, but also to cooperate
         to limit the amount of disruption such hosts can cause to the
         shared communication facility.

      1.2.3  Error Logging

         The Internet includes a great variety of host and gateway
         systems, each implementing many protocols and protocol layers,
         and some of these contain bugs and mis-features in their
         Internet protocol software.  As a result of complexity,
         diversity, and distribution of function, the diagnosis of user
         problems is often very difficult.

         Problem diagnosis will be aided if host implementations include
         a carefully designed facility for logging erroneous or
         "strange" protocol events.  It is important to include as much
         diagnostic information as possible when an error is logged.  In
         particular, it is often useful to record the header(s) of a
         packet that caused an error.  However, care must be taken to
         ensure that error logging does not consume prohibitive amounts
         of resources or otherwise interfere with the operation of the

         There is a tendency for abnormal but harmless protocol events
         to overflow error logging files; this can be avoided by using a
         "circular" log, or by enabling logging only while diagnosing a
         known failure.  It may be useful to filter and count duplicate
         successive messages.  One strategy that seems to work well is:
         (1) always count abnormalities and make such counts accessible
         through the management protocol (see Section 6.3); and (2)
         allow the logging of a great variety of events to be
         selectively enabled.  For example, it might useful to be able
         to "log everything" or to "log everything for host X".

         Note that different managements may have differing policies
         about the amount of error logging that they want normally
         enabled in a host.  Some will say, "if it doesn't hurt me, I
         don't want to know about it", while others will want to take a
         more watchful and aggressive attitude about detecting and
         removing protocol abnormalities.

      1.2.4  Configuration

         It would be ideal if a host implementation of the Internet
         protocol suite could be entirely self-configuring.  This would
         allow the whole suite to be implemented in ROM or cast into
         silicon, it would simplify diskless workstations, and it would

RFC1123                       INTRODUCTION                  October 1989

         be an immense boon to harried LAN administrators as well as
         system vendors.  We have not reached this ideal; in fact, we
         are not even close.

         At many points in this document, you will find a requirement
         that a parameter be a configurable option.  There are several
         different reasons behind such requirements.  In a few cases,
         there is current uncertainty or disagreement about the best
         value, and it may be necessary to update the recommended value
         in the future.  In other cases, the value really depends on
         external factors -- e.g., the size of the host and the
         distribution of its communication load, or the speeds and
         topology of nearby networks -- and self-tuning algorithms are
         unavailable and may be insufficient.  In some cases,
         configurability is needed because of administrative

         Finally, some configuration options are required to communicate
         with obsolete or incorrect implementations of the protocols,
         distributed without sources, that unfortunately persist in many
         parts of the Internet.  To make correct systems coexist with
         these faulty systems, administrators often have to "mis-
         configure" the correct systems.  This problem will correct
         itself gradually as the faulty systems are retired, but it
         cannot be ignored by vendors.

         When we say that a parameter must be configurable, we do not
         intend to require that its value be explicitly read from a
         configuration file at every boot time.  We recommend that
         implementors set up a default for each parameter, so a
         configuration file is only necessary to override those defaults
         that are inappropriate in a particular installation.  Thus, the
         configurability requirement is an assurance that it will be
         POSSIBLE to override the default when necessary, even in a
         binary-only or ROM-based product.

         This document requires a particular value for such defaults in
         some cases.  The choice of default is a sensitive issue when
         the configuration item controls the accommodation to existing
         faulty systems.  If the Internet is to converge successfully to
         complete interoperability, the default values built into
         implementations must implement the official protocol, not
         "mis-configurations" to accommodate faulty implementations.
         Although marketing considerations have led some vendors to
         choose mis-configuration defaults, we urge vendors to choose
         defaults that will conform to the standard.

         Finally, we note that a vendor needs to provide adequate

RFC1123                       INTRODUCTION                  October 1989

         documentation on all configuration parameters, their limits and

   1.3  Reading this Document

      1.3.1  Organization

         In general, each major section is organized into the following

         (1)  Introduction

         (2)  Protocol Walk-Through -- considers the protocol
              specification documents section-by-section, correcting
              errors, stating requirements that may be ambiguous or
              ill-defined, and providing further clarification or

         (3)  Specific Issues -- discusses protocol design and
              implementation issues that were not included in the walk-

         (4)  Interfaces -- discusses the service interface to the next
              higher layer.

         (5)  Summary -- contains a summary of the requirements of the

         Under many of the individual topics in this document, there is
         parenthetical material labeled "DISCUSSION" or
         "IMPLEMENTATION".  This material is intended to give
         clarification and explanation of the preceding requirements
         text.  It also includes some suggestions on possible future
         directions or developments.  The implementation material
         contains suggested approaches that an implementor may want to

         The summary sections are intended to be guides and indexes to
         the text, but are necessarily cryptic and incomplete.  The
         summaries should never be used or referenced separately from
         the complete RFC.

      1.3.2  Requirements

         In this document, the words that are used to define the
         significance of each particular requirement are capitalized.
         These words are:

RFC1123                       INTRODUCTION                  October 1989

         *    "MUST"

              This word or the adjective "REQUIRED" means that the item
              is an absolute requirement of the specification.

         *    "SHOULD"

              This word or the adjective "RECOMMENDED" means that there
              may exist valid reasons in particular circumstances to
              ignore this item, but the full implications should be
              understood and the case carefully weighed before choosing
              a different course.

         *    "MAY"

              This word or the adjective "OPTIONAL" means that this item
              is truly optional.  One vendor may choose to include the
              item because a particular marketplace requires it or
              because it enhances the product, for example; another
              vendor may omit the same item.

         An implementation is not compliant if it fails to satisfy one
         or more of the MUST requirements for the protocols it
         implements.  An implementation that satisfies all the MUST and
         all the SHOULD requirements for its protocols is said to be
         "unconditionally compliant"; one that satisfies all the MUST
         requirements but not all the SHOULD requirements for its
         protocols is said to be "conditionally compliant".

      1.3.3  Terminology

         This document uses the following technical terms:

              A segment is the unit of end-to-end transmission in the
              TCP protocol.  A segment consists of a TCP header followed
              by application data.  A segment is transmitted by
              encapsulation in an IP datagram.

              This term is used by some application layer protocols
              (particularly SMTP) for an application data unit.

              A [UDP] datagram is the unit of end-to-end transmission in
              the UDP protocol.

RFC1123                       INTRODUCTION                  October 1989

              A host is said to be multihomed if it has multiple IP
              addresses to connected networks.

   1.4  Acknowledgments

      This document incorporates contributions and comments from a large
      group of Internet protocol experts, including representatives of
      university and research labs, vendors, and government agencies.
      It was assembled primarily by the Host Requirements Working Group
      of the Internet Engineering Task Force (IETF).

      The Editor would especially like to acknowledge the tireless
      dedication of the following people, who attended many long
      meetings and generated 3 million bytes of electronic mail over the
      past 18 months in pursuit of this document: Philip Almquist, Dave
      Borman (Cray Research), Noel Chiappa, Dave Crocker (DEC), Steve
      Deering (Stanford), Mike Karels (Berkeley), Phil Karn (Bellcore),
      John Lekashman (NASA), Charles Lynn (BBN), Keith McCloghrie (TWG),
      Paul Mockapetris (ISI), Thomas Narten (Purdue), Craig Partridge
      (BBN), Drew Perkins (CMU), and James Van Bokkelen (FTP Software).

      In addition, the following people made major contributions to the
      effort: Bill Barns (Mitre), Steve Bellovin (AT&T), Mike Brescia
      (BBN), Ed Cain (DCA), Annette DeSchon (ISI), Martin Gross (DCA),
      Phill Gross (NRI), Charles Hedrick (Rutgers), Van Jacobson (LBL),
      John Klensin (MIT), Mark Lottor (SRI), Milo Medin (NASA), Bill
      Melohn (Sun Microsystems), Greg Minshall (Kinetics), Jeff Mogul
      (DEC), John Mullen (CMC), Jon Postel (ISI), John Romkey (Epilogue
      Technology), and Mike StJohns (DCA).  The following also made
      significant contributions to particular areas: Eric Allman
      (Berkeley), Rob Austein (MIT), Art Berggreen (ACC), Keith Bostic
      (Berkeley), Vint Cerf (NRI), Wayne Hathaway (NASA), Matt Korn
      (IBM), Erik Naggum (Naggum Software, Norway), Robert Ullmann
      (Prime Computer), David Waitzman (BBN), Frank Wancho (USA), Arun
      Welch (Ohio State), Bill Westfield (Cisco), and Rayan Zachariassen

      We are grateful to all, including any contributors who may have
      been inadvertently omitted from this list.

RFC1123              APPLICATIONS LAYER -- GENERAL          October 1989


   This section contains general requirements that may be applicable to
   all application-layer protocols.

   2.1  Host Names and Numbers

      The syntax of a legal Internet host name was specified in RFC-952
      [DNS:4].  One aspect of host name syntax is hereby changed: the
      restriction on the first character is relaxed to allow either a
      letter or a digit.  Host software MUST support this more liberal

      Host software MUST handle host names of up to 63 characters and
      SHOULD handle host names of up to 255 characters.

      Whenever a user inputs the identity of an Internet host, it SHOULD
      be possible to enter either (1) a host domain name or (2) an IP
      address in dotted-decimal ("#.#.#.#") form.  The host SHOULD check
      the string syntactically for a dotted-decimal number before
      looking it up in the Domain Name System.

           This last requirement is not intended to specify the complete
           syntactic form for entering a dotted-decimal host number;
           that is considered to be a user-interface issue.  For
           example, a dotted-decimal number must be enclosed within
           "[ ]" brackets for SMTP mail (see Section 5.2.17).  This
           notation could be made universal within a host system,
           simplifying the syntactic checking for a dotted-decimal

           If a dotted-decimal number can be entered without such
           identifying delimiters, then a full syntactic check must be
           made, because a segment of a host domain name is now allowed
           to begin with a digit and could legally be entirely numeric
           (see Section  However, a valid host name can never
           have the dotted-decimal form #.#.#.#, since at least the
           highest-level component label will be alphabetic.

   2.2  Using Domain Name Service

      Host domain names MUST be translated to IP addresses as described
      in Section 6.1.

      Applications using domain name services MUST be able to cope with
      soft error conditions.  Applications MUST wait a reasonable
      interval between successive retries due to a soft error, and MUST

RFC1123              APPLICATIONS LAYER -- GENERAL          October 1989

      allow for the possibility that network problems may deny service
      for hours or even days.

      An application SHOULD NOT rely on the ability to locate a WKS
      record containing an accurate listing of all services at a
      particular host address, since the WKS RR type is not often used
      by Internet sites.  To confirm that a service is present, simply
      attempt to use it.

   2.3  Applications on Multihomed hosts

      When the remote host is multihomed, the name-to-address
      translation will return a list of alternative IP addresses.  As
      specified in Section, this list should be in order of
      decreasing preference.  Application protocol implementations
      SHOULD be prepared to try multiple addresses from the list until
      success is obtained.  More specific requirements for SMTP are
      given in Section 5.3.4.

      When the local host is multihomed, a UDP-based request/response
      application SHOULD send the response with an IP source address
      that is the same as the specific destination address of the UDP
      request datagram.  The "specific destination address" is defined
      in the "IP Addressing" section of the companion RFC [INTRO:1].

      Similarly, a server application that opens multiple TCP
      connections to the same client SHOULD use the same local IP
      address for all.

   2.4  Type-of-Service

      Applications MUST select appropriate TOS values when they invoke
      transport layer services, and these values MUST be configurable.
      Note that a TOS value contains 5 bits, of which only the most-
      significant 3 bits are currently defined; the other two bits MUST
      be zero.

           As gateway algorithms are developed to implement Type-of-
           Service, the recommended values for various application
           protocols may change.  In addition, it is likely that
           particular combinations of users and Internet paths will want
           non-standard TOS values.  For these reasons, the TOS values
           must be configurable.

           See the latest version of the "Assigned Numbers" RFC
           [INTRO:5] for the recommended TOS values for the major
           application protocols.

RFC1123              APPLICATIONS LAYER -- GENERAL          October 1989


                                               |          | | | |S| |
                                               |          | | | |H| |F
                                               |          | | | |O|M|o
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                                               |          |M|O| |D|T|n
                                               |          |U|U|M| | |o
                                               |          |S|L|A|N|N|t
                                               |          |T|D|Y|O|O|t
FEATURE                                        |SECTION   | | | |T|T|e
                                               |          | | | | | |
User interfaces:                               |          | | | | | |
  Allow host name to begin with digit          |2.1       |x| | | | |
  Host names of up to 635 characters           |2.1       |x| | | | |
  Host names of up to 255 characters           |2.1       | |x| | | |
  Support dotted-decimal host numbers          |2.1       | |x| | | |
  Check syntactically for dotted-dec first     |2.1       | |x| | | |
                                               |          | | | | | |
Map domain names per Section 6.1               |2.2       |x| | | | |
Cope with soft DNS errors                      |2.2       |x| | | | |
   Reasonable interval between retries         |2.2       |x| | | | |
   Allow for long outages                      |2.2       |x| | | | |
Expect WKS records to be available             |2.2       | | | |x| |
                                               |          | | | | | |
Try multiple addr's for remote multihomed host |2.3       | |x| | | |
UDP reply src addr is specific dest of request |2.3       | |x| | | |
Use same IP addr for related TCP connections   |2.3       | |x| | | |
Specify appropriate TOS values                 |2.4       |x| | | | |
  TOS values configurable                      |2.4       |x| | | | |
  Unused TOS bits zero                         |2.4       |x| | | | |
                                               |          | | | | | |
                                               |          | | | | | |

RFC1123                  REMOTE LOGIN -- TELNET             October 1989



      Telnet is the standard Internet application protocol for remote
      login.  It provides the encoding rules to link a user's
      keyboard/display on a client ("user") system with a command
      interpreter on a remote server system.  A subset of the Telnet
      protocol is also incorporated within other application protocols,
      e.g., FTP and SMTP.

      Telnet uses a single TCP connection, and its normal data stream
      ("Network Virtual Terminal" or "NVT" mode) is 7-bit ASCII with
      escape sequences to embed control functions.  Telnet also allows
      the negotiation of many optional modes and functions.

      The primary Telnet specification is to be found in RFC-854
      [TELNET:1], while the options are defined in many other RFCs; see
      Section 7 for references.


      3.2.1  Option Negotiation: RFC-854, pp. 2-3

         Every Telnet implementation MUST include option negotiation and
         subnegotiation machinery [TELNET:2].

         A host MUST carefully follow the rules of RFC-854 to avoid
         option-negotiation loops.  A host MUST refuse (i.e, reply
         WONT/DONT to a DO/WILL) an unsupported option.  Option
         negotiation SHOULD continue to function (even if all requests
         are refused) throughout the lifetime of a Telnet connection.

         If all option negotiations fail, a Telnet implementation MUST
         default to, and support, an NVT.

              Even though more sophisticated "terminals" and supporting
              option negotiations are becoming the norm, all
              implementations must be prepared to support an NVT for any
              user-server communication.

      3.2.2  Telnet Go-Ahead Function: RFC-854, p. 5, and RFC-858

         On a host that never sends the Telnet command Go Ahead (GA),
         the Telnet Server MUST attempt to negotiate the Suppress Go
         Ahead option (i.e., send "WILL Suppress Go Ahead").  A User or
         Server Telnet MUST always accept negotiation of the Suppress Go

RFC1123                  REMOTE LOGIN -- TELNET             October 1989

         Ahead option.

         When it is driving a full-duplex terminal for which GA has no
         meaning, a User Telnet implementation MAY ignore GA commands.

              Half-duplex ("locked-keyboard") line-at-a-time terminals
              for which the Go-Ahead mechanism was designed have largely
              disappeared from the scene.  It turned out to be difficult
              to implement sending the Go-Ahead signal in many operating
              systems, even some systems that support native half-duplex
              terminals.  The difficulty is typically that the Telnet
              server code does not have access to information about
              whether the user process is blocked awaiting input from
              the Telnet connection, i.e., it cannot reliably determine
              when to send a GA command.  Therefore, most Telnet Server
              hosts do not send GA commands.

              The effect of the rules in this section is to allow either
              end of a Telnet connection to veto the use of GA commands.

              There is a class of half-duplex terminals that is still
              commercially important: "data entry terminals," which
              interact in a full-screen manner.  However, supporting
              data entry terminals using the Telnet protocol does not
              require the Go Ahead signal; see Section 3.3.2.

      3.2.3  Control Functions: RFC-854, pp. 7-8

         The list of Telnet commands has been extended to include EOR
         (End-of-Record), with code 239 [TELNET:9].

         Both User and Server Telnets MAY support the control functions
         EOR, EC, EL, and Break, and MUST support AO, AYT, DM, IP, NOP,
         SB, and SE.

         A host MUST be able to receive and ignore any Telnet control
         functions that it does not support.

              Note that a Server Telnet is required to support the
              Telnet IP (Interrupt Process) function, even if the server
              host has an equivalent in-stream function (e.g., Control-C
              in many systems).  The Telnet IP function may be stronger
              than an in-stream interrupt command, because of the out-
              of-band effect of TCP urgent data.

              The EOR control function may be used to delimit the

RFC1123                  REMOTE LOGIN -- TELNET             October 1989

              stream.  An important application is data entry terminal
              support (see Section 3.3.2).  There was concern that since
              EOR had not been defined in RFC-854, a host that was not
              prepared to correctly ignore unknown Telnet commands might
              crash if it received an EOR.  To protect such hosts, the
              End-of-Record option [TELNET:9] was introduced; however, a
              properly implemented Telnet program will not require this

      3.2.4  Telnet "Synch" Signal: RFC-854, pp. 8-10

         When it receives "urgent" TCP data, a User or Server Telnet
         MUST discard all data except Telnet commands until the DM (and
         end of urgent) is reached.

         When it sends Telnet IP (Interrupt Process), a User Telnet
         SHOULD follow it by the Telnet "Synch" sequence, i.e., send as
         TCP urgent data the sequence "IAC IP IAC DM".  The TCP urgent
         pointer points to the DM octet.

         When it receives a Telnet IP command, a Server Telnet MAY send
         a Telnet "Synch" sequence back to the user, to flush the output
         stream.  The choice ought to be consistent with the way the
         server operating system behaves when a local user interrupts a

         When it receives a Telnet AO command, a Server Telnet MUST send
         a Telnet "Synch" sequence back to the user, to flush the output

         A User Telnet SHOULD have the capability of flushing output
         when it sends a Telnet IP; see also Section 3.4.5.

              There are three possible ways for a User Telnet to flush
              the stream of server output data:

              (1)  Send AO after IP.

                   This will cause the server host to send a "flush-
                   buffered-output" signal to its operating system.
                   However, the AO may not take effect locally, i.e.,
                   stop terminal output at the User Telnet end, until
                   the Server Telnet has received and processed the AO
                   and has sent back a "Synch".

              (2)  Send DO TIMING-MARK [TELNET:7] after IP, and discard
                   all output locally until a WILL/WONT TIMING-MARK is

RFC1123                  REMOTE LOGIN -- TELNET             October 1989

                   received from the Server Telnet.

