Network Working Group M. Patrick
Request for Comments: 3046 Motorola BCS
Category: Standards Track January 2001 DHCP Relay Agent Information 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 (C) The Internet Society (2001). All Rights Reserved.
Newer high-speed public Internet access technologies call for a
high-speed modem to have a local area network (LAN) attachment to one
or more customer premise hosts. It is advantageous to use the
Dynamic Host Configuration Protocol (DHCP) as defined in RFC 2131 to
assign customer premise host IP addresses in this environment.
However, a number of security and scaling problems arise with such
"public" DHCP use. This document describes a new DHCP option to
address these issues. This option extends the set of DHCP options as
defined in RFC 2132.
The new option is called the Relay Agent Information option and is
inserted by the DHCP relay agent when forwarding client-originated
DHCP packets to a DHCP server. Servers recognizing the Relay Agent
Information option may use the information to implement IP address or
other parameter assignment policies. The DHCP Server echoes the
option back verbatim to the relay agent in server-to-client replies,
and the relay agent strips the option before forwarding the reply to
The "Relay Agent Information" option is organized as a single DHCP
option that contains one or more "sub-options" that convey
information known by the relay agent. The initial sub-options are
defined for a relay agent that is co-located in a public circuit
access unit. These include a "circuit ID" for the incoming circuit,
and a "remote ID" which provides a trusted identifier for the remote
Table of Contents
1 Introduction........................................... 21.1 High-Speed Circuit Switched Data Networks.............. 21.2 DHCP Relay Agent in the Circuit Access Equipment....... 42.0 Relay Agent Information Option......................... 52.1 Agent Operation........................................ 62.1.1 Reforwarded DHCP requests............................ 72.2 Server Operation....................................... 73.0 Relay Agent Information Suboptions..................... 83.1 Agent Circuit ID....................................... 83.2 Agent Remote ID........................................ 94.0 Issues Resolved........................................ 95.0 Security Considerations................................ 106.0 IANA Considerations.................................... 117.0 Intellectual Property Notice........................... 128.0 References............................................. 129.0 Glossary............................................... 1310.0 Author's Address...................................... 1311.0 Full Copyright Statement ............................. 141 Introduction
1.1 High-Speed Circuit Switched Data Networks
Public Access to the Internet is usually via a circuit switched data
network. Today, this is primarily implemented with dial-up modems
connecting to a Remote Access Server. But higher speed circuit
access networks also include ISDN, ATM, Frame Relay, and Cable Data
Networks. All of these networks can be characterized as a "star"
topology where multiple users connect to a "circuit access unit" via
switched or permanent circuits.
With dial-up modems, only a single host PC attempts to connect to the
central point. The PPP protocol is widely used to assign IP
addresses to be used by the single host PC.
The newer high-speed circuit technologies, however, frequently
provide a LAN interface (especially Ethernet) to one or more host
PCs. It is desirable to support centralized assignment of the IP
addresses of host computers connecting on such circuits via DHCP.
The DHCP server can be, but usually is not, co-implemented with the
centralized circuit concentration access device. The DHCP server is
often connected as a separate server on the "Central LAN" to which
the central access device (or devices) attach.
A common physical model for high-speed Internet circuit access is
shown in Figure 1, below.
Central | Circuit |-- ckt 1--- Modem1-- Host-|- Host A
LAN | | Access | Lan |- Host B
| | Unit 1 | |- Host C
|-----| |-- |
| |(relay agent) |...
+---------+ | +---------------+
| DHCP |--|
| Server | |
+---------+ | | Circuit |-- ckt 1--- Modem2-- Host--- Host D
| Other | | | Access | Lan
| Servers |--|-----| Unit 2 |
| (Web, | | | |-- ckt 2--- Modem3-- Host--- Host E
| DNS) | | |(relay agent) |... Lan
| | +---------------+
Figure 1: DHCP High Speed Circuit Access Model
Note that in this model, the "modem" connects to a LAN at the user
site, rather than to a single host. Multiple hosts are implemented
at this site. Although it is certainly possible to implement a full
IP router at the user site, this requires a relatively expensive
piece of equipment (compared to typical modem costs). Furthermore, a
router requires an IP address not only for every host, but for the
router itself. Finally, a user-side router requires a dedicated
Logical IP Subnet (LIS) for each user. While this model is
appropriate for relatively small corporate networking environments,
it is not appropriate for large, public accessed networks. In this
scenario, it is advantageous to implement an IP networking model that
does not allocate an IP address for the modem (or other networking
equipment device at the user site), and especially not an entire LIS
for the user side LAN.
Note that using this method to obtain IP addresses means that IP
addresses can only be obtained while communication to the central
site is available. Some host lan installations may use a local DHCP
server or other methods to obtain IP addresses for in-house use.
