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Versions: 00 01 02 03 04 05 RFC 4361
DHC Working Group Ted Lemon
INTERNET DRAFT Nominum
Expires: January 2006 Bill Sommerfeld
Internet Draft Sun Microsystems
Document: <draft-ietf-dhc-3315id-for-v4-05.txt>
Updates: 2131, 2132, 3315
Category: Standards Track June, 2005
Node-Specific Client Identifiers for DHCPv4
Status of this Memo
By submitting this Internet-Draft, each author represents that any
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This document is a submission by the Dynamic Host Configuration
Working Group of the Internet Engineering Task Force (IETF). Comments
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Abstract
This document specifies the format that is to be used for encoding
DHCPv4 client identifiers, so that those identifiers will be inter-
changeable with identifiers used in the DHCPv6 protocol. This
document also addresses and corrects some problems in RFC2131 and
RFC2132 with respect to the handling of DHCP client identifiers.
1. Introduction
This document specifies the way in which DHCPv4 [RFC2131] clients
should identify themselves. DHCPv4 client implementations that
conform to this specification use a DHCPv6-style DHCP Unique
Identifier (DUID) [RFC3315] encapsulated in a DHCPv4 client
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identifier option. This supersedes the behavior 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
behalf of the client will be handled more 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.
2. 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 [RFC2119].
3. Applicability
This document updates RFC2131 and RFC2132. This document also
specifies behavior that is required of DHCPv4 and DHCPv6 clients
that are intended to operate in a dual-stack configuration.
Finally, this document recommends behavior for host configurations
where more than one DHCP client must operate in sequence in order
to fully configure the client - e.g., a network boot loader and the
operating system it loads.
DHCPv4 clients and servers that are implemented according to this
document should be implemented as if the changes specified in
section 6.3 and 6.4 have been made to RFC2131 and RFC2132. DHCPv4
clients should, in addition, follow the behavior specified in
section 6.1. DHCPv6 clients should follow the behavior specified
in section 6.2. DHCPv4 servers should additionally follow the
behavior specified in section 6.3.
4. Problem Statement
4.1. 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. One result of this recommendation
is that when the network interface hardware on a client computer
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is replaced, 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
DHCPv4 server.
4.2. Clients can accidentally present multiple identities
Consider a DHCPv4 client that has two network interfaces, one of
which is wired and one of which is wireless. The DHCPv4 client
will succeed in configuring either zero, one, or two network
interfaces. Under the current specification, each network
interface will receive a different IP address. The DHCPv4 server
will treat each network interface as a completely independent
DHCPv4 client, on a completely independent host.
Thus, when the client presents some information to be updated in a
network directory service, such as the DNS, the name that is
presented will be the same on both interfaces, but the identity
that is presented will be different. What will happen is that one
of the two interfaces will get the name, and will retain that name
as long as it has a valid lease, even if it loses its connection to
the network, while the other network interface will never get the
name. In some cases, this will achieve the desired result - when
only one network interface is connected, sometimes its IP address
will be published. In some cases, the one connected interface's IP
address will not be the one that is published. When there are two
interfaces, sometimes the correct one will be published, and
sometimes not.
This is likely to be a particular problem with modern laptops,
which usually have built-in wireless ethernet and wired ethernet.
When the user is near a wired outlet, he or she may want the
additional speed and privacy provided by a wired connection, but
that same user may unplug from the wired network and wander around,
still connected to the wireless network. When a transition like
this happens, under the current scheme, if the address of the wired
interface is the one that gets published, this client will be seen
by hosts attempting to connect to it as if it has intermittent
connectivity, even though it actually has continuous network
connectivity through the wireless port.
Another common case of a duplicate identity being presented occurs
when a boot monitor such as a PXE loader specifies one DHCP client
identifier, and then the operating system loaded by the boot loader
specifies a different identity.
4.3. RFC2131/2132 and RFC3315 identifiers are incompatible
The 'client identifier' option is used by DHCPv4 clients and
servers to identify clients. In some cases, the value of the
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'client identifier' option is used to mediate access to resources
(for example, the client's domain name, as published through the
DHCPv4 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.
4.4. RFC2131 does not require the use of a client identifier
RFC2131 allows the DHCPv4 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 sections 4.2 and 4.3.
5. Requirements
In order to address the problems stated in section 4, DHCPv4 client
identifiers must have the following characteristics:
- They must be persistent, in the sense that a particular host's
client identifier must not change simply because a piece of
network hardware is added or removed.
- It must be possible for the client to represent itself as having
more than one network identity - for example so that a client
with two network interfaces can express to the DHCPv4 server that
these two network interfaces are to receive different IP
addresses, even if they happen to be connected to the same link.
- In cases where the DHCPv4 client is expressing more than one
network identity at the same time, it must nevertheless be
possible for the DHCPv4 server to determine that the two network
identities belong to the same host.
- In some cases it may be desirable for a DHCP client to present
the same identity on two interfaces, so that if they both happen
to be connected to the same network, they will both receive the
same IP address. In such cases, it must be possible for the
client to use exactly the same identifier for each interface.
- DHCPv4 servers that do not conform to this specification, but that
are compliant with the older client identifier specification,
must correctly handle client identifiers sent by clients that
conform to this specification.
- DHCPv4 servers that do conform to this specification must
interoperate correctly with DHCPv4 clients that do not conform to
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this specification, except that when configuring such clients,
behaviors such as those described in section two may occur.
- The use by DHCPv4 clients of the chaddr field of the DHCPv4 packet
as an identifier must be deprecated.
