RFC 3736

Stateless Dynamic Host Configuration Protocol (DHCP) Service for IPv6

📅 published April 2004, from ietf

Stateless Dynamic Host Configuration Protocol (DHCP) Service for IPv6 🎀

okay bestie, the title block is giving: this is RFC 3736 from the Network Working Group, written by R. Droms at Cisco Systems 💅. It's on the Standards Track and dated April 2004 📅. The header doesn't list anything it obsoletes or updates, so no drama there ✨.

Status of this Memo 📜

This document specifies an Internet standards track protocol for the Internet community, and it wants discussion and suggestions for improvements 🫶. For the standardization state and status of this protocol, check the current edition of the "Internet Official Protocol Standards" (STD 1). Distribution of this memo is unlimited 🌈.

Copyright (C) The Internet Society (2004). All Rights Reserved.

Abstract 🦋

Stateless Dynamic Host Configuration Protocol service for IPv6 (DHCPv6) is how nodes grab configuration info, like the addresses of DNS recursive name servers 🌸. The big perk is that it doesn't need to keep any dynamic state for individual clients, so it's lowkey so chill ✨. A node that uses stateless DHCP must have gotten its IPv6 addresses some other way, usually stateless address autoconfiguration 💜. This document explains which parts of RFC 3315 must be implemented in each kind of DHCP agent so that agent can support stateless DHCP 🔥.

1. Introduction 🧚

Nodes that already have IPv6 addresses from somewhere else, like stateless address autoconfiguration [6] or manual configuration, can use stateless DHCP to get other config info, such as a list of DNS recursive name servers or SIP servers 💖. A stateless DHCP server only hands out configuration information and does not do any address assignment 🍓. It's called "stateless" because it doesn't have to keep any dynamic state for individual clients 🫧.

The DHCP specification [1] defines more than 10 protocol messages and 20 options, but only a subset of those are required for stateless DHCP service 🤯. This document spells out which messages and options defined in RFC 3315 are required for stateless DHCP service. It's meant to guide interoperable implementation of clients and servers that use stateless DHCP service 🎀.

The operation of relay agents is the same for stateless and stateful DHCP service, and it's described in the DHCP specification 🌈.

Section 4 of this document lists the sections of the DHCP document that an implementor should read for an overview of the DHCP specification and the basic requirements of a DHCP service 📚. Section 5 lists the specific messages and options that are required for stateless DHCP service 💅. Section 6 describes how stateless and stateful DHCP servers work together to serve clients that need address assignment and clients that only need stateless service 👑.

2. Terminology 🩷

Throughout this document, "DHCP" means DHCP for IPv6 🌷.

This document uses the terminology defined in RFC 2460 [2], the DHCP specification [1], and the DHCP DNS configuration options specification [3] 🧠.

"Stateless DHCP" is the use of DHCP to provide configuration information to clients without the server having to maintain dynamic state about those DHCP clients 💜.

3. Overview 🌟

This document assumes that a node using stateless DHCP configuration is not using DHCP for address assignment, and that the node has worked out at least a link-local address, as described in section 5.3 of RFC 2461 [4] 🦋.

To get configuration parameters through stateless DHCP, a node sends the DHCP Information-request message 📨. DHCP servers answer with a Reply message that carries configuration parameters for the node 💌. That Reply can carry things like a list of DNS recursive name servers [3] and SIP servers [5] 🫶.

This document doesn't apply to the function of DHCP relay agents as described in RFC 3315. One network element can provide both DHCP server and DHCP relay service 🍭. For example, it can give stateless DHCP service to hosts that want stateless DHCP, while relaying messages from hosts that want address assignment through DHCP to another DHCP server 🔀.

