Network Working Group                                         K. Kinnear
Request for Comments: 3527                                      M. Stapp
Category: Standards Track                                     R. Johnson
                                                           J. Kumarasamy
                                                           Cisco Systems
                                                              April 2003
Network Working Group                                         K. Kinnear
Request for Comments: 3527                                      M. Stapp
Category: Standards Track                                     R. Johnson
                                                           J. Kumarasamy
                                                           Cisco Systems
                                                              April 2003

Link Selection sub-option for the Relay Agent Information Option for DHCPv4


Status of this Memo


This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.

本文件规定了互联网社区的互联网标准跟踪协议,并要求进行讨论和提出改进建议。有关本协议的标准化状态和状态,请参考当前版本的“互联网官方协议标准”(STD 1)。本备忘录的分发不受限制。

Copyright Notice


Copyright (C) The Internet Society (2003). All Rights Reserved.




This document describes the link selection sub-option of the relay-agent-information option for the Dynamic Host Configuration Protocol (DHCPv4). The giaddr specifies an IP address which determines both a subnet, and thereby a link on which a Dynamic Host Configuration Protocol (DHCP) client resides as well as an IP address that can be used to communicate with the relay agent. The subnet-selection option allows the functions of the giaddr to be split so that when one entity is performing as a DHCP proxy, it can specify the subnet/link from which to allocate an IP address, which is different from the IP address with which it desires to communicate with the DHCP server. Analogous situations exist where the relay agent needs to specify the subnet/link on which a DHCP client resides, which is different from an IP address that can be used to communicate with the relay agent.


1. Introduction
1. 介绍

In RFC 2131, the giaddr specifies an IP address which determines a subnet (and from there a link) on which a DHCP client resides as well as an IP address which can be used to communicate with the relay agent. The subnet-selection option [RFC 3011] allows these functions of the giaddr to be split, so that when one entity is performing as a

在RFC 2131中,giaddr指定一个IP地址,该地址确定DHCP客户端所在的子网(以及从该子网到链路)以及一个可用于与中继代理通信的IP地址。子网选择选项[RFC 3011]允许拆分giaddr的这些功能,以便在一个实体作为

DHCP proxy, it can specify the subnet/link from which to allocate an IP address that is different from the IP address with which it desires to communicate with the DHCP server.


Analogous situations exist where the relay agent needs to specify the subnet/link on which a DHCP client resides, which is different from an IP address that can be used to communicate with the relay agent. Consider the following architecture:


      +--------+         +---------------+
      |  DHCP  |     IP x|               |IP y
      | Server |-.......-|  Relay Agent  |----+------------+
      +--------+         |               |    |            |
                         +---------------+    |         +------+
                                              |         |Modem |
                                              |         +------+
                                              |          |    |
                                           +-----+  +-----+ +-----+
                                           |Host1|  |Host2| |Host3|
                                           +-----+  +-----+ +-----+
      +--------+         +---------------+
      |  DHCP  |     IP x|               |IP y
      | Server |-.......-|  Relay Agent  |----+------------+
      +--------+         |               |    |            |
                         +---------------+    |         +------+
                                              |         |Modem |
                                              |         +------+
                                              |          |    |
                                           +-----+  +-----+ +-----+
                                           |Host1|  |Host2| |Host3|
                                           +-----+  +-----+ +-----+

In the usual approach, the relay agent would put IP address Y into the giaddr of any packets that it forwarded to the DHCP server. However, if for any reason, IP address Y is not accessible from the DHCP server, this approach will fail. There are several reasons why IP y might be inaccessible from the DHCP server:

在通常的方法中,中继代理将IP地址Y放入它转发到DHCP服务器的任何数据包的giaddr中。但是,如果由于任何原因,无法从DHCP服务器访问IP地址Y,此方法将失败。IP y无法从DHCP服务器访问的原因有几个:

o There might be some firewall capability in the network element in which the relay agent resides that does not allow the DHCP server to access the relay agent via IP y.

o 中继代理所在的网元中可能存在某些防火墙功能,不允许DHCP服务器通过IP y访问中继代理。

o There might not be an IP y. An example would be the case where there was only one host and this was a point to point link.

o 可能没有IP地址。例如,只有一个主机,这是一个点对点链接。

In any of these or other cases, the relay agent needs to be able to communicate to the DHCP server the subnet/link from which to allocate an IP address. The IP address, which will communicate to the DHCP server the subnet/link information, cannot be used as a way to communicate between the DHCP server and the relay agent.


Since the relay agent can modify the client's DHCP DHCPREQUEST in only two ways, the giaddr and the relay-agent-info option, there is a need to extend the relay-agent-info option with a new sub-option, the link-selection sub-option, to allow separation of the specification of the subnet/link from the IP address to use when communicating with the relay agent.

由于中继代理只能以两种方式修改客户端的DHCP DHCPREQUEST,即giaddr和中继代理信息选项,因此需要使用新的子选项(链路选择子选项)扩展中继代理信息选项,允许在与中继代理通信时,将子网/链路的规范与要使用的IP地址分离。

2. Terminology
2. 术语

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 BCP 14, RFC 2119 [RFC 2119].

