DHC Working Group Ted Lemon
INTERNET DRAFT Nominum
Expires: August 2004 Bill Sommerfeld
Internet Draft Sun Microsystems
Document: <draft-ietf-dhc-3315id-for-v4-02.txt>
Category: Standards Track February, 2004
Node-Specific Client Identifiers for DHCPv4
Status of this Memo
This document is a submission by the Dynamic Host Configuration
Working Group of the Internet Engineering Task Force (IETF). Comments
should be submitted to the dhcwg@ietf.org mailing list.
Distribution of this memo is unlimited.
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups. Note that other groups may also distribute
working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at
any time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at:
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at:
http://www.ietf.org/shadow.html.
Abstract
This document specifies the format that is to be used for encoding
DHCPv4 [RFC2131 and RFC2132] client identifiers, so that those
identifiers will be interchangeable with identifiers used in the
DHCPv6 protocol [RFC3315].
Introduction
This document specifies the way in which DHCPv4 clients should
identify themselves. DHCPv4 client implementations that conform to
this specification use a DHCPv6-style DHCP Unique Identifier (DUID)
encapsulated in a DHCPv4 client identifier option. This supersedes
the behaviour specified in RFC2131 and RFC2132.
The reason for making this change is that as we make the transition
from IPv4 to IPv6, there will be network devices that must use both
DHCPv4 and DHCPv6. Users of these devices will have a smoother
network experience if the devices identify themselves consistently,
regardless of the version of DHCP they are using at any given
moment. Most obviously, DNS updates made by the DHCP server on
Lemon & Sommerfeld Expires August, 2004 [Page 1]
Internet-Draft Node-specific Identifiers for DHCPv4 February 2004
behalf of the client will not be handled correctly. This change
also addresses certain limitations in the functioning of
RFC2131/2132-style DHCP client identifiers.
This document first describes the problem to be solved. It then
states the new technique that is to be used to solve the problem.
Finally, it describes the specific changes that one would have to
make to RFC2131 and RFC2132 in order for those documents not to
contradict what is described in this document.
1.0 Applicability
This document updates RFC2131 and RFC2132. DHCPv4 servers
implementations SHOULD conform to this document. DHCPv4 clients on
network devices that are expected to support DHCPv6 in the future
SHOULD conform to this document. This document makes no changes to
the behavior of DHCPv6 clients or servers.
DHCPv4 clients and servers that are implemented according to this
document should be implemented as if the changes specified in
section 4.3 and 4.4 have been made to RFC2131 and RFC2132.
2.0 Problem Statement
2.1. Client identities are ephemeral
RFC2132 recommends that client identifiers be generated by using
the permanent link-layer address of the network interface that the
client is trying to configure. In cases where a network interface
is removed from the client computer and replaced with a different
network interface with a different permanent link-layer address,
the identity of the client changes. The client loses its IP
address and any other resources associated with its old identifier
- for example, its domain name as published through the DHCP
server.
2.2. Clients can accidentally present multiple identities
Consider a DHCP client that has two network interfaces, one of
which is wired and one of which is wireless. There are three
interesting cases here:
(a) Each network interface is attached to a different link.
(b) Both network interface are connected to the same link.
(c) Only one network interface is attached to a link.
Case (a) is problematic, and is beyond the scope of this document.
Briefly, in case (a), there is no obvious way to choose which of
the two network interfaces represents the published identity of the
client, and since the two network interfaces are connected to
different network links, this could make a significant
difference. Also, if, as is likely, the two devices use two
different identifiers, but wish to be identified as the same
Lemon & Sommerfeld Expires August, 2004 [Page 2]
Internet-Draft Node-specific Identifiers for DHCPv4 February 2004
client in the sense of the domain name on which their A record is
published, they will compete for which interface identity gets the
single available published identity, and there is no obvious way
to write a DHCP client that produces the right result.
Cases (b) and (c) are very common in practice, because many
devices such as laptop computers that are popular at the time of
this writing have both wireless and wired network interfaces that
are installed simultaneously. Both wired and wireless have
advantages - wired has the advantage of speed, and wireless the
advantage of mobility.
So it seems likely that there will be devices that are in states
(b) and (c) frequently, and indeed frequently make transitions
between these states. If the DHCP client that configures these
devices uses the link-layer address of each device as an
identifier, the two devices will appear to the DHCP server to be
different nodes, and thus will be assigned different IP addresses.
As in state (a), in state (b) only one of the two devices will be
be able to acquire the public identity of the client, although this
is less of a problem in case (b) because both interfaces are at
least connected to the same network link. Furthermore, if a device
in state (b) makes the transition to state (c), it is quite
possible that the lease for the device that has lost connectivity
will remain valid for some time. If the public identity of the
client is associated with this now-defunct interface the client
will not be reachable through its published domain name.