1.2 DHCP Relay Agent in the Circuit Access Unit
It is desirable to use DHCP to assign the IP addresses for public
high-speed circuit access. A number of circuit access units (e.g.,
RAS's, cable modem termination systems, ADSL access units, etc)
connect to a LAN (or local internet) to which is attached a DHCP
For scaling and security reasons, it is advantageous to implement a
"router hop" at the circuit access unit, much like high-capacity
RAS's do today. The circuit access equipment acts as both a router
to the circuits and as the DHCP relay agent.
The advantages of co-locating the DHCP relay agent with the circuit
access equipment are:
DHCP broadcast replies can be routed to only the proper circuit,
avoiding, say, the replication of the DCHP reply broadcast onto
thousands of access circuits;
The same mechanism used to identify the remote connection of the
circuit (e.g., a user ID requested by a Remote Access Server acting
as the circuit access equipment) may be used as a host identifier by
DHCP, and used for parameter assignment. This includes centralized
assignment of IP addresses to hosts. This provides a secure remote
ID from a trusted source -- the relay agent.
A number of issues arise when forwarding DHCP requests from hosts
connecting publicly accessed high-speed circuits with LAN connections
at the host. Many of these are security issues arising from DHCP
client requests from untrusted sources. How does the relay agent
know to which circuit to forward replies? How does the system
prevent DHCP IP exhaustion attacks? This is when an attacker
requests all available IP addresses from a DHCP server by sending
requests with fabricated client MAC addresses. How can an IP address
or LIS be permanently assigned to a particular user or modem? How
does one prevent "spoofing" of client identifier fields used to
assign IP addresses? How does one prevent denial of service by
"spoofing" other client's MAC addresses?
All of these issues may be addressed by having the circuit access
equipment, which is a trusted component, add information to DHCP
client requests that it forwards to the DHCP server.
2.0 Relay Agent Information Option
This document defines a new DHCP Option called the Relay Agent
Information Option. It is a "container" option for specific agent-
supplied sub-options. The format of the Relay Agent Information
Code Len Agent Information Field
| 82 | N | i1 | i2 | i3 | i4 | | iN |
The length N gives the total number of octets in the Agent
Information Field. The Agent Information field consists of a
sequence of SubOpt/Length/Value tuples for each sub-option, encoded
in the following manner:
SubOpt Len Sub-option Value
| 1 | N | s1 | s2 | s3 | s4 | | sN |
SubOpt Len Sub-option Value
| 2 | N | i1 | i2 | i3 | i4 | | iN |
No "pad" sub-option is defined, and the Information field shall NOT
be terminated with a 255 sub-option. The length N of the DHCP Agent
Information Option shall include all bytes of the sub-option
code/length/value tuples. Since at least one sub-option must be
defined, the minimum Relay Agent Information length is two (2). The
length N of the sub-options shall be the number of octets in only
that sub-option's value field. A sub-option length may be zero. The
sub-options need not appear in sub-option code order.
The initial assignment of DHCP Relay Agent Sub-options is as follows:
DHCP Agent Sub-Option Description
1 Agent Circuit ID Sub-option
2 Agent Remote ID Sub-option
2.1 Agent Operation
Overall adding of the DHCP relay agent option SHOULD be configurable,
and SHOULD be disabled by default. Relay agents SHOULD have separate
configurables for each sub-option to control whether it is added to
A DHCP relay agent adding a Relay Agent Information field SHALL add
it as the last option (but before 'End Option' 255, if present) in
the DHCP options field of any recognized BOOTP or DHCP packet
forwarded from a client to a server.
Relay agents receiving a DHCP packet from an untrusted circuit with
giaddr set to zero (indicating that they are the first-hop router)
but with a Relay Agent Information option already present in the
packet SHALL discard the packet and increment an error count. A
trusted circuit may contain a trusted downstream (closer to client)
network element (bridge) between the relay agent and the client that
MAY add a relay agent option but not set the giaddr field. In this
case, the relay agent does NOT add a "second" relay agent option, but
forwards the DHCP packet per normal DHCP relay agent operations,
setting the giaddr field as it deems appropriate.
The mechanisms for distinguishing between "trusted" and "untrusted"
circuits are specific to the type of circuit termination equipment,
and may involve local administration. For example, a Cable Modem
Termination System may consider upstream packets from most cable
modems as "untrusted", but an ATM switch terminating VCs switched
through a DSLAM may consider such VCs as "trusted" and accept a relay
agent option added by the DSLAM.
Relay agents MAY have a configurable for the maximum size of the DHCP
packet to be created after appending the Agent Information option.
Packets which, after appending the Relay Agent Information option,
would exceed this configured maximum size shall be forwarded WITHOUT
adding the Agent Information option. An error counter SHOULD be
incremented in this case. In the absence of this configurable, the
agent SHALL NOT increase a forwarded DHCP packet size to exceed the
MTU of the interface on which it is forwarded.
