DHC Working Grop Michael Patrick
<draft-ietf-dhc-agent-options-00.txt> Motorola ISG
December 11, 1996
DHCP Agent-Supplied Options
Status of this Memo
This document is an Internet Draft. Internet Drafts are working
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Abstract
Newer high-speed public Internet access technologies call for a
high-speed modem to have a LAN attachment to one or more user hosts.
It is advantageous to use DHCP to assign user host IP addresses in
this environment, but a number of security and scaling problems arise
with such "public" DHCP use. This draft calls for the definition of
three options which are added by the DHCP relay agent when forwarding
public DHCP requests:
-- Agent Circuit ID
-- Agent Remote ID
-- Agent Subnet Mask
These options solve the identified problems.
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Table of Contents
1 Introduction...........................................2
1.1 High-Speed Circuit Switched Data Networks..............2
1.2 Bridge vs. Router Models...............................3
1.3 DHCP Relay Agent in the Circuit Access Equipment.......4
2.0 Agent-Supplied Options.................................6
2.1 Agent Circuit ID.......................................6
2.2 Agent Remote ID........................................7
2.3 Agent Subnet Mask......................................7
3.0 Issues Resolved........................................8
4.0 References.............................................10
5.0 Author's Address.......................................10
1 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 central access point.
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.
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+---------------+
Central | DHCP |-- ckt 1--- Modem1-- Host--- Host A
LAN | | Relay | Lan +- Host B
| | Agent | +- Host C
|-----| 1 |--
| | |...
+---------+ | +---------------+
| DHCP |--|
| Server | |
+---------+ |
|
| +---------------+
+---------+ | | DHCP |-- ckt 1--- Modem2-- Host--- Host D
| Other | | | Relay | Lan
| Servers |--|-----| Agent |
| (Web, | | | 2 |-- ckt 2--- Modem3-- Host--- Host E
| DNS) | | | |... 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.
1.2 Bridge vs. Router Models
For relay agents implementing a "bridge" model, i.e. where the
service offered is to bridge the Host LAN with the Central LAN, the
DHCP broadcast from the host is forwarded at the data link layer to
the DHCP server. The central access device performs proxy ARP for
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hosts, and DHCP proceeds as if the host and the DHCP server are on
the same IP subnetwork.
It is envisioned, however, that public access devices must support
tens of thousands of hosts connecting into the service, e.g. through
a cable data service. In this environment, it is unreasonable to
require all hosts and servers on the central LAN to maintain ARP
tables with tens of thousands of entries. A this scale, then, it is
necessary to provides a "Routing" model of public Internet access,
where one or more Logical IP Subnets (LISs) are implemented for
connecting hosts. The connecting hosts must be assigned, via DHCP,
host addresses from a set of LISs.
The Routing access model requires at some point between the Host and
the Central LAN a Logical IP Subnet (LIS) interface be identified,
and that the Host be assigned an IP address on that LIS. The
choices in use today by LAN-attached modems include:
1. Modem LIS: A LIS is defined for all hosts connected to the
Modem. This describes "dial-up" routers, ISDN routers, IP
routing Frame Relay Access Devices, and some cable data modems.
2. Router LIS: The circuit attachment equipment provides only
bridging between an intermediate LAN and the Host LAN. A
traditional router implements a LIS interface to the
intermediate LAN, routing it to the central LAN. This
characterizes traditional dial-up Remote Access Servers, and
some cable data head-end equipment.
3. Virtual LIS: The circuit attachment equipment provides
"virtual" LISs, and permits hosts connecting on its circuits to
belong to any of them. This characterizes switching hubs, and
some cable data network routers.
1.3 DHCP Relay Agent in the Circuit Access Equipment
This document advocates the co-located implementation of a DHCP relay
agent with the circuit access equipment. In the Modem LIS and Virtual
LIS model, the circuit access equipment acts as both a router to the
circuits and as the DHCP relay agent. It is appropriate for any of
the circuit access routing models discussed above, although is most
appropriate for the Modem LIS and Virtual LIS models.
The advantages of co-locating the DHCP relay agent with the circuit
access equipment are:
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1. 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;
2. The same mechanism use to identify the remote connection of the
circuit (e.g. a user ID requested by a Remote Access Server
acting as teh circuit access equiment) may be used as a host
indentifier by DHCP, and used for parameter assignment. This
includes centralized static 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 DHCP server know from which LIS to assign an IP
address?
- 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 identifer 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 Agent-Supplied Options
This document describes options which are added by the DHCP relay
agent. It also specifies handling of those options by the DHCP
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server. The option IDs shown have been assigned by IANA, and are in
decimal format.
