Subscriber-ID Suboption for the Dynamic Host Configuration Protocol (DHCP) Relay Agent Option
A DHCP relay agent suboption that carries a stable subscriber ID, whatever network path the customer uses.
Johnson, R., Palaniappan, T., and M. Stapp, "Subscriber-ID Suboption for the Dynamic Host Configuration Protocol (DHCP) Relay Agent Option", RFC 3993, DOI 10.17487/RFC3993, March 2005, <https://www.rfc-editor.org/info/rfc3993>.
@misc{rfc3993,
series = {Request for Comments},
number = 3993,
howpublished = {RFC 3993},
publisher = {RFC Editor},
doi = {10.17487/RFC3993},
url = {https://www.rfc-editor.org/info/rfc3993},
author = {R. Johnson and T. Palaniappan and M. Stapp},
title = {{Subscriber-ID Suboption for the Dynamic Host Configuration Protocol (DHCP) Relay Agent Option}},
pagetotal = 7,
year = 2005,
month = mar,
} RFC text
Contents
Network Working Group R. Johnson
Request for Comments: 3993 T. Palaniappan
Category: Standards Track M. Stapp
Cisco Systems, Inc.
March 2005
Subscriber-ID Suboption for the
Dynamic Host Configuration Protocol (DHCP) Relay Agent Option
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.
Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This memo defines a new Subscriber-ID suboption for the Dynamic Host
Configuration Protocol's (DHCP) relay agent information option. The
suboption allows a DHCP relay agent to associate a stable
"Subscriber-ID" with DHCP client messages in a way that is
independent of the client and of the underlying physical network
infrastructure.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Requirements Terminology . . . . . . . . . . . . . . . . . . . 2
3. The Subscriber-ID Suboption . . . . . . . . . . . . . . . . . 2
3.1. Suboption Format . . . . . . . . . . . . . . . . . . . . 3
4. Relay Agent Behavior . . . . . . . . . . . . . . . . . . . . . 3
5. DHCP Server Behavior . . . . . . . . . . . . . . . . . . . . . 4
6. Security Considerations . . . . . . . . . . . . . . . . . . . 4
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 5
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 5
9.1. Normative References . . . . . . . . . . . . . . . . . . 5
9.2. Informative References . . . . . . . . . . . . . . . . . 5
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 6
Full Copyright Statement . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
DHCP (RFC 2131 [2]) provides IP addresses and configuration
information for IPv4 clients. It includes a relay agent capability
in which processes within the network infrastructure receive
broadcast messages from clients and forward them to DHCP servers as
unicast messages. In network environments such as DOCSIS data-over-
cable and xDSL, it has proven useful for the relay agent to add
information to the DHCP message before forwarding it, by using the
relay agent information option (RFC 3046 [3]).
Servers that recognize the relay agent option echo it back in their
replies, and some of the information that relays add may be used to
help an edge device efficiently return replies to clients. The
information that relays supply can also be used in the server's
decision making about the addresses and configuration parameters that
the client should receive.
In many service provider environments, it is desirable to associate
some provider-specific information with clients' DHCP messages. This
is often done by using the relay agent information option. RFC 3046
defines Remote-ID and Circuit-ID suboptions that are used to carry
such information. The values of those suboptions, however, are
usually based on a network resource such as an IP address of a
network access device, an ATM Virtual Circuit identifier, or a DOCSIS
cable-modem identifier. As a result, the values carried in these
suboptions are dependent on the physical network configuration. If a
client connects to the service provider network through different
paths, different values are carried in network-dependent suboptions.
2. Requirements Terminology
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 [1].
3. The Subscriber-ID Suboption
In complex service provider environments, connecting a customer's
DHCP configuration and administrative information is necessary. The
Subscriber-ID suboption carries a value that can be independent of
the physical network configuration through which the subscriber is
connected. This value complements, and might well be used in
addition to, the network-based relay agent option suboptions
discussed in Section 2. The "subscriber-id" assigned by the provider
is intended to be stable as customers connect through different
paths, and as network changes occur.
The Subscriber-ID information allows the service provider to
assign/activate subscriber-specific actions; e.g., assignment of host
IP address and subnet mask, DNS configuration, or trigger accounting.
