Network Working Group J. Reynolds
Request for Comments: 1497 ISI
Obsoletes: 1395, 1084, 1048 August 1993
Updates: 951
BOOTP Vendor Information Extensions
Status of this Memo
This memo is a status report on the vendor information extensions
used in the Bootstrap Protocol (BOOTP). Distribution of this memo is
unlimited.
Introduction
This RFC is a slight revision and extension of RFC-1048 by Philip
Prindeville, who should be credited with the original work in this
memo. This memo will be updated as additional tags are are defined.
This edition introduces Tag 18 for Extension Path.
As workstations and personal computers proliferate on the Internet,
the administrative complexity of maintaining a network is increased
by an order of magnitude. The assignment of local network resources
to each client represents one such difficulty. In most environments,
delegating such responsibility to the user is not plausible and,
indeed, the solution is to define the resources in uniform terms, and
to automate their assignment.
The basic Bootstrap Protocol [RFC-951] dealt with the issue of
assigning an internet address to a client, as well as a few other
resources. The protocol included provisions for vendor-defined
resource information.
This memo defines a (potentially) vendor-independent interpretation
of this resource information.
Overview of BOOTP
While the Reverse Address Resolution (RARP) Protocol [RFC-903] may be
used to assign an IP address to a local network hardware address, it
provides only part of the functionality needed. Though this protocol
can be used in conjunction with other supplemental protocols (the
Resource Location Protocol [RFC-887], the Domain Name System [RFC-
1034]), a more integrated solution may be desirable.
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RFC 1497 BOOTP Extensions August 1993
Bootstrap Protocol (BOOTP) is a UDP/IP-based protocol that allows a
booting host to configure itself dynamically, and more significantly,
without user supervision. It provides a means to assign a host its
IP address, a file from which to download a boot program from some
server, that server's address, and (if present) the address of an
Internet gateway.
One obvious advantage of this procedure is the centralized management
of network addresses, which eliminates the need for per-host unique
configuration files. In an environment with several hundred hosts,
maintaining local configuration information and operating system
versions specific to each host might otherwise become chaotic. By
categorizing hosts into classes and maintaining configuration
information and boot programs for each class, the complexity of this
chore may be reduced in magnitude.
BOOTP Vendor Information Format
The full description of the BOOTP request/reply packet format may be
found in [RFC-951]. The rest of this document will concern itself
with the last field of the packet, a 64 octet area reserved for
vendor information, to be used in a hitherto unspecified fashion. A
generalized use of this area for giving information useful to a wide
class of machines, operating systems, and configurations follows. In
situations where a single BOOTP server is to be used among
heterogeneous clients in a single site, a generic class of data may
be used.
Vendor Information "Magic Cookie"
As suggested in [RFC-951], the first four bytes of this field have
been assigned to the magic cookie, which identifies the mode in
which the succeeding data is to be interpreted. The value of the
magic cookie is the 4 octet dotted decimal 99.130.83.99 (or
hexadecimal number 63.82.53.63) in network byte order.
Format of Individual Fields
The vendor information field has been implemented as a free
format, with extendable tagged sub-fields. These sub-fields are
length tagged (with exceptions; see below), allowing clients not
implementing certain types to correctly skip fields they cannot
interpret. Lengths are exclusive of the tag and length octets;
all multi-byte quantities are in network byte-order.
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Fixed Length Data
The fixed length data are comprised of two formats. Those that
have no data consist of a single tag octet and are implicitly
of one-octet length, while those that contain data consist of
one tag octet, one length octet, and length octets of data.
Pad Field (Tag: 0, Data: None)
May be used to align subsequent fields to word boundaries
required by the target machine (i.e., 32-bit quantities such
as IP addresses on 32-bit boundaries).
Subnet Mask Field (Tag: 1, Data: 4 subnet mask bytes)
Specifies the net and local subnet mask as per the standard
on subnetting [RFC-950]. For convenience, this field must
precede the GATEWAY field (below), if present.
Time Offset Field (Tag: 2, Data: 4 time offset bytes)
Specifies the time offset of the local subnet in seconds
from Coordinated Universal Time (UTC); signed 32-bit
integer.
End Field (Tag: 255, Data: None)
Specifies end of usable data in the vendor information area.