2.1 Agent Circuit ID
This option MAY be added by DHCP relay agents which terminate
switched or permanent circuits. It encodes a agent-local identifier
of the circuit from which a DHCP discover/request packet was
received. It is intended for use by agents in relaying DHCP
responses back to the proper circuit. Possible uses of this field
include
- Router interface number
- Switching Hub port number
- Remote Access Server port number
- Frame Relay DLCI
- ATM virtual circuit number
- Cable Data virtual circuit number
The format of the Agent Circuit ID may be further standardized by
IETF working groups responsible for IP communication on that type of
circuit. In the absence of such standardization, the format may
proprietary to the relay agent vendor.
DHCP relay agents SHALL NOT modify any existing Agent Circuit ID
field which may be in a received DHCP Discover/Request; such an
option may have been added by a circuit bridge.
DHCP servers supporting this option MUST return the option value
unchanged in all Offer and Ack responses. Servers MAY use the
information for IP and other parameter assignment policies, but care
should be taken due to the potential proprietary format. The DHCP
server MAY report the Agent Circuit ID value of current leases in
statistical reports (including its MIB) and in logs.
Code Len Circuit ID
+------+------+------+------+------+------+------+------+--
| 82 | n | c1 | c2 | c3 | c4 | c5 | c6 | ...
+------+------+------+------+------+------+------+------+--
2.2 Agent Remote ID
This 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
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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 format of the Agent Remote ID will depend on the type of circuit
connected to the relay agent, and further specification of this field
may be standardized by the IETF working groups responsible for IP
communications on those circuit types. The only requirement is that
the remote ID be administered as globally unique.
DHCP servers supporting this option MUST return the option value
unchanged in all Offer and Ack responses. DHCP servers MAY use this
option to select parameters specific to particular users, hosts, or
subscriber modems. 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 Offer/Ack reply.
DHCP relay agents SHALL NOT modify any existing Agent Remote ID field
in received broadcasted DHCP Discover/Ack packets; such a field may
have been added by a circuit bridge.
Code Len Agent Remote ID
+------+------+------+------+------+------+------+------+--
| 83 | n | r1 | r2 | r3 | r4 | r5 | r6 | ...
+------+------+------+------+------+------+------+------+--
2.3 Agent Subnet Mask
This option MAY be added by DHCP relay agents which terminate
multiple Logical IP Subnets. It provides the server with the subnet
mask for the LIS on which the relay agent received the request. (The
giaddr field provides the agent's IP host address on that LIS.)
DHCP servers supporting this option MAY copy the Agent Subnet mask
value into the Client Subnet Mask (option 1) parameter returned to
the host, and SHOULD have a configurable option to do so. DHCP
Servers SHOULD NOT return the Agent Subnet Mask option in the
response.
This option is intended to avoid the duplicate configuration in both
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the relay agent and the server of the agent's circuit subnet masks.
A DHCP relay agent terminating a public data switched network may
have thousands of such configured circuits and masks.
DHCP relay agents SHALL remove any incoming Agent Subnet Mask options
on received broadcasted DHCP Discover/Request packets from clients.
This option is only appropriately added by the relay agent
implementing a LIS interface.
Code Len Agent Subnet Mask
+------+------+------+------+------+------+
| 84 | 4 | m1 | m2 | m3 | m4 |
+------+------+------+------+------+------+
3.0 Issues Resolved
Broadcast Forwarding
The circuit access equipment forwards the normally broadcasted
DHCP response only on the circuit indicated in the Agent Circuit
ID.
DHCP LIS selection
The DHCP server MAY use the "Gateway IP Address" to select the
Logical IP subnet from which to assign IP addresses. It may select
either the Subnet Mask provided by the forwarding relay agent, or
use local configuration information to select the Subnet Mask
based on giaddr (or other info in the DHCP request).
Note that the DHCP server SHOULD use the "giaddr" field of the
relayed DHCP request for the Router Option reported to the host.
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)
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The DHCP server SHOULD implement policies that restrict the number
of IP addresses to be assigned to a single remote ID.
Static Assignment
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.
IP Spoofing
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
other hosts.
Client Identifer Spoofing
By using the agent-supplied Agent Remote ID option, the untrusted
and as-yet unstandardized client identifer 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 on a
different remote ID.
4.0 References
[1] Droms, R. "Dynamic Host Configuration Protocol", RFC 1531
[2] Alexander,S. and Droms, R., "DHCP Options and BOOTP Vendor Extension"
RFC 1533.
5.0 Author's Address
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Michael Patrick
Motorola Information Systems Group
20 Cabot Blvd., MS M4-30
Mansfield, MA 02048
Phone: (508) 261-5707
Email: mpatrick@dma.isg.mot.com
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