Network Working Group M. Stapp

Network Working Group M. Stapp
Request for Comments: 4703 B. Volz
Category: Standards Track Cisco Systems, Inc.
October 2006

Resolution of Fully Qualified Domain Name (FQDN) Conflicts
among Dynamic Host Configuration Protocol (DHCP) Clients

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 (2006).

Abstract

The Dynamic Host Configuration Protocol (DHCP) provides a mechanism
for host configuration that includes dynamic assignment of IP
addresses and fully qualified domain names. To maintain accurate
name-to-IP-address and IP-address-to-name mappings in the DNS, these
dynamically assigned addresses and fully qualified domain names
(FQDNs) require updates to the DNS. This document identifies
situations in which conflicts in the use of fully qualified domain
names may arise among DHCP clients and servers, and it describes a
strategy for the use of the DHCID DNS resource record (RR) in
resolving those conflicts.

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Table of Contents

1. Introduction ....................................................3
2. Terminology .....................................................3
3. Issues with DNS Update in DHCP Environments .....................4
3.1. Client Misconfiguration ....................................4
3.2. Multiple DHCP Servers ......................................5
4. Use of the DHCID RR .............................................5
5. Procedures for Performing DNS Updates ...........................6
5.1. Error Return Codes .........................................6
5.2. Dual IPv4/IPv6 Client Considerations .......................6
5.3. Adding A and/or AAAA RRs to DNS ............................7
5.3.1. Initial DHCID RR Request ............................7
5.3.2. DNS UPDATE When FQDN in Use .........................7
5.3.3. FQDN in Use by Another Client .......................8
5.4. Adding PTR RR Entries to DNS ...............................8
5.5. Removing Entries from DNS ..................................9
5.6. Updating Other RRs ........................................10
6. Security Considerations ........................................10
7. Acknowledgements ...............................................11
8. References .....................................................11
8.1. Normative References ......................................11
8.2. Informative References ....................................11

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1. Introduction

"The Client FQDN Option" [8] includes a description of the operation
of [4] clients and servers that use the DHCPv4 client FQDN option.
"The DHCPv6 Client FQDN Option" [9] includes a description of the
operation of [5] clients and servers that use the DHCPv6 client FQDN
option. Through the use of the client FQDN option, DHCP clients and
servers can negotiate the client's FQDN and the allocation of
responsibility for updating the DHCP client's A and/or AAAA RRs.
This document identifies situations in which conflicts in the use of
FQDNs may arise among DHCP clients and servers, and it describes a
strategy for the use of the DHCID DNS resource record [2] in
resolving those conflicts.

In any case, whether a site permits all, some, or no DHCP servers and
clients to perform DNS updates ([3], [10]) into the zones that it
controls is entirely a matter of local administrative policy. This
document does not require any specific administrative policy, and
does not propose one. The range of possible policies is very broad,
from sites where only the DHCP servers have been given credentials
that the DNS servers will accept, to sites where each individual DHCP
client has been configured with credentials that allow the client to
modify its own FQDN. Compliant implementations MAY support some or
all of these possibilities. Furthermore, this specification applies
only to DHCP client and server processes; it does not apply to other
processes that initiate DNS updates.

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 [1].

This document assumes familiarity with DNS terminology defined in [6]
and DHCP terminology defined in [4] and [5].

FQDN, or Fully Qualified Domain Name, is the full name of a system,
rather than just its hostname. For example, "venera" is a hostname,
and "venera.isi.edu" is an FQDN. See [7].

DOCSIS, or Data-Over-Cable Service Interface Specifications, is
defined by CableLabs.

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3. Issues with DNS Update in DHCP Environments

There are two DNS update situations that require special
consideration in DHCP environments: cases where more than one DHCP
client has been configured with the same FQDN, and cases where more
than one DHCP server has been given authority to perform DNS updates
in a zone. In these cases, it is possible for DNS records to be
modified in inconsistent ways unless the updaters have a mechanism
that allows them to detect anomalous situations. If DNS updaters can
detect these situations, site administrators can configure the
updaters' behavior so that the site's policies can be enforced. This
specification describes a mechanism designed to allow updaters to
detect these situations and suggests that DHCP implementations use
this mechanism by default.

