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
DHCPv4中继代理信息选项的链路选择子选项
Status of this Memo
本备忘录的状况
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
版权公告
Copyright (C) The Internet Society (2003). All Rights Reserved.
版权所有（C）互联网协会（2003年）。版权所有。
Abstract
摘要
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.
本文档描述了动态主机配置协议（DHCPv4）的中继代理信息选项的链路选择子选项。giaddr指定一个IP地址，该地址确定子网，从而确定动态主机配置协议（DHCP）客户端所在的链路，以及可用于与中继代理通信的IP地址。子网选择选项允许拆分giaddr的功能，以便当一个实体作为DHCP代理执行时，它可以指定分配IP地址的子网/链路，该地址不同于它希望与DHCP服务器通信的IP地址。存在类似情况，中继代理需要指定DHCP客户端所在的子网/链路，这与可用于与中继代理通信的IP地址不同。
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.
DHCP代理，它可以指定从中分配IP地址的子网/链路，该IP地址不同于它希望与DHCP服务器通信的IP地址。
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:
存在类似情况，中继代理需要指定DHCP客户端所在的子网/链路，这与可用于与中继代理通信的IP地址不同。考虑下面的体系结构：
+--------+ +---------------+
| 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.
在任何这些或其他情况下，中继代理都需要能够与DHCP服务器通信要从中分配IP地址的子网/链路。将与DHCP服务器通信子网/链路信息的IP地址不能用作DHCP服务器和中继代理之间通信的方式。
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。
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:
本文件使用以下术语：
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.
DHCP客户端是使用DHCP获取配置参数（如网络地址）的Internet主机。
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.
DHCP服务器是向DHCP客户端返回配置参数的Internet主机。
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.
链路是一种通信设施或介质，节点可通过它在链路层（即IPv4正下方的层）进行通信。例如以太网络（简单或桥接）；PPP链接；X.25、帧中继或ATM网络；互联网（或更高）层的“隧道”，如IPv4或IPv6本身上的隧道。
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.
子网（在本文档中）由可路由地址范围组成。它可能是同时存在于链接上的多个链接之一。
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.
链路选择子选项由任何希望为其正在中继的DHCP客户端请求指定子网/链路但需要子网/链路规范不同于DHCP服务器与中继代理通信时应使用的IP地址的DHCP中继代理使用。
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).
在需要此子选项的场景中，中继代理会在设置giaddr值时（即，在中继到DHCP服务器的所有消息上）添加此子选项。
When the DHCP server is allocating an address and this sub-option is present, then the DHCP server MUST allocate the address on either:
当DHCP服务器正在分配地址且存在此子选项时，DHCP服务器必须在以下任一位置分配地址：
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:
子选项的格式为：
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.
使用此子选项的中继代理必须假定接收子选项的服务器支持该子选项，并使用子选项中的可用信息正确分配IP地址。使用此子选项的中继代理不得根据此子选项是否出现在服务器的响应数据包中而采取不同的操作。
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.
使用管理技术确保使用此子选项的任何中继代理只向支持此子选项的服务器发送数据包，这一点很重要。
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.
支持此子选项不需要更改DHCP服务器的操作或功能，只需要选择要分配地址的子网（和链接）。例如，对未知子网的DHCPDISCOVER的处理应保持不变。
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.
当存在此子选项且服务器支持此子选项时，服务器不得提供不在请求的子网或与该子网关联的链路（网段）上的地址。
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.
DHCP服务器向其发送回复的IP地址必须与不存在此子选项时选择的IP地址相同。
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.
此处记录的子网选择选项或链路选择子选项的存在将允许恶意DHCP客户端执行比不使用这些选项时更完整的地址池耗尽攻击，因为客户端将不再仅限于攻击其本地子网上的地址池。
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]。
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。
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。
[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。
[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。
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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: kkinnear@cisco.com
电话：（978）936-0000电子邮件：kkinnear@cisco.com
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: mjs@cisco.com
电话：（978）936-0000电子邮件：mjs@cisco.com
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: jayk@cisco.com
电话：（408）526-4000电子邮件：jayk@cisco.com
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: raj@cisco.com
电话：（408）526-4000电子邮件：raj@cisco.com
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确认
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