                   Since the DO TIMING-MARK will be processed after the
                   IP at the server, the reply to it should be in the
                   right place in the output data stream.  However, the
                   TIMING-MARK will not send a "flush buffered output"
                   signal to the server operating system.  Whether or
                   not this is needed is dependent upon the server

              (3)  Do both.

              The best method is not entirely clear, since it must
              accommodate a number of existing server hosts that do not
              follow the Telnet standards in various ways.  The safest
              approach is probably to provide a user-controllable option
              to select (1), (2), or (3).

      3.2.5  NVT Printer and Keyboard: RFC-854, p. 11

         In NVT mode, a Telnet SHOULD NOT send characters with the
         high-order bit 1, and MUST NOT send it as a parity bit.
         Implementations that pass the high-order bit to applications
         SHOULD negotiate binary mode (see Section 3.2.6).

              Implementors should be aware that a strict reading of
              RFC-854 allows a client or server expecting NVT ASCII to
              ignore characters with the high-order bit set.  In
              general, binary mode is expected to be used for
              transmission of an extended (beyond 7-bit) character set
              with Telnet.

              However, there exist applications that really need an 8-
              bit NVT mode, which is currently not defined, and these
              existing applications do set the high-order bit during
              part or all of the life of a Telnet connection.  Note that
              binary mode is not the same as 8-bit NVT mode, since
              binary mode turns off end-of-line processing.  For this
              reason, the requirements on the high-order bit are stated
              as SHOULD, not MUST.

              RFC-854 defines a minimal set of properties of a "network
              virtual terminal" or NVT; this is not meant to preclude
              additional features in a real terminal.  A Telnet
              connection is fully transparent to all 7-bit ASCII
              characters, including arbitrary ASCII control characters.

RFC1123                  REMOTE LOGIN -- TELNET             October 1989

              For example, a terminal might support full-screen commands
              coded as ASCII escape sequences; a Telnet implementation
              would pass these sequences as uninterpreted data.  Thus,
              an NVT should not be conceived as a terminal type of a
              highly-restricted device.

      3.2.6  Telnet Command Structure: RFC-854, p. 13

         Since options may appear at any point in the data stream, a
         Telnet escape character (known as IAC, with the value 255) to
         be sent as data MUST be doubled.

      3.2.7  Telnet Binary Option: RFC-856

         When the Binary option has been successfully negotiated,
         arbitrary 8-bit characters are allowed.  However, the data
         stream MUST still be scanned for IAC characters, any embedded
         Telnet commands MUST be obeyed, and data bytes equal to IAC
         MUST be doubled.  Other character processing (e.g., replacing
         CR by CR NUL or by CR LF) MUST NOT be done.  In particular,
         there is no end-of-line convention (see Section 3.3.1) in
         binary mode.

              The Binary option is normally negotiated in both
              directions, to change the Telnet connection from NVT mode
              to "binary mode".

              The sequence IAC EOR can be used to delimit blocks of data
              within a binary-mode Telnet stream.

      3.2.8  Telnet Terminal-Type Option: RFC-1091

         The Terminal-Type option MUST use the terminal type names
         officially defined in the Assigned Numbers RFC [INTRO:5], when
         they are available for the particular terminal.  However, the
         receiver of a Terminal-Type option MUST accept any name.

              RFC-1091 [TELNET:10] updates an earlier version of the
              Terminal-Type option defined in RFC-930.  The earlier
              version allowed a server host capable of supporting
              multiple terminal types to learn the type of a particular
              client's terminal, assuming that each physical terminal
              had an intrinsic type.  However, today a "terminal" is
              often really a terminal emulator program running in a PC,
              perhaps capable of emulating a range of terminal types.
              Therefore, RFC-1091 extends the specification to allow a

RFC1123                  REMOTE LOGIN -- TELNET             October 1989

              more general terminal-type negotiation between User and
              Server Telnets.


      3.3.1  Telnet End-of-Line Convention

         The Telnet protocol defines the sequence CR LF to mean "end-
         of-line".  For terminal input, this corresponds to a command-
         completion or "end-of-line" key being pressed on a user
         terminal; on an ASCII terminal, this is the CR key, but it may
         also be labelled "Return" or "Enter".

         When a Server Telnet receives the Telnet end-of-line sequence
         CR LF as input from a remote terminal, the effect MUST be the
         same as if the user had pressed the "end-of-line" key on a
         local terminal.  On server hosts that use ASCII, in particular,
         receipt of the Telnet sequence CR LF must cause the same effect
         as a local user pressing the CR key on a local terminal.  Thus,
         CR LF and CR NUL MUST have the same effect on an ASCII server
         host when received as input over a Telnet connection.

         A User Telnet MUST be able to send any of the forms: CR LF, CR
         NUL, and LF.  A User Telnet on an ASCII host SHOULD have a
         user-controllable mode to send either CR LF or CR NUL when the
         user presses the "end-of-line" key, and CR LF SHOULD be the

         The Telnet end-of-line sequence CR LF MUST be used to send
         Telnet data that is not terminal-to-computer (e.g., for Server
         Telnet sending output, or the Telnet protocol incorporated
         another application protocol).

              To allow interoperability between arbitrary Telnet clients
              and servers, the Telnet protocol defined a standard
              representation for a line terminator.  Since the ASCII
              character set includes no explicit end-of-line character,
              systems have chosen various representations, e.g., CR, LF,
              and the sequence CR LF.  The Telnet protocol chose the CR
              LF sequence as the standard for network transmission.

              Unfortunately, the Telnet protocol specification in RFC-
              854 [TELNET:1] has turned out to be somewhat ambiguous on
              what character(s) should be sent from client to server for
              the "end-of-line" key.  The result has been a massive and
              continuing interoperability headache, made worse by
              various faulty implementations of both User and Server

RFC1123                  REMOTE LOGIN -- TELNET             October 1989


              Although the Telnet protocol is based on a perfectly
              symmetric model, in a remote login session the role of the
              user at a terminal differs from the role of the server
              host.  For example, RFC-854 defines the meaning of CR, LF,
              and CR LF as output from the server, but does not specify
              what the User Telnet should send when the user presses the
              "end-of-line" key on the terminal; this turns out to be
              the point at issue.

              When a user presses the "end-of-line" key, some User
              Telnet implementations send CR LF, while others send CR
              NUL (based on a different interpretation of the same
              sentence in RFC-854).  These will be equivalent for a
              correctly-implemented ASCII server host, as discussed
              above.  For other servers, a mode in the User Telnet is

              The existence of User Telnets that send only CR NUL when
              CR is pressed creates a dilemma for non-ASCII hosts: they
              can either treat CR NUL as equivalent to CR LF in input,
              thus precluding the possibility of entering a "bare" CR,
              or else lose complete interworking.

              Suppose a user on host A uses Telnet to log into a server
              host B, and then execute B's User Telnet program to log
              into server host C.  It is desirable for the Server/User
              Telnet combination on B to be as transparent as possible,
              i.e., to appear as if A were connected directly to C.  In
              particular, correct implementation will make B transparent
              to Telnet end-of-line sequences, except that CR LF may be
              translated to CR NUL or vice versa.

              To understand Telnet end-of-line issues, one must have at
              least a general model of the relationship of Telnet to the
              local operating system.  The Server Telnet process is
              typically coupled into the terminal driver software of the
              operating system as a pseudo-terminal.  A Telnet end-of-
              line sequence received by the Server Telnet must have the
              same effect as pressing the end-of-line key on a real
              locally-connected terminal.

              Operating systems that support interactive character-at-
              a-time applications (e.g., editors) typically have two
              internal modes for their terminal I/O: a formatted mode,
              in which local conventions for end-of-line and other

RFC1123                  REMOTE LOGIN -- TELNET             October 1989

              formatting rules have been applied to the data stream, and
              a "raw" mode, in which the application has direct access
              to every character as it was entered.  A Server Telnet
              must be implemented in such a way that these modes have
              the same effect for remote as for local terminals.  For
              example, suppose a CR LF or CR NUL is received by the
              Server Telnet on an ASCII host.  In raw mode, a CR
              character is passed to the application; in formatted mode,
              the local system's end-of-line convention is used.

      3.3.2  Data Entry Terminals

              In addition to the line-oriented and character-oriented
              ASCII terminals for which Telnet was designed, there are
              several families of video display terminals that are
              sometimes known as "data entry terminals" or DETs.  The
              IBM 3270 family is a well-known example.

              Two Internet protocols have been designed to support
              generic DETs: SUPDUP [TELNET:16, TELNET:17], and the DET
              option [TELNET:18, TELNET:19].  The DET option drives a
              data entry terminal over a Telnet connection using (sub-)
              negotiation.  SUPDUP is a completely separate terminal
              protocol, which can be entered from Telnet by negotiation.
              Although both SUPDUP and the DET option have been used
              successfully in particular environments, neither has
              gained general acceptance or wide implementation.

              A different approach to DET interaction has been developed
              for supporting the IBM 3270 family through Telnet,
              although the same approach would be applicable to any DET.
              The idea is to enter a "native DET" mode, in which the
              native DET input/output stream is sent as binary data.
              The Telnet EOR command is used to delimit logical records
              (e.g., "screens") within this binary stream.

              The rules for entering and leaving native DET mode are as

              o    The Server uses the Terminal-Type option [TELNET:10]
                   to learn that the client is a DET.

              o    It is conventional, but not required, that both ends
                   negotiate the EOR option [TELNET:9].

              o    Both ends negotiate the Binary option [TELNET:3] to

RFC1123                  REMOTE LOGIN -- TELNET             October 1989

                   enter native DET mode.

              o    When either end negotiates out of binary mode, the
                   other end does too, and the mode then reverts to
                   normal NVT.

      3.3.3  Option Requirements

         Every Telnet implementation MUST support the Binary option
         [TELNET:3] and the Suppress Go Ahead option [TELNET:5], and
         SHOULD support the Echo [TELNET:4], Status [TELNET:6], End-of-
         Record [TELNET:9], and Extended Options List [TELNET:8]

         A User or Server Telnet SHOULD support the Window Size Option
         [TELNET:12] if the local operating system provides the
         corresponding capability.

              Note that the End-of-Record option only signifies that a
              Telnet can receive a Telnet EOR without crashing;
              therefore, every Telnet ought to be willing to accept
              negotiation of the End-of-Record option.  See also the
              discussion in Section 3.2.3.

      3.3.4  Option Initiation

         When the Telnet protocol is used in a client/server situation,
         the server SHOULD initiate negotiation of the terminal
         interaction mode it expects.

              The Telnet protocol was defined to be perfectly
              symmetrical, but its application is generally asymmetric.
              Remote login has been known to fail because NEITHER side
              initiated negotiation of the required non-default terminal
              modes.  It is generally the server that determines the
              preferred mode, so the server needs to initiate the
              negotiation; since the negotiation is symmetric, the user
              can also initiate it.

         A client (User Telnet) SHOULD provide a means for users to
         enable and disable the initiation of option negotiation.

              A user sometimes needs to connect to an application
              service (e.g., FTP or SMTP) that uses Telnet for its

RFC1123                  REMOTE LOGIN -- TELNET             October 1989

              control stream but does not support Telnet options.  User
              Telnet may be used for this purpose if initiation of
              option negotiation is  disabled.

      3.3.5  Telnet Linemode Option

              An important new Telnet option, LINEMODE [TELNET:12], has
              been proposed.  The LINEMODE option provides a standard
              way for a User Telnet and a Server Telnet to agree that
              the client rather than the server will perform terminal
              character processing.  When the client has prepared a
              complete line of text, it will send it to the server in
              (usually) one TCP packet.  This option will greatly
              decrease the packet cost of Telnet sessions and will also
              give much better user response over congested or long-
              delay networks.

              The LINEMODE option allows dynamic switching between local
              and remote character processing.  For example, the Telnet
              connection will automatically negotiate into single-
              character mode while a full screen editor is running, and
              then return to linemode when the editor is finished.

              We expect that when this RFC is released, hosts should
              implement the client side of this option, and may
              implement the server side of this option.  To properly
              implement the server side, the server needs to be able to
              tell the local system not to do any input character
              processing, but to remember its current terminal state and
              notify the Server Telnet process whenever the state
              changes.  This will allow password echoing and full screen
              editors to be handled properly, for example.


      3.4.1  Character Set Transparency

         User Telnet implementations SHOULD be able to send or receive
         any 7-bit ASCII character.  Where possible, any special
         character interpretations by the user host's operating system
         SHOULD be bypassed so that these characters can conveniently be
         sent and received on the connection.

         Some character value MUST be reserved as "escape to command
         mode"; conventionally, doubling this character allows it to be
         entered as data.  The specific character used SHOULD be user

RFC1123                  REMOTE LOGIN -- TELNET             October 1989

         On binary-mode connections, a User Telnet program MAY provide
         an escape mechanism for entering arbitrary 8-bit values, if the
         host operating system doesn't allow them to be entered directly
         from the keyboard.

              The transparency issues are less pressing on servers, but
              implementors should take care in dealing with issues like:
              masking off parity bits (sent by an older, non-conforming
              client) before they reach programs that expect only NVT
              ASCII, and properly handling programs that request 8-bit
              data streams.

      3.4.2  Telnet Commands

         A User Telnet program MUST provide a user the capability of
         entering any of the Telnet control functions IP, AO, or AYT,
         and SHOULD provide the capability of entering EC, EL, and

      3.4.3  TCP Connection Errors

         A User Telnet program SHOULD report to the user any TCP errors
         that are reported by the transport layer (see "TCP/Application
         Layer Interface" section in [INTRO:1]).

      3.4.4  Non-Default Telnet Contact Port

         A User Telnet program SHOULD allow the user to optionally
         specify a non-standard contact port number at the Server Telnet

      3.4.5  Flushing Output

         A User Telnet program SHOULD provide the user the ability to
         specify whether or not output should be flushed when an IP is
         sent; see Section 3.2.4.

         For any output flushing scheme that causes the User Telnet to
         flush output locally until a Telnet signal is received from the
         Server, there SHOULD be a way for the user to manually restore
         normal output, in case the Server fails to send the expected

RFC1123                  REMOTE LOGIN -- TELNET             October 1989


                                                 |        | | | |S| |
                                                 |        | | | |H| |F
                                                 |        | | | |O|M|o
                                                 |        | |S| |U|U|o
                                                 |        | |H| |L|S|t
                                                 |        |M|O| |D|T|n
                                                 |        |U|U|M| | |o
                                                 |        |S|L|A|N|N|t
                                                 |        |T|D|Y|O|O|t
FEATURE                                          |SECTION | | | |T|T|e
                                                 |        | | | | | |
Option Negotiation                               |3.2.1   |x| | | | |
  Avoid negotiation loops                        |3.2.1   |x| | | | |
  Refuse unsupported options                     |3.2.1   |x| | | | |
  Negotiation OK anytime on connection           |3.2.1   | |x| | | |
  Default to NVT                                 |3.2.1   |x| | | | |
  Send official name in Term-Type option         |3.2.8   |x| | | | |
  Accept any name in Term-Type option            |3.2.8   |x| | | | |
  Implement Binary, Suppress-GA options          |3.3.3   |x| | | | |
  Echo, Status, EOL, Ext-Opt-List options        |3.3.3   | |x| | | |
  Implement Window-Size option if appropriate    |3.3.3   | |x| | | |
  Server initiate mode negotiations              |3.3.4   | |x| | | |
  User can enable/disable init negotiations      |3.3.4   | |x| | | |
                                                 |        | | | | | |
Go-Aheads                                        |        | | | | | |
  Non-GA server negotiate SUPPRESS-GA option     |3.2.2   |x| | | | |
  User or Server accept SUPPRESS-GA option       |3.2.2   |x| | | | |
  User Telnet ignore GA's                        |3.2.2   | | |x| | |
                                                 |        | | | | | |
Control Functions                                |        | | | | | |
  Support SE NOP DM IP AO AYT SB                 |3.2.3   |x| | | | |
  Support EOR EC EL Break                        |3.2.3   | | |x| | |
  Ignore unsupported control functions           |3.2.3   |x| | | | |
  User, Server discard urgent data up to DM      |3.2.4   |x| | | | |
  User Telnet send "Synch" after IP, AO, AYT     |3.2.4   | |x| | | |
  Server Telnet reply Synch to IP                |3.2.4   | | |x| | |
  Server Telnet reply Synch to AO                |3.2.4   |x| | | | |
  User Telnet can flush output when send IP      |3.2.4   | |x| | | |
                                                 |        | | | | | |
Encoding                                         |        | | | | | |
  Send high-order bit in NVT mode                |3.2.5   | | | |x| |
  Send high-order bit as parity bit              |3.2.5   | | | | |x|
  Negot. BINARY if pass high-ord. bit to applic  |3.2.5   | |x| | | |
  Always double IAC data byte                    |3.2.6   |x| | | | |

RFC1123                  REMOTE LOGIN -- TELNET             October 1989

  Double IAC data byte in binary mode            |3.2.7   |x| | | | |
  Obey Telnet cmds in binary mode                |3.2.7   |x| | | | |
  End-of-line, CR NUL in binary mode             |3.2.7   | | | | |x|
                                                 |        | | | | | |
End-of-Line                                      |        | | | | | |
  EOL at Server same as local end-of-line        |3.3.1   |x| | | | |
  ASCII Server accept CR LF or CR NUL for EOL    |3.3.1   |x| | | | |
  User Telnet able to send CR LF, CR NUL, or LF  |3.3.1   |x| | | | |
    ASCII user able to select CR LF/CR NUL       |3.3.1   | |x| | | |
    User Telnet default mode is CR LF            |3.3.1   | |x| | | |
  Non-interactive uses CR LF for EOL             |3.3.1   |x| | | | |
                                                 |        | | | | | |
User Telnet interface                            |        | | | | | |
  Input & output all 7-bit characters            |3.4.1   | |x| | | |
  Bypass local op sys interpretation             |3.4.1   | |x| | | |
  Escape character                               |3.4.1   |x| | | | |
     User-settable escape character              |3.4.1   | |x| | | |
  Escape to enter 8-bit values                   |3.4.1   | | |x| | |
  Can input IP, AO, AYT                          |3.4.2   |x| | | | |
  Can input EC, EL, Break                        |3.4.2   | |x| | | |
  Report TCP connection errors to user           |3.4.3   | |x| | | |
  Optional non-default contact port              |3.4.4   | |x| | | |
  Can spec: output flushed when IP sent          |3.4.5   | |x| | | |
  Can manually restore output mode               |3.4.5   | |x| | | |
                                                 |        | | | | | |

RFC1123                   FILE TRANSFER -- FTP              October 1989



      4.1.1  INTRODUCTION

         The File Transfer Protocol FTP is the primary Internet standard
         for file transfer.  The current specification is contained in
         RFC-959 [FTP:1].

