RFC 5859

TFTP Server Address Option for DHCPv4

📅 published June 2010, from ietf

RFC 5859: TFTP Server Address Option for DHCPv4 🎀

okay bestie, here's the header tea 🫶

Series: Internet Engineering Task Force (IETF) 🌸
Request for Comments: 5859
Author: R. Johnson, Cisco Systems, Inc. 💅
Category: Informational
Date: June 2010
ISSN: 2070-1721
Obsoletes/Updates: none listed in the header 🙈

Abstract

So this memo is all about documenting how the "TFTP Server Address" option is already being used out in the wild ✨ The option number currently in use is 150. This memo documents the current usage of the option in agreement with RFC 3942, which says that any pre-existing usages of option numbers in the range 128-223 should be documented, and the Dynamic Host Configuration working group will try to officially assign those numbers to those options. The option is defined for DHCPv4 and only works with IPv4 addresses 🦋

Status of This Memo

This one is NOT an Internet Standards Track spec, it's just published for info purposes 📚

This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Not all documents approved by the IESG are a candidate for any level of Internet Standard; see Section 2 of RFC 5741.

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc5859.

Copyright Notice

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November 10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s) controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate it into languages other than English.

1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.  Conventions . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3.  TFTP Server Address Option Definition . . . . . . . . . . . . . 4
4.  Security Considerations . . . . . . . . . . . . . . . . . . . . 5
5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
6.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 5
  6.1.  Normative References  . . . . . . . . . . . . . . . . . . . 5
  6.2.  Informative References  . . . . . . . . . . . . . . . . . . 6

1. Introduction ✨

Okay so picture this 💖 Voice over IP (VoIP) devices, like IP phones, need to download their configuration from a configuration server on the network. There are two commonly accepted ways to find this server via DHCP: the "sname" field in the DHCP header [RFC2131], and the "TFTP Server Name" option (66) [RFC2132]. The thing is, both of these only give you the TFTP server's hostname. That hostname then has to be translated into an IP address, and the usual way to do that would be DNS [RFC1034].

Here's the problem 🙈 That means the firmware in a VoIP device (which might have limited flash, memory, and/or processing resources) would need to implement the DNS protocol just to do the translation. It also adds an extra, totally unnecessary point of failure, because the device would depend on the DNS server infrastructure just to boot up and talk to its call agent.

To get DNS out of the failure picture and keep the firmware in these VoIP devices to a minimum, the "VoIP Configuration Server Address" option (150) was introduced. This option lets the DHCP server pass one or more IP addresses of the VoIP configuration server(s) instead of a hostname, so the device can use the info directly 🦋

There are some other reasons for this option too: (1) the "siaddr" field isn't configurable on some DHCP servers; (2) the "siaddr" field only allows for one IPv4 address, and it's desirable to be able to configure multiple IP addresses for redundancy; (3) some DHCP servers have been found to fill in their own IPv4 address as siaddr; (4) some customers were already using the "siaddr" field for other purposes; and finally (5) the configuration server may use a protocol other than TFTP to serve configuration files, which makes the "TFTP Server Name" option (66) inappropriate.

If other download server address info also shows up in the response packet, like "sname" and "TFTP Server Name", it's up to the device to decide which piece of info to use 💅

2. Conventions 🎀

Here are the key words, and they're the only ones in caps for a reason 👑

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119].

3. TFTP Server Address Option Definition 🧚

The TFTP Server Address option is a DHCP option [RFC2132] ✨ The option contains one or more IPv4 addresses that the client MAY use. Right now it's used for downloading configuration from a VoIP server via TFTP, but the option may be used for other things besides contacting a VoIP configuration server.

The format of the option is:

           Code   Len   IPv4 Configuration Server Address(es)
          +-----+-----+-----+-----+-----+-----+
          | 150 |  n  |     IPv4 address      | ...
          +-----+-----+-----+-----+-----+-----+

                              Figure 1

The option minimum length (n) is 4.

The "Len" field must specify a length that is an integral multiple of 4 octets (4, 8, 12, etc.). If an option comes in where this isn't the case, the option information MUST be ignored, but further option processing may continue. Dividing this "Len" value by 4 will give you the number of IPv4 VoIP configuration server addresses that are specified in the option 🔥

The option MUST NOT be specified by the DHCP client, since it's only meant to be returned from the DHCP server. If the DHCP client wants to receive this info from the server, it needs to include the number 150 in the "DHCP Parameter List" option (55).

Server addresses SHOULD be listed in order of preference, and the client SHOULD use the addresses sequentially, but it may be configured to use addresses randomly. The client may use as many or as few of the addresses provided as it likes. For example, if the client can only accept two configuration server addresses, it may ignore any other addresses provided after the second address 💜

Each TFTP server address that is being used by the client should be tried a total of four times with a 4-second wait time before moving on to the next address ⭐

When this option shows up along with the TFTP Server Name option (66) [RFC2132], this option SHOULD have priority over option 66. Main character energy, no notes 👑

There's currently no defined IPv6 DHCP equivalent for this option 🌈

4. Security Considerations 💀

⚠️ Okay bestie, this section is serious, so read it with your whole chest 💀

A rogue DHCP server could use this option to coerce a client into downloading configuration data from an alternate configuration server, and thus gain control of the device's configuration. That said, this is no more of a security threat than similar attacks using other DHCP options that specify server names or addresses, and there are a lot of those. If this is a concern, DHCP authentication may be used, but even secure delivery of an address over DHCP does not protect the subsequent insecure download over TFTP. TFTP itself provides no authentication or access control mechanisms, so even if DHCP messages were authenticated, downloading the configuration would still be insecure, unless some object-level security mechanisms were used.

Where security concerns are an issue, it's suggested that configuration files should be signed by a trusted agent. Configuration files may also be encrypted based on a configuration parameter on the DHCP client device. In other words, there are various methods to ensure the integrity of configuration data independent from ensuring the integrity of this DHCP option or even DHCP itself. The full extent of such options is far too broad to be addressed in this document 🙈

Message authentication in DHCP for intradomain use, where the out-of-band exchange of a shared secret is feasible, is defined in [RFC3118]. Potential exposures to attack are discussed in Section 7 of the DHCP protocol specification [RFC2131].

5. IANA Considerations 🌸

IANA has assigned DHCP option number 150, in accordance with [RFC3942].

6. References 📚

6.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, March 1997.
[RFC2132] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor Extensions", RFC 2132, March 1997.
[RFC3942] Volz, B., "Reclassifying Dynamic Host Configuration Protocol version 4 (DHCPv4) Options", RFC 3942, November 2004.

6.2. Informative References

[RFC1034] Mockapetris, P., "Domain names - concepts and facilities", STD 13, RFC 1034, November 1987.
[RFC3118] Droms, R. and W. Arbaugh, "Authentication for DHCP Messages", RFC 3118, June 2001.

Author's Address 🫶

Richard A. Johnson
Cisco Systems, Inc.
170 W. Tasman Dr.
San Jose, CA  95134
USA

Phone: +1 408 526 4000
EMail: raj@cisco.com

That's the whole RFC, bestie 🌈💖✨

📦 where this page comes from

ietf: rfc5859 2010-06
💖 girlified by claude-haiku-5-5 on 2026-10-17: a language model's rewrite of the original, under its terms