TitleBootstrap Protocol
AuthorW.J. Croft, J. Gilmore
DateSeptember 1985
Format:TXT, HTML
Updated byRFC1395, RFC1497, RFC1532, RFC1542, RFC5494

Network Working Group                   Bill Croft (Stanford University)
Request for Comments: 951                John Gilmore (Sun Microsystems)
                                                          September 1985

                       BOOTSTRAP PROTOCOL (BOOTP)

1. Status of this Memo

   This RFC suggests a proposed protocol for the ARPA-Internet
   community, and requests discussion and suggestions for improvements.
   Distribution of this memo is unlimited.

2. Overview

   This RFC describes an IP/UDP bootstrap protocol (BOOTP) which allows
   a diskless client machine to discover its own IP address, the address
   of a server host, and the name of a file to be loaded into memory and
   executed.  The bootstrap operation can be thought of as consisting of
   TWO PHASES.  This RFC describes the first phase, which could be
   labeled 'address determination and bootfile selection'.  After this
   address and filename information is obtained, control passes to the
   second phase of the bootstrap where a file transfer occurs.  The file
   transfer will typically use the TFTP protocol [9], since it is
   intended that both phases reside in PROM on the client.  However
   BOOTP could also work with other protocols such as SFTP [3] or
   FTP [6].

   We suggest that the client's PROM software provide a way to do a
   complete bootstrap without 'user' interaction.  This is the type of
   boot that would occur during an unattended power-up.  A mechanism
   should be provided for the user to manually supply the necessary
   address and filename information to bypass the BOOTP protocol and
   enter the file transfer phase directly.  If non-volatile storage is
   available, we suggest keeping default settings there and bypassing
   the BOOTP protocol unless these settings cause the file transfer
   phase to fail.  If the cached information fails, the bootstrap should
   fall back to phase 1 and use BOOTP.

   Here is a brief outline of the protocol:

      1. A single packet exchange is performed.  Timeouts are used to
      retransmit until a reply is received.  The same packet field
      layout is used in both directions.  Fixed length fields of maximum
      reasonable length are used to simplify structure definition and

      2. An 'opcode' field exists with two values.  The client
      broadcasts a 'bootrequest' packet.  The server then answers with a
      'bootreply' packet.  The bootrequest contains the client's
      hardware address and its IP address, if known.

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Bootstrap Protocol

      3. The request can optionally contain the name of the server the
      client wishes to respond.  This is so the client can force the
      boot to occur from a specific host (e.g. if multiple versions of
      the same bootfile exist or if the server is in a far distant
      net/domain).  The client does not have to deal with name / domain
      services; instead this function is pushed off to the BOOTP server.

      4. The request can optionally contain the 'generic' filename to be
      booted.  For example 'unix' or 'ethertip'.  When the server sends
      the bootreply, it replaces this field with the fully qualified
      path name of the appropriate boot file.  In determining this name,
      the server may consult his own database correlating the client's
      address and filename request, with a particular boot file
      customized for that client.  If the bootrequest filename is a null
      string, then the server returns a filename field indicating the
      'default' file to be loaded for that client.

      5. In the case of clients who do not know their IP addresses, the
      server must also have a database relating hardware address to IP
      address.  This client IP address is then placed into a field in
      the bootreply.

      6. Certain network topologies (such as Stanford's) may be such
      that a given physical cable does not have a TFTP server directly
      attached to it (e.g. all the gateways and hosts on a certain cable
      may be diskless).  With the cooperation of neighboring gateways,
      BOOTP can allow clients to boot off of servers several hops away,
      through these gateways.  See the section 'Booting Through
      Gateways' below.  This part of the protocol requires no special
      action on the part of the client.  Implementation is optional and
      requires a small amount of additional code in gateways and

3. Packet Format

   All numbers shown are decimal, unless indicated otherwise.  The BOOTP
   packet is enclosed in a standard IP [8] UDP [7] datagram.  For
   simplicity it is assumed that the BOOTP packet is never fragmented.
   Any numeric fields shown are packed in 'standard network byte order',
   i.e. high order bits are sent first.

   In the IP header of a bootrequest, the client fills in its own IP
   source address if known, otherwise zero.  When the server address is
   unknown, the IP destination address will be the 'broadcast address'  This address means 'broadcast on the local cable,
   (I don't know my net number)' [4].