         FTP uses separate simultaneous TCP connections for control and
         for data transfer.  The FTP protocol includes many features,
         some of which are not commonly implemented.  However, for every
         feature in FTP, there exists at least one implementation.  The
         minimum implementation defined in RFC-959 was too small, so a
         somewhat larger minimum implementation is defined here.

         Internet users have been unnecessarily burdened for years by
         deficient FTP implementations.  Protocol implementors have
         suffered from the erroneous opinion that implementing FTP ought
         to be a small and trivial task.  This is wrong, because FTP has
         a user interface, because it has to deal (correctly) with the
         whole variety of communication and operating system errors that
         may occur, and because it has to handle the great diversity of
         real file systems in the world.

  LOCAL Type: RFC-959 Section

            An FTP program MUST support TYPE I ("IMAGE" or binary type)
            as well as TYPE L 8 ("LOCAL" type with logical byte size 8).
            A machine whose memory is organized into m-bit words, where
            m is not a multiple of 8, MAY also support TYPE L m.

                 The command "TYPE L 8" is often required to transfer
                 binary data between a machine whose memory is organized
                 into (e.g.) 36-bit words and a machine with an 8-bit
                 byte organization.  For an 8-bit byte machine, TYPE L 8
                 is equivalent to IMAGE.

                 "TYPE L m" is sometimes specified to the FTP programs
                 on two m-bit word machines to ensure the correct
                 transfer of a native-mode binary file from one machine
                 to the other.  However, this command should have the
                 same effect on these machines as "TYPE I".

RFC1123                   FILE TRANSFER -- FTP              October 1989

  Telnet Format Control: RFC-959 Section

            A host that makes no distinction between TYPE N and TYPE T
            SHOULD implement TYPE T to be identical to TYPE N.

                 This provision should ease interoperation with hosts
                 that do make this distinction.

                 Many hosts represent text files internally as strings
                 of ASCII characters, using the embedded ASCII format
                 effector characters (LF, BS, FF, ...) to control the
                 format when a file is printed.  For such hosts, there
                 is no distinction between "print" files and other
                 files.  However, systems that use record structured
                 files typically need a special format for printable
                 files (e.g., ASA carriage control).   For the latter
                 hosts, FTP allows a choice of TYPE N or TYPE T.
  Page Structure: RFC-959 Section and Appendix I

            Implementation of page structure is NOT RECOMMENDED in
            general. However, if a host system does need to implement
            FTP for "random access" or "holey" files, it MUST use the
            defined page structure format rather than define a new
            private FTP format.
  Data Structure Transformations: RFC-959 Section 3.1.2

            An FTP transformation between record-structure and file-
            structure SHOULD be invertible, to the extent possible while
            making the result useful on the target host.

                 RFC-959 required strict invertibility between record-
                 structure and file-structure, but in practice,
                 efficiency and convenience often preclude it.
                 Therefore, the requirement is being relaxed.  There are
                 two different objectives for transferring a file:
                 processing it on the target host, or just storage.  For
                 storage, strict invertibility is important.  For
                 processing, the file created on the target host needs
                 to be in the format expected by application programs on
                 that host.

                 As an example of the conflict, imagine a record-
                 oriented operating system that requires some data files
                 to have exactly 80 bytes in each record.  While STORing

RFC1123                   FILE TRANSFER -- FTP              October 1989

                 a file on such a host, an FTP Server must be able to
                 pad each line or record to 80 bytes; a later retrieval
                 of such a file cannot be strictly invertible.
  Data Connection Management: RFC-959 Section 3.3

            A User-FTP that uses STREAM mode SHOULD send a PORT command
            to assign a non-default data port before each transfer
            command is issued.

                 This is required because of the long delay after a TCP
                 connection is closed until its socket pair can be
                 reused, to allow multiple transfers during a single FTP
                 session.  Sending a port command can avoided if a
                 transfer mode other than stream is used, by leaving the
                 data transfer connection open between transfers.
  PASV Command: RFC-959 Section 4.1.2

            A server-FTP MUST implement the PASV command.

            If multiple third-party transfers are to be executed during
            the same session, a new PASV command MUST be issued before
            each transfer command, to obtain a unique port pair.

                 The format of the 227 reply to a PASV command is not
                 well standardized.  In particular, an FTP client cannot
                 assume that the parentheses shown on page 40 of RFC-959
                 will be present (and in fact, Figure 3 on page 43 omits
                 them).  Therefore, a User-FTP program that interprets
                 the PASV reply must scan the reply for the first digit
                 of the host and port numbers.

                 Note that the host number h1,h2,h3,h4 is the IP address
                 of the server host that is sending the reply, and that
                 p1,p2 is a non-default data transfer port that PASV has
  LIST and NLST Commands: RFC-959 Section 4.1.3

            The data returned by an NLST command MUST contain only a
            simple list of legal pathnames, such that the server can use
            them directly as the arguments of subsequent data transfer
            commands for the individual files.

            The data returned by a LIST or NLST command SHOULD use an

RFC1123                   FILE TRANSFER -- FTP              October 1989

            implied TYPE AN, unless the current type is EBCDIC, in which
            case an implied TYPE EN SHOULD be used.

                 Many FTP clients support macro-commands that will get
                 or put files matching a wildcard specification, using
                 NLST to obtain a list of pathnames.  The expansion of
                 "multiple-put" is local to the client, but "multiple-
                 get" requires cooperation by the server.

                 The implied type for LIST and NLST is designed to
                 provide compatibility with existing User-FTPs, and in
                 particular with multiple-get commands.
  SITE Command: RFC-959 Section 4.1.3

            A Server-FTP SHOULD use the SITE command for non-standard
            features, rather than invent new private commands or
            unstandardized extensions to existing commands.
  STOU Command: RFC-959 Section 4.1.3

            The STOU command stores into a uniquely named file.  When it
            receives an STOU command, a Server-FTP MUST return the
            actual file name in the "125 Transfer Starting" or the "150
            Opening Data Connection" message that precedes the transfer
            (the 250 reply code mentioned in RFC-959 is incorrect).  The
            exact format of these messages is hereby defined to be as

                125 FILE: pppp
                150 FILE: pppp

            where pppp represents the unique pathname of the file that
            will be written.
  Telnet End-of-line Code: RFC-959, Page 34

            Implementors MUST NOT assume any correspondence between READ
            boundaries on the control connection and the Telnet EOL
            sequences (CR LF).

                 Thus, a server-FTP (or User-FTP) must continue reading
                 characters from the control connection until a complete
                 Telnet EOL sequence is encountered, before processing
                 the command (or response, respectively).  Conversely, a
                 single READ from the control connection may include

RFC1123                   FILE TRANSFER -- FTP              October 1989

                 more than one FTP command.
  FTP Replies: RFC-959 Section 4.2, Page 35

            A Server-FTP MUST send only correctly formatted replies on
            the control connection.  Note that RFC-959 (unlike earlier
            versions of the FTP spec) contains no provision for a
            "spontaneous" reply message.

            A Server-FTP SHOULD use the reply codes defined in RFC-959
            whenever they apply.  However, a server-FTP MAY use a
            different reply code when needed, as long as the general
            rules of Section 4.2 are followed. When the implementor has
            a choice between a 4xx and 5xx reply code, a Server-FTP
            SHOULD send a 4xx (temporary failure) code when there is any
            reasonable possibility that a failed FTP will succeed a few
            hours later.

            A User-FTP SHOULD generally use only the highest-order digit
            of a 3-digit reply code for making a procedural decision, to
            prevent difficulties when a Server-FTP uses non-standard
            reply codes.

            A User-FTP MUST be able to handle multi-line replies.  If
            the implementation imposes a limit on the number of lines
            and if this limit is exceeded, the User-FTP MUST recover,
            e.g., by ignoring the excess lines until the end of the
            multi-line reply is reached.

            A User-FTP SHOULD NOT interpret a 421 reply code ("Service
            not available, closing control connection") specially, but
            SHOULD detect closing of the control connection by the

                 Server implementations that fail to strictly follow the
                 reply rules often cause FTP user programs to hang.
                 Note that RFC-959 resolved ambiguities in the reply
                 rules found in earlier FTP specifications and must be

                 It is important to choose FTP reply codes that properly
                 distinguish between temporary and permanent failures,
                 to allow the successful use of file transfer client
                 daemons.  These programs depend on the reply codes to
                 decide whether or not to retry a failed transfer; using
                 a permanent failure code (5xx) for a temporary error
                 will cause these programs to give up unnecessarily.

RFC1123                   FILE TRANSFER -- FTP              October 1989

                 When the meaning of a reply matches exactly the text
                 shown in RFC-959, uniformity will be enhanced by using
                 the RFC-959 text verbatim.  However, a Server-FTP
                 implementor is encouraged to choose reply text that
                 conveys specific system-dependent information, when
  Connections: RFC-959 Section 5.2

            The words "and the port used" in the second paragraph of
            this section of RFC-959 are erroneous (historical), and they
            should be ignored.

            On a multihomed server host, the default data transfer port
            (L-1) MUST be associated with the same local IP address as
            the corresponding control connection to port L.

            A user-FTP MUST NOT send any Telnet controls other than
            SYNCH and IP on an FTP control connection. In particular, it
            MUST NOT attempt to negotiate Telnet options on the control
            connection.  However, a server-FTP MUST be capable of
            accepting and refusing Telnet negotiations (i.e., sending

                 Although the RFC says: "Server- and User- processes
                 should follow the conventions for the Telnet
                 protocol...[on the control connection]", it is not the
                 intent that Telnet option negotiation is to be
  Minimum Implementation; RFC-959 Section 5.1

            The following commands and options MUST be supported by
            every server-FTP and user-FTP, except in cases where the
            underlying file system or operating system does not allow or
            support a particular command.

                 Type: ASCII Non-print, IMAGE, LOCAL 8
                 Mode: Stream
                 Structure: File, Record*
                    USER, PASS, ACCT,
                    PORT, PASV,
                    TYPE, MODE, STRU,
                    RETR, STOR, APPE,
                    RNFR, RNTO, DELE,
                    CWD,  CDUP, RMD,  MKD,  PWD,

RFC1123                   FILE TRANSFER -- FTP              October 1989

                    LIST, NLST,
                    SYST, STAT,
                    HELP, NOOP, QUIT.

            *Record structure is REQUIRED only for hosts whose file
            systems support record structure.

                 Vendors are encouraged to implement a larger subset of
                 the protocol.  For example, there are important
                 robustness features in the protocol (e.g., Restart,
                 ABOR, block mode) that would be an aid to some Internet
                 users but are not widely implemented.

                 A host that does not have record structures in its file
                 system may still accept files with STRU R, recording
                 the byte stream literally.

      4.1.3  SPECIFIC ISSUES
  Non-standard Command Verbs

            FTP allows "experimental" commands, whose names begin with
            "X".  If these commands are subsequently adopted as
            standards, there may still be existing implementations using
            the "X" form.  At present, this is true for the directory

                RFC-959   "Experimental"

                  MKD        XMKD
                  RMD        XRMD
                  PWD        XPWD
                  CDUP       XCUP
                  CWD        XCWD

            All FTP implementations SHOULD recognize both forms of these
            commands, by simply equating them with extra entries in the
            command lookup table.

                 A User-FTP can access a server that supports only the
                 "X" forms by implementing a mode switch, or
                 automatically using the following procedure: if the
                 RFC-959 form of one of the above commands is rejected
                 with a 500 or 502 response code, then try the
                 experimental form; any other response would be passed
                 to the user.

RFC1123                   FILE TRANSFER -- FTP              October 1989

  Idle Timeout

            A Server-FTP process SHOULD have an idle timeout, which will
            terminate the process and close the control connection if
            the server is inactive (i.e., no command or data transfer in
            progress) for a long period of time.  The idle timeout time
            SHOULD be configurable, and the default should be at least 5

            A client FTP process ("User-PI" in RFC-959) will need
            timeouts on responses only if it is invoked from a program.

                 Without a timeout, a Server-FTP process may be left
                 pending indefinitely if the corresponding client
                 crashes without closing the control connection.
  Concurrency of Data and Control

                 The intent of the designers of FTP was that a user
                 should be able to send a STAT command at any time while
                 data transfer was in progress and that the server-FTP
                 would reply immediately with status -- e.g., the number
                 of bytes transferred so far.  Similarly, an ABOR
                 command should be possible at any time during a data

                 Unfortunately, some small-machine operating systems
                 make such concurrent programming difficult, and some
                 other implementers seek minimal solutions, so some FTP
                 implementations do not allow concurrent use of the data
                 and control connections.  Even such a minimal server
                 must be prepared to accept and defer a STAT or ABOR
                 command that arrives during data transfer.
  FTP Restart Mechanism

            The description of the 110 reply on pp. 40-41 of RFC-959 is
            incorrect; the correct description is as follows.  A restart
            reply message, sent over the control connection from the
            receiving FTP to the User-FTP, has the format:

                110 MARK ssss = rrrr


            *    ssss is a text string that appeared in a Restart Marker

RFC1123                   FILE TRANSFER -- FTP              October 1989

                 in the data stream and encodes a position in the
                 sender's file system;

            *    rrrr encodes the corresponding position in the
                 receiver's file system.

            The encoding, which is specific to a particular file system
            and network implementation, is always generated and
            interpreted by the same system, either sender or receiver.

            When an FTP that implements restart receives a Restart
            Marker in the data stream, it SHOULD force the data to that
            point to be written to stable storage before encoding the
            corresponding position rrrr.  An FTP sending Restart Markers
            MUST NOT assume that 110 replies will be returned
            synchronously with the data, i.e., it must not await a 110
            reply before sending more data.

            Two new reply codes are hereby defined for errors
            encountered in restarting a transfer:

              554 Requested action not taken: invalid REST parameter.

                 A 554 reply may result from a FTP service command that
                 follows a REST command.  The reply indicates that the
                 existing file at the Server-FTP cannot be repositioned
                 as specified in the REST.

              555 Requested action not taken: type or stru mismatch.

                 A 555 reply may result from an APPE command or from any
                 FTP service command following a REST command.  The
                 reply indicates that there is some mismatch between the
                 current transfer parameters (type and stru) and the
                 attributes of the existing file.

                 Note that the FTP Restart mechanism requires that Block
                 or Compressed mode be used for data transfer, to allow
                 the Restart Markers to be included within the data
                 stream.  The frequency of Restart Markers can be low.

                 Restart Markers mark a place in the data stream, but
                 the receiver may be performing some transformation on
                 the data as it is stored into stable storage.  In
                 general, the receiver's encoding must include any state
                 information necessary to restart this transformation at
                 any point of the FTP data stream.  For example, in TYPE

RFC1123                   FILE TRANSFER -- FTP              October 1989

                 A transfers, some receiver hosts transform CR LF
                 sequences into a single LF character on disk.   If a
                 Restart Marker happens to fall between CR and LF, the
                 receiver must encode in rrrr that the transfer must be
                 restarted in a "CR has been seen and discarded" state.

                 Note that the Restart Marker is required to be encoded
                 as a string of printable ASCII characters, regardless
                 of the type of the data.

                 RFC-959 says that restart information is to be returned
                 "to the user".  This should not be taken literally.  In
                 general, the User-FTP should save the restart
                 information (ssss,rrrr) in stable storage, e.g., append
                 it to a restart control file.  An empty restart control
                 file should be created when the transfer first starts
                 and deleted automatically when the transfer completes
                 successfully.  It is suggested that this file have a
                 name derived in an easily-identifiable manner from the
                 name of the file being transferred and the remote host
                 name; this is analogous to the means used by many text
                 editors for naming "backup" files.

                 There are three cases for FTP restart.

                 (1)  User-to-Server Transfer

                      The User-FTP puts Restart Markers <ssss> at
                      convenient places in the data stream.  When the
                      Server-FTP receives a Marker, it writes all prior
                      data to disk, encodes its file system position and
                      transformation state as rrrr, and returns a "110
                      MARK ssss = rrrr" reply over the control
                      connection.  The User-FTP appends the pair
                      (ssss,rrrr) to its restart control file.

                      To restart the transfer, the User-FTP fetches the
                      last (ssss,rrrr) pair from the restart control
                      file, repositions its local file system and
                      transformation state using ssss, and sends the
                      command "REST rrrr" to the Server-FTP.

                 (2)  Server-to-User Transfer

                      The Server-FTP puts Restart Markers <ssss> at
                      convenient places in the data stream.  When the
                      User-FTP receives a Marker, it writes all prior
                      data to disk, encodes its file system position and

RFC1123                   FILE TRANSFER -- FTP              October 1989

                      transformation state as rrrr, and appends the pair
                      (rrrr,ssss) to its restart control file.

                      To restart the transfer, the User-FTP fetches the
                      last (rrrr,ssss) pair from the restart control
                      file, repositions its local file system and
                      transformation state using rrrr, and sends the
                      command "REST ssss" to the Server-FTP.

                 (3)  Server-to-Server ("Third-Party") Transfer

                      The sending Server-FTP puts Restart Markers <ssss>
                      at convenient places in the data stream.  When it
                      receives a Marker, the receiving Server-FTP writes
                      all prior data to disk, encodes its file system
                      position and transformation state as rrrr, and
                      sends a "110 MARK ssss = rrrr" reply over the
                      control connection to the User.  The User-FTP
                      appends the pair (ssss,rrrr) to its restart
                      control file.

                      To restart the transfer, the User-FTP fetches the
                      last (ssss,rrrr) pair from the restart control
                      file, sends "REST ssss" to the sending Server-FTP,
                      and sends "REST rrrr" to the receiving Server-FTP.


         This section discusses the user interface for a User-FTP
  Pathname Specification

            Since FTP is intended for use in a heterogeneous
            environment, User-FTP implementations MUST support remote
            pathnames as arbitrary character strings, so that their form
            and content are not limited by the conventions of the local
            operating system.

                 In particular, remote pathnames can be of arbitrary
                 length, and all the printing ASCII characters as well
                 as space (0x20) must be allowed.  RFC-959 allows a
                 pathname to contain any 7-bit ASCII character except CR
                 or LF.

RFC1123                   FILE TRANSFER -- FTP              October 1989

  "QUOTE" Command

            A User-FTP program MUST implement a "QUOTE" command that
            will pass an arbitrary character string to the server and
            display all resulting response messages to the user.

            To make the "QUOTE" command useful, a User-FTP SHOULD send
            transfer control commands to the server as the user enters
            them, rather than saving all the commands and sending them
            to the server only when a data transfer is started.

                 The "QUOTE" command is essential to allow the user to
                 access servers that require system-specific commands
                 (e.g., SITE or ALLO), or to invoke new or optional
                 features that are not implemented by the User-FTP.  For
                 example, "QUOTE" may be used to specify "TYPE A T" to
                 send a print file to hosts that require the
                 distinction, even if the User-FTP does not recognize
                 that TYPE.
  Displaying Replies to User

            A User-FTP SHOULD display to the user the full text of all
            error reply messages it receives.  It SHOULD have a
            "verbose" mode in which all commands it sends and the full
            text and reply codes it receives are displayed, for
            diagnosis of problems.
  Maintaining Synchronization

            The state machine in a User-FTP SHOULD be forgiving of
            missing and unexpected reply messages, in order to maintain
            command synchronization with the server.