This suboption is de-coupled from the access network's physical
structure, so subscriber moves from one access-point to another, for
example, would not require reconfiguration at the service provider's
DHCP servers.
The Subscriber-ID is an ASCII string; the encoding of the string is
defined in Section 3.1. The semantic contents of the Subscriber-ID
string are, of course, provider-specific. This specification does
not establish any semantic requirements on the data in the string.
3.1. Suboption Format
This memo defines a new DHCP relay agent option suboption that
carries a "Subscriber-ID" value. The value is an ASCII string. The
suboption takes a form similar to that of many other relay
information option suboptions:
0 1 2 3 4 5
+-----+-----+-----+-----+-----+----+--
|Code | Len | Subscriber-ID string ...
+-----+-----+-----+-----+-----+----+--
The Code for the suboption is 6.
The one-octet Len field is the length of the ID string, in octets.
The minimum length of the ID string is 1 octet.
The "Subscriber-ID" is an NVT ASCII [4] string. The string MUST NOT
be NULL terminated, as the length is specified in the "Len" field.
4. Relay Agent Behavior
DHCP relay agents MAY be configured to include a Subscriber-ID
suboption if they include a relay agent information option in relayed
DHCP messages. The subscriber-id strings themselves are assigned and
configured through mechanisms that are outside the scope of this
memo.
5. DHCP Server Behavior
This suboption provides additional information to the DHCP server.
If it is configured to support this option, the DHCP server may use
this information in addition to other relay agent option data and
other options included in the DHCP client messages in order to assign
an IP address and/or other configuration parameters to the client.
There is no special additional processing for this suboption.
6. Security Considerations
Message authentication in DHCP for intradomain use where the out-of-
band exchange of a shared secret is feasible is defined in RFC 3118
[5]. Potential exposures to attacks are discussed in section 7 of
the DHCP protocol specification in RFC 2131 [2].
The DHCP relay agent option depends on a trusted relationship between
the DHCP relay agent and the server, as described in section 5 of RFC
3046. Fraudulent relay agent option data could potentially lead to
theft-of-service or exhaustion of limited resources (like IP
addresses) by unauthorized clients. A host that tampered with relay
agent data associated with another host's DHCP messages could deny
service to that host, or interfere with its operation by leading the
DHCP server to assign it inappropriate configuration parameters.
While the introduction of fraudulent relay agent options can be
prevented by a perimeter defense that blocks these options unless the
relay agent is trusted, a deeper defense using authentication for
relay agent options via the Authentication Suboption [6] or IPSec [7]
SHOULD be deployed as well.
There are several data fields in a DHCP message conveying information
that may identify an individual host on the network. These include
the chaddr, the client-id option, and the hostname and client-fqdn
options. Depending on the type of identifier selected, the
Subscriber-ID suboption may also convey information that identifies a
specific host or a specific user on the network. In practice, this
information isn't exposed outside the internal service-provider
network, where DHCP messages are usually confined. Administrators
who configure data that's going to be used in DHCP Subscriber-ID
suboptions should be careful to use identifiers that are appropriate
for the types of networks they administer. If DHCP messages travel
outside the service-provider's own network, or if the suboption
values may become visible to other users, that may raise privacy
concerns for the access provider or service provider.
7. IANA Considerations
IANA has assigned a value of 6 from the DHCP Relay Agent Information
Option [3] suboption codes for the Subscriber-ID Suboption described
in this document.
8. Acknowledgements
This document is the result of work done within Cisco Systems.
Thanks especially to Andy Sudduth for his review comments.
9. References
9.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
March 1997.
[3] Patrick, M., "DHCP Relay Agent Information Option", RFC 3046,
January 2001.
[4] Postel, J. and J. Reynolds, "Telnet Protocol Specification", STD
8, RFC 854, May 1983.
9.2. Informative References
[5] Droms, R. and W. Arbaugh, "Authentication for DHCP Messages",
RFC 3118, June 2001.
[6] Stapp, M., "The Authentication Suboption for the DHCP Relay
Agent Option", Work in Progress.
[7] Droms, R., "Authentication of Relay Agent Options Using IPSec",
Work in Progress.
Authors' Addresses
Richard Johnson
Cisco Systems, Inc.