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Bootstrap Protocol

   The UDP header contains source and destination port numbers.  The
   BOOTP protocol uses two reserved port numbers, 'BOOTP client' (68)
   and 'BOOTP server' (67).  The client sends requests using 'BOOTP
   server' as the destination port; this is usually a broadcast.  The
   server sends replies using 'BOOTP client' as the destination port;
   depending on the kernel or driver facilities in the server, this may
   or may not be a broadcast (this is explained further in the section
   titled 'Chicken/Egg issues' below).  The reason TWO reserved ports
   are used, is to avoid 'waking up' and scheduling the BOOTP server
   daemons, when a bootreply must be broadcast to a client.  Since the
   server and other hosts won't be listening on the 'BOOTP client' port,
   any such incoming broadcasts will be filtered out at the kernel
   level.  We could not simply allow the client to pick a 'random' port
   number for the UDP source port field; since the server reply may be
   broadcast, a randomly chosen port number could confuse other hosts
   that happened to be listening on that port.

   The UDP length field is set to the length of the UDP plus BOOTP
   portions of the packet.  The UDP checksum field can be set to zero by
   the client (or server) if desired, to avoid this extra overhead in a
   PROM implementation.  In the 'Packet Processing' section below the
   phrase '[UDP checksum.]' is used whenever the checksum might be

      -----   -----   -----------

         op      1       packet op code / message type.
                         1 = BOOTREQUEST, 2 = BOOTREPLY

         htype   1       hardware address type,
                         see ARP section in "Assigned Numbers" RFC.
                         '1' = 10mb ethernet

         hlen    1       hardware address length
                         (eg '6' for 10mb ethernet).

         hops    1       client sets to zero,
                         optionally used by gateways
                         in cross-gateway booting.

         xid     4       transaction ID, a random number,
                         used to match this boot request with the
                         responses it generates.

         secs    2       filled in by client, seconds elapsed since
                         client started trying to boot.

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Bootstrap Protocol

         --      2       unused

         ciaddr  4       client IP address;
                         filled in by client in bootrequest if known.

         yiaddr  4       'your' (client) IP address;
                         filled by server if client doesn't
                         know its own address (ciaddr was 0).

         siaddr  4       server IP address;
                         returned in bootreply by server.

         giaddr  4       gateway IP address,
                         used in optional cross-gateway booting.

         chaddr  16      client hardware address,
                         filled in by client.

         sname   64      optional server host name,
                         null terminated string.

         file    128     boot file name, null terminated string;
                         'generic' name or null in bootrequest,
                         fully qualified directory-path
                         name in bootreply.

         vend    64      optional vendor-specific area,
                         e.g. could be hardware type/serial on request,
                         or 'capability' / remote file system handle
                         on reply.  This info may be set aside for use
                         by a third phase bootstrap or kernel.

4. Chicken / Egg Issues

   How can the server send an IP datagram to the client, if the client
   doesnt know its own IP address (yet)?  Whenever a bootreply is being
   sent, the transmitting machine performs the following operations:

      1. If the client knows its own IP address ('ciaddr' field is
      nonzero), then the IP can be sent 'as normal', since the client
      will respond to ARPs [5].

      2. If the client does not yet know its IP address (ciaddr zero),
      then the client cannot respond to ARPs sent by the transmitter of
      the bootreply.  There are two options:

         a. If the transmitter has the necessary kernel or driver hooks

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Bootstrap Protocol

         to 'manually' construct an ARP address cache entry, then it can
         fill in an entry using the 'chaddr' and 'yiaddr' fields.  Of
         course, this entry should have a timeout on it, just like any
         other entry made by the normal ARP code itself.  The
         transmitter of the bootreply can then simply send the bootreply
         to the client's IP address.  UNIX (4.2 BSD) has this

         b. If the transmitter lacks these kernel hooks, it can simply
         send the bootreply to the IP broadcast address on the
         appropriate interface.  This is only one additional broadcast
         over the previous case.

5. Client Use of ARP

   The client PROM must contain a simple implementation of ARP, e.g. the
   address cache could be just one entry in size.  This will allow a
   second-phase-only boot (TFTP) to be performed when the client knows
   the IP addresses and bootfile name.

   Any time the client is expecting to receive a TFTP or BOOTP reply, it
   should be prepared to answer an ARP request for its own IP to
   hardware address mapping (if known).

   Since the bootreply will contain (in the hardware encapsulation) the
   hardware source address of the server/gateway, the client MAY be able
   to avoid sending an ARP request for the server/gateway IP address to
   be used in the following TFTP phase.  However this should be treated
   only as a special case, since it is desirable to still allow a
   second-phase-only boot as described above.