3.1. Client Misconfiguration

Administrators may wish to maintain a one-to-one relationship between
active DHCP clients and FQDNs, and to maintain consistency between a
client's A, AAAA, and PTR RRs. Clients that are not represented in
the DNS, or clients that inadvertently share an FQDN with another
client may encounter inconsistent behavior or may not be able to
obtain access to network resources. Whether each DHCP client is
configured with an FQDN by its administrator or whether the DHCP
server is configured to distribute the clients' FQDN, the consistency
of the DNS data is entirely dependent on the accuracy of the
configuration procedure. Sites that deploy [10] may configure
credentials for each client and its assigned FQDN in a way that is
more error-resistant, as both the FQDN and credentials must match.

Consider an example in which two DHCP clients in the "example.com"
network are both configured with the hostname "foo". The clients are
permitted to perform their own DNS updates. The first client, client
A, is configured via DHCP. It adds an A RR to "foo.example.com", and
its DHCP server adds a PTR RR corresponding to its assigned IP
address. When the second client, client B, boots, it is also
configured via DHCP, and it also begins to update "foo.example.com".

At this point, the "example.com" administrators may wish to establish
some policy about DHCP clients' FQDNs. If the policy is that each
client that boots should replace any existing A RR that matches its
FQDN, Client B can proceed, though Client A may encounter problems.
In this example, Client B replaces the A RR associated with
"foo.example.com". Client A must have some way to recognize that the
RR associated with "foo.example.com" now contains information for
Client B, so that it can avoid modifying the RR. When Client A's
assigned IP address expires, for example, it should not remove an RR
that reflects Client B's DHCP-assigned IP address.

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If the policy is that the first DHCP client with a given FQDN should
be the only client associated with that FQDN, Client B needs to be
able to determine if it is not the client associated with
"foo.example.com". It could be that Client A booted first, and that
Client B should choose another FQDN. Or it could be that B has
booted on a new subnet and received a new IP address assignment, in
which case B should update the DNS with its new IP address. It must
either retain persistent state about the last IP address it was
assigned (in addition to its current IP address) or it must have some
other way to detect that it was the last updater of "foo.example.com"
in order to implement the site's policy.

3.2. Multiple DHCP Servers

It is possible to arrange for DHCP servers to perform A and/or AAAA
RR updates on behalf of their clients. If a single DHCP server
manages all of the DHCP clients at a site, it can maintain a database
of the FQDNs in use and can check that database before assigning an
FQDN to a client. Such a database is necessarily proprietary,
however, and the approach does not work once more than one DHCP
server is deployed.

When multiple DHCP servers are deployed, the servers require a way to
coordinate the identities of DHCP clients. Consider an example in
which DHCPv4 Client A boots, obtains an IP address from Server S1,
presenting the hostname "foo" in a Client FQDN option [8] in its
DHCPREQUEST message. Server S1 updates the FQDN "foo.example.com",
adding an A RR containing the IP address assigned to A. The client
then moves to another subnet, served by Server S2. When Client A
boots on the new subnet, Server S2 will assign it a new IP address
and will attempt to add an A RR containing the newly assigned IP
address to the FQDN "foo.example.com". At this point, without some
communication mechanism that S2 can use to ask S1 (and every other
DHCP server that updates the zone) about the client, S2 has no way to
know whether Client A is currently associated with the FQDN, or
whether A is a different client configured with the same FQDN. If
the servers cannot distinguish between these situations, they cannot
enforce the site's naming policies.

4. Use of the DHCID RR

A solution to both of these problems is for the updater (a DHCP
client or DHCP server) to be able to determine which DHCP client has
been associated with an FQDN, in order to offer administrators the
opportunity to configure updater behavior.

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For this purpose, a DHCID RR, specified in [2], is used to associate
client identification information with an FQDN and the A, AAAA, and
PTR RRs associated with that FQDN. When either a client or server
adds A, AAAA, or PTR RRs for a client, it also adds a DHCID RR that
specifies a unique client identity, based on data from the client's
DHCP message. In this model, only one client is associated with a
given FQDN at a time.