本文件中的关键词“必须”、“不得”、“必需”、“应”、“不应”、“应”、“不应”、“建议”、“可”和“可选”应按照BCP 14、RFC 2119[RFC 2119]中的说明进行解释。

This document uses the following terms:


o "DHCP client"

o “DHCP客户端”

A DHCP client is an Internet host using DHCP to obtain configuration parameters such as a network address.


o "DHCP relay agent"

o “DHCP中继代理”

A DHCP relay agent is a third-party agent that transfers BOOTP and DHCP messages between clients and servers residing on different subnets, per [RFC 951] and [RFC 1542].

DHCP中继代理是第三方代理,根据[RFC 951]和[RFC 1542],在驻留在不同子网上的客户端和服务器之间传输BOOTP和DHCP消息。

o "DHCP server"

o “DHCP服务器”

A DHCP server is an Internet host that returns configuration parameters to DHCP clients.


o "link"

o “链接”

A link is a communications facility or medium over which nodes can communicate at the link layer, i.e., the layer immediately below IPv4. Examples are Ethernets (simple or bridged); PPP links; X.25, Frame Relay, or ATM networks; and internet (or higher) layer "tunnels", such as tunnels over IPv4 or IPv6 itself.


o "subnet"

o “子网”

A subnet (for the purposes of this document) consists of a routable address range. It may be one of several that exist on a link at the same time.


3. Link selection sub-option definition
3. 链接选择子选项定义

The link-selection sub-option is used by any DHCP relay agent that desires to specify a subnet/link for a DHCP client request that it is relaying but needs the subnet/link specification to be different from the IP address the DHCP server should use when communicating with the relay agent.


The sub-option contains a single IP address that is an address contained in a subnet. The value for the subnet address is determined by taking any IP address on the subnet and ANDing that address with the subnet mask (i.e., the network and subnet bits are left alone and the remaining (address) bits are set to zero). This determines a single subnet, and when allocating an IP address, all of the other related subnets on the same link will also be considered in the same way as currently specified for the processing of the giaddr in [RFC 2131, Section 4.3.1, first group of bullets, bullet 4].

子选项包含一个IP地址,该地址包含在子网中。子网地址的值通过获取子网上的任何IP地址并将该地址与子网掩码进行and运算来确定(即,网络和子网位保持不变,其余(地址)位设置为零)。这决定了一个子网,当分配IP地址时,同一链路上的所有其他相关子网也将按照[RFC 2131,第4.3.1节,第一组项目符号,项目符号4]中当前为处理giaddr规定的方式考虑。

In scenarios where this sub-option is needed, the relay agent adds it whenever it sets the giaddr value (i.e., on all messages relayed to the DHCP server).


When the DHCP server is allocating an address and this sub-option is present, then the DHCP server MUST allocate the address on either:


o the subnet specified in the link-selection sub-option, or;

o 链路选择子选项中指定的子网,或;

o a subnet on the same link (also known as a network segment) as the subnet specified by the link-selection sub-option.

o 与链路选择子选项指定的子网位于同一链路(也称为网段)上的子网。

The format of the sub-option is:


             SubOpt   Len     subnet IP address
            |  5   |   4  |  a1  |  a2  |  a3  |  a4  |
             SubOpt   Len     subnet IP address
            |  5   |   4  |  a1  |  a2  |  a3  |  a4  |

A relay agent which uses this sub-option MUST assume that the server receiving the sub-option supports the sub-option and uses the information available in the sub-option to correctly allocate an IP address. A relay agent which uses this sub-option MUST NOT take different actions based on whether this sub-option appears or does not appear in the response packet from the server.


It is important to ensure, using administrative techniques, that any relay agent employing this sub-option is directed to only send packets to a server that supports this sub-option.


Support for this sub-option does not require changes to operations or features of the DHCP server other than to select the subnet (and link) on which to allocate an address. For example, the handling of DHCPDISCOVER for an unknown subnet should continue to operate unchanged.


In the event that a DHCP server receives a packet that contains both a subnet-selection option [RFC 3011], as well as a link-selection sub-option, the information contained in the link-selection sub-option MUST be used to control the allocation of an IP address in preference to the information contained in the subnet-selection option.

如果DHCP服务器接收到包含子网选择选项[RFC 3011]和链路选择子选项的数据包,则必须使用链路选择子选项中包含的信息来控制IP地址的分配,而不是子网选择选项中包含的信息。

When this sub-option is present and the server supports this sub-option, the server MUST NOT offer an address that is not on the requested subnet or the link (network segment) with which that subnet is associated.


The IP address to which a DHCP server sends a reply MUST be the same as it would choose when this sub-option is not present.


4. Security Considerations
4. 安全考虑

Potential attacks on DHCP are discussed in section 7 of the DHCP protocol specification [RFC 2131], as well as in the DHCP authentication specification [RFC 3118].

