Network Working Group R. Johnson
Internet-Draft Cisco Systems, Inc.
Expires: August 7, 2005 February 6, 2005
TFTP Server Address DHCP Option
draft-raj-dhc-tftp-addr-option-00.txt
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Copyright (C) The Internet Society (2005).
Abstract
This memo defines the "TFTP Server Address" option as it is currently
in use by Cisco Systems. The option number currently in use is 150.
This memo documents the current usage of the option in agreement with
RFC-3942 [7] , which declares that any pre-existing usages of option
numbers in the range 128 - 223 should be documented and the working
group will try to officially assign those numbers to those options.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. TFTP Server Address Option Definition . . . . . . . . . . . . 4
3. Security Considerations . . . . . . . . . . . . . . . . . . . 5
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
5. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 6
Intellectual Property and Copyright Statements . . . . . . . . 7
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1. Introduction
Small devices, such as IP phones, have a need for downloading their
configuration from a TFTP server on the network. There are commonly
accepted methods to discover this server via DHCP; the "sname" field
in the DHCP header [2] , the "TFTP Server name" option (#66) [3] .
Both of these sources of information, however, contain the TFTP
server's hostname. That hostname must then be translated to an IP
address. The usual method to accomplish this would be DNS. [4] This
means the firmware in a small "network appliance" device would need
to implement the DNS protocol in order to perform this translation.
This unacceptably increases the firmware code size.
In order to keep the firmware in a small "network appliance" to a
minimum, it was decided that it would be best to introduce the "TFTP
Server Address" option (#150). This option allows the DHCP server to
pass the IP address of the TFTP server instead of the hostname, thus
making the information directly usable by the "network appliance".
In cases where both "sname" or "TFTP Server name" appear in a DHCP
response packet along with the "TFTP Server address" option in the
same packet, it is left to the device to decide which piece of
information to use.
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2. TFTP Server Address Option Definition
The TFTP Server Address option is a DHCP option [3]. The option
contains the IPv4 address of the TFTP server which the client should
use (if needed).
The format of the option is:
Code Len IPv4 TFTP Server Address(es)
+-----+-----+-----+-----+-----+-----+
| 150 | n | IPv4 address | ...
+-----+-----+-----+-----+-----+-----+
Figure 1
The option minimum length (n) is 4.
The "Len" field must specify a length which is an integral multiple
of 4 octets (4, 8, 12, etc.). If an option is received where this is
not the case, the option information SHOULD be ignored. Dividing
this "Len" value by 4 will give number of IPv4 TFTP server addresses
which are specified in the option.
The option SHOULD NOT be specified by the DHCP Client.
Server addresses SHOULD be listed in order of preference.
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3. Security Considerations
A rogue DHCP Server could use this option in order to coerce a Client
into downloading configuration from an alternate TFTP and thus gain
control of the device's configuration. This is easier done with the
TFTP Server Address option than it was with the TFTP Server Name
option, because in the later case the attack would need to control
DNS responses as well as inserting the rogue DHCP option information.
If this may be a concern, then either DHCP Authentication may be
used, or simply make use of the TFTP Server Name option instead.
Message authentication in DHCP for intradomain use where the out-of-
band exchange of a shared secret is feasible is defined in [5].
Potential exposures to attack are discussed in section 7 of the DHCP
protocol specification in [2].
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4. IANA Considerations
This option is in current usage with the number 150. As per
RFC-3942, this already existing number assignment should simply be
made "official" by IANA, unless there is a conflict with some other
usage.
5 References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", RFC 2119, BCP 14, March 1997.
[2] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
March 1997.
[3] Droms, R. and S. Alexander, "DHCP Options and BOOTP Vendor
Extensions", RFC 2132, March 1997.
[4] Mockapetris, P., "Domain Names - Concepts and Facilities", RFC
1034, November 1987.
[5] Droms, R., "Authentication for DHCP Messages", RFC 3118, June
2001.
[6] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", RFC 2434, October 1998.
[7] Volz, B., "Reclassifying Dynamic Host Configuration Protocol
version 4 (DHCPv4) Options", RFC 3942, November 2004.
Author's Address
Richard A. Johnson
Cisco Systems, Inc.
170 W. Tasman Dr.
San Jose, CA 95134
US
Phone: +1 408 526 4000
EMail: raj@cisco.com
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