The Relay Agent Information option echoed by a server MUST be removed
by either the relay agent or the trusted downstream network element
which added it when forwarding a server-to-client response back to
The agent SHALL NOT add an "Option Overload" option to the packet or
use the "file" or "sname" fields for adding Relay Agent Information
option. It SHALL NOT parse or remove Relay Agent Information options
that may appear in the sname or file fields of a server-to-client
packet forwarded through the agent.
The operation of relay agents for specific sub-options is specified
with that sub-option.
Relay agents are NOT required to monitor or modify client-originated
DHCP packets addressed to a server unicast address. This includes
the DHCP-REQUEST sent when entering the RENEWING state.
Relay agents MUST NOT modify DHCP packets that use the IPSEC
Authentication Header or IPSEC Encapsulating Security Payload .
2.1.1 Reforwarded DHCP requests
A DHCP relay agent may receive a client DHCP packet forwarded from a
BOOTP/DHCP relay agent closer to the client. Such a packet will have
giaddr as non-zero, and may or may not already have a DHCP Relay
Agent option in it.
Relay agents configured to add a Relay Agent option which receive a
client DHCP packet with a nonzero giaddr SHALL discard the packet if
the giaddr spoofs a giaddr address implemented by the local agent
Otherwise, the relay agent SHALL forward any received DHCP packet
with a valid non-zero giaddr WITHOUT adding any relay agent options.
Per RFC 2131, it shall also NOT modify the giaddr value.
2.2 Server Operation
DHCP servers unaware of the Relay Agent Information option will
ignore the option upon receive and will not echo it back on
responses. This is the specified server behavior for unknown
DHCP servers claiming to support the Relay Agent Information option
SHALL echo the entire contents of the Relay Agent Information option
in all replies. Servers SHOULD copy the Relay Agent Information
option as the last DHCP option in the response. Servers SHALL NOT
place the echoed Relay Agent Information option in the overloaded
sname or file fields. If a server is unable to copy a full Relay
Agent Information field into a response, it SHALL send the response
without the Relay Information Field, and SHOULD increment an error
counter for the situation.
The operation of DHCP servers for specific sub-options is specified
with that sub-option.
Note that DHCP relay agents are not required to monitor unicast DHCP
messages sent directly between the client and server (i.e., those
that aren't sent via a relay agent). However, some relay agents MAY
chose to do such monitoring and add relay agent options.
Consequently, servers SHOULD be prepared to handle relay agent
options in unicast messages, but MUST NOT expect them to always be
3.0 Relay Agent Information Sub-options
3.1 Agent Circuit ID Sub-option
This sub-option MAY be added by DHCP relay agents which terminate
switched or permanent circuits. It encodes an agent-local identifier
of the circuit from which a DHCP client-to-server packet was
received. It is intended for use by agents in relaying DHCP
responses back to the proper circuit. Possible uses of this field
- Router interface number
- Switching Hub port number
- Remote Access Server port number
- Frame Relay DLCI
- ATM virtual circuit number
- Cable Data virtual circuit number
Servers MAY use the Circuit ID for IP and other parameter assignment
policies. The Circuit ID SHOULD be considered an opaque value, with
policies based on exact string match only; that is, the Circuit ID
SHOULD NOT be internally parsed by the server.
The DHCP server SHOULD report the Agent Circuit ID value of current
leases in statistical reports (including its MIB) and in logs. Since
the Circuit ID is local only to a particular relay agent, a circuit
ID should be qualified with the giaddr value that identifies the
SubOpt Len Circuit ID
| 1 | n | c1 | c2 | c3 | c4 | c5 | c6 | ...
3.2 Agent Remote ID Sub-option
This sub-option MAY be added by DHCP relay agents which terminate
switched or permanent circuits and have mechanisms to identify the
remote host end of the circuit. The Remote ID field may be used to
encode, for instance:
-- a "caller ID" telephone number for dial-up connection
-- a "user name" prompted for by a Remote Access Server
-- a remote caller ATM address
-- a "modem ID" of a cable data modem
-- the remote IP address of a point-to-point link
-- a remote X.25 address for X.25 connections
The remote ID MUST be globally unique.
DHCP servers MAY use this option to select parameters specific to
particular users, hosts, or subscriber modems. The option SHOULD be
considered an opaque value, with policies based on exact string match
only; that is, the option SHOULD NOT be internally parsed by the
The relay agent MAY use this field in addition to or instead of the
Agent Circuit ID field to select the circuit on which to forward the
DHCP reply (e.g., Offer, Ack, or Nak). DHCP servers SHOULD report
this value in any reports or MIBs associated with a particular
SubOpt Len Agent Remote ID
| 2 | n | r1 | r2 | r3 | r4 | r5 | r6 | ...