170 W. Tasman Dr.
San Jose, CA 95134
USA
Phone: 408.526.4000
EMail: raj@cisco.com
Theyn Palaniappan
Cisco Systems, Inc.
170 W. Tasman Dr.
San Jose, CA 95134
USA
Phone: 408.526.4000
EMail: athenmoz@cisco.com
Mark Stapp
Cisco Systems, Inc.
1414 Massachusetts Ave.
Boxborough, MA 01719
USA
Phone: 978.936.0000
EMail: mjs@cisco.com
Full Copyright Statement
Copyright (C) The Internet Society (2005).
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AI ELI5
On this page
§1Why a Subscriber-ID Is Needed
DHCP (RFC 2131) gives IPv4 clients addresses and configuration. A relay agent receives broadcast messages from clients and forwards them to DHCP servers as unicast. In networks such as DOCSIS data-over-cable and xDSL, relays add information using the relay agent information option (RFC 3046). Servers that recognize it echo it back, and may use it in deciding which addresses and parameters a client gets.
RFC 3046 defines the Remote-ID and Circuit-ID suboptions. Their values usually come from a network resource, such as the IP address of an access device, an ATM Virtual Circuit identifier or a DOCSIS cable-modem identifier. So they depend on the physical network, and a client that connects by a different path produces different values.
The old labels say which wire a request came in on. If the customer plugs into a different wire, the label changes, which is confusing for the provider.
§3What the Subscriber-ID Means
The Subscriber-ID suboption carries a value that can be independent of the physical network the subscriber uses. It complements the network-based suboptions and might be used alongside them. The provider-assigned subscriber-id is meant to stay stable as customers connect through different paths and as the network changes.
It lets the provider assign or activate subscriber-specific actions, such as assigning a host IP address and subnet mask, DNS configuration, or triggering accounting. Because it is decoupled from the access network's structure, a subscriber moving from one access point to another would not require reconfiguration at the provider's DHCP servers.
The meaning of the string is provider-specific. The RFC sets no semantic requirements on its contents.
It is a membership number. Wherever you sit, you show the same number, so the provider knows you and does not need to redo your settings.
§3.1The Suboption Format
The suboption looks like many other relay information suboptions: a code, a length, and the value.
The Code is 6. The one-octet Len field is the length of the ID string in octets, and the minimum length of the string is 1 octet. The Subscriber-ID itself is an NVT ASCII string. It MUST NOT be NULL terminated, because the length is already given in the Len field.
It is a small package: a number saying what kind it is, a number saying how long it is, then the customer's name tag in plain letters.
§4Relay Agent and Server Behavior
DHCP relay agents MAY be configured to include a Subscriber-ID suboption if they include a relay agent information option in relayed DHCP messages. How the subscriber-id strings are assigned and configured is outside the scope of the RFC.
On the server side, the suboption is extra information. A server configured to support it may use it, together with other relay agent option data and the options in the client's messages, to assign an IP address or other configuration parameters. There is no special additional processing for this suboption.
The helper in the middle may stick the tag on the request. The provider's server can read the tag as one more hint when deciding what settings to hand out.
§6Security and Privacy
The relay agent option depends on a trusted relationship between the relay agent and the server. Fraudulent relay agent option data could lead to theft of service or exhaustion of limited resources like IP addresses by unauthorized clients. A host that tampered with another host's relay data could deny it service or lead the server to assign it inappropriate parameters.
A perimeter defense can block these options unless the relay is trusted, but the RFC says a deeper defense using the Authentication Suboption or IPSec SHOULD be deployed as well. Message authentication for DHCP is defined in RFC 3118.
Depending on the identifier chosen, a Subscriber-ID may identify a specific host or user. In practice DHCP messages are usually confined to the provider's internal network, but if they leave it or values become visible to other users, privacy concerns may arise. Administrators should pick appropriate identifiers.
If someone fakes the tag, they could get free service or use up addresses, so the relay and server must trust each other. Tags should also not reveal private things about a person.
§7IANA Assignment
IANA assigned the value 6 from the DHCP Relay Agent Information Option suboption codes to the Subscriber-ID suboption. That is the only IANA action in the RFC.
The official number list now says that number 6 means the customer tag, so everyone uses the same number.