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rfc:rfc951

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
    parsing.
    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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RFC 951 September 1985 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
    servers.

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'
 255.255.255.255.  This address means 'broadcast on the local cable,
 (I don't know my net number)' [4].

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RFC 951 September 1985 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
 verified/computed.
    FIELD   BYTES   DESCRIPTION
    -----   -----   -----------
       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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RFC 951 September 1985 Bootstrap Protocol

  1. - 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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RFC 951 September 1985 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
       capability.
       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
 above.

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RFC 951 September 1985 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 255.255.255.255.  (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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RFC 951 September 1985 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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RFC 951 September 1985 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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RFC 951 September 1985 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
    server.

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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RFC 951 September 1985 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'
 string.
 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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RFC 951 September 1985 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
    homedirectory
    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
    /usr/boot
    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     36.19.0.5
    burr            1 02.60.8c.34.11.78     36.44.0.12
    101-gateway     1 02.60.8c.23.ab.35     36.44.0.32      gate 101
    mjh-gateway     1 02.60.8c.12.32.bc     36.42.0.64      gate mjh
    welch-tipa      1 02.60.8c.22.65.32     36.47.0.14      tip
    welch-tipb      1 02.60.8c.12.15.c8     36.46.0.12      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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RFC 951 September 1985 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.

REFERENCES

 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.

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