6. Comparison to RARP

   An earlier protocol, Reverse Address Resolution Protocol (RARP) [1]
   was proposed to allow a client to determine its IP address, given
   that it knew its hardware address.  However RARP had the disadvantage
   that it was a hardware link level protocol (not IP/UDP based).  This
   means that RARP could only be implemented on hosts containing special
   kernel or driver modifications to access these 'raw' packets.  Since
   there are many network kernels existent now, with each source
   maintained by different organizations, a boot protocol that does not
   require kernel modifications is a decided advantage.

   BOOTP provides this hardware to IP address lookup function, in
   addition to the other useful features described in the sections

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Bootstrap Protocol

7. Packet Processing

   7.1. Client Transmission

      Before setting up the packet for the first time, it is a good idea
      to clear the entire packet buffer to all zeros; this will place
      all fields in their default state.  The client then creates a
      packet with the following fields.

      The IP destination address is set to  (the
      broadcast address) or to the server's IP address (if known).  The
      IP source address and 'ciaddr' are set to the client's IP address
      if known, else 0.  The UDP header is set with the proper length;
      source port = 'BOOTP client' port destination port = 'BOOTP
      server' port.

      'op' is set to '1', BOOTREQUEST.  'htype' is set to the hardware
      address type as assigned in the ARP section of the "Assigned
      Numbers" RFC. 'hlen' is set to the length of the hardware address,
      e.g. '6' for 10mb ethernet.

      'xid' is set to a 'random' transaction id.  'secs' is set to the
      number of seconds that have elapsed since the client has started
      booting.  This will let the servers know how long a client has
      been trying.  As the number gets larger, certain servers may feel
      more 'sympathetic' towards a client they don't normally service.
      If a client lacks a suitable clock, it could construct a rough
      estimate using a loop timer.  Or it could choose to simply send
      this field as always a fixed value, say 100 seconds.

      If the client knows its IP address, 'ciaddr' (and the IP source
      address) are set to this value.  'chaddr' is filled in with the
      client's hardware address.

      If the client wishes to restrict booting to a particular server
      name, it may place a null-terminated string in 'sname'.  The name
      used should be any of the allowable names or nicknames of the
      desired host.

      The client has several options for filling the 'file' name field.
      If left null, the meaning is 'I want to boot the default file for
      my machine'.  A null file name can also mean 'I am only interested
      in finding out client/server/gateway IP addresses, I dont care
      about file names'.

      The field can also be a 'generic' name such as 'unix' or

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Bootstrap Protocol

      'gateway'; this means 'boot the named program configured for my
      machine'.  Finally the field can be a fully directory qualified
      path name.

      The 'vend' field can be filled in by the client with
      vendor-specific strings or structures.  For example the machine
      hardware type or serial number may be placed here.  However the
      operation of the BOOTP server should not DEPEND on this
      information existing.

      If the 'vend' field is used, it is recommended that a 4 byte
      'magic number' be the first item within 'vend'.  This lets a
      server determine what kind of information it is seeing in this
      field.  Numbers can be assigned by the usual 'magic number'
      process --you pick one and it's magic.  A different magic number
      could be used for bootreply's than bootrequest's to allow the
      client to take special action with the reply information.

      [UDP checksum.]

   7.2. Client Retransmission Strategy

      If no reply is received for a certain length of time, the client
      should retransmit the request.  The time interval must be chosen
      carefully so as not to flood the network.  Consider the case of a
      cable containing 100 machines that are just coming up after a
      power failure.  Simply retransmitting the request every four
      seconds will inundate the net.

      As a possible strategy, you might consider backing off
      exponentially, similar to the way ethernet backs off on a
      collision.  So for example if the first packet is at time 0:00,
      the second would be at :04, then :08, then :16, then :32, then
      :64.  You should also randomize each time; this would be done
      similar to the ethernet specification by starting with a mask and
      'and'ing that with with a random number to get the first backoff.
      On each succeeding backoff, the mask is increased in length by one
      bit.  This doubles the average delay on each backoff.

      After the 'average' backoff reaches about 60 seconds, it should be
      increased no further, but still randomized.

      Before each retransmission, the client should update the 'secs'
      field. [UDP checksum.]

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Bootstrap Protocol

   7.3. Server Receives BOOTREQUEST

      [UDP checksum.]  If the UDP destination port does not match the
      'BOOTP server' port, discard the packet.

      If the server name field (sname) is null (no particular server
      specified), or sname is specified and matches our name or
      nickname, then continue with packet processing.