DHCP协议规范[RFC 2131]第7节以及DHCP身份验证规范[RFC 3118]中讨论了对DHCP的潜在攻击。

The link-selection sub-option allows a relay agent to specify the subnet/link on which to allocate an address for a DHCP client. Given that the subnet-selection option already exists [RFC 3011], no fundamental new security issues are raised by the existence of the link-selection sub-option specified in this document beyond those implied by the subnet-selection option [RFC 3011].

链路选择子选项允许中继代理指定要为DHCP客户端分配地址的子网/链路。鉴于子网选择选项[RFC 3011]已经存在,除了子网选择选项[RFC 3011]隐含的安全问题外,本文档中指定的链路选择子选项的存在不会引发任何新的基本安全问题。

The existence of either the subnet-selection option or link-selection sub-option documented here would allow a malicious DHCP client to perform a more complete address-pool exhaustion attack than could be performed without the use of these options, since the client would no longer be restricted to attacking address-pools on just its local subnet.


There is some minor protection against this form of attack using this sub-option that is not present for the subnet-selection option, in that a trusted relay agent that supports the relay-agent-info option MUST discard a packet it receives with a zero giaddr and a relay-agent-info option when that packet arrives on an "untrusted" circuit [RFC 3046, section 2.1].

使用子网选择选项中不存在的此子选项可以对这种形式的攻击提供一些较小的保护,因为支持中继代理信息选项的受信任中继代理必须在数据包到达“不受信任”电路时丢弃使用零giaddr和中继代理信息选项接收的数据包[RFC 3046,第2.1节]。

5. IANA Considerations
5. IANA考虑

IANA has assigned a value of 5 from the DHCP Relay Agent sub-options space [RFC 3046] for the link-selection sub-option defined in Section 3.

IANA已从DHCP中继代理子选项空间[RFC 3046]为第3节中定义的链路选择子选项分配了值5。

6. Acknowledgments
6. 致谢

Eric Rosen helped the authors to understand the need for this sub-option. Much of this document was borrowed, with only minimal modifications, from the document describing the subnet-selection option [RFC 3011].

Eric Rosen帮助作者理解了该子选项的必要性。本文档的大部分内容都是从描述子网选择选项[RFC 3011]的文档中借来的,只做了很少的修改。

7. References
7. 工具书类
7.1. Normative References
7.1. 规范性引用文件

[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.

[RFC 2119]Bradner,S.,“RFC中用于表示需求水平的关键词”,BCP 14,RFC 2119,1997年3月。

[RFC 2131] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, March 1997.

[RFC 2131]Droms,R.,“动态主机配置协议”,RFC 2131,1997年3月。

[RFC 3011] Waters, G. "The IPv4 Subnet Selection Option for DHCP", RFC 3011, November 2000.

[RFC 3011]Waters,G.“DHCP的IPv4子网选择选项”,RFC 301119000年11月。

[RFC 3046] Patrick, M., "DHCP Relay Agent Information Option", RFC 3046, January 2001.

[RFC 3046]Patrick,M.,“DHCP中继代理信息选项”,RFC 3046,2001年1月。

7.2. Informative References
7.2. 资料性引用

[RFC 951] Croft, W. and J. Gilmore, "Bootstrap Protocol", RFC 951, September 1985.

[RFC 951]Croft,W.和J.Gilmore,“引导协议”,RFC 9511985年9月。

[RFC 1542] Wimer, W., "Clarifications and Extensions for the Bootstrap Protocol", RFC 1542, October 1993.

[RFC 1542]Wimer,W.,“引导协议的澄清和扩展”,RFC 1542,1993年10月。

8. Intellectual Property Statement
8. 知识产权声明

The IETF takes no position regarding the validity or scope of any intellectual property 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; neither does it represent that it has made any effort to identify any such rights. Information on the IETF's procedures with respect to rights in standards-track and standards-related documentation can be found in BCP-11. Copies of claims of rights made available for publication and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementors or users of this specification can be obtained from the IETF Secretariat.


The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights which may cover technology that may be required to practice this standard. Please address the information to the IETF Executive Director.


9. Authors' Addresses
9. 作者地址

Kim Kinnear Cisco Systems 1414 Massachusetts Ave Boxborough, Ma. 01719

Kim Kinnear思科系统公司马萨诸塞州伯斯堡大道1414号,马萨诸塞州。01719

Phone: (978) 936-0000 EMail:


Mark Stapp Cisco Systems 1414 Massachusetts Ave Boxborough, Ma. 01719

马萨诸塞州伯斯堡马萨诸塞大道1414号Mark Stapp Cisco Systems。01719

Phone: (978) 936-0000 EMail:


Jay Kumarasamy Cisco Systems 170 W. Tasman Dr. San Jose, CA 95134

Jay Kumarasamy Cisco Systems 170 W.Tasman博士,加利福尼亚州圣何塞,邮编95134

Phone: (408) 526-4000 EMail:


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

Richard Johnson Cisco Systems 170 W.Tasman Dr.圣何塞,加利福尼亚州95134

Phone: (408) 526-4000 EMail:


10. Full Copyright Statement
10. 完整版权声明

Copyright (C) The Internet Society (2003). All Rights Reserved.


This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English.


The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns.






Funding for the RFC Editor function is currently provided by the Internet Society.