By associating this ownership information with each FQDN, cooperating
DNS updaters may determine whether their client is currently
associated with a particular FQDN and implement the appropriately
configured administrative policy. In addition, DHCP clients that
currently have FQDNs may move from one DHCP server to another without
losing their FQDNs.

The specific algorithm utilizing the DHCID RR to signal client
ownership is explained below. The algorithm only works in the case
where the updating entities all cooperate -- this approach is
advisory only and is not a substitute for DNS security, nor is it
replaced by DNS security.

5. Procedures for Performing DNS Updates

5.1. Error Return Codes

Certain RCODEs defined in [3] indicate that the destination DNS
server cannot perform an update, i.e., FORMERR, SERVFAIL, REFUSED,
NOTIMP. If one of these RCODEs is returned, the updater MUST
terminate its update attempt. Other RCODEs [13] may indicate that
there are problems with the key being used and may mean to try a
different key, if available, or to terminate the operation. Because
some errors may indicate a misconfiguration of the updater or the DNS
server, the updater MAY attempt to signal to its administrator that
an error has occurred, e.g., through a log message.

5.2. Dual IPv4/IPv6 Client Considerations

At the time of publication of this document, a small minority of DHCP
clients support both IPv4 and IPv6. We anticipate, however, that a
transition will take place over a period of time, and more sites will
have dual-stack clients present. IPv6 clients require updates of
AAAA RRs; IPv4 client require updates of A RRs. The administrators
of mixed deployments will likely wish to permit a single FQDN to
contain A and AAAA RRs from the same client.

Sites that wish to permit a single FQDN to contain both A and AAAA
RRs MUST make use of DHCPv4 clients and servers that support using
the DHCP Unique Identifier (DUID) for DHCPv4 client identifiers such

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that this DUID is used in computing the RDATA of the DHCID RR by both
DHCPv4 and DHCPv6 for the client; see [11]. Otherwise, a dual-stack
client that uses older-style DHCPv4 client identifiers (see [4] and
[12]) will only be able to have either its A or AAAA records in DNS
under a single FQDN because of the DHCID RR conflicts that result.

5.3. Adding A and/or AAAA RRs to DNS

When a DHCP client or server intends to update A and/or AAAA RRs, it
starts with the UPDATE request in Section 5.3.1.

As the update sequence below can result in loops, implementers SHOULD
limit the total number of attempts for a single transaction.

5.3.1. Initial DHCID RR Request

The updater prepares a DNS UPDATE request that includes as a
prerequisite the assertion that the FQDN does not exist. The update
section of the request attempts to add the new FQDN and its IP
address mapping (A and/or AAAA RRs) and the DHCID RR with its unique
client identity.

If the UPDATE request succeeds, the A and/or AAAA RR update is now
complete (and a client updater is finished, while a server would then
proceed to perform a PTR RR update).

If the response to the UPDATE returns YXDOMAIN, the updater can now
conclude that the intended FQDN is in use and proceeds to
Section 5.3.2.

If any other status is returned, the updater SHOULD NOT attempt an
update (see Section 5.1).

5.3.2. DNS UPDATE When FQDN in Use

The updater next attempts to confirm that the FQDN is not being used
by some other client by preparing an UPDATE request in which there
are two prerequisites. The first prerequisite is that the FQDN
exists. The second is that the desired FQDN has attached to it a
DHCID RR whose contents match the client identity. The update
section of the UPDATE request contains:

1. A delete of any existing A RRs on the FQDN if this is an A update
or an AAAA update and the updater does not desire A records on
the FQDN, or if this update is adding an A and the updater only
desires a single IP address on the FQDN.

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2. A delete of the existing AAAA RRs on the FQDN if the updater does
not desire AAAA records on the FQDN, or if this update is adding
an AAAA and the updater only desires a single IP address on the
FQDN.

3. An add (or adds) of the A RR that matches the DHCP binding if
this is an A update.

4. Adds of the AAAA RRs that match the DHCP bindings if this is an
AAAA update.

Whether A or AAAA RRs are deleted depends on the updater or updater's
policy. For example, if the updater is the client or configured as
the only DHCP server for the link on which the client is located, the
updater may find it beneficial to delete all A and/or AAAA RRs and
then add the current set of A and/or AAAA RRs, if any, for the
client.