2.3. RFC2131/2132 and RFC3315 identifiers are incompatible
The 'client identifier' option is used by DHCP clients and servers
to identify clients. In some cases, the value of the 'client
identifier' option is used to mediate access to resources (for
example, the client's domain name, as published through the DHCP
server). RFC2132 and RFC3315 specify different methods for
deriving client identifiers. These methods guarantee that the
DHCPv4 and DHCPv6 identifier will be different. This means that
mediation of access to resources using these identifiers will not
work correctly in cases where a node may be configured using DHCPv4
in some cases and DHCPv6 in other cases.
2.4. RFC2131 does not require the use of a client identifier
RFC2131 allows the DHCP server to identify clients either by
using the client identifier option sent by the client, or, if the
client did not send one, the client's link-layer address. Like
the client identifier format recommended by RFC2131, this suffers
from the problems previously described in (2) and (3).
Lemon & Sommerfeld Expires August, 2004 [Page 3]
Internet-Draft Node-specific Identifiers for DHCPv4 February 2004
3. Solutions
The solution to problem 2.1 is to use a DHCP client identifier
that is persistent - not tied to a particular piece of removable
network hardware. Then, when network hardware is swapped in and
out, the client identifier does not change, and thus the client has
a consistent IP address and consistent use of whatever resources
have been associated with its identifier.
This creates a new problem in case 2.2, however: if the two
network interfaces are connected to the same link and use the same
identifier, then the server's IP address assignment algorithm will
assign the same IP address to both interfaces. But if the DHCP
client state machines configuring the two interfaces are
sufficiently out of sync, the DHCPDISCOVER from the slower
interface may be sent after the DHCPACK for the faster
interface. In this case, the DHCP server will detect a conflict
and abandon the IP address, because the faster interface is
responding to ICMP echo requests. So we can't just use the same
identifier on every interface.
The solution to problem 2.3 is to use the DHCP Unique Identifier as
defined in RFC3315 as a client identifier. The DUID provides
several different ways of producing persistent DHCP client
identifiers, at least one of which is likely to be appropriate for
any particular sort of network device. This is also a valid way of
addressing problem 2.1.
To finish addressing problem 2.2, we modify the solution slightly.
In addition to the DUID, RFC3315 defines an Identity Association ID
(IAID). The IAID, in combination with the DUID, identifies a
particular identity with which to associate an IP address. So a
DHCP client has a single DUID, but has one IAID for each interface.
The DHCP server associates IP addresses with the combination of
(DUID, IAID), but uses the DUID to identify the client as a whole.
The solution to problem (2.4) is to deprecate the use of the
contents of the chaddr field in the DHCP packet as a means of
identifying the client.
4. Implementation Requirements
Here we specify changes to the behavior of DHCP clients and
servers. We also specify changes to the wording in RFC2131 and
RFC2132. DHCP clients, servers and relay agents that conform to
this specification must implement RFC2131 and RFC2132 with the
wording changes specified in sections 4.3 and 4.4.
4.1. DHCP Client behavior
DHCP clients conforming to this specification MUST use stable DHCP
node identifiers in the dhcp-client-identifier option. DHCP
clients MUST NOT use client identifiers based solely on layer two
Lemon & Sommerfeld Expires August, 2004 [Page 4]
Internet-Draft Node-specific Identifiers for DHCPv4 February 2004
addresses that are hard-wired to the layer two device (e.g., the
ethernet MAC address) as suggested in RFC2131, except as allowed in
section 9.2 of RFC3315. DHCP clients MUST send a 'client
identifier' option containing an Identity Association Unique
Identifier, as defined in section 10 of RFC3315, and a DHCP Unique
Identifier, as defined in section 9 of RFC3315. These together
constitute an RFC3315-style binding identifier.
The general format of the DHCPv4 'client identifier' option is
defined in section 9.14 of RFC2132.
To send an RFC3315-style binding identifiier in a DHCPv4 'client
identifier' option, the type of the 'client identifier' option is
set to 255. The type field is immediately followed by the IAID,
which is an opaque 32-bit quantity. The IAID is immediately
followed by the DUID, which consumes the remaining contents of the
'client identifier' option. The format of the 'client
identifier' option is as follows:
Code Len Type IAID DUID
+----+----+-----+----+----+----+----+----+----+---
| 61 | n | 255 | i1 | i2 | i3 | i4 | d1 | d2 |...
+----+----+-----+----+----+----+----+----+----+---
Any DHCPv4 or DHCPv6 client that conforms to this specification
SHOULD provide a means by which an operator can learn what DUID the
client has chosen. Such clients SHOULD also provide a means by
which the operator can configure the DUID. A device that is
normally configured with both a DHCPv4 and DHCPv6 client SHOULD
automatically use the same DUID for DHCPv4 and DHCPv6 without any
operator intervention.
DHCP clients that support more than one network interface SHOULD
use the same DUID on every interface. DHCP clients that support
more than one network interface SHOULD use a different IAID on
each interface.
4.2. DHCPv4 Server behavior
This document does not require any change to DHCPv4 or DHCPv6
servers that follow RFC2131 and RFC2132. However, some DHCPv4
servers can be configured not to conform to RFC2131 and RFC2131, in
the sense that they ignore the 'client identifier' option and use
the client's hardware address instead.