The rest of this field should be filled with PAD zero)
octets.
Variable Length Data
The variable length data has a single format; it consists of
one tag octet, one length octet, and length octets of data.
Gateway Field (Tag: 3, Data: N address bytes)
Specifies the IP addresses of N/4 gateways for this subnet.
If one of many gateways is preferred, that should be first.
Time Server Field (Tag: 4, Data: N address bytes)
Specifies the IP addresses of N/4 time servers [RFC-868].
IEN-116 Name Server Field (Tag: 5, Data: N address bytes)
Specifies the IP addresses of N/4 name servers [IEN-116].
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RFC 1497 BOOTP Extensions August 1993
Domain Name Server Field (Tag: 6, Data: N address bytes)
Specifies the IP addresses of N/4 domain name servers RFC-
1034].
Log Server Field (Tag: 7, Data: N address bytes)
Specifies the IP addresses of N/4 MIT-LCS UDP log server
[LOGGING].
Cookie/Quote Server Field (Tag: 8, Data: N address bytes)
Specifies the IP addresses of N/4 Quote of the Day servers
[RFC-865].
LPR Server Field (Tag: 9, Data: N address bytes)
Specifies the IP addresses of N/4 Berkeley 4BSD printer
servers [LPD].
Impress Server Field (Tag: 10, Data: N address bytes)
Specifies the IP addresses of N/4 Impress network image
servers [IMAGEN].
RLP Server Field (Tag: 11, Data: N address bytes)
Specifies the IP addresses of N/4 Resource Location Protocol
(RLP) servers [RFC-887].
Hostname (Tag: 12, Data: N bytes of hostname)
Specifies the name of the client. The name may or may not
domain qualified: this is a site-specific issue.
Boot File Size (Tag: 13, Data: 2)
A two octet value (in network order) specifying the number
of 512 octet blocks in the default boot file. Informs BOOTP
client how large the BOOTP file image is.
Merit Dump File (Tag: 14, Data: N bytes of filename)
Name of a file to dump core of this client to.
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Domain Name (Tag: 15, Data: N bytes of domain name)
Specifies the domain name of the client for Domain Name
Server (DNS) resolution [RFC-1034].
Swap Server (Tag: 16, Data: 4 address bytes)
An IP address to hold the IP address of a swap server.
Root Path (Tag: 17, Data: N bytes of path name)
A string to specify a pathname to mount as a root disk.
Extensions Path (Tag: 18, Data: N bytes of path name)
A string to specify a file, retrievable via TFTP, which
contains information which can be interpreted in the same
way as the 64-octet vendor-extension field within the BOOTP
response, with the following exceptions:
- the length of the file is unconstrained;
- all references to Tag 18 (i.e., instances of the
BOOTP Extensions Path field) within the file are
ignored.
Reserved Fields (Tag: 128-254, Data: N bytes of undefined
content)
Specifies additional site-specific information, to be
interpreted on an implementation-specific basis. This
should follow all data with the preceding generic tags 0-
127).
Extensions
Additional generic data fields may be registered by contacting:
Internet Assigned Numbers Authority (IANA)
Information Sciences Institute
University of Southern California
4676 Admiralty Way
Marina del Rey, California 90292-6695
or by email as: iana@isi.edu
Implementation specific use of undefined generic types (those in the
range 19-127) may conflict with other implementations, and
registration is required.
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RFC 1497 BOOTP Extensions August 1993
When selecting information to put into the vendor specific area, care
should be taken to not exceed the 64 byte length restriction.
Nonessential information (such as host name and quote of the day
server) may be excluded, which may later be located with a more
appropriate service protocol, such as RLP or the WKS resource-type of
the domain name system. Indeed, even RLP servers may be discovered
using a broadcast request to locate a local RLP server.
Comparison to Alternative Approaches
Extending BOOTP to provide more configuration information than the
minimum required by boot PROMs may not be necessary. Rather than
having each module in a host (e.g., the time module, the print
spooler, the domain name resolver) broadcast to the BOOTP server to
obtain the addresses of required servers, it would be better for each
of them to multicast directly to the particular server group of
interest, possibly using "expanding ring" multicasts.