If the UPDATE request succeeds, the updater can conclude that the
current client was the last client associated with the FQDN, and that
the FQDN now contains the updated A and/or AAAA RRs. The update is
now complete (and a client updater is finished, while a server would
then proceed to perform a PTR RR update).

If the response to the UPDATE request returns NXDOMAIN, the FQDN is
no longer in use, and the updater proceeds back to Section 5.3.1.

If the response to the UPDATE request returns NXRRSET, there are two
possibilities: there are no DHCID RRs for the FQDN, or the DHCID RR
does not match. In either case, the updater proceeds to
Section 5.3.3.

5.3.3. FQDN in Use by Another Client

As the FQDN appears to be in use by another client or is not
associated with any client, the updater SHOULD either choose another
FQDN and restart the update process with this new FQDN or terminate
the update with a failure.

Techniques that may be considered to disambiguate FQDNs include
adding some suffix or prefix to the hostname portion of the FQDN or
randomly generating a hostname.

5.4. Adding PTR RR Entries to DNS

The DHCP server submits a DNS UPDATE request that deletes all of the
PTR RRs associated with the client's assigned IP address and adds a
PTR RR whose data is the client's (possibly disambiguated) FQDN. The

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server MAY also add a DHCID RR as specified in Section 4, in which
case it would include a delete of all of the DHCID RRs associated
with the client's assigned IP address and would add a DHCID RR for
the client.

There is no need to validate the DHCID RR for PTR updates as the DHCP
server (or servers) only assigns an address to a single client at a
time.

5.5. Removing Entries from DNS

The most important consideration in removing DNS entries is to be
sure that an entity removing a DNS entry is only removing an entry
that it added, or for which an administrator has explicitly assigned
it responsibility.

When an address' lease time or valid lifetime expires or a DHCP
client issues a DHCPRELEASE [4] or Release [5] request, the DHCP
server SHOULD delete the PTR RR that matches the DHCP binding, if one
was successfully added. The server's UPDATE request SHOULD assert
that the domain name (PTRDNAME field) in the PTR record matches the
FQDN of the client whose address has expired or been released and
should delete all RRs for the FQDN.

The entity chosen to handle the A or AAAA records for this client
(either the client or the server) SHOULD delete the A or AAAA records
that were added when the address was assigned to the client.
However, the updater should only remove the DHCID RR if there are no
A or AAAA RRs remaining for the client.

In order to perform this A or AAAA RR delete, the updater prepares an
UPDATE request that contains a prerequisite that asserts that the
DHCID RR exists whose data is the client identity described in
Section 4 and contains an update section that deletes the client's
specific A or AAAA RR.

If the UPDATE request succeeds, the updater prepares a second UPDATE
request that contains three prerequisites and an update section that
deletes all RRs for the FQDN. The first prerequisite asserts that
the DHCID RR exists whose data is the client identity described in
Section 4. The second prerequisite asserts that there are no A RRs.
The third prerequisite asserts that there are no AAAA RRs.

If either request fails, the updater MUST NOT delete the FQDN. It
may be that the client whose address has expired has moved to another
network and obtained an address from a different server, which has
caused the client's A or AAAA RR to be replaced. Or, the DNS data
may have been removed or altered by an administrator.

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5.6. Updating Other RRs

The procedures described in this document only cover updates to the
A, AAAA, PTR, and DHCID RRs. Updating other types of RRs is outside
the scope of this document.

6. Security Considerations

Administrators should be wary of permitting unsecured DNS updates to
zones, whether or not they are exposed to the global Internet. Both
DHCP clients and servers SHOULD use some form of update request
authentication (e.g., TSIG [13]) when performing DNS updates.

Whether a DHCP client may be responsible for updating an FQDN-to-IP-
address mapping, or whether this is the responsibility of the DHCP
server, is a site-local matter. The choice between the two
alternatives may be based on the security model that is used with the
Dynamic DNS Update protocol (e.g., only a client may have sufficient
credentials to perform updates to the FQDN-to-IP-address mapping for
its FQDN).