4. Basic Requirements for Implementation of DHCP 🧩

Several sections of the DHCP specification give background info or define parts of the spec that are common to all implementations 🙈:

1-4:   give an introduction to DHCP and an overview of DHCP message
       flows

5:     defines constants used throughout the protocol specification

6, 7:  illustrate the format of DHCP messages

8:     describes the representation of Domain Names

9:     defines the "DHCP unique identifier" (DUID)

13-16: describe DHCP message transmission, retransmission, and
       validation

21:    describes authentication for DHCP

5. Implementation of Stateless DHCP 💖

The client says it wants configuration info by sending an Information-request message that includes an Option Request option, which lists the options it wants to get from the DHCP server 🎀. For example, if the client wants a list of DNS recursive name servers, it names the DNS Recursive Name Server option in the Information-request message 🌸. The server works out the right configuration parameters for the client based on its configuration policies, then replies with a Reply message holding the requested parameters 💅. In this example, the server would reply with DNS configuration parameters 📮.

As described in section 18.1.5 of RFC 3315, a node may include a Client Identifier option in the Information-request message to identify itself to a server ✨. The reason is that the server administrator may want to customize the server's response to each node based on the node's identity 🪪.

RFC 3315 doesn't define any mechanisms for controlling when a host uses an Information-request message to get updated configuration parameters 🕰️. The DHC WG has taken on developing such a mechanism or mechanisms, which will be published as Standards-track RFC(s) 🛠️.

RFC 3315 also doesn't give any guidance on when a host might send an Information-request message to get updated configuration parameters after it has moved to a new link 🚶‍♀️. The DHC WG is reviewing a related document, "Detection of Network Attachment (DNA) in IPv4" [8], which describes how a host using IPv4 can figure out when to use DHCPv4 👀. Either the DHC WG or a WG formed from the DNA BOF will develop a similar document for IPv6 🌈.

5.1. Messages Required for Stateless DHCP Service 📬

Clients and servers implement the following messages for stateless DHCP service. The section numbers in this list refer to the DHCP specification 🧾:

  • Information-request: a DHCP client sends this to a server to request configuration parameters (sections 18.1.5 and 18.2.5) 📤
  • Reply: a DHCP server sends this to a client, containing configuration parameters (sections 18.2.6 and 18.2.8) 📥

Servers and relay agents also implement the following messages for stateless DHCP service. The section numbers in this list refer to the DHCP specification 🔁:

  • Relay-forward: a DHCP relay agent sends this to carry the client message to a server (section 15.13) 🚀
  • Relay-reply: a DHCP server sends this to carry a response message back to the relay agent (section 15.14) 🪽

5.2. Options Required for Stateless DHCP Service 🔑

Clients and servers implement the following options for stateless DHCP service. The section numbers in this list refer to the DHCP specification 🗂️:

  • Option Request: specifies the configuration information that the client is requesting from the server (section 22.7) 🙋‍♀️
  • Status Code: used to indicate completion status or other status information (section 22.13) ✅
  • Server Identifier: used to identify the server responding to a client request (section 22.3) 🆔

Servers and relay agents also implement the following options for stateless DHCP service. The section numbers in this list refer to the DHCP specification 🧩:

  • Client message: a DHCP relay agent sends this in a Relay-forward message to carry the client message to a server (section 20) 📦
  • Server message: a DHCP server sends this in a Relay-reply message to carry a response message to the relay agent (section 20) 🎁
  • Interface-ID: sent by the DHCP relay agent and returned by the server to identify the interface to use when forwarding a message to the client (section 22.18) 🛣️

5.3. Options Used for Configuration Information 🎀

Clients and servers use the following options to pass configuration information to clients. Note that other options for configuration information may be specified in future Internet Standards 🔮:

  • DNS Recursive Name Servers: specifies the DNS recursive name servers [7] the client uses for name resolution. See "DNS Configuration options for DHCPv6" [3] 🌐
  • DNS search list: specifies the domain names to be searched during name resolution. See "DNS Configuration options for DHCPv6" [3] 🔍
  • SIP Servers: specifies the SIP servers the client uses to get a list of domain names of IPv6 addresses that can be mapped to one or more SIP outbound proxy servers [5] 📞

5.4. Other Options Used in Stateless DHCP 🌸

Clients and servers may implement the following options for stateless DHCP service. The section numbers in this list refer to the DHCP specification 🫧:

  • Preference: a DHCP server sends this to indicate the preference level for the server (section 22.8) ⭐
  • Elapsed time: a DHCP client sends this to indicate the time since the client began the DHCP configuration process (section 22.9) ⏱️
  • User Class: a DHCP client sends this to give the server extra info for selecting configuration parameters for the client (section 22.15) 👤
  • Vendor Class: a DHCP client sends this to give the server extra info about the client vendor and hardware for selecting configuration parameters for the client (section 22.16) 🏷️
  • Vendor-specific Information: used to pass information to clients in options defined by vendors (section 22.17) 🏭
  • Client Identifier: a DHCP client sends this to identify itself (section 22.2). Clients are not required to send this option. Servers send the option back if it is included in a message from a client 🪞
  • Authentication: used to provide authentication of DHCP messages (section 21) 🔐

6. Interaction with DHCP for Address Assignment 🎀

okay so picture this: some networks have clients using stateless address autoconfiguration plus DHCP just for DNS config 💖, AND clients using DHCP for stateful address config 🦋. depending on how the relay agents are set up, DHCP servers that are only meant for stateless config might get messages from the stateful crowd 🙈

a DHCP server that can only hand out stateless config through the Information-request/Reply exchange will discard any other DHCP messages it gets 💅. like, specifically, it throws away everything except Information-Request or Relay-forward ✨, and it does NOT join any stateful address configuration exchanges 🙅‍♀️. if there are other DHCP servers set up for stateful address assignment, one of those servers handles the address assignment 🧚 so everybody's taken care of, periodt 👑

7. Security Considerations 🔥

okay this section is SERIOUS bestie 😭 but it's also kind of a relief? stateless DHCP service is a proper subset of the DHCP service in the DHCP spec, RFC 3315 [1] 🫶. so stateless DHCP adds no extra security considerations beyond what's covered in sections 21, 22.11, and 23 of the DHCP specification [1] ✨

BUT (and this is a big but 💀) the config info a node gets through stateless DHCP can be used to pull off spoofing, man-in-the-middle, denial-of-service, and other attacks 😱. the specs for each option that carries config info go into more detail about these attacks 📖. and good news: Authenticated DHCP, described in sections 21 and 22.11 of the DHCP specification [1], can be used to avoid attacks mounted through the stateless DHCP service 🔐 so lock it down, bestie 💜

8. Acknowledgments 🌸

Jim Bound, Ted Lemon, and Bernie Volz reviewed this document and gave editorial suggestions, and they're literally the main characters here 🫶 thanks also to Peter Barany, Tim Chown, Christian Huitema, Tatuya Jinmei, Pekka Savola, and Juha Wiljakka for their review and comments 💖

9. References 📚

9.1. Normative References

[1] Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins, C. and M. Carney, "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)", RFC 3315, July 2003.

[2] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", RFC 2460, December 1998.

9.2. Informative References

[3] Droms, R., Ed., "DNS Configuration options for Dynamic Host Configuration Protocol for IPv6 (DHCPv6)", RFC 3646, December 2003.

[4] Narten, T., Nordmark, E. and W. Simpson, "Neighbor Discovery for IP Version 6 (IPv6)", RFC 2461, December 1998.

[5] Schulzrinne, H. and B. Volz, "Dynamic Host Configuration Protocol (DHCPv6) Options for Session Initiation Protocol (SIP) Servers", RFC 3319, July 2003.

[6] Thomson, S. and T. Narten, "IPv6 Stateless Address Autoconfiguration", RFC 2462, December 1998.

[7] Mockapetris, P., "Domain names - concepts and facilities", STD 13, RFC 1034, November 1987.

[8] Aboba, B., "Detection of Network Attachment (DNA) in IPv4", Work in Progress.

10. Author's Address 🖊️

Ralph Droms
Cisco Systems
1414 Massachusetts Avenue
Boxborough, MA  01719
USA

Phone: +1 978 497 4733
EMail: rdroms@cisco.com

Copyright (C) The Internet Society (2004). 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.

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