RFC1123                   FILE TRANSFER -- FTP              October 1989


                                           |               | | | |S| |
                                           |               | | | |H| |F
                                           |               | | | |O|M|o
                                           |               | |S| |U|U|o
                                           |               | |H| |L|S|t
                                           |               |M|O| |D|T|n
                                           |               |U|U|M| | |o
                                           |               |S|L|A|N|N|t
                                           |               |T|D|Y|O|O|t
FEATURE                                    |SECTION        | | | |T|T|e
Implement TYPE T if same as TYPE N         |        | |x| | | |
File/Record transform invertible if poss.  |        | |x| | | |
User-FTP send PORT cmd for stream mode     |        | |x| | | |
Server-FTP implement PASV                  |        |x| | | | |
  PASV is per-transfer                     |        |x| | | | |
NLST reply usable in RETR cmds             |        |x| | | | |
Implied type for LIST and NLST             |        | |x| | | |
SITE cmd for non-standard features         |        | |x| | | |
STOU cmd return pathname as specified      |        |x| | | | |
Use TCP READ boundaries on control conn.   |       | | | | |x|
                                           |               | | | | | |
Server-FTP send only correct reply format  |       |x| | | | |
Server-FTP use defined reply code if poss. |       | |x| | | |
  New reply code following Section 4.2     |       | | |x| | |
User-FTP use only high digit of reply      |       | |x| | | |
User-FTP handle multi-line reply lines     |       |x| | | | |
User-FTP handle 421 reply specially        |       | | | |x| |
                                           |               | | | | | |
Default data port same IP addr as ctl conn |       |x| | | | |
User-FTP send Telnet cmds exc. SYNCH, IP   |       | | | | |x|
User-FTP negotiate Telnet options          |       | | | | |x|
Server-FTP handle Telnet options           |       |x| | | | |
Handle "Experimental" directory cmds       |        | |x| | | |
Idle timeout in server-FTP                 |        | |x| | | |
    Configurable idle timeout              |        | |x| | | |
Receiver checkpoint data at Restart Marker |        | |x| | | |
Sender assume 110 replies are synchronous  |        | | | | |x|
                                           |               | | | | | |
Support TYPE:                              |               | | | | | |
  ASCII - Non-Print (AN)                   |       |x| | | | |
  ASCII - Telnet (AT) -- if same as AN     |        | |x| | | |
  ASCII - Carriage Control (AC)            |959  | | |x| | |
  EBCDIC - (any form)                      |959    | | |x| | |
  IMAGE                                    |        |x| | | | |
  LOCAL 8                                  |        |x| | | | |

RFC1123                   FILE TRANSFER -- FTP              October 1989

  LOCAL m                                  |        | | |x| | |2
                                           |               | | | | | |
Support MODE:                              |               | | | | | |
  Stream                                   |       |x| | | | |
  Block                                    |959 3.4.2      | | |x| | |
                                           |               | | | | | |
Support STRUCTURE:                         |               | | | | | |
  File                                     |       |x| | | | |
  Record                                   |       |x| | | | |3
  Page                                     |        | | | |x| |
                                           |               | | | | | |
Support commands:                          |               | | | | | |
  USER                                     |       |x| | | | |
  PASS                                     |       |x| | | | |
  ACCT                                     |       |x| | | | |
  CWD                                      |       |x| | | | |
  CDUP                                     |       |x| | | | |
  SMNT                                     |959 5.3.1      | | |x| | |
  REIN                                     |959 5.3.1      | | |x| | |
  QUIT                                     |       |x| | | | |
                                           |               | | | | | |
  PORT                                     |       |x| | | | |
  PASV                                     |        |x| | | | |
  TYPE                                     |       |x| | | | |1
  STRU                                     |       |x| | | | |1
  MODE                                     |       |x| | | | |1
                                           |               | | | | | |
  RETR                                     |       |x| | | | |
  STOR                                     |       |x| | | | |
  STOU                                     |959 5.3.1      | | |x| | |
  APPE                                     |       |x| | | | |
  ALLO                                     |959 5.3.1      | | |x| | |
  REST                                     |959 5.3.1      | | |x| | |
  RNFR                                     |       |x| | | | |
  RNTO                                     |       |x| | | | |
  ABOR                                     |959 5.3.1      | | |x| | |
  DELE                                     |       |x| | | | |
  RMD                                      |       |x| | | | |
  MKD                                      |       |x| | | | |
  PWD                                      |       |x| | | | |
  LIST                                     |       |x| | | | |
  NLST                                     |       |x| | | | |
  SITE                                     |        | | |x| | |
  STAT                                     |       |x| | | | |
  SYST                                     |       |x| | | | |
  HELP                                     |       |x| | | | |
  NOOP                                     |       |x| | | | |
                                           |               | | | | | |

RFC1123                   FILE TRANSFER -- FTP              October 1989

User Interface:                            |               | | | | | |
  Arbitrary pathnames                      |        |x| | | | |
  Implement "QUOTE" command                |        |x| | | | |
  Transfer control commands immediately    |        | |x| | | |
  Display error messages to user           |        | |x| | | |
    Verbose mode                           |        | |x| | | |
  Maintain synchronization with server     |        | |x| | | |


(1)  For the values shown earlier.

(2)  Here m is number of bits in a memory word.

(3)  Required for host with record-structured file system, optional

RFC1123                  FILE TRANSFER -- TFTP              October 1989


      4.2.1  INTRODUCTION

         The Trivial File Transfer Protocol TFTP is defined in RFC-783

         TFTP provides its own reliable delivery with UDP as its
         transport protocol, using a simple stop-and-wait acknowledgment
         system.  Since TFTP has an effective window of only one 512
         octet segment, it can provide good performance only over paths
         that have a small delay*bandwidth product.  The TFTP file
         interface is very simple, providing no access control or

         TFTP's most important application is bootstrapping a host over
         a local network, since it is simple and small enough to be
         easily implemented in EPROM [BOOT:1, BOOT:2].  Vendors are
         urged to support TFTP for booting.


         The TFTP specification [TFTP:1] is written in an open style,
         and does not fully specify many parts of the protocol.
  Transfer Modes: RFC-783, Page 3

            The transfer mode "mail" SHOULD NOT be supported.
  UDP Header: RFC-783, Page 17

            The Length field of a UDP header is incorrectly defined; it
            includes the UDP header length (8).

      4.2.3  SPECIFIC ISSUES
  Sorcerer's Apprentice Syndrome

            There is a serious bug, known as the "Sorcerer's Apprentice
            Syndrome," in the protocol specification.  While it does not
            cause incorrect operation of the transfer (the file will
            always be transferred correctly if the transfer completes),
            this bug may cause excessive retransmission, which may cause
            the transfer to time out.

            Implementations MUST contain the fix for this problem: the
            sender (i.e., the side originating the DATA packets) must
            never resend the current DATA packet on receipt of a

RFC1123                  FILE TRANSFER -- TFTP              October 1989

            duplicate ACK.

                 The bug is caused by the protocol rule that either
                 side, on receiving an old duplicate datagram, may
                 resend the current datagram.  If a packet is delayed in
                 the network but later successfully delivered after
                 either side has timed out and retransmitted a packet, a
                 duplicate copy of the response may be generated.  If
                 the other side responds to this duplicate with a
                 duplicate of its own, then every datagram will be sent
                 in duplicate for the remainder of the transfer (unless
                 a datagram is lost, breaking the repetition).  Worse
                 yet, since the delay is often caused by congestion,
                 this duplicate transmission will usually causes more
                 congestion, leading to more delayed packets, etc.

                 The following example may help to clarify this problem.

                     TFTP A                  TFTP B

                 (1)  Receive ACK X-1
                      Send DATA X
                 (2)                          Receive DATA X
                                              Send ACK X
                        (ACK X is delayed in network,
                         and  A times out):
                 (3)  Retransmit DATA X

                 (4)                          Receive DATA X again
                                              Send ACK X again
                 (5)  Receive (delayed) ACK X
                      Send DATA X+1
                 (6)                          Receive DATA X+1
                                              Send ACK X+1
                 (7)  Receive ACK X again
                      Send DATA X+1 again
                 (8)                          Receive DATA X+1 again
                                              Send ACK X+1 again
                 (9)  Receive ACK X+1
                      Send DATA X+2
                 (10)                         Receive DATA X+2
                                              Send ACK X+3
                 (11) Receive ACK X+1 again
                      Send DATA X+2 again
                 (12)                         Receive DATA X+2 again
                                              Send ACK X+3 again

RFC1123                  FILE TRANSFER -- TFTP              October 1989

                 Notice that once the delayed ACK arrives, the protocol
                 settles down to duplicate all further packets
                 (sequences 5-8 and 9-12).  The problem is caused not by
                 either side timing out, but by both sides
                 retransmitting the current packet when they receive a

                 The fix is to break the retransmission loop, as
                 indicated above.  This is analogous to the behavior of
                 TCP.  It is then possible to remove the retransmission
                 timer on the receiver, since the resent ACK will never
                 cause any action; this is a useful simplification where
                 TFTP is used in a bootstrap program.  It is OK to allow
                 the timer to remain, and it may be helpful if the
                 retransmitted ACK replaces one that was genuinely lost
                 in the network.  The sender still requires a retransmit
                 timer, of course.
  Timeout Algorithms

            A TFTP implementation MUST use an adaptive timeout.

                 TCP retransmission algorithms provide a useful base to
                 work from.  At least an exponential backoff of
                 retransmission timeout is necessary.

            A variety of non-standard extensions have been made to TFTP,
            including additional transfer modes and a secure operation
            mode (with passwords).  None of these have been
  Access Control

            A server TFTP implementation SHOULD include some
            configurable access control over what pathnames are allowed
            in TFTP operations.
  Broadcast Request

            A TFTP request directed to a broadcast address SHOULD be
            silently ignored.

                 Due to the weak access control capability of TFTP,
                 directed broadcasts of TFTP requests to random networks

RFC1123                  FILE TRANSFER -- TFTP              October 1989

                 could create a significant security hole.


                                                 |        | | | |S| |
                                                 |        | | | |H| |F
                                                 |        | | | |O|M|o
                                                 |        | |S| |U|U|o
                                                 |        | |H| |L|S|t
                                                 |        |M|O| |D|T|n
                                                 |        |U|U|M| | |o
                                                 |        |S|L|A|N|N|t
                                                 |        |T|D|Y|O|O|t
FEATURE                                          |SECTION | | | |T|T|e
Fix Sorcerer's Apprentice Syndrome               | |x| | | | |
Transfer modes:                                  |        | | | | | |
  netascii                                       |RFC-783 |x| | | | |
  octet                                          |RFC-783 |x| | | | |
  mail                                           | | | | |x| |
  extensions                                     | | | |x| | |
Use adaptive timeout                             | |x| | | | |
Configurable access control                      | | |x| | | |
Silently ignore broadcast request                | | |x| | | |

RFC1123                  MAIL -- SMTP & RFC-822             October 1989



      In the TCP/IP protocol suite, electronic mail in a format
      specified in RFC-822 [SMTP:2] is transmitted using the Simple Mail
      Transfer Protocol (SMTP) defined in RFC-821 [SMTP:1].

      While SMTP has remained unchanged over the years, the Internet
      community has made several changes in the way SMTP is used.  In
      particular, the conversion to the Domain Name System (DNS) has
      caused changes in address formats and in mail routing.  In this
      section, we assume familiarity with the concepts and terminology
      of the DNS, whose requirements are given in Section 6.1.

      RFC-822 specifies the Internet standard format for electronic mail
      messages.  RFC-822 supercedes an older standard, RFC-733, that may
      still be in use in a few places, although it is obsolete.  The two
      formats are sometimes referred to simply by number ("822" and

      RFC-822 is used in some non-Internet mail environments with
      different mail transfer protocols than SMTP, and SMTP has also
      been adapted for use in some non-Internet environments.  Note that
      this document presents the rules for the use of SMTP and RFC-822
      for the Internet environment only; other mail environments that
      use these protocols may be expected to have their own rules.


      This section covers both RFC-821 and RFC-822.

      The SMTP specification in RFC-821 is clear and contains numerous
      examples, so implementors should not find it difficult to
      understand.  This section simply updates or annotates portions of
      RFC-821 to conform with current usage.

      RFC-822 is a long and dense document, defining a rich syntax.
      Unfortunately, incomplete or defective implementations of RFC-822
      are common.  In fact, nearly all of the many formats of RFC-822
      are actually used, so an implementation generally needs to
      recognize and correctly interpret all of the RFC-822 syntax.

      5.2.1  The SMTP Model: RFC-821 Section 2

              Mail is sent by a series of request/response transactions
              between a client, the "sender-SMTP," and a server, the

RFC1123                  MAIL -- SMTP & RFC-822             October 1989

              "receiver-SMTP".  These transactions pass (1) the message
              proper, which is composed of header and body, and (2) SMTP
              source and destination addresses, referred to as the

              The SMTP programs are analogous to Message Transfer Agents
              (MTAs) of X.400.  There will be another level of protocol
              software, closer to the end user, that is responsible for
              composing and analyzing RFC-822 message headers; this
              component is known as the "User Agent" in X.400, and we
              use that term in this document.  There is a clear logical
              distinction between the User Agent and the SMTP
              implementation, since they operate on different levels of
              protocol.  Note, however, that this distinction is may not
              be exactly reflected the structure of typical
              implementations of Internet mail.  Often there is a
              program known as the "mailer" that implements SMTP and
              also some of the User Agent functions; the rest of the
              User Agent functions are included in a user interface used
              for entering and reading mail.

              The SMTP envelope is constructed at the originating site,
              typically by the User Agent when the message is first
              queued for the Sender-SMTP program.  The envelope
              addresses may be derived from information in the message
              header, supplied by the user interface (e.g., to implement
              a bcc: request), or derived from local configuration
              information (e.g., expansion of a mailing list).  The SMTP
              envelope cannot in general be re-derived from the header
              at a later stage in message delivery, so the envelope is
              transmitted separately from the message itself using the
              MAIL and RCPT commands of SMTP.

              The text of RFC-821 suggests that mail is to be delivered
              to an individual user at a host.  With the advent of the
              domain system and of mail routing using mail-exchange (MX)
              resource records, implementors should now think of
              delivering mail to a user at a domain, which may or may
              not be a particular host.  This DOES NOT change the fact
              that SMTP is a host-to-host mail exchange protocol.

      5.2.2  Canonicalization: RFC-821 Section 3.1

         The domain names that a Sender-SMTP sends in MAIL and RCPT
         commands MUST have been  "canonicalized," i.e., they must be
         fully-qualified principal names or domain literals, not
         nicknames or domain abbreviations.  A canonicalized name either
         identifies a host directly or is an MX name; it cannot be a

RFC1123                  MAIL -- SMTP & RFC-822             October 1989


      5.2.3  VRFY and EXPN Commands: RFC-821 Section 3.3

         A receiver-SMTP MUST implement VRFY and SHOULD implement EXPN
         (this requirement overrides RFC-821).  However, there MAY be
         configuration information to disable VRFY and EXPN in a
         particular installation; this might even allow EXPN to be
         disabled for selected lists.

         A new reply code is defined for the VRFY command:

              252 Cannot VRFY user (e.g., info is not local), but will
                  take message for this user and attempt delivery.

              SMTP users and administrators make regular use of these
              commands for diagnosing mail delivery problems.  With the
              increasing use of multi-level mailing list expansion
              (sometimes more than two levels), EXPN has been
              increasingly important for diagnosing inadvertent mail
              loops.  On the other hand,  some feel that EXPN represents
              a significant privacy, and perhaps even a security,

      5.2.4  SEND, SOML, and SAML Commands: RFC-821 Section 3.4

         An SMTP MAY implement the commands to send a message to a
         user's terminal: SEND, SOML, and SAML.

              It has been suggested that the use of mail relaying
              through an MX record is inconsistent with the intent of
              SEND to deliver a message immediately and directly to a
              user's terminal.  However, an SMTP receiver that is unable
              to write directly to the user terminal can return a "251
              User Not Local" reply to the RCPT following a SEND, to
              inform the originator of possibly deferred delivery.

      5.2.5  HELO Command: RFC-821 Section 3.5

         The sender-SMTP MUST ensure that the <domain> parameter in a
         HELO command is a valid principal host domain name for the
         client host.  As a result, the receiver-SMTP will not have to
         perform MX resolution on this name in order to validate the
         HELO parameter.

         The HELO receiver MAY verify that the HELO parameter really

RFC1123                  MAIL -- SMTP & RFC-822             October 1989

         corresponds to the IP address of the sender.  However, the
         receiver MUST NOT refuse to accept a message, even if the
         sender's HELO command fails verification.

              Verifying the HELO parameter requires a domain name lookup
              and may therefore take considerable time.  An alternative
              tool for tracking bogus mail sources is suggested below
              (see "DATA Command").

              Note also that the HELO argument is still required to have
              valid <domain> syntax, since it will appear in a Received:
              line; otherwise, a 501 error is to be sent.

              When HELO parameter validation fails, a suggested
              procedure is to insert a note about the unknown
              authenticity of the sender into the message header (e.g.,
              in the "Received:"  line).

      5.2.6  Mail Relay: RFC-821 Section 3.6

         We distinguish three types of mail (store-and-) forwarding:

         (1)  A simple forwarder or "mail exchanger" forwards a message
              using private knowledge about the recipient; see section
              3.2 of RFC-821.

         (2)  An SMTP mail "relay" forwards a message within an SMTP
              mail environment as the result of an explicit source route
              (as defined in section 3.6 of RFC-821).  The SMTP relay
              function uses the "@...:" form of source route from RFC-
              822 (see Section 5.2.19 below).

         (3)  A mail "gateway" passes a message between different
              environments.  The rules for mail gateways are discussed
              below in Section 5.3.7.

         An Internet host that is forwarding a message but is not a
         gateway to a different mail environment (i.e., it falls under
         (1) or (2)) SHOULD NOT alter any existing header fields,
         although the host will add an appropriate Received: line as
         required in Section 5.2.8.

         A Sender-SMTP SHOULD NOT send a RCPT TO: command containing an
         explicit source route using the "@...:" address form.  Thus,
         the relay function defined in section  3.6 of RFC-821 should
         not be used.

RFC1123                  MAIL -- SMTP & RFC-822             October 1989

              The intent is to discourage all source routing and to
              abolish explicit source routing for mail delivery within
              the Internet environment.  Source-routing is unnecessary;
              the simple target address "user@domain" should always
              suffice.  This is the result of an explicit architectural
              decision to use universal naming rather than source
              routing for mail.  Thus, SMTP provides end-to-end
              connectivity, and the DNS provides globally-unique,
              location-independent names.  MX records handle the major
              case where source routing might otherwise be needed.

         A receiver-SMTP MUST accept the explicit source route syntax in
         the envelope, but it MAY implement the relay function as
         defined in section 3.6 of RFC-821.  If it does not implement
         the relay function, it SHOULD attempt to deliver the message
         directly to the host to the right of the right-most "@" sign.