4.0 Issues Resolved
The DHCP relay agent option resolves several issues in an environment
in which untrusted hosts access the internet via a circuit based
public network. This resolution assumes that all DHCP protocol
traffic by the public hosts traverse the DHCP relay agent and that
the IP network between the DHCP relay agent and the DHCP server is
The circuit access equipment forwards the normally broadcasted
DHCP response only on the circuit indicated in the Agent Circuit
DHCP Address Exhaustion
In general, the DHCP server may be extended to maintain a database
with the "triplet" of
(client IP address, client MAC address, client remote ID)
The DHCP server SHOULD implement policies that restrict the number
of IP addresses to be assigned to a single remote ID.
The DHCP server may use the remote ID to select the IP address to
be assigned. It may permit static assignment of IP addresses to
particular remote IDs, and disallow an address request from an
unauthorized remote ID.
The circuit access device may associate the IP address assigned by
a DHCP server in a forwarded DHCP Ack packet with the circuit to
which it was forwarded. The circuit access device MAY prevent
forwarding of IP packets with source IP addresses -other than-
those it has associated with the receiving circuit. This prevents
simple IP spoofing attacks on the Central LAN, and IP spoofing of
Client Identifier Spoofing
By using the agent-supplied Agent Remote ID option, the untrusted
and as-yet unstandardized client identifier field need not be used
by the DHCP server.
MAC Address Spoofing
By associating a MAC address with an Agent Remote ID, the DHCP
server can prevent offering an IP address to an attacker spoofing
the same MAC address on a different remote ID.
5.0 Security Considerations
DHCP as currently defined provides no authentication or security
mechanisms. Potential exposures to attack are discussed in section 7
of the DHCP protocol specification in RFC 2131 .
This document introduces mechanisms to address several security
attacks on the operation of IP address assignment, including IP
spoofing, Client ID spoofing, MAC address spoofing, and DHCP server
address exhaustion. It relies on an implied trusted relationship
between the DHCP Relay Agent and the DHCP server, with an assumed
untrusted DHCP client. It introduces a new identifer, the "Remote
ID", that is also assumed to be trusted. The Remote ID is provided
by the access network or modem and not by client premise equipment.
Cryptographic or other techniques to authenticate the remote ID are
certainly possible and encouraged, but are beyond the scope of this
This option is targeted towards environments in which the network
infrastructure -- the relay agent, the DHCP server, and the entire
network in which those two devices reside -- is trusted and secure.
As used in this document, the word "trusted" implies that
unauthorized DHCP traffic cannot enter the trusted network except
through secured and trusted relay agents and that all devices
internal to the network are secure and trusted. Potential deployers
of this option should give careful consideration to the potential
security vulnerabilities that are present in this model before
deploying this option in actual networks.
Note that any future mechanisms for authenticating DHCP client to
server communications must take care to omit the DHCP Relay Agent
option from server authentication calculations. This was the
principal reason for organizing the DHCP Relay Agent Option as a
single option with sub-options, and for requiring the relay agent to
remove the option before forwarding to the client.
While it is beyond the scope of this document to specify the general
forwarding algorithm of public data circuit access units, note that
automatic reforwarding of IP or ARP broadcast packets back downstream
exposes serious IP security risks. For example, if an upstream
broadcast DHCP-DISCOVER or DHCP-REQUEST were re-broadcast back
downstream, any public host may easily spoof the desired DHCP server.
6.0 IANA Considerations
IANA is required to maintain a new number space of "DHCP Relay Agent
Sub-options", located in the BOOTP-DHCP Parameters Registry. The
initial sub-options are described in section 2.0 of this document.
IANA assigns future DHCP Relay Agent Sub-options with a "IETF
Consensus" policy as described in RFC 2434 . Future proposed
sub-options are to be referenced symbolically in the Internet-Drafts
that describe them, and shall be assigned numeric codes by IANA when
approved for publication as an RFC.
7.0 Intellectual Property Notices
This section contains two notices as required by  for standards
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 has been notified of intellectual property rights claimed in
regard to some or all of the specification contained in this
document. For more information consult the online list of claimed
 Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
 Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
Extension", RFC 2132, March 1997.
 Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, RFC 2434, October 1998.
 Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
 Bradner, S., "The Internet Standards Process -- Revision 3", BCP
9, RFC 2026, October 1996.
 Kent, S. and R. Atkinson, "Security Architecture for the
Internet Protocol", RFC 2401, November 1998.
DSLAM Digital Subscriber Link Access Multiplexer
IANA Internet Assigned Numbers Authority
LIS Logical IP Subnet
MAC Message Authentication Code
RAS Remote Access Server
10.0 Author's Address
Motorola Broadband Communications Sector
20 Cabot Blvd., MS M4-30
Mansfield, MA 02048
Phone: (508) 261-5707
11.0 Full Copyright Statement
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