The multicast approach has the following advantages over the BOOTP
approach:
- It eliminates dependency on a third party (the BOOTP server) that
may be temporarily unavailable or whose database may be incorrect or
incomplete. Multicasting directly to the desired services will
locate those servers that are currently available, and only those.
- It reduces the administrative chore of keeping the (probably
replicated) BOOTP database up-to-date and consistent. This is
especially important in an environment with a growing number of
services and an evolving population of servers.
- In some cases, it reduces the amount of packet traffic and/or the
delay required to get the desired information. For example, the
current time can be obtained by a single multicast to a time server
group which evokes replies from those time servers that are
currently up. The BOOTP approach would require a broadcast to the
BOOTP server, a reply from the BOOTP server, one or more unicasts to
time servers (perhaps waiting for long timeouts if the initially
chosen server(s) are down), and finally a reply from a server.
One apparent advantage of the proposed BOOTP extensions is that they
provide a uniform way to locate servers. However, the multicast
approach could also be implemented in a consistent way across
multiple services. The V System naming protocol is a good example of
this; character string pathnames are used to name any number of
resources (i.e., not just files) and a standard subroutine library
looks after multicasting to locate the resources, caching the
discovered locations, and detecting stale cache data.
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Another apparent advantage of the BOOTP approach is that it allows an
administrator to easily control which hosts use which servers. The
multicast approach favors more distributed control over resource
allocation, where each server decides which hosts it will serve,
using whatever level of authentication is appropriate for the
particular service. For example, time servers usually don't care who
they serve (i.e., administrative control via the BOOTP database is
unnecessary), whereas file servers usually require strong
authentication (i.e., administrative control via the BOOTP database
is insufficient).
The main drawback of the multicast approach, of course, is that IP
multicasting is not widely implemented, and there is a need to locate
existing services which do not understand IP multicasts.
The BOOTP approach may be most efficient in the case that all the
information needed by the client host is returned by a single BOOTP
reply and each program module simply reads the information it needs
from a local table filled in by the BOOTP reply.
Acknowledgments
The following people provided helpful comments on the first edition
of this memo: Drew Perkins, of Carnagie Mellon University, Bill
Croft, of Stanford University, and co-author of BOOTP, and Steve
Deering, also of Stanford University, for contributing the
"Comparison to Alternative Approaches" section.
References
[RFC-951] Croft, B., and J. Gilmore, "Bootstrap Protocol (BOOTP)",
RFC 951, Stanford and SUN Microsystems, September 1985.
[RFC-903] Finlayson, R., Mann, T., Mogul, J., and M. Theimer, "A
Reverse Address Resolution Protocol", STD 38, RFC 903,
Stanford, June 1984.
[RFC-887] Accetta, M., "Resource Location Protocol", RFC 887, CMU,
December 1983.
[RFC-1034] Mockapetris, P., "Domain Names - Concepts and
Facilities", STD 13, RFC 1034, USC/Information Sciences
Institute, November 1987.
[RFC-950] Mogul, J., and J. Postel, "Internet Standard Subnetting
Procedure", STD 5, RFC 950, USC/Information Sciences
Institute, August 1985.
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[RFC-868] Postel, J., "Time Protocol", STD 26, RFC 868,
USC/Information Sciences Institute, May 1983.
[IEN-116] Postel, J., "Internet Name Server", USC/Information
Sciences Institute, August 1979.
[LOGGING] Clark, D., "Logging and Status Protocol", Massachusetts
Institute of Technology Laboratory for Computer Science,
Cambridge, Massachusetts, 1981.
[RFC-865] Postel, J., "Quote of the Day Protocol", STD 23, RFC 865,
USC/Information Sciences Institute, May 1983.
[LPD] Campbell, R., "4.2BSD Line Printer Spooler Manual", UNIX
Programmer's Manual, Vol II, University of California at
Berkeley, Computer Science Division, July 1983.
[IMAGEN] "Image Server XT Programmer's Guide", Imagen Corporation,
Santa Clara, California, August 1986.
Security Considerations
Security issues are not discussed in this memo.
Author's Address:
Joyce K. Reynolds
Information Sciences Institute
University of Southern California
4676 Admiralty Way
Marina del Rey, CA 90292
Phone: (310) 822-1511
EMail: jkrey@isi.edu
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