              For example, suppose a host that does not implement the
              relay function receives a message with the SMTP command:
              "RCPT TO:<@ALPHA,@BETA:joe@GAMMA>", where ALPHA, BETA, and
              GAMMA represent domain names.  Rather than immediately
              refusing the message with a 550 error reply as suggested
              on page 20 of RFC-821, the host should try to forward the
              message to GAMMA directly, using: "RCPT TO:<joe@GAMMA>".
              Since this host does not support relaying, it is not
              required to update the reverse path.

              Some have suggested that source routing may be needed
              occasionally for manually routing mail around failures;
              however, the reality and importance of this need is
              controversial.  The use of explicit SMTP mail relaying for
              this purpose is discouraged, and in fact it may not be
              successful, as many host systems do not support it.  Some
              have used the "%-hack" (see Section 5.2.16) for this

      5.2.7  RCPT Command: RFC-821 Section 4.1.1

         A host that supports a receiver-SMTP MUST support the reserved
         mailbox "Postmaster".

         The receiver-SMTP MAY verify RCPT parameters as they arrive;
         however, RCPT responses MUST NOT be delayed beyond a reasonable
         time (see Section 5.3.2).

         Therefore, a "250 OK" response to a RCPT does not necessarily

RFC1123                  MAIL -- SMTP & RFC-822             October 1989

         imply that the delivery address(es) are valid.  Errors found
         after message acceptance will be reported by mailing a
         notification message to an appropriate address (see Section

              The set of conditions under which a RCPT parameter can be
              validated immediately is an engineering design choice.
              Reporting destination mailbox errors to the Sender-SMTP
              before mail is transferred is generally desirable to save
              time and network bandwidth, but this advantage is lost if
              RCPT verification is lengthy.

              For example, the receiver can verify immediately any
              simple local reference, such as a single locally-
              registered mailbox.  On the other hand, the "reasonable
              time" limitation generally implies deferring verification
              of a mailing list until after the message has been
              transferred and accepted, since verifying a large mailing
              list can take a very long time.  An implementation might
              or might not choose to defer validation of addresses that
              are non-local and therefore require a DNS lookup.  If a
              DNS lookup is performed but a soft domain system error
              (e.g., timeout) occurs, validity must be assumed.

      5.2.8  DATA Command: RFC-821 Section 4.1.1

         Every receiver-SMTP (not just one that "accepts a message for
         relaying or for final delivery" [SMTP:1]) MUST insert a
         "Received:" line at the beginning of a message.  In this line,
         called a "time stamp line" in RFC-821:

         *    The FROM field SHOULD contain both (1) the name of the
              source host as presented in the HELO command and (2) a
              domain literal containing the IP address of the source,
              determined from the TCP connection.

         *    The ID field MAY contain an "@" as suggested in RFC-822,
              but this is not required.

         *    The FOR field MAY contain a list of <path> entries when
              multiple RCPT commands have been given.

         An Internet mail program MUST NOT change a Received: line that
         was previously added to the message header.

RFC1123                  MAIL -- SMTP & RFC-822             October 1989

              Including both the source host and the IP source address
              in the Received: line may provide enough information for
              tracking illicit mail sources and eliminate a need to
              explicitly verify the HELO parameter.

              Received: lines are primarily intended for humans tracing
              mail routes, primarily of diagnosis of faults.  See also
              the discussion under 5.3.7.

         When the receiver-SMTP makes "final delivery" of a message,
         then it MUST pass the MAIL FROM: address from the SMTP envelope
         with the message, for use if an error notification message must
         be sent later (see Section 5.3.3).  There is an analogous
         requirement when gatewaying from the Internet into a different
         mail environment; see Section 5.3.7.

              Note that the final reply to the DATA command depends only
              upon the successful transfer and storage of the message.
              Any problem with the destination address(es) must either
              (1) have been reported in an SMTP error reply to the RCPT
              command(s), or (2) be reported in a later error message
              mailed to the originator.

              The MAIL FROM: information may be passed as a parameter or
              in a Return-Path: line inserted at the beginning of the

      5.2.9  Command Syntax: RFC-821 Section 4.1.2

         The syntax shown in RFC-821 for the MAIL FROM: command omits
         the case of an empty path:  "MAIL FROM: <>" (see RFC-821 Page
         15).  An empty reverse path MUST be supported.

      5.2.10  SMTP Replies:  RFC-821 Section 4.2

         A receiver-SMTP SHOULD send only the reply codes listed in
         section 4.2.2 of RFC-821 or in this document.  A receiver-SMTP
         SHOULD use the text shown in examples in RFC-821 whenever

         A sender-SMTP MUST determine its actions only by the reply
         code, not by the text (except for 251 and 551 replies); any
         text, including no text at all, must be acceptable.  The space
         (blank) following the reply code is considered part of the
         text.  Whenever possible, a sender-SMTP SHOULD test only the

RFC1123                  MAIL -- SMTP & RFC-822             October 1989

         first digit of the reply code, as specified in Appendix E of

              Interoperability problems have arisen with SMTP systems
              using reply codes that are not listed explicitly in RFC-
              821 Section 4.3 but are legal according to the theory of
              reply codes explained in Appendix E.

      5.2.11  Transparency: RFC-821 Section 4.5.2

         Implementors MUST be sure that their mail systems always add
         and delete periods to ensure message transparency.

      5.2.12  WKS Use in MX Processing: RFC-974, p. 5

         RFC-974 [SMTP:3] recommended that the domain system be queried
         for WKS ("Well-Known Service") records, to verify that each
         proposed mail target does support SMTP.  Later experience has
         shown that WKS is not widely supported, so the WKS step in MX
         processing SHOULD NOT be used.

      The following are notes on RFC-822, organized by section of that

      5.2.13  RFC-822 Message Specification: RFC-822 Section 4

         The syntax shown for the Return-path line omits the possibility
         of a null return path, which is used to prevent looping of
         error notifications (see Section 5.3.3).  The complete syntax

             return = "Return-path"  ":" route-addr
                    / "Return-path"  ":" "<" ">"

         The set of optional header fields is hereby expanded to include
         the Content-Type field defined in RFC-1049 [SMTP:7].  This
         field "allows mail reading systems to automatically identify
         the type of a structured message body and to process it for
         display accordingly".  [SMTP:7]  A User Agent MAY support this

      5.2.14  RFC-822 Date and Time Specification: RFC-822 Section 5

         The syntax for the date is hereby changed to:

            date = 1*2DIGIT month 2*4DIGIT

RFC1123                  MAIL -- SMTP & RFC-822             October 1989

         All mail software SHOULD use 4-digit years in dates, to ease
         the transition to the next century.

         There is a strong trend towards the use of numeric timezone
         indicators, and implementations SHOULD use numeric timezones
         instead of timezone names.  However, all implementations MUST
         accept either notation.  If timezone names are used, they MUST
         be exactly as defined in RFC-822.

         The military time zones are specified incorrectly in RFC-822:
         they count the wrong way from UT (the signs are reversed).  As
         a result, military time zones in RFC-822 headers carry no

         Finally, note that there is a typo in the definition of "zone"
         in the syntax summary of appendix D; the correct definition
         occurs in Section 3 of RFC-822.

      5.2.15  RFC-822 Syntax Change: RFC-822 Section 6.1

         The syntactic definition of "mailbox" in RFC-822 is hereby
         changed to:

            mailbox =  addr-spec            ; simple address
                    / [phrase] route-addr   ; name & addr-spec

         That is, the phrase preceding a route address is now OPTIONAL.
         This change makes the following header field legal, for

             From: <craig@nnsc.nsf.net>

      5.2.16  RFC-822  Local-part: RFC-822 Section 6.2

         The basic mailbox address specification has the form: "local-
         part@domain".  Here "local-part", sometimes called the "left-
         hand side" of the address, is domain-dependent.

         A host that is forwarding the message but is not the
         destination host implied by the right-hand side "domain" MUST
         NOT interpret or modify the "local-part" of the address.

         When mail is to be gatewayed from the Internet mail environment
         into a foreign mail environment (see Section 5.3.7), routing
         information for that foreign environment MAY be embedded within
         the "local-part" of the address.  The gateway will then
         interpret this local part appropriately for the foreign mail

RFC1123                  MAIL -- SMTP & RFC-822             October 1989

              Although source routes are discouraged within the Internet
              (see Section 5.2.6), there are non-Internet mail
              environments whose delivery mechanisms do depend upon
              source routes.  Source routes for extra-Internet
              environments can generally be buried in the "local-part"
              of the address (see Section 5.2.16) while mail traverses
              the Internet.  When the mail reaches the appropriate
              Internet mail gateway, the gateway will interpret the
              local-part and build the necessary address or route for
              the target mail environment.

              For example, an Internet host might send mail to:
              "a!b!c!user@gateway-domain".  The complex local part
              "a!b!c!user" would be uninterpreted within the Internet
              domain, but could be parsed and understood by the
              specified mail gateway.

              An embedded source route is sometimes encoded in the
              "local-part" using "%" as a right-binding routing
              operator.  For example, in:


              the "%" convention implies that the mail is to be routed
              from "relay1" through "relay2", "relay3", and finally to
              "user" at "domain".  This is commonly known as the "%-
              hack".  It is suggested that "%" have lower precedence
              than any other routing operator (e.g., "!") hidden in the
              local-part; for example, "a!b%c" would be interpreted as

              Only the target host (in this case, "relay1") is permitted
              to analyze the local-part "user%domain%relay3%relay2".

      5.2.17  Domain Literals: RFC-822 Section 6.2.3

         A mailer MUST be able to accept and parse an Internet domain
         literal whose content ("dtext"; see RFC-822) is a dotted-
         decimal host address.  This satisfies the requirement of
         Section 2.1 for the case of mail.

         An SMTP MUST accept and recognize a domain literal for any of
         its own IP addresses.

RFC1123                  MAIL -- SMTP & RFC-822             October 1989

      5.2.18  Common Address Formatting Errors: RFC-822 Section 6.1

         Errors in formatting or parsing 822 addresses are unfortunately
         common.  This section mentions only the most common errors.  A
         User Agent MUST accept all valid RFC-822 address formats, and
         MUST NOT generate illegal address syntax.

         o    A common error is to leave out the semicolon after a group

         o    Some systems fail to fully-qualify domain names in
              messages they generate.  The right-hand side of an "@"
              sign in a header address field MUST be a fully-qualified
              domain name.

              For example, some systems fail to fully-qualify the From:
              address; this prevents a "reply" command in the user
              interface from automatically constructing a return

                   Although RFC-822 allows the local use of abbreviated
                   domain names within a domain, the application of
                   RFC-822 in Internet mail does not allow this.  The
                   intent is that an Internet host must not send an SMTP
                   message header containing an abbreviated domain name
                   in an address field.  This allows the address fields
                   of the header to be passed without alteration across
                   the Internet, as required in Section 5.2.6.

         o    Some systems mis-parse multiple-hop explicit source routes
              such as:


         o    Some systems over-qualify domain names by adding a
              trailing dot to some or all domain names in addresses or
              message-ids.  This violates RFC-822 syntax.

      5.2.19  Explicit Source Routes: RFC-822 Section 6.2.7

         Internet host software SHOULD NOT create an RFC-822 header
         containing an address with an explicit source route, but MUST
         accept such headers for compatibility with earlier systems.


RFC1123                  MAIL -- SMTP & RFC-822             October 1989

              In an understatement, RFC-822 says "The use of explicit
              source routing is discouraged".  Many hosts implemented
              RFC-822 source routes incorrectly, so the syntax cannot be
              used unambiguously in practice.  Many users feel the
              syntax is ugly.  Explicit source routes are not needed in
              the mail envelope for delivery; see Section 5.2.6.  For
              all these reasons, explicit source routes using the RFC-
              822 notations are not to be used in Internet mail headers.

              As stated in Section 5.2.16, it is necessary to allow an
              explicit source route to be buried in the local-part of an
              address, e.g., using the "%-hack", in order to allow mail
              to be gatewayed into another environment in which explicit
              source routing is necessary.  The vigilant will observe
              that there is no way for a User Agent to detect and
              prevent the use of such implicit source routing when the
              destination is within the Internet.  We can only
              discourage source routing of any kind within the Internet,
              as unnecessary and undesirable.


      5.3.1  SMTP Queueing Strategies

         The common structure of a host SMTP implementation includes
         user mailboxes, one or more areas for queueing messages in
         transit, and one or more daemon processes for sending and
         receiving mail.  The exact structure will vary depending on the
         needs of the users on the host and the number and size of
         mailing lists supported by the host.  We describe several
         optimizations that have proved helpful, particularly for
         mailers supporting high traffic levels.

         Any queueing strategy MUST include:

         o    Timeouts on all activities.  See Section 5.3.2.

         o    Never sending error messages in response to error

 Sending Strategy

            The general model of a sender-SMTP is one or more processes
            that periodically attempt to transmit outgoing mail.  In a
            typical system, the program that composes a message has some
            method for requesting immediate attention for a new piece of
            outgoing mail, while mail that cannot be transmitted

RFC1123                  MAIL -- SMTP & RFC-822             October 1989

            immediately MUST be queued and periodically retried by the
            sender.  A mail queue entry will include not only the
            message itself but also the envelope information.

            The sender MUST delay retrying a particular destination
            after one attempt has failed.  In general, the retry
            interval SHOULD be at least 30 minutes; however, more
            sophisticated and variable strategies will be beneficial
            when the sender-SMTP can determine the reason for non-

            Retries continue until the message is transmitted or the
            sender gives up; the give-up time generally needs to be at
            least 4-5 days.  The parameters to the retry algorithm MUST
            be configurable.

            A sender SHOULD keep a list of hosts it cannot reach and
            corresponding timeouts, rather than just retrying queued
            mail items.

                 Experience suggests that failures are typically
                 transient (the target system has crashed), favoring a
                 policy of two connection attempts in the first hour the
                 message is in the queue, and then backing off to once
                 every two or three hours.

                 The sender-SMTP can shorten the queueing delay by
                 cooperation with the receiver-SMTP.  In particular, if
                 mail is received from a particular address, it is good
                 evidence that any mail queued for that host can now be

                 The strategy may be further modified as a result of
                 multiple addresses per host (see Section 5.3.4), to
                 optimize delivery time vs. resource usage.

                 A sender-SMTP may have a large queue of messages for
                 each unavailable destination host, and if it retried
                 all these messages in every retry cycle, there would be
                 excessive Internet overhead and the daemon would be
                 blocked for a long period.  Note that an SMTP can
                 generally determine that a delivery attempt has failed
                 only after a timeout of a minute or more; a one minute
                 timeout per connection will result in a very large
                 delay if it is repeated for dozens or even hundreds of
                 queued messages.

RFC1123                  MAIL -- SMTP & RFC-822             October 1989

            When the same message is to be delivered to several users on
            the same host, only one copy of the message SHOULD be
            transmitted.  That is, the sender-SMTP should use the
            command sequence: RCPT, RCPT,... RCPT, DATA instead of the
            sequence: RCPT, DATA, RCPT, DATA,... RCPT, DATA.
            Implementation of this efficiency feature is strongly urged.

            Similarly, the sender-SMTP MAY support multiple concurrent
            outgoing mail transactions to achieve timely delivery.
            However, some limit SHOULD be imposed to protect the host
            from devoting all its resources to mail.

            The use of the different addresses of a multihomed host is
            discussed below.
  Receiving strategy

            The receiver-SMTP SHOULD attempt to keep a pending listen on
            the SMTP port at all times.  This will require the support
            of multiple incoming TCP connections for SMTP.  Some limit
            MAY be imposed.

                 When the receiver-SMTP receives mail from a particular
                 host address, it could notify the sender-SMTP to retry
                 any mail pending for that host address.

      5.3.2  Timeouts in SMTP

         There are two approaches to timeouts in the sender-SMTP:  (a)
         limit the time for each SMTP command separately, or (b) limit
         the time for the entire SMTP dialogue for a single mail
         message.  A sender-SMTP SHOULD use option (a), per-command
         timeouts.  Timeouts SHOULD be easily reconfigurable, preferably
         without recompiling the SMTP code.

              Timeouts are an essential feature of an SMTP
              implementation.  If the timeouts are too long (or worse,
              there are no timeouts), Internet communication failures or
              software bugs in receiver-SMTP programs can tie up SMTP
              processes indefinitely.  If the timeouts are too short,
              resources will be wasted with attempts that time out part
              way through message delivery.

              If option (b) is used, the timeout has to be very large,
              e.g., an hour, to allow time to expand very large mailing
              lists.  The timeout may also need to increase linearly

RFC1123                  MAIL -- SMTP & RFC-822             October 1989

              with the size of the message, to account for the time to
              transmit a very large message.  A large fixed timeout
              leads to two problems:  a failure can still tie up the
              sender for a very long time, and very large messages may
              still spuriously time out (which is a wasteful failure!).

              Using the recommended option (a), a timer is set for each
              SMTP command and for each buffer of the data transfer.
              The latter means that the overall timeout is inherently
              proportional to the size of the message.

         Based on extensive experience with busy mail-relay hosts, the
         minimum per-command timeout values SHOULD be as follows:

         o    Initial 220 Message: 5 minutes

              A Sender-SMTP process needs to distinguish between a
              failed TCP connection and a delay in receiving the initial
              220 greeting message.  Many receiver-SMTPs will accept a
              TCP connection but delay delivery of the 220 message until
              their system load will permit more mail to be processed.

         o    MAIL Command: 5 minutes

         o    RCPT Command: 5 minutes

              A longer timeout would be required if processing of
              mailing lists and aliases were not deferred until after
              the message was accepted.

         o    DATA Initiation: 2 minutes

              This is while awaiting the "354 Start Input" reply to a
              DATA command.

         o    Data Block: 3 minutes

              This is while awaiting the completion of each TCP SEND
              call transmitting a chunk of data.

         o    DATA Termination: 10 minutes.

              This is while awaiting the "250 OK" reply. When the
              receiver gets the final period terminating the message
              data, it typically performs processing to deliver the
              message to a user mailbox.  A spurious timeout at this
              point would be very wasteful, since the message has been

RFC1123                  MAIL -- SMTP & RFC-822             October 1989

              successfully sent.

         A receiver-SMTP SHOULD have a timeout of at least 5 minutes
         while it is awaiting the next command from the sender.

      5.3.3  Reliable Mail Receipt

         When the receiver-SMTP accepts a piece of mail (by sending a
         "250 OK" message in response to DATA), it is accepting
         responsibility for delivering or relaying the message.  It must
         take this responsibility seriously, i.e., it MUST NOT lose the
         message for frivolous reasons, e.g., because the host later
         crashes or because of a predictable resource shortage.

         If there is a delivery failure after acceptance of a message,
         the receiver-SMTP MUST formulate and mail a notification
         message.  This notification MUST be sent using a null ("<>")
         reverse path in the envelope; see Section 3.6 of RFC-821.  The
         recipient of this notification SHOULD be the address from the
         envelope return path (or the Return-Path: line).  However, if
         this address is null ("<>"),  the receiver-SMTP MUST NOT send a
         notification.  If the address is an explicit source route, it
         SHOULD be stripped down to its final hop.