- DHCPv4 client identifiers used by dual-stack hosts that also use
DHCPv6 must use the same host identification string for both
DHCPv4 and DHCPv6 - for example, a DHCPv4 server that uses the
client's identity to update the DNS on behalf of a DHCPv4 client
must register the same client identity in the DNS that would be
registered by the DHCPv6 server on behalf of the DHCPv6 client
running on that host, and vice versa.
In order to satisfy all but the last of these requirements, we need
to construct a DHCPv4 client identifier out of two parts. One part
must be unique to the host on which the client is running. The
other must be unique to the network identity being presented. The
DHCP Unique Identifier (DUID) and Identity Association Identifier
(IAID) specified in RFC3315 satisfy these requirements.
In order to satisfy the last requirement, we must use the DUID to
identify the DHCPv4 client. So, taking all the requirements
together, the DUID and IAID described in RFC3315 are the only
possible solution.
By following these rules, a compliant DHCPv4 client will
interoperate correctly with both compliant and non-complient DHCPv4
servers. A non-compliant DHCPv4 client will also interoperate
correctly with a compliant DHCPv4 server. If either server or
client is not compliant, the goals stated in the draft are not met,
but there is no loss of functionality.
6. Implementation
Here we specify changes to the behavior of DHCPv4 clients and
servers. We also specify changes to the wording in RFC2131 and
RFC2132. DHCPv4 clients, servers and relay agents that conform to
this specification must implement RFC2131 and RFC2132 with the
wording changes specified in sections 6.3 and 6.4.
6.1. DHCPv4 client behavior
DHCPv4 clients conforming to this specification MUST use stable
DHCPv4 node identifiers in the dhcp-client-identifier option.
DHCPv4 clients MUST NOT use client identifiers based solely on
layer two 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. DHCPv4 clients MUST send a
'client identifier' option containing an Identity Association
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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 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 by both a DHCPv4 and DHCPv6 client SHOULD automatically
use the same DUID for DHCPv4 and DHCPv6 without any operator
intervention.
DHCPv4 clients that support more than one network interface SHOULD
use the same DUID on every interface. DHCPv4 clients that support
more than one network interface SHOULD use a different IAID on
each interface.
A DHCPv4 client that generates a DUID and that has stable storage
MUST retain this DUID for use in subsequent DHCPv4 messages, even
after an operating system reboot.
6.2 DHCPv6 client behavior
Any 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 by both a DHCPv4 and DHCPv6 client SHOULD automatically
use the same DUID for DHCPv4 and DHCPv6 without any operator
intervention.
6.3. DHCPv4 server behavior
This document does not require any change to DHCPv4 or DHCPv6
servers that follow RFC2131 and RFC2132. However, some DHCPv4
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servers can be configured not to conform to RFC2131 and RFC2132, in
the sense that they ignore the 'client identifier' option and use
the client's hardware address instead.
DHCPv4 servers that conform to this specification MUST use the
'client identifier' option to identify the client if the client
sends it.
DHCPv4 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
case where the DHCPv4 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.
6.4. 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. The client MUST use the same 'client identifier' option
for all messages."
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".
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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 DHCPv4 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. DHCPv4
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.
6.5. 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 four-byte IA_ID field, followed
by the DUID for the client as defined in RF3315, section 9." The
text "its minimum length is 2" in the following paragraph is deleted.
7. Notes on DHCP clients in multi-stage network booting
In some cases a single device may actually run more than one DHCP
client in sequence, in the process of loading an operating system
over the network. In such cases, it may be that the first stage
boot uses a different client identifier, or no client identifier,
than the subsequent stage or stages.
The effect of this, under the DHCPv4 protocol, is that the two (in
some cases more than two!) boot stages will present different
identities. A DHCPv4 server will therefore allocate two different
IP addresses to the two different boot stages.
Some DHCP servers work around this problem for the common case
where the boot PROM presents no client identifier, and the
operating system DHCP client presents a client identifier
constructed from the MAC address of the network interface - both
are treated as the same identifier. This prevents the consumption
of an extra IP address.
A compliant DHCPv4 client does not use a client identifier
constructed from the MAC address of the network interface, because
network interfaces are not stable. So a compliant DHCPv4 client
can't be supported by a simple hack like the one described
previously; this may have some significant impact at some sites.
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We can't state the solution to this problem as a set of
requirements, because the circumstances in which this occurs vary
too widely. However, we can make some suggestions.
First, we suggest that DHCP clients in network boot loaders request
short lease times, so that their IP addresses are not retained.
Such clients should send a DHCPRELEASE message to the DHCP server
before moving on to the next stage of the boot process. Such
clients should provide a way for the operating system DHCP client
to configure a DUID to use in subsequent boots. DHCP clients in
the final stage should, where possible, configure the DUID used by
the boot PROM to be the same as the DUID used by the operating
system.
Secondly, implementors of DHCPv4 clients that are expected to only
be used in a multi-stage network boot configuration, and that are
not expected ever to network boot using DHCPv6, and that have a MAC
address that can't be easily changed, may not need to implement the
changes described in this specification. There is some danger in
making this assumption--the first solution suggested is definitely
better. A compromise might be to have the final-stage DHCP client
detect whether it is running on legacy hardware; if it is, it uses
the old identifier; if it is not, it follows the scheme described
in the previous paragraph.
8. 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.
9. IANA Considerations
None.
10. 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
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
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11. Informative References
[RFC3118] Droms, R., Arbaugh, W., "Authentication for DHCP
Messages", RFC3118, June, 2001
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
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