DHCP servers that conform to this specification MUST use the
'client identifier' option to identify the client if the client
sends it.
DHCP servers MAY use administrator-supplied values for chaddr and
htype to identify the client in the case where the administrator is
assigning a fixed IP address to the client, even if the client
sends an client identifier option. This is ONLY permitted in the
Lemon & Sommerfeld Expires August, 2004 [Page 5]
Internet-Draft Node-specific Identifiers for DHCPv4 February 2004
case where the DHCP server administrator has provided the values
for chaddr and htype, because in this case if it causes a problem,
the administrator can correct the problem by removing the offending
configuration information.
4.3. Changes from RFC2131
In section 2 of RFC2131, on page 9, the text that reads "; for
example, the 'client identifier' may contain a hardware address,
identical to the contents of the 'chaddr' field, or it may contain
another type of identifier, such as a DNS name" is deleted.
In section 4.2 of RFC2131, the text "The client MAY choose to
explicitly provide the identifier through the 'client identifier'
option. If the client supplies a 'client identifier', the client
MUST use the same 'client identifier' in all subsequent messages,
and the server MUST use that identifier to identify the client. If
the client does not provide a 'client identifier' option, the
server MUST use the contents of the 'chaddr' field to identify the
client." is replaced by the text "The client MUST explicitly
provide a client identifier through the 'client identifier'
option."
In the same section, the text "Use of 'chaddr' as the client's
unique identifier may cause unexpected results, as that identifier
may be associated with a hardware interface that could be moved to
a new client. Some sites may choose to use a manufacturer's serial
number as the 'client identifier', to avoid unexpected changes in a
clients network address due to transfer of hardware interfaces
among computers. Sites may also choose to use a DNS name as the
'client identifier', causing address leases to be associated with
the DNS name rather than a specific hardware box." is replaced by
the text "The DHCP client MUST NOT rely on the 'chaddr' field to
identify it."
In section 4.4.1 of RFC2131, the text "The client MAY include a
different unique identifier" is replaced with "The client MUST
include a unique identifier".
In sections 3.1, item 4 and 6, 3.2 item 3 and 4, and 4.3.1, where
RFC2131 says that 'chaddr' may be used instead of the 'client
identifier' option, the text that says "or 'chaddr'" and "'chaddr'
or" is deleted.
Note that these changes do not relieve the DHCP server of the
obligation to use 'chaddr' as an identifier if the client does not
send a 'client identifier' option. Rather, they oblige clients
that conform with this document to send a 'client identifier'
option, and not rely on 'chaddr' for identification. DHCP servers
MUST use 'chaddr' as an identifier in cases where 'client
identifier' is not sent, in order to support old clients that do
not conform with this document.
Lemon & Sommerfeld Expires August, 2004 [Page 6]
Internet-Draft Node-specific Identifiers for DHCPv4 February 2004
4.4. Changes from RFC2132
The text in section 9.14, beginning with "The client identifier MAY
consist of" through "that meet this requirement for uniqueness." is
replaced with "the client identifier consists of a type field whose
value is normally 255, followed by a two-byte type field, followed
by the contents of the identifier. The two-byte type field and the
format of the contents of the identifier are defined in RF3315,
section 9." The text "its minimum length is 2" in the following
paragraph is deleted.
5. Security Considerations
This document raises no new security issues. Potential exposure to
attack in the DHCPv4 protocol are discussed in section 7 of the
DHCP protocol specification [RFC2131] and in Authentication for
DHCP messages [RFC3118]. Potential exposure to attack in the
DHCPv6 protocol is discussed in section 23 of RFC3315.
6. IANA Considerations
This document defines no new name spaces that need to be
administered by the IANA. This document deprecates all 'client
identifier' type codes other than 255, and thus there is no need
for the IANA to track possible values for the type field of the
'client identifier' option.
7. Normative References
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
March 1997.
[RFC2132] S. Alexander, R. Droms, "DHCP Options and BOOTP Vendor
Extensions", RFC2132, March, 1997
[RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
Carney, M., "Dynamic Host Configuration Protocol for
IPv6 (DHCPV6)", July, 2003
8. Informative References
[RFC3118] Droms, R., Arbaugh, W., "Authentication for DHCP
Messages", RFC3118, June, 2001
Lemon & Sommerfeld Expires August, 2004 [Page 7]
Internet-Draft Node-specific Identifiers for DHCPv4 February 2004
Author's Addresses
Ted Lemon
Nominum
2385 Bay Road
Redwood City, CA 94063 USA
+1 650 381 6000
mellon@nominum.com
Bill Sommerfeld
Sun Microsystems
1 Network Drive
Burlington, MA 01824
+1 781 442 3458
sommerfeld@sun.com
Full Copyright Statement
"Copyright (C) 2003, 2004 The Internet Society. 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.
This document and the information contained herein is provided on
an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE."
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
Lemon & Sommerfeld Expires August, 2004 [Page 8]
Internet-Draft Node-specific Identifiers for DHCPv4 February 2004