              For example, suppose that an error notification must be
              sent for a message that arrived with:
              "MAIL FROM:<@a,@b:user@d>".  The notification message
              should be sent to: "RCPT TO:<user@d>".

              Some delivery failures after the message is accepted by
              SMTP will be unavoidable.  For example, it may be
              impossible for the receiver-SMTP to validate all the
              delivery addresses in RCPT command(s) due to a "soft"
              domain system error or because the target is a mailing
              list (see earlier discussion of RCPT).

         To avoid receiving duplicate messages as the result of
         timeouts, a receiver-SMTP MUST seek to minimize the time
         required to respond to the final "." that ends a message
         transfer.  See RFC-1047 [SMTP:4] for a discussion of this

      5.3.4  Reliable Mail Transmission

         To transmit a message, a sender-SMTP determines the IP address
         of the target host from the destination address in the
         envelope.  Specifically, it maps the string to the right of the

RFC1123                  MAIL -- SMTP & RFC-822             October 1989

         "@" sign into an IP address.  This mapping or the transfer
         itself may fail with a soft error, in which case the sender-
         SMTP will requeue the outgoing mail for a later retry, as
         required in Section

         When it succeeds, the mapping can result in a list of
         alternative delivery addresses rather than a single address,
         because of (a) multiple MX records, (b) multihoming, or both.
         To provide reliable mail transmission, the sender-SMTP MUST be
         able to try (and retry) each of the addresses in this list in
         order, until a delivery attempt succeeds.  However, there MAY
         also be a configurable limit on the number of alternate
         addresses that can be tried.  In any case, a host SHOULD try at
         least two addresses.

         The following information is to be used to rank the host

         (1)  Multiple MX Records -- these contain a preference
              indication that should be used in sorting.  If there are
              multiple destinations with the same preference and there
              is no clear reason to favor one (e.g., by address
              preference), then the sender-SMTP SHOULD pick one at
              random to spread the load across multiple mail exchanges
              for a specific organization; note that this is a
              refinement of the procedure in [DNS:3].

         (2)  Multihomed host -- The destination host (perhaps taken
              from the preferred MX record) may be multihomed, in which
              case the domain name resolver will return a list of
              alternative IP addresses.  It is the responsibility of the
              domain name resolver interface (see Section below)
              to have ordered this list by decreasing preference, and
              SMTP MUST try them in the order presented.

              Although the capability to try multiple alternative
              addresses is required, there may be circumstances where
              specific installations want to limit or disable the use of
              alternative addresses.  The question of whether a sender
              should attempt retries using the different addresses of a
              multihomed host has been controversial.  The main argument
              for using the multiple addresses is that it maximizes the
              probability of timely delivery, and indeed sometimes the
              probability of any delivery; the counter argument is that
              it may result in unnecessary resource use.

              Note that resource use is also strongly determined by the

RFC1123                  MAIL -- SMTP & RFC-822             October 1989

              sending strategy discussed in Section 5.3.1.

      5.3.5  Domain Name Support

         SMTP implementations MUST use the mechanism defined in Section
         6.1 for mapping between domain names and IP addresses.  This
         means that every Internet SMTP MUST include support for the
         Internet DNS.

         In particular, a sender-SMTP MUST support the MX record scheme
         [SMTP:3].  See also Section 7.4 of [DNS:2] for information on
         domain name support for SMTP.

      5.3.6  Mailing Lists and Aliases

         An SMTP-capable host SHOULD support both the alias and the list
         form of address expansion for multiple delivery.  When a
         message is delivered or forwarded to each address of an
         expanded list form, the return address in the envelope
         ("MAIL FROM:") MUST be changed to be the address of a person
         who administers the list, but the message header MUST be left
         unchanged; in particular, the "From" field of the message is

              An important mail facility is a mechanism for multi-
              destination delivery of a single message, by transforming
              or "expanding" a pseudo-mailbox address into a list of
              destination mailbox addresses.  When a message is sent to
              such a pseudo-mailbox (sometimes called an "exploder"),
              copies are forwarded or redistributed to each mailbox in
              the expanded list.  We classify such a pseudo-mailbox as
              an "alias" or a "list", depending upon the expansion

              (a)  Alias

                   To expand an alias, the recipient mailer simply
                   replaces the pseudo-mailbox address in the envelope
                   with each of the expanded addresses in turn; the rest
                   of the envelope and the message body are left
                   unchanged.  The message is then delivered or
                   forwarded to each expanded address.

              (b)  List

                   A mailing list may be said to operate by
                   "redistribution" rather than by "forwarding".  To

RFC1123                  MAIL -- SMTP & RFC-822             October 1989

                   expand a list, the recipient mailer replaces the
                   pseudo-mailbox address in the envelope with each of
                   the expanded addresses in turn. The return address in
                   the envelope is changed so that all error messages
                   generated by the final deliveries will be returned to
                   a list administrator, not to the message originator,
                   who generally has no control over the contents of the
                   list and will typically find error messages annoying.

      5.3.7  Mail Gatewaying

         Gatewaying mail between different mail environments, i.e.,
         different mail formats and protocols, is complex and does not
         easily yield to standardization.  See for example [SMTP:5a],
         [SMTP:5b].  However, some general requirements may be given for
         a gateway between the Internet and another mail environment.

         (A)  Header fields MAY be rewritten when necessary as messages
              are gatewayed across mail environment boundaries.

                   This may involve interpreting the local-part of the
                   destination address, as suggested in Section 5.2.16.

                   The other mail systems gatewayed to the Internet
                   generally use a subset of RFC-822 headers, but some
                   of them do not have an equivalent to the SMTP
                   envelope.  Therefore, when a message leaves the
                   Internet environment, it may be necessary to fold the
                   SMTP envelope information into the message header.  A
                   possible solution would be to create new header
                   fields to carry the envelope information (e.g., "X-
                   SMTP-MAIL:" and "X-SMTP-RCPT:"); however, this would
                   require changes in mail programs in the foreign

         (B)  When forwarding a message into or out of the Internet
              environment, a gateway MUST prepend a Received: line, but
              it MUST NOT alter in any way a Received: line that is
              already in the header.

                   This requirement is a subset of the general
                   "Received:" line requirement of Section 5.2.8; it is
                   restated here for emphasis.

                   Received: fields of messages originating from other

RFC1123                  MAIL -- SMTP & RFC-822             October 1989

                   environments may not conform exactly to RFC822.
                   However, the most important use of Received: lines is
                   for debugging mail faults, and this debugging can be
                   severely hampered by well-meaning gateways that try
                   to "fix" a Received: line.

                   The gateway is strongly encouraged to indicate the
                   environment and protocol in the "via" clauses of
                   Received field(s) that it supplies.

         (C)  From the Internet side, the gateway SHOULD accept all
              valid address formats in SMTP commands and in RFC-822
              headers, and all valid RFC-822 messages.  Although a
              gateway must accept an RFC-822 explicit source route
              ("@...:" format) in either the RFC-822 header or in the
              envelope, it MAY or may not act on the source route; see
              Sections 5.2.6 and 5.2.19.

                   It is often tempting to restrict the range of
                   addresses accepted at the mail gateway to simplify
                   the translation into addresses for the remote
                   environment.  This practice is based on the
                   assumption that mail users have control over the
                   addresses their mailers send to the mail gateway.  In
                   practice, however, users have little control over the
                   addresses that are finally sent; their mailers are
                   free to change addresses into any legal RFC-822

         (D)  The gateway MUST ensure that all header fields of a
              message that it forwards into the Internet meet the
              requirements for Internet mail.  In particular, all
              addresses in "From:", "To:", "Cc:", etc., fields must be
              transformed (if necessary) to satisfy RFC-822 syntax, and
              they must be effective and useful for sending replies.

         (E)  The translation algorithm used to convert mail from the
              Internet protocols to another environment's protocol
              SHOULD try to ensure that error messages from the foreign
              mail environment are delivered to the return path from the
              SMTP envelope, not to the sender listed in the "From:"
              field of the RFC-822 message.

                   Internet mail lists usually place the address of the
                   mail list maintainer in the envelope but leave the

RFC1123                  MAIL -- SMTP & RFC-822             October 1989

                   original message header intact (with the "From:"
                   field containing the original sender).  This yields
                   the behavior the average recipient expects: a reply
                   to the header gets sent to the original sender, not
                   to a mail list maintainer; however, errors get sent
                   to the maintainer (who can fix the problem) and not
                   the sender (who probably cannot).

         (F)  Similarly, when forwarding a message from another
              environment into the Internet, the gateway SHOULD set the
              envelope return path in accordance with an error message
              return address, if any, supplied by the foreign

      5.3.8  Maximum Message Size

         Mailer software MUST be able to send and receive messages of at
         least 64K bytes in length (including header), and a much larger
         maximum size is highly desirable.

              Although SMTP does not define the maximum size of a
              message, many systems impose implementation limits.

              The current de facto minimum limit in the Internet is 64K
              bytes.  However, electronic mail is used for a variety of
              purposes that create much larger messages.  For example,
              mail is often used instead of FTP for transmitting ASCII
              files, and in particular to transmit entire documents.  As
              a result, messages can be 1 megabyte or even larger.  We
              note that the present document together with its lower-
              layer companion contains 0.5 megabytes.

RFC1123                  MAIL -- SMTP & RFC-822             October 1989


                                               |          | | | |S| |
                                               |          | | | |H| |F
                                               |          | | | |O|M|o
                                               |          | |S| |U|U|o
                                               |          | |H| |L|S|t
                                               |          |M|O| |D|T|n
                                               |          |U|U|M| | |o
                                               |          |S|L|A|N|N|t
                                               |          |T|D|Y|O|O|t
FEATURE                                        |SECTION   | | | |T|T|e
                                               |          | | | | | |
RECEIVER-SMTP:                                 |          | | | | | |
  Implement VRFY                               |5.2.3     |x| | | | |
  Implement EXPN                               |5.2.3     | |x| | | |
    EXPN, VRFY configurable                    |5.2.3     | | |x| | |
  Implement SEND, SOML, SAML                   |5.2.4     | | |x| | |
  Verify HELO parameter                        |5.2.5     | | |x| | |
    Refuse message with bad HELO               |5.2.5     | | | | |x|
  Accept explicit src-route syntax in env.     |5.2.6     |x| | | | |
  Support "postmaster"                         |5.2.7     |x| | | | |
  Process RCPT when received (except lists)    |5.2.7     | | |x| | |
      Long delay of RCPT responses             |5.2.7     | | | | |x|
                                               |          | | | | | |
  Add Received: line                           |5.2.8     |x| | | | |
      Received: line include domain literal    |5.2.8     | |x| | | |
  Change previous Received: line               |5.2.8     | | | | |x|
  Pass Return-Path info (final deliv/gwy)      |5.2.8     |x| | | | |
  Support empty reverse path                   |5.2.9     |x| | | | |
  Send only official reply codes               |5.2.10    | |x| | | |
  Send text from RFC-821 when appropriate      |5.2.10    | |x| | | |
  Delete "." for transparency                  |5.2.11    |x| | | | |
  Accept and recognize self domain literal(s)  |5.2.17    |x| | | | |
                                               |          | | | | | |
  Error message about error message            |5.3.1     | | | | |x|
  Keep pending listen on SMTP port             |   | |x| | | |
  Provide limit on recv concurrency            |   | | |x| | |
  Wait at least 5 mins for next sender cmd     |5.3.2     | |x| | | |
  Avoidable delivery failure after "250 OK"    |5.3.3     | | | | |x|
  Send error notification msg after accept     |5.3.3     |x| | | | |
    Send using null return path                |5.3.3     |x| | | | |
    Send to envelope return path               |5.3.3     | |x| | | |
    Send to null address                       |5.3.3     | | | | |x|
    Strip off explicit src route               |5.3.3     | |x| | | |
  Minimize acceptance delay (RFC-1047)         |5.3.3     |x| | | | |

RFC1123                  MAIL -- SMTP & RFC-822             October 1989

                                               |          | | | | | |
SENDER-SMTP:                                   |          | | | | | |
  Canonicalized domain names in MAIL, RCPT     |5.2.2     |x| | | | |
  Implement SEND, SOML, SAML                   |5.2.4     | | |x| | |
  Send valid principal host name in HELO       |5.2.5     |x| | | | |
  Send explicit source route in RCPT TO:       |5.2.6     | | | |x| |
  Use only reply code to determine action      |5.2.10    |x| | | | |
  Use only high digit of reply code when poss. |5.2.10    | |x| | | |
  Add "." for transparency                     |5.2.11    |x| | | | |
                                               |          | | | | | |
  Retry messages after soft failure            |   |x| | | | |
    Delay before retry                         |   |x| | | | |
    Configurable retry parameters              |   |x| | | | |
    Retry once per each queued dest host       |   | |x| | | |
  Multiple RCPT's for same DATA                |   | |x| | | |
  Support multiple concurrent transactions     |   | | |x| | |
    Provide limit on concurrency               |   | |x| | | |
                                               |          | | | | | |
  Timeouts on all activities                   |5.3.1     |x| | | | |
    Per-command timeouts                       |5.3.2     | |x| | | |
    Timeouts easily reconfigurable             |5.3.2     | |x| | | |
    Recommended times                          |5.3.2     | |x| | | |
  Try alternate addr's in order                |5.3.4     |x| | | | |
    Configurable limit on alternate tries      |5.3.4     | | |x| | |
    Try at least two alternates                |5.3.4     | |x| | | |
  Load-split across equal MX alternates        |5.3.4     | |x| | | |
  Use the Domain Name System                   |5.3.5     |x| | | | |
    Support MX records                         |5.3.5     |x| | | | |
    Use WKS records in MX processing           |5.2.12    | | | |x| |
                                               |          | | | | | |
MAIL FORWARDING:                               |          | | | | | |
  Alter existing header field(s)               |5.2.6     | | | |x| |
  Implement relay function: 821/section 3.6    |5.2.6     | | |x| | |
    If not, deliver to RHS domain              |5.2.6     | |x| | | |
  Interpret 'local-part' of addr               |5.2.16    | | | | |x|
                                               |          | | | | | |
MAILING LISTS AND ALIASES                      |          | | | | | |
  Support both                                 |5.3.6     | |x| | | |
  Report mail list error to local admin.       |5.3.6     |x| | | | |
                                               |          | | | | | |
MAIL GATEWAYS:                                 |          | | | | | |
  Embed foreign mail route in local-part       |5.2.16    | | |x| | |
  Rewrite header fields when necessary         |5.3.7     | | |x| | |
  Prepend Received: line                       |5.3.7     |x| | | | |
  Change existing Received: line               |5.3.7     | | | | |x|
  Accept full RFC-822 on Internet side         |5.3.7     | |x| | | |
  Act on RFC-822 explicit source route         |5.3.7     | | |x| | |

RFC1123                  MAIL -- SMTP & RFC-822             October 1989

  Send only valid RFC-822 on Internet side     |5.3.7     |x| | | | |
  Deliver error msgs to envelope addr          |5.3.7     | |x| | | |
  Set env return path from err return addr     |5.3.7     | |x| | | |
                                               |          | | | | | |
USER AGENT -- RFC-822                          |          | | | | | |
  Allow user to enter <route> address          |5.2.6     | | | |x| |
  Support RFC-1049 Content Type field          |5.2.13    | | |x| | |
  Use 4-digit years                            |5.2.14    | |x| | | |
  Generate numeric timezones                   |5.2.14    | |x| | | |
  Accept all timezones                         |5.2.14    |x| | | | |
  Use non-num timezones from RFC-822           |5.2.14    |x| | | | |
  Omit phrase before route-addr                |5.2.15    | | |x| | |
  Accept and parse dot.dec. domain literals    |5.2.17    |x| | | | |
  Accept all RFC-822 address formats           |5.2.18    |x| | | | |
  Generate invalid RFC-822 address format      |5.2.18    | | | | |x|
  Fully-qualified domain names in header       |5.2.18    |x| | | | |
  Create explicit src route in header          |5.2.19    | | | |x| |
  Accept explicit src route in header          |5.2.19    |x| | | | |
                                               |          | | | | | |
Send/recv at least 64KB messages               |5.3.8     |x| | | | |

RFC1123               SUPPORT SERVICES -- DOMAINS           October 1989



      6.1.1 INTRODUCTION

         Every host MUST implement a resolver for the Domain Name System
         (DNS), and it MUST implement a mechanism using this DNS
         resolver to convert host names to IP addresses and vice-versa
         [DNS:1, DNS:2].

         In addition to the DNS, a host MAY also implement a host name
         translation mechanism that searches a local Internet host
         table.  See Section for more information on this

              Internet host name translation was originally performed by
              searching local copies of a table of all hosts.  This
              table became too large to update and distribute in a
              timely manner and too large to fit into many hosts, so the
              DNS was invented.

              The DNS creates a distributed database used primarily for
              the translation between host names and host addresses.
              Implementation of DNS software is required.  The DNS
              consists of two logically distinct parts: name servers and
              resolvers (although implementations often combine these
              two logical parts in the interest of efficiency) [DNS:2].

              Domain name servers store authoritative data about certain
              sections of the database and answer queries about the
              data.  Domain resolvers query domain name servers for data
              on behalf of user processes.  Every host therefore needs a
              DNS resolver; some host machines will also need to run
              domain name servers.  Since no name server has complete
              information, in general it is necessary to obtain
              information from more than one name server to resolve a


         An implementor must study references [DNS:1] and [DNS:2]
         carefully.  They provide a thorough description of the theory,
         protocol, and implementation of the domain name system, and
         reflect several years of experience.

RFC1123               SUPPORT SERVICES -- DOMAINS           October 1989

  Resource Records with Zero TTL: RFC-1035 Section 3.2.1

            All DNS name servers and resolvers MUST properly handle RRs
            with a zero TTL: return the RR to the client but do not
            cache it.

                 Zero TTL values are interpreted to mean that the RR can
                 only be used for the transaction in progress, and
                 should not be cached; they are useful for extremely
                 volatile data.
  QCLASS Values: RFC-1035 Section 3.2.5

            A query with "QCLASS=*" SHOULD NOT be used unless the
            requestor is seeking data from more than one class.  In
            particular, if the requestor is only interested in Internet
            data types, QCLASS=IN MUST be used.
  Unused Fields: RFC-1035 Section 4.1.1

            Unused fields in a query or response message MUST be zero.
  Compression: RFC-1035 Section 4.1.4

            Name servers MUST use compression in responses.

                 Compression is essential to avoid overflowing UDP
                 datagrams; see Section
  Misusing Configuration Info: RFC-1035 Section 6.1.2

            Recursive name servers and full-service resolvers generally
            have some configuration information containing hints about
            the location of root or local name servers.  An
            implementation MUST NOT include any of these hints in a

                 Many implementors have found it convenient to store
                 these hints as if they were cached data, but some
                 neglected to ensure that this "cached data" was not
                 included in responses.  This has caused serious
                 problems in the Internet when the hints were obsolete
                 or incorrect.

