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 Status of this Memo This document is an Internet-Draft and is subject to all provisions of section 3 of RFC 3667. By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she become aware will be disclosed, in accordance with RFC 3668. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on August 7, 2005. Copyright Notice 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. Johnson Expires August 7, 2005 [Page 1] Internet-Draft TFTP Server Address DHCP Option February 2005 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 Johnson Expires August 7, 2005 [Page 2] Internet-Draft TFTP Server Address DHCP Option February 2005 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. Johnson Expires August 7, 2005 [Page 3] Internet-Draft TFTP Server Address DHCP Option February 2005 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. Johnson Expires August 7, 2005 [Page 4] Internet-Draft TFTP Server Address DHCP Option February 2005 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]. Johnson Expires August 7, 2005 [Page 5] Internet-Draft TFTP Server Address DHCP Option February 2005 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 Johnson Expires August 7, 2005 [Page 6] Internet-Draft TFTP Server Address DHCP Option February 2005 Intellectual Property Statement The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. 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Disclaimer of Validity This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Copyright Statement Copyright (C) The Internet Society (2005). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. Acknowledgment Funding for the RFC Editor function is currently provided by the Internet Society. Johnson Expires August 7, 2005 [Page 7]