      If the sname field is specified, but does not match 'us', then
      there are several options:

         1. You may choose to simply discard this packet.

         2. If a name lookup on sname shows it to be on this same cable,
         discard the packet.

         3. If sname is on a different net, you may choose to forward
         the packet to that address.  If so, check the 'giaddr' (gateway
         address) field.  If 'giaddr' is zero, fill it in with my
         address or the address of a gateway that can be used to get to
         that net.  Then forward the packet.

      If the client IP address (ciaddr) is zero, then the client does
      not know its own IP address.  Attempt to lookup the client
      hardware address (chaddr, hlen, htype) in our database.  If no
      match is found, discard the packet.  Otherwise we now have an IP
      address for this client; fill it into the 'yiaddr' (your IP
      address) field.

      We now check the boot file name field (file).  The field will be
      null if the client is not interested in filenames, or wants the
      default bootfile.  If the field is non-null, it is used as a
      lookup key in a database, along with the client's IP address.  If
      there is a default file or generic file (possibly indexed by the
      client address) or a fully-specified path name that matches, then
      replace the 'file' field with the fully-specified path name of the
      selected boot file.  If the field is non-null and no match was
      found, then the client is asking for a file we dont have; discard
      the packet, perhaps some other BOOTP server will have it.

      The 'vend' vendor-specific data field should now be checked and if
      a recognized type of data is provided, client-specific actions
      should be taken, and a response placed in the 'vend' data field of
      the reply packet.  For example, a workstation client could provide

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Bootstrap Protocol

      an authentication key and receive from the server a capability for
      remote file access, or a set of configuration options, which can
      be passed to the operating system that will shortly be booted in.

      Place my (server) IP address in the 'siaddr' field.  Set the 'op'
      field to BOOTREPLY.  The UDP destination port is set to 'BOOTP
      client'.  If the client address 'ciaddr' is nonzero, send the
      packet there; else if the gateway address 'giaddr' is nonzero, set
      the UDP destination port to 'BOOTP server' and send the packet to
      'giaddr'; else the client is on one of our cables but it doesnt
      know its own IP address yet --use a method described in the 'Egg'
      section above to send it to the client. If 'Egg' is used and we
      have multiple interfaces on this host, use the 'yiaddr' (your IP
      address) field to figure out which net (cable/interface) to send
      the packet to.  [UDP checksum.]

   7.4. Server/Gateway Receives BOOTREPLY

      [UDP checksum.]  If 'yiaddr' (your [the client's] IP address)
      refers to one of our cables, use one of the 'Egg' methods above to
      forward it to the client.  Be sure to send it to the 'BOOTP
      client' UDP destination port.

   7.5. Client Reception

      Don't forget to process ARP requests for my own IP address (if I
      know it).  [UDP checksum.]  The client should discard incoming
      packets that: are not IP/UDPs addressed to the boot port; are not
      BOOTREPLYs; do not match my IP address (if I know it) or my
      hardware address; do not match my transaction id.  Otherwise we
      have received a successful reply. 'yiaddr' will contain my IP
      address, if I didnt know it before.  'file' is the name of the
      file name to TFTP 'read request'.  The server address is in
      'siaddr'.  If 'giaddr' (gateway address) is nonzero, then the
      packets should be forwarded there first, in order to get to the

8. Booting Through Gateways

   This part of the protocol is optional and requires some additional
   code in cooperating gateways and servers, but it allows cross-gateway
   booting.  This is mainly useful when gateways are diskless machines.
   Gateways containing disks (e.g. a UNIX machine acting as a gateway),
   might as well run their own BOOTP/TFTP servers.

   Gateways listening to broadcast BOOTREQUESTs may decide to forward or
   rebroadcast these requests 'when appropriate'.  For example, the

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Bootstrap Protocol

   gateway could have, as part of his configuration tables, a list of
   other networks or hosts to receive a copy of any broadcast
   BOOTREQUESTs.  Even though a 'hops' field exists, it is a poor idea
   to simply globally rebroadcast the requests, since broadcast loops
   will almost certainly occur.

   The forwarding could begin immediately, or wait until the 'secs'
   (seconds client has been trying) field passes a certain threshold.

   If a gateway does decide to forward the request, it should look at
   the 'giaddr' (gateway IP address) field.  If zero, it should plug its
   own IP address (on the receiving cable) into this field.  It may also
   use the 'hops' field to optionally control how far the packet is
   reforwarded. Hops should be incremented on each forwarding.  For
   example, if hops passes '3', the packet should probably be discarded.
   [UDP checksum.]