RFC1123               SUPPORT SERVICES -- DOMAINS           October 1989

      6.1.3  SPECIFIC ISSUES
  Resolver Implementation

            A name resolver SHOULD be able to multiplex concurrent
            requests if the host supports concurrent processes.

            In implementing a DNS resolver, one of two different models
            MAY optionally be chosen: a full-service resolver, or a stub

            (A)  Full-Service Resolver

                 A full-service resolver is a complete implementation of
                 the resolver service, and is capable of dealing with
                 communication failures, failure of individual name
                 servers, location of the proper name server for a given
                 name, etc.  It must satisfy the following requirements:

                 o    The resolver MUST implement a local caching
                      function to avoid repeated remote access for
                      identical requests, and MUST time out information
                      in the cache.

                 o    The resolver SHOULD be configurable with start-up
                      information pointing to multiple root name servers
                      and multiple name servers for the local domain.
                      This insures that the resolver will be able to
                      access the whole name space in normal cases, and
                      will be able to access local domain information
                      should the local network become disconnected from
                      the rest of the Internet.

            (B)  Stub Resolver

                 A "stub resolver" relies on the services of a recursive
                 name server on the connected network or a "nearby"
                 network.  This scheme allows the host to pass on the
                 burden of the resolver function to a name server on
                 another host.  This model is often essential for less
                 capable hosts, such as PCs, and is also recommended
                 when the host is one of several workstations on a local
                 network, because it allows all of the workstations to
                 share the cache of the recursive name server and hence
                 reduce the number of domain requests exported by the
                 local network.

RFC1123               SUPPORT SERVICES -- DOMAINS           October 1989

                 At a minimum, the stub resolver MUST be capable of
                 directing its requests to redundant recursive name
                 servers.  Note that recursive name servers are allowed
                 to restrict the sources of requests that they will
                 honor, so the host administrator must verify that the
                 service will be provided.  Stub resolvers MAY implement
                 caching if they choose, but if so, MUST timeout cached

  Transport Protocols

            DNS resolvers and recursive servers MUST support UDP, and
            SHOULD support TCP, for sending (non-zone-transfer) queries.
            Specifically, a DNS resolver or server that is sending a
            non-zone-transfer query MUST send a UDP query first.  If the
            Answer section of the response is truncated and if the
            requester supports TCP, it SHOULD try the query again using

            DNS servers MUST be able to service UDP queries and SHOULD
            be able to service TCP queries.  A name server MAY limit the
            resources it devotes to TCP queries, but it SHOULD NOT
            refuse to service a TCP query just because it would have
            succeeded with UDP.

            Truncated responses MUST NOT be saved (cached) and later
            used in such a way that the fact that they are truncated is

                 UDP is preferred over TCP for queries because UDP
                 queries have much lower overhead, both in packet count
                 and in connection state.  The use of UDP is essential
                 for heavily-loaded servers, especially the root
                 servers.  UDP also offers additional robustness, since
                 a resolver can attempt several UDP queries to different
                 servers for the cost of a single TCP query.

                 It is possible for a DNS response to be truncated,
                 although this is a very rare occurrence in the present
                 Internet DNS.  Practically speaking, truncation cannot
                 be predicted, since it is data-dependent.  The
                 dependencies include the number of RRs in the answer,
                 the size of each RR, and the savings in space realized
                 by the name compression algorithm.  As a rule of thumb,
                 truncation in NS and MX lists should not occur for
                 answers containing 15 or fewer RRs.

RFC1123               SUPPORT SERVICES -- DOMAINS           October 1989

                 Whether it is possible to use a truncated answer
                 depends on the application.  A mailer must not use a
                 truncated MX response, since this could lead to mail

                 Responsible practices can make UDP suffice in the vast
                 majority of cases.  Name servers must use compression
                 in responses.  Resolvers must differentiate truncation
                 of the Additional section of a response (which only
                 loses extra information) from truncation of the Answer
                 section (which for MX records renders the response
                 unusable by mailers).  Database administrators should
                 list only a reasonable number of primary names in lists
                 of name servers, MX alternatives, etc.

                 However, it is also clear that some new DNS record
                 types defined in the future will contain information
                 exceeding the 512 byte limit that applies to UDP, and
                 hence will require TCP.  Thus, resolvers and name
                 servers should implement TCP services as a backup to
                 UDP today, with the knowledge that they will require
                 the TCP service in the future.

            By private agreement, name servers and resolvers MAY arrange
            to use TCP for all traffic between themselves.  TCP MUST be
            used for zone transfers.

            A DNS server MUST have sufficient internal concurrency that
            it can continue to process UDP queries while awaiting a
            response or performing a zone transfer on an open TCP
            connection [DNS:2].

            A server MAY support a UDP query that is delivered using an
            IP broadcast or multicast address.  However, the Recursion
            Desired bit MUST NOT be set in a query that is multicast,
            and MUST be ignored by name servers receiving queries via a
            broadcast or multicast address.  A host that sends broadcast
            or multicast DNS queries SHOULD send them only as occasional
            probes, caching the IP address(es) it obtains from the
            response(s) so it can normally send unicast queries.

                 Broadcast or (especially) IP multicast can provide a
                 way to locate nearby name servers without knowing their
                 IP addresses in advance.  However, general broadcasting
                 of recursive queries can result in excessive and
                 unnecessary load on both network and servers.

RFC1123               SUPPORT SERVICES -- DOMAINS           October 1989

  Efficient Resource Usage

            The following requirements on servers and resolvers are very
            important to the health of the Internet as a whole,
            particularly when DNS services are invoked repeatedly by
            higher level automatic servers, such as mailers.

            (1)  The resolver MUST implement retransmission controls to
                 insure that it does not waste communication bandwidth,
                 and MUST impose finite bounds on the resources consumed
                 to respond to a single request.  See [DNS:2] pages 43-
                 44 for specific recommendations.

            (2)  After a query has been retransmitted several times
                 without a response, an implementation MUST give up and
                 return a soft error to the application.

            (3)  All DNS name servers and resolvers SHOULD cache
                 temporary failures, with a timeout period of the order
                 of minutes.

                      This will prevent applications that immediately
                      retry soft failures (in violation of Section 2.2
                      of this document) from generating excessive DNS

            (4)  All DNS name servers and resolvers SHOULD cache
                 negative responses that indicate the specified name, or
                 data of the specified type, does not exist, as
                 described in [DNS:2].

            (5)  When a DNS server or resolver retries a UDP query, the
                 retry interval SHOULD be constrained by an exponential
                 backoff algorithm, and SHOULD also have upper and lower

                      A measured RTT and variance (if available) should
                      be used to calculate an initial retransmission
                      interval.  If this information is not available, a
                      default of no less than 5 seconds should be used.
                      Implementations may limit the retransmission
                      interval, but this limit must exceed twice the
                      Internet maximum segment lifetime plus service
                      delay at the name server.

            (6)  When a resolver or server receives a Source Quench for

RFC1123               SUPPORT SERVICES -- DOMAINS           October 1989

                 a query it has issued, it SHOULD take steps to reduce
                 the rate of querying that server in the near future.  A
                 server MAY ignore a Source Quench that it receives as
                 the result of sending a response datagram.

                      One recommended action to reduce the rate is to
                      send the next query attempt to an alternate
                      server, if there is one available.  Another is to
                      backoff the retry interval for the same server.

  Multihomed Hosts

            When the host name-to-address function encounters a host
            with multiple addresses, it SHOULD rank or sort the
            addresses using knowledge of the immediately connected
            network number(s) and any other applicable performance or
            history information.

                 The different addresses of a multihomed host generally
                 imply different Internet paths, and some paths may be
                 preferable to others in performance, reliability, or
                 administrative restrictions.  There is no general way
                 for the domain system to determine the best path.  A
                 recommended approach is to base this decision on local
                 configuration information set by the system

                 The following scheme has been used successfully:

                 (a)  Incorporate into the host configuration data a
                      Network-Preference List, that is simply a list of
                      networks in preferred order.  This list may be
                      empty if there is no preference.

                 (b)  When a host name is mapped into a list of IP
                      addresses, these addresses should be sorted by
                      network number, into the same order as the
                      corresponding networks in the Network-Preference
                      List.  IP addresses whose networks do not appear
                      in the Network-Preference List should be placed at
                      the end of the list.

RFC1123               SUPPORT SERVICES -- DOMAINS           October 1989


            DNS software MUST support all well-known, class-independent
            formats [DNS:2], and SHOULD be written to minimize the
            trauma associated with the introduction of new well-known
            types and local experimentation with non-standard types.

                 The data types and classes used by the DNS are
                 extensible, and thus new types will be added and old
                 types deleted or redefined.  Introduction of new data
                 types ought to be dependent only upon the rules for
                 compression of domain names inside DNS messages, and
                 the translation between printable (i.e., master file)
                 and internal formats for Resource Records (RRs).

                 Compression relies on knowledge of the format of data
                 inside a particular RR.  Hence compression must only be
                 used for the contents of well-known, class-independent
                 RRs, and must never be used for class-specific RRs or
                 RR types that are not well-known.  The owner name of an
                 RR is always eligible for compression.

                 A name server may acquire, via zone transfer, RRs that
                 the server doesn't know how to convert to printable
                 format.  A resolver can receive similar information as
                 the result of queries.  For proper operation, this data
                 must be preserved, and hence the implication is that
                 DNS software cannot use textual formats for internal

                 The DNS defines domain name syntax very generally -- a
                 string of labels each containing up to 63 8-bit octets,
                 separated by dots, and with a maximum total of 255
                 octets.  Particular applications of the DNS are
                 permitted to further constrain the syntax of the domain
                 names they use, although the DNS deployment has led to
                 some applications allowing more general names.  In
                 particular, Section 2.1 of this document liberalizes
                 slightly the syntax of a legal Internet host name that
                 was defined in RFC-952 [DNS:4].
  Status of RR Types

            Name servers MUST be able to load all RR types except MD and
            MF from configuration files.  The MD and MF types are
            obsolete and MUST NOT be implemented; in particular, name
            servers MUST NOT load these types from configuration files.

RFC1123               SUPPORT SERVICES -- DOMAINS           October 1989

                 The RR types MB, MG, MR, NULL, MINFO and RP are
                 considered experimental, and applications that use the
                 DNS cannot expect these RR types to be supported by
                 most domains.  Furthermore these types are subject to

                 The TXT and WKS RR types have not been widely used by
                 Internet sites; as a result, an application cannot rely
                 on the the existence of a TXT or WKS RR in most

            DNS software may need to operate in environments where the
            root servers or other servers are unavailable due to network
            connectivity or other problems.  In this situation, DNS name
            servers and resolvers MUST continue to provide service for
            the reachable part of the name space, while giving temporary
            failures for the rest.

                 Although the DNS is meant to be used primarily in the
                 connected Internet, it should be possible to use the
                 system in networks which are unconnected to the
                 Internet.  Hence implementations must not depend on
                 access to root servers before providing service for
                 local names.
  Local Host Table

                 A host may use a local host table as a backup or
                 supplement to the DNS.  This raises the question of
                 which takes precedence, the DNS or the host table; the
                 most flexible approach would make this a configuration

                 Typically, the contents of such a supplementary host
                 table will be determined locally by the site.  However,
                 a publically-available table of Internet hosts is
                 maintained by the DDN Network Information Center (DDN
                 NIC), with a format documented in [DNS:4].  This table
                 can be retrieved from the DDN NIC using a protocol
                 described in [DNS:5].  It must be noted that this table
                 contains only a small fraction of all Internet hosts.
                 Hosts using this protocol to retrieve the DDN NIC host
                 table should use the VERSION command to check if the

RFC1123               SUPPORT SERVICES -- DOMAINS           October 1989

                 table has changed before requesting the entire table
                 with the ALL command.  The VERSION identifier should be
                 treated as an arbitrary string and tested only for
                 equality; no numerical sequence may be assumed.

                 The DDN NIC host table includes administrative
                 information that is not needed for host operation and
                 is therefore not currently included in the DNS
                 database; examples include network and gateway entries.
                 However, much of this additional information will be
                 added to the DNS in the future.  Conversely, the DNS
                 provides essential services (in particular, MX records)
                 that are not available from the DDN NIC host table.

  DNS Administration

            This document is concerned with design and implementation
            issues in host software, not with administrative or
            operational issues.  However, administrative issues are of
            particular importance in the DNS, since errors in particular
            segments of this large distributed database can cause poor
            or erroneous performance for many sites.  These issues are
            discussed in [DNS:6] and [DNS:7].
  DNS User Interface

            Hosts MUST provide an interface to the DNS for all
            application programs running on the host.  This interface
            will typically direct requests to a system process to
            perform the resolver function [DNS:1, 6.1:2].

            At a minimum, the basic interface MUST support a request for
            all information of a specific type and class associated with
            a specific name, and it MUST return either all of the
            requested information, a hard error code, or a soft error
            indication.  When there is no error, the basic interface
            returns the complete response information without
            modification, deletion, or ordering, so that the basic
            interface will not need to be changed to accommodate new
            data types.

                 The soft error indication is an essential part of the
                 interface, since it may not always be possible to
                 access particular information from the DNS; see Section

RFC1123               SUPPORT SERVICES -- DOMAINS           October 1989

            A host MAY provide other DNS interfaces tailored to
            particular functions, transforming the raw domain data into
            formats more suited to these functions.  In particular, a
            host MUST provide a DNS interface to facilitate translation
            between host addresses and host names.
 Interface Abbreviation Facilities

            User interfaces MAY provide a method for users to enter
            abbreviations for commonly-used names.  Although the
            definition of such methods is outside of the scope of the
            DNS specification, certain rules are necessary to insure
            that these methods allow access to the entire DNS name space
            and to prevent excessive use of Internet resources.

            If an abbreviation method is provided, then:

            (a)  There MUST be some convention for denoting that a name
                 is already complete, so that the abbreviation method(s)
                 are suppressed.  A trailing dot is the usual method.

            (b)  Abbreviation expansion MUST be done exactly once, and
                 MUST be done in the context in which the name was

                 For example, if an abbreviation is used in a mail
                 program for a destination, the abbreviation should be
                 expanded into a full domain name and stored in the
                 queued message with an indication that it is already
                 complete.  Otherwise, the abbreviation might be
                 expanded with a mail system search list, not the
                 user's, or a name could grow due to repeated
                 canonicalizations attempts interacting with wildcards.

            The two most common abbreviation methods are:

            (1)  Interface-level aliases

                 Interface-level aliases are conceptually implemented as
                 a list of alias/domain name pairs. The list can be
                 per-user or per-host, and separate lists can be
                 associated with different functions, e.g. one list for
                 host name-to-address translation, and a different list
                 for mail domains.  When the user enters a name, the
                 interface attempts to match the name to the alias
                 component of a list entry, and if a matching entry can

RFC1123               SUPPORT SERVICES -- DOMAINS           October 1989

                 be found, the name is replaced by the domain name found
                 in the pair.

                 Note that interface-level aliases and CNAMEs are
                 completely separate mechanisms; interface-level aliases
                 are a local matter while CNAMEs are an Internet-wide
                 aliasing mechanism which is a required part of any DNS

            (2)  Search Lists

                 A search list is conceptually implemented as an ordered
                 list of domain names.  When the user enters a name, the
                 domain names in the search list are used as suffixes to
                 the user-supplied name, one by one, until a domain name
                 with the desired associated data is found, or the
                 search list is exhausted.  Search lists often contain
                 the name of the local host's parent domain or other
                 ancestor domains.  Search lists are often per-user or

                 It SHOULD be possible for an administrator to disable a
                 DNS search-list facility.  Administrative denial may be
                 warranted in some cases, to prevent abuse of the DNS.

                 There is danger that a search-list mechanism will
                 generate excessive queries to the root servers while
                 testing whether user input is a complete domain name,
                 lacking a final period to mark it as complete.  A
                 search-list mechanism MUST have one of, and SHOULD have
                 both of, the following two provisions to prevent this:

                 (a)  The local resolver/name server can implement
                      caching  of negative responses (see Section

                 (b)  The search list expander can require two or more
                      interior dots in a generated domain name before it
                      tries using the name in a query to non-local
                      domain servers, such as the root.

                      The intent of this requirement is to avoid
                      excessive delay for the user as the search list is
                      tested, and more importantly to prevent excessive
                      traffic to the root and other high-level servers.
                      For example, if the user supplied a name "X" and
                      the search list contained the root as a component,

RFC1123               SUPPORT SERVICES -- DOMAINS           October 1989

                      a query would have to consult a root server before
                      the next search list alternative could be tried.
                      The resulting load seen by the root servers and
                      gateways near the root would be multiplied by the
                      number of hosts in the Internet.

                      The negative caching alternative limits the effect
                      to the first time a name is used.  The interior
                      dot rule is simpler to implement but can prevent
                      easy use of some top-level names.