Whether a DHCP server is always responsible for updating the FQDN-
to-IP-address mapping (in addition to updating the IP-to-FQDN
mapping), regardless of the wishes of an individual DHCP client, is
also a site-local matter. The choice between the two alternatives
may be based on the security model that is being used with dynamic
DNS updates. In cases where a DHCP server is performing DNS updates
on behalf of a client, the DHCP server should be sure of the FQDN to
use for the client, and of the identity of the client.

Currently, it is difficult for DHCP servers to develop much
confidence in the identities of their clients, given the absence of
entity authentication from the DHCP protocol itself. There are many
ways for a DHCP server to develop an FQDN to use for a client, but
only in certain relatively rare circumstances will the DHCP server
know for certain the identity of the client. If [14] becomes widely
deployed, this may become more customary.

One example of a situation that offers some extra assurances is when
the DHCP client is connected to a network through a DOCSIS cable
modem, and the Cable Modem Termination System (head-end) of the cable
modem ensures that MAC address spoofing simply does not occur.
Another example of a configuration that might be trusted is when
clients obtain network access via a network access server using PPP.
The Network Access Server (NAS) itself might be obtaining IP
addresses via DHCP, encoding client identification into the DHCP
client-id option. In this case, the NAS as well as the DHCP server
might be operating within a trusted environment, in which case the

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DHCP server could be configured to trust that the user authentication
and authorization processing of the NAS was sufficient, and would
therefore trust the client identification encoded within the DHCP
client-id.

7. Acknowledgements

Many thanks to Mark Beyer, Jim Bound, Ralph Droms, Robert Elz, Peter
Ford, Olafur Gudmundsson, Edie Gunter, Andreas Gustafsson, David W.
Hankins, R. Barr Hibbs, Kim Kinnear, Stuart Kwan, Ted Lemon, Ed
Lewis, Michael Lewis, Josh Littlefield, Michael Patton, Pekka Savola,
and Glenn Stump for their review and comments.

8. References

8.1. Normative References

[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.

[2] Stapp, M., Lemon, T., and A. Gustafsson, "A DNS Resource Record
(RR) for Encoding Dynamic Host Configuration Protocol (DHCP)
Information (DHCID RR), RFC 4701, October 2006.

[3] Vixie, P., Thomson, S., Rekhter, Y., and J. Bound, "Dynamic
Updates in the Domain Name System (DNS UPDATE)", RFC 2136,
April 1997.

[4] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
March 1997.

[5] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., and M.
Carney, "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)",
RFC 3315, July 2003.

8.2. Informative References

[6] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.

[7] Malkin, G., "Internet Users' Glossary", FYI 18, RFC 1983,
August 1996.

[8] Stapp, M., Volz, B., and Y. Rekhter, "The Dynamic Host
Configuration Protocol (DHCP) Client Fully Qualified Domain
Name (FQDN) Option", RFC 4702, October 2006.

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[9] Volz, B., "The Dynamic Host Configuration Protocol for IPv6
(DHCPv6) Client Fully Qualified Domain Name (FQDN) Option", RFC
4704, October 2006.

[10] Wellington, B., "Secure Domain Name System (DNS) Dynamic
Update", RFC 3007, November 2000.

[11] Lemon, T. and B. Sommerfeld, "Node-specific Client Identifiers
for Dynamic Host Configuration Protocol Version Four (DHCPv4)",
RFC 4361, February 2006.

[12] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
Extensions", RFC 2132, March 1997.

[13] Vixie, P., Gudmundsson, O., Eastlake, D., and B. Wellington,
"Secret Key Transaction Authentication for DNS (TSIG)",
RFC 2845, May 2000.

[14] Droms, R. and W. Arbaugh, "Authentication for DHCP Messages",
RFC 3118, June 2001.

Authors' Addresses

Mark Stapp
Cisco Systems, Inc.
1414 Massachusetts Ave.
Boxborough, MA 01719
USA

Phone: 978.936.1535
EMail: [email protected]

Bernie Volz
Cisco Systems, Inc.
1414 Massachusetts Ave.
Boxborough, MA 01719
USA

Phone: 978.936.0382
EMail: [email protected]

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