   Here we have recommended placing this special forwarding function in
   the gateways.  But that does not have to be the case.  As long as
   some 'BOOTP forwarding agent' exists on the net with the booting
   client, the agent can do the forwarding when appropriate.  Thus this
   service may or may not be co-located with the gateway.

   In the case of a forwarding agent not located in the gateway, the
   agent could save himself some work by plugging the broadcast address
   of the interface receiving the bootrequest into the 'giaddr' field.
   Thus the reply would get forwarded using normal gateways, not
   involving the forwarding agent.  Of course the disadvantage here is
   that you lose the ability to use the 'Egg' non-broadcast method of
   sending the reply, causing extra overhead for every host on the
   client cable.

9. Sample BOOTP Server Database

   As a suggestion, we show a sample text file database that the BOOTP
   server program might use.  The database has two sections, delimited
   by a line containing an percent in column 1.  The first section
   contains a 'default directory' and mappings from generic names to
   directory/pathnames.  The first generic name in this section is the
   'default file' you get when the bootrequest contains a null 'file'

   The second section maps hardware addresstype/address into an
   ipaddress. Optionally you can also overide the default generic name
   by supplying a ipaddress specific genericname.  A 'suffix' item is
   also an option; if supplied, any generic names specified by the
   client will be accessed by first appending 'suffix' to the 'pathname'

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Bootstrap Protocol

   appropriate to that generic name.  If that file is not found, then
   the plain 'pathname' will be tried.  This 'suffix' option allows a
   whole set of custom generics to be setup without a lot of effort.
   Below is shown the general format; fields are delimited by one or
   more spaces or tabs; trailing empty fields may be omitted; blank
   lines and lines beginning with '#' are ignored.

      # comment line

      genericname1    pathname1
      genericname2    pathname2

      % end of generic names, start of address mappings

      hostname1 hardwaretype hardwareaddr1 ipaddr1 genericname suffix
      hostname2 hardwaretype hardwareaddr2 ipaddr2 genericname suffix

   Here is a specific example.  Note the 'hardwaretype' number is the
   same as that shown in the ARP section of the 'Assigned Numbers' RFC.
   The 'hardwaretype' and 'ipaddr' numbers are in decimal;
   'hardwareaddr' is in hex.

      # last updated by smith

      vmunix          vmunix
      tip             ethertip
      watch           /usr/diag/etherwatch
      gate            gate.

      % end of generic names, start of address mappings

      hamilton        1 02.60.8c.06.34.98
      burr            1 02.60.8c.34.11.78
      101-gateway     1 02.60.8c.23.ab.35      gate 101
      mjh-gateway     1 02.60.8c.12.32.bc      gate mjh
      welch-tipa      1 02.60.8c.22.65.32      tip
      welch-tipb      1 02.60.8c.12.15.c8      tip

   In the example above, if 'mjh-gateway' does a default boot, it will
   get the file '/usr/boot/gate.mjh'.

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Bootstrap Protocol

10. Acknowledgements

   Ross Finlayson (et. al.) produced two earlier RFC's discussing TFTP
   bootstraping [2] using RARP [1].

   We would also like to acknowledge the previous work and comments of
   Noel Chiappa, Bob Lyon, Jeff Mogul, Mark Lewis, and David Plummer.


   1.  Ross Finlayson, Timothy Mann, Jeffrey Mogul, Marvin Theimer.  A
       Reverse Address Resolution Protocol.  RFC 903, NIC, June, 1984.

   2.  Ross Finlayson.  Bootstrap Loading using TFTP.  RFC 906, NIC,
       June, 1984.

   3.  Mark Lottor.  Simple File Transfer Protocol.  RFC 913, NIC,
       September, 1984.

   4.  Jeffrey Mogul.  Broadcasting Internet Packets.  RFC 919, NIC,
       October, 1984.

   5.  David Plummer.  An Ethernet Address Resolution Protocol.  RFC
       826, NIC, September, 1982.

   6.  Jon Postel.  File Transfer Protocol.  RFC 765, NIC, June, 1980.

   7.  Jon Postel.  User Datagram Protocol.  RFC 768, NIC, August, 1980.

   8.  Jon Postel.  Internet Protocol.  RFC 791, NIC, September, 1981.

   9.  K. R. Sollins, Noel Chiappa.  The TFTP Protocol.  RFC 783, NIC,
       June, 1981.