                                               |           | | | |S| |
                                               |           | | | |H| |F
                                               |           | | | |O|M|o
                                               |           | |S| |U|U|o
                                               |           | |H| |L|S|t
                                               |           |M|O| |D|T|n
                                               |           |U|U|M| | |o
                                               |           |S|L|A|N|N|t
                                               |           |T|D|Y|O|O|t
FEATURE                                        |SECTION    | | | |T|T|e
GENERAL ISSUES                                 |           | | | | | |
                                               |           | | | | | |
Implement DNS name-to-address conversion       |6.1.1      |x| | | | |
Implement DNS address-to-name conversion       |6.1.1      |x| | | | |
Support conversions using host table           |6.1.1      | | |x| | |
Properly handle RR with zero TTL               |    |x| | | | |
Use QCLASS=* unnecessarily                     |    | |x| | | |
  Use QCLASS=IN for Internet class             |    |x| | | | |
Unused fields zero                             |    |x| | | | |
Use compression in responses                   |    |x| | | | |
                                               |           | | | | | |
Include config info in responses               |    | | | | |x|
Support all well-known, class-indep. types     |    |x| | | | |
Easily expand type list                        |    | |x| | | |
Load all RR types (except MD and MF)           |    |x| | | | |
Load MD or MF type                             |    | | | | |x|
Operate when root servers, etc. unavailable    |    |x| | | | |
RESOLVER ISSUES:                               |           | | | | | |
                                               |           | | | | | |
Resolver support multiple concurrent requests  |    | |x| | | |
Full-service resolver:                         |    | | |x| | |
  Local caching                                |    |x| | | | |

RFC1123               SUPPORT SERVICES -- DOMAINS           October 1989

  Information in local cache times out         |    |x| | | | |
  Configurable with starting info              |    | |x| | | |
Stub resolver:                                 |    | | |x| | |
  Use redundant recursive name servers         |    |x| | | | |
  Local caching                                |    | | |x| | |
  Information in local cache times out         |    |x| | | | |
Support for remote multi-homed hosts:          |           | | | | | |
  Sort multiple addresses by preference list   |    | |x| | | |
                                               |           | | | | | |
TRANSPORT PROTOCOLS:                           |           | | | | | |
                                               |           | | | | | |
Support UDP queries                            |    |x| | | | |
Support TCP queries                            |    | |x| | | |
  Send query using UDP first                   |    |x| | | | |1
  Try TCP if UDP answers are truncated         |    | |x| | | |
Name server limit TCP query resources          |    | | |x| | |
  Punish unnecessary TCP query                 |    | | | |x| |
Use truncated data as if it were not           |    | | | | |x|
Private agreement to use only TCP              |    | | |x| | |
Use TCP for zone transfers                     |    |x| | | | |
TCP usage not block UDP queries                |    |x| | | | |
Support broadcast or multicast queries         |    | | |x| | |
  RD bit set in query                          |    | | | | |x|
  RD bit ignored by server is b'cast/m'cast    |    |x| | | | |
  Send only as occasional probe for addr's     |    | |x| | | |
RESOURCE USAGE:                                |           | | | | | |
                                               |           | | | | | |
Transmission controls, per [DNS:2]             |    |x| | | | |
  Finite bounds per request                    |    |x| | | | |
Failure after retries => soft error            |    |x| | | | |
Cache temporary failures                       |    | |x| | | |
Cache negative responses                       |    | |x| | | |
Retries use exponential backoff                |    | |x| | | |
  Upper, lower bounds                          |    | |x| | | |
Client handle Source Quench                    |    | |x| | | |
Server ignore Source Quench                    |    | | |x| | |
USER INTERFACE:                                |           | | | | | |
                                               |           | | | | | |
All programs have access to DNS interface      |    |x| | | | |
Able to request all info for given name        |    |x| | | | |
Returns complete info or error                 |    |x| | | | |
Special interfaces                             |    | | |x| | |
  Name<->Address translation                   |    |x| | | | |
                                               |           | | | | | |
Abbreviation Facilities:                       |    | | |x| | |

RFC1123               SUPPORT SERVICES -- DOMAINS           October 1989

  Convention for complete names                |    |x| | | | |
  Conversion exactly once                      |    |x| | | | |
  Conversion in proper context                 |    |x| | | | |
  Search list:                                 |    | | |x| | |
    Administrator can disable                  |    | |x| | | |
    Prevention of excessive root queries       |    |x| | | | |
      Both methods                             |    | |x| | | |

1.   Unless there is private agreement between particular resolver and
     particular server.

RFC1123            SUPPORT SERVICES -- INITIALIZATION       October 1989


      6.2.1  INTRODUCTION

         This section discusses the initialization of host software
         across a connected network, or more generally across an
         Internet path.  This is necessary for a diskless host, and may
         optionally be used for a host with disk drives.  For a diskless
         host, the initialization process is called "network booting"
         and is controlled by a bootstrap program located in a boot ROM.

         To initialize a diskless host across the network, there are two
         distinct phases:

         (1)  Configure the IP layer.

              Diskless machines often have no permanent storage in which
              to store network configuration information, so that
              sufficient configuration information must be obtained
              dynamically to support the loading phase that follows.
              This information must include at least the IP addresses of
              the host and of the boot server.  To support booting
              across a gateway, the address mask and a list of default
              gateways are also required.

         (2)  Load the host system code.

              During the loading phase, an appropriate file transfer
              protocol is used to copy the system code across the
              network from the boot server.

         A host with a disk may perform the first step, dynamic
         configuration.  This is important for microcomputers, whose
         floppy disks allow network configuration information to be
         mistakenly duplicated on more than one host.  Also,
         installation of new hosts is much simpler if they automatically
         obtain their configuration information from a central server,
         saving administrator time and decreasing the probability of

      6.2.2  REQUIREMENTS
  Dynamic Configuration

            A number of protocol provisions have been made for dynamic

            o    ICMP Information Request/Reply messages

RFC1123            SUPPORT SERVICES -- INITIALIZATION       October 1989

                 This obsolete message pair was designed to allow a host
                 to find the number of the network it is on.
                 Unfortunately, it was useful only if the host already
                 knew the host number part of its IP address,
                 information that hosts requiring dynamic configuration
                 seldom had.

            o    Reverse Address Resolution Protocol (RARP) [BOOT:4]

                 RARP is a link-layer protocol for a broadcast medium
                 that allows a host to find its IP address given its
                 link layer address.  Unfortunately, RARP does not work
                 across IP gateways and therefore requires a RARP server
                 on every network.  In addition, RARP does not provide
                 any other configuration information.

            o    ICMP Address Mask Request/Reply messages

                 These ICMP messages allow a host to learn the address
                 mask for a particular network interface.

            o    BOOTP Protocol [BOOT:2]

                 This protocol allows a host to determine the IP
                 addresses of the local host and the boot server, the
                 name of an appropriate boot file, and optionally the
                 address mask and list of default gateways.  To locate a
                 BOOTP server, the host broadcasts a BOOTP request using
                 UDP.  Ad hoc gateway extensions have been used to
                 transmit the BOOTP broadcast through gateways, and in
                 the future the IP Multicasting facility will provide a
                 standard mechanism for this purpose.

            The suggested approach to dynamic configuration is to use
            the BOOTP protocol with the extensions defined in "BOOTP
            Vendor Information Extensions" RFC-1084 [BOOT:3].  RFC-1084
            defines some important general (not vendor-specific)
            extensions.  In particular, these extensions allow the
            address mask to be supplied in BOOTP; we RECOMMEND that the
            address mask be supplied in this manner.

                 Historically, subnetting was defined long after IP, and
                 so a separate mechanism (ICMP Address Mask messages)
                 was designed to supply the address mask to a host.
                 However, the IP address mask and the corresponding IP
                 address conceptually form a pair, and for operational

RFC1123            SUPPORT SERVICES -- INITIALIZATION       October 1989

                 simplicity they ought to be defined at the same time
                 and by the same mechanism, whether a configuration file
                 or a dynamic mechanism like BOOTP.

                 Note that BOOTP is not sufficiently general to specify
                 the configurations of all interfaces of a multihomed
                 host.  A multihomed host must either use BOOTP
                 separately for each interface, or configure one
                 interface using BOOTP to perform the loading, and
                 perform the complete initialization from a file later.

                 Application layer configuration information is expected
                 to be obtained from files after loading of the system
  Loading Phase

            A suggested approach for the loading phase is to use TFTP
            [BOOT:1] between the IP addresses established by BOOTP.

            TFTP to a broadcast address SHOULD NOT be used, for reasons
            explained in Section

RFC1123              SUPPORT SERVICES -- MANAGEMENT         October 1989


      6.3.1  INTRODUCTION

         The Internet community has recently put considerable effort
         into the development of network management protocols.  The
         result has been a two-pronged approach [MGT:1, MGT:6]:  the
         Simple Network Management Protocol (SNMP) [MGT:4] and the
         Common Management Information Protocol over TCP (CMOT) [MGT:5].

         In order to be managed using SNMP or CMOT, a host will need to
         implement an appropriate management agent.  An Internet host
         SHOULD include an agent for either SNMP or CMOT.

         Both SNMP and CMOT operate on a Management Information Base
         (MIB) that defines a collection of management values.  By
         reading and setting these values, a remote application may
         query and change the state of the managed system.

         A standard MIB [MGT:3] has been defined for use by both
         management protocols, using data types defined by the Structure
         of Management Information (SMI) defined in [MGT:2].  Additional
         MIB variables can be introduced under the "enterprises" and
         "experimental" subtrees of the MIB naming space [MGT:2].

         Every protocol module in the host SHOULD implement the relevant
         MIB variables.  A host SHOULD implement the MIB variables as
         defined in the most recent standard MIB, and MAY implement
         other MIB variables when appropriate and useful.


         The MIB is intended to cover both hosts and gateways, although
         there may be detailed differences in MIB application to the two
         cases.  This section contains the appropriate interpretation of
         the MIB for hosts.  It is likely that later versions of the MIB
         will include more entries for host management.

         A managed host must implement the following groups of MIB
         object definitions: System, Interfaces, Address Translation,
         IP, ICMP, TCP, and UDP.

         The following specific interpretations apply to hosts:

         o    ipInHdrErrors

              Note that the error "time-to-live exceeded" can occur in a
              host only when it is forwarding a source-routed datagram.

RFC1123              SUPPORT SERVICES -- MANAGEMENT         October 1989

         o    ipOutNoRoutes

              This object counts datagrams discarded because no route
              can be found.  This may happen in a host if all the
              default gateways in the host's configuration are down.

         o    ipFragOKs, ipFragFails, ipFragCreates

              A host that does not implement intentional fragmentation
              (see "Fragmentation" section of [INTRO:1]) MUST return the
              value zero for these three objects.

         o    icmpOutRedirects

              For a host, this object MUST always be zero, since hosts
              do not send Redirects.

         o    icmpOutAddrMaskReps

              For a host, this object MUST always be zero, unless the
              host is an authoritative source of address mask

         o    ipAddrTable

              For a host, the "IP Address Table" object is effectively a
              table of logical interfaces.

         o    ipRoutingTable

              For a host, the "IP Routing Table" object is effectively a
              combination of the host's Routing Cache and the static
              route table described in "Routing Outbound Datagrams"
              section of [INTRO:1].

              Within each ipRouteEntry, ipRouteMetric1...4 normally will
              have no meaning for a host and SHOULD always be -1, while
              ipRouteType will normally have the value "remote".

              If destinations on the connected network do not appear in
              the Route Cache (see "Routing Outbound Datagrams section
              of [INTRO:1]), there will be no entries with ipRouteType
              of "direct".

              The current MIB does not include Type-of-Service in an
              ipRouteEntry, but a future revision is expected to make

RFC1123              SUPPORT SERVICES -- MANAGEMENT         October 1989

              this addition.

              We also expect the MIB to be expanded to allow the remote
              management of applications (e.g., the ability to partially
              reconfigure mail systems).  Network service applications
              such as mail systems should therefore be written with the
              "hooks" for remote management.


                                               |           | | | |S| |
                                               |           | | | |H| |F
                                               |           | | | |O|M|o
                                               |           | |S| |U|U|o
                                               |           | |H| |L|S|t
                                               |           |M|O| |D|T|n
                                               |           |U|U|M| | |o
                                               |           |S|L|A|N|N|t
                                               |           |T|D|Y|O|O|t
FEATURE                                        |SECTION    | | | |T|T|e
Support SNMP or CMOT agent                     |6.3.1      | |x| | | |
Implement specified objects in standard MIB    |6.3.1      | |x| | | |

RFC1123              SUPPORT SERVICES -- MANAGEMENT         October 1989


   This section lists the primary references with which every
   implementer must be thoroughly familiar.  It also lists some
   secondary references that are suggested additional reading.


   [INTRO:1] "Requirements for Internet Hosts -- Communication Layers,"
        IETF Host Requirements Working Group, R. Braden, Ed., RFC-1122,
        October 1989.

   [INTRO:2]  "DDN Protocol Handbook," NIC-50004, NIC-50005, NIC-50006,
        (three volumes), SRI International, December 1985.

   [INTRO:3]  "Official Internet Protocols," J. Reynolds and J. Postel,
        RFC-1011, May 1987.

        This document is republished periodically with new RFC numbers;
        the latest version must be used.

   [INTRO:4]  "Protocol Document Order Information," O. Jacobsen and J.
        Postel, RFC-980, March 1986.

   [INTRO:5]  "Assigned Numbers," J. Reynolds and J. Postel, RFC-1010,
        May 1987.

        This document is republished periodically with new RFC numbers;
        the latest version must be used.


   [TELNET:1]  "Telnet Protocol Specification," J. Postel and J.
        Reynolds, RFC-854, May 1983.

   [TELNET:2]  "Telnet Option Specification," J. Postel and J. Reynolds,
        RFC-855, May 1983.

   [TELNET:3]  "Telnet Binary Transmission," J. Postel and J. Reynolds,
        RFC-856, May 1983.

   [TELNET:4]  "Telnet Echo Option," J. Postel and J. Reynolds, RFC-857,
        May 1983.

   [TELNET:5]  "Telnet Suppress Go Ahead Option," J. Postel and J.

RFC1123              SUPPORT SERVICES -- MANAGEMENT         October 1989

        Reynolds, RFC-858, May 1983.

   [TELNET:6]  "Telnet Status Option," J. Postel and J. Reynolds, RFC-
        859, May 1983.

   [TELNET:7]  "Telnet Timing Mark Option," J. Postel and J. Reynolds,
        RFC-860, May 1983.

   [TELNET:8]  "Telnet Extended Options List," J. Postel and J.
        Reynolds, RFC-861, May 1983.

   [TELNET:9]  "Telnet End-Of-Record Option," J. Postel, RFC-855,
        December 1983.

   [TELNET:10] "Telnet Terminal-Type Option," J. VanBokkelen, RFC-1091,
        February 1989.

        This document supercedes RFC-930.

   [TELNET:11] "Telnet Window Size Option," D. Waitzman, RFC-1073,
        October 1988.

   [TELNET:12] "Telnet Linemode Option," D. Borman, RFC-1116, August

   [TELNET:13] "Telnet Terminal Speed Option," C. Hedrick, RFC-1079,
        December 1988.

   [TELNET:14] "Telnet Remote Flow Control Option," C. Hedrick, RFC-
        1080, November 1988.


   [TELNET:15] "Telnet Protocol," MIL-STD-1782, U.S. Department of
        Defense, May 1984.

        This document is intended to describe the same protocol as RFC-
        854.  In case of conflict, RFC-854 takes precedence, and the
        present document takes precedence over both.

   [TELNET:16] "SUPDUP Protocol," M. Crispin, RFC-734, October 1977.

   [TELNET:17] "Telnet SUPDUP Option," M. Crispin, RFC-736, October

   [TELNET:18] "Data Entry Terminal Option," J. Day, RFC-732, June 1977.

RFC1123              SUPPORT SERVICES -- MANAGEMENT         October 1989

   [TELNET:19] "TELNET Data Entry Terminal option -- DODIIS
        Implementation," A. Yasuda and T. Thompson, RFC-1043, February


   [FTP:1]  "File Transfer Protocol," J. Postel and J. Reynolds, RFC-
        959, October 1985.

   [FTP:2]  "Document File Format Standards," J. Postel, RFC-678,
        December 1974.

   [FTP:3]  "File Transfer Protocol," MIL-STD-1780, U.S. Department of
        Defense, May 1984.

        This document is based on an earlier version of the FTP
        specification (RFC-765) and is obsolete.


   [TFTP:1]  "The TFTP Protocol Revision 2," K. Sollins, RFC-783, June


   [SMTP:1]  "Simple Mail Transfer Protocol," J. Postel, RFC-821, August

   [SMTP:2]  "Standard For The Format of ARPA Internet Text Messages,"
        D. Crocker, RFC-822, August 1982.

        This document obsoleted an earlier specification, RFC-733.

   [SMTP:3]  "Mail Routing and the Domain System," C. Partridge, RFC-
        974, January 1986.

        This RFC describes the use of MX records, a mandatory extension
        to the mail delivery process.

   [SMTP:4]  "Duplicate Messages and SMTP," C. Partridge, RFC-1047,
        February 1988.

RFC1123              SUPPORT SERVICES -- MANAGEMENT         October 1989

   [SMTP:5a]  "Mapping between X.400 and RFC 822," S. Kille, RFC-987,
        June 1986.

   [SMTP:5b]  "Addendum to RFC-987," S. Kille, RFC-???, September 1987.

        The two preceding RFC's define a proposed standard for
        gatewaying mail between the Internet and the X.400 environments.

   [SMTP:6]  "Simple Mail Transfer Protocol,"  MIL-STD-1781, U.S.
        Department of Defense, May 1984.

        This specification is intended to describe the same protocol as
        does RFC-821.  However, MIL-STD-1781 is incomplete; in
        particular, it does not include MX records [SMTP:3].

   [SMTP:7]  "A Content-Type Field for Internet Messages," M. Sirbu,
        RFC-1049, March 1988.


   [DNS:1]  "Domain Names - Concepts and Facilities," P. Mockapetris,
        RFC-1034, November 1987.

        This document and the following one obsolete RFC-882, RFC-883,
        and RFC-973.

   [DNS:2]  "Domain Names - Implementation and Specification," RFC-1035,
        P. Mockapetris, November 1987.

   [DNS:3]  "Mail Routing and the Domain System," C. Partridge, RFC-974,
        January 1986.

   [DNS:4]  "DoD Internet Host Table Specification," K. Harrenstein,
        RFC-952, M. Stahl, E. Feinler, October 1985.


   [DNS:5]  "Hostname Server," K. Harrenstein, M. Stahl, E. Feinler,
        RFC-953, October 1985.

   [DNS:6]  "Domain Administrators Guide," M. Stahl, RFC-1032, November

RFC1123              SUPPORT SERVICES -- MANAGEMENT         October 1989

   [DNS:7]  "Domain Administrators Operations Guide," M. Lottor, RFC-
        1033, November 1987.

   [DNS:8]  "The Domain Name System Handbook," Vol. 4 of Internet
        Protocol Handbook, NIC 50007, SRI Network Information Center,
        August 1989.


   [BOOT:1] "Bootstrap Loading Using TFTP," R. Finlayson, RFC-906, June

   [BOOT:2] "Bootstrap Protocol (BOOTP)," W. Croft and J. Gilmore, RFC-
        951, September 1985.

   [BOOT:3] "BOOTP Vendor Information Extensions," J. Reynolds, RFC-
        1084, December 1988.

        Note: this RFC revised and obsoleted RFC-1048.

   [BOOT:4] "A Reverse Address Resolution Protocol," R. Finlayson, T.
        Mann, J. Mogul, and M. Theimer, RFC-903, June 1984.


   [MGT:1]  "IAB Recommendations for the Development of Internet Network
        Management Standards," V. Cerf, RFC-1052, April 1988.

   [MGT:2]  "Structure and Identification of Management Information for
        TCP/IP-based internets," M. Rose and K. McCloghrie, RFC-1065,
        August 1988.

   [MGT:3]  "Management Information Base for Network Management of
        TCP/IP-based internets," M. Rose and K. McCloghrie, RFC-1066,
        August 1988.

   [MGT:4]  "A Simple Network Management Protocol," J. Case, M. Fedor,
        M. Schoffstall, and C. Davin, RFC-1098, April 1989.

   [MGT:5]  "The Common Management Information Services and Protocol
        over TCP/IP," U. Warrier and L. Besaw, RFC-1095, April 1989.

   [MGT:6]  "Report of the Second Ad Hoc Network Management Review
        Group," V. Cerf, RFC-1109, August 1989.

RFC1123              SUPPORT SERVICES -- MANAGEMENT         October 1989

Security Considerations

   There are many security issues in the application and support
   programs of host software, but a full discussion is beyond the scope
   of this RFC.  Security-related issues are mentioned in sections
   concerning TFTP (Sections 4.2.1,,, the SMTP VRFY and
   EXPN commands (Section 5.2.3), the SMTP HELO command (5.2.5), and the
   SMTP DATA command (Section 5.2.8).

Author's Address

   Robert Braden
   USC/Information Sciences Institute
   4676 Admiralty Way
   Marina del Rey, CA 90292-6695

   Phone: (213) 822 1511

   EMail: Braden@ISI.EDU