DHC Working Group M. Stapp
Internet-Draft Cisco Systems, Inc.
Expires: April 26, 2004 October 27, 2003
Resolution of DNS Name Conflicts Among DHCP Clients
<draft-ietf-dhc-ddns-resolution-06.txt>
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Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
DHCP provides a powerful mechanism for IP host configuration.
However, the configuration capability provided by DHCP does not
include updating DNS, and specifically updating the name to address
and address to name mappings maintained in the DNS. This document
describes techniques for the resolution of DNS name conflicts among
DHCP clients.
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Table of Contents
1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Issues with DNS Update in DHCP Environments . . . . . . . . 3
3.1 Client Mis-Configuration . . . . . . . . . . . . . . . . . . 4
3.2 Multiple DHCP Servers . . . . . . . . . . . . . . . . . . . 5
4. Use of the DHCID RR . . . . . . . . . . . . . . . . . . . . 5
5. DNS RR TTLs . . . . . . . . . . . . . . . . . . . . . . . . 6
6. Procedures for performing DNS updates . . . . . . . . . . . 6
6.1 Error Return Codes . . . . . . . . . . . . . . . . . . . . . 6
6.2 Dual IPv4/IPv6 Client Considerations . . . . . . . . . . . . 6
6.3 Adding A RRs to DNS . . . . . . . . . . . . . . . . . . . . 7
6.3.1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
6.3.2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
6.3.3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
6.4 Adding PTR RR Entries to DNS . . . . . . . . . . . . . . . . 8
6.5 Removing Entries from DNS . . . . . . . . . . . . . . . . . 8
6.6 Updating Other RRs . . . . . . . . . . . . . . . . . . . . . 9
7. Security Considerations . . . . . . . . . . . . . . . . . . 9
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10
References . . . . . . . . . . . . . . . . . . . . . . . . . 10
References . . . . . . . . . . . . . . . . . . . . . . . . . 11
Author's Address . . . . . . . . . . . . . . . . . . . . . . 11
Full Copyright Statement . . . . . . . . . . . . . . . . . . 12
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1. 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[1].
2. Introduction
"The Client FQDN Option"[6] includes a description of the operation
of DHCP[7] clients and servers that use the 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 or AAAA RR. This document
identifies situations in which conflicts in the use of FQDNs may
arise among DHCP clients, and describes a strategy for the use of
the DHCID DNS resource record[4] in resolving those conflicts.
In any case, whether a site permits all, some, or no DHCP servers
and clients to perform DNS updates (RFC 2136[5], RFC 3007[11]) 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
domain name. 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.
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 anomolous situations. If DNS updaters can
detect these situations, site administrators can configure the
updaters' behavior so that the site's policies can be enforced. We
use the term "Name Conflict" to refer to cases where more than one
DHCP client wishes to be associated with a single FQDN. This
specification describes a mechanism designed to allow updaters to
detect these situations, and suggests that DHCP implementations use
this mechanism by default.
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3.1 Client Mis-Configuration
At many (though not all) sites, administrators wish to maintain a
one-to-one relationship between active DHCP clients and domain
names, and to maintain consistency between a host's A and PTR RRs.
Hosts that are not represented in the DNS, or hosts which
inadvertently share an FQDN with another host may encounter
inconsistent behavior or may not be able to obtain access to network
resources. Whether each DHCP client is configured with a domain name
by its administrator or whether the DHCP server is configured to
distribute the clients' names, the consistency of the DNS data is
entirely dependent on the accuracy of the configuration procedure.
Sites that deploy Secure DNS[10] may configure credentials for each
host and its assigned name in a way that is more error-resistant,
but this level of pre-configuration is still rare in DHCP
environments.
Consider an example in which two DHCP clients in the "org.nil"
network are both configured with the name "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.org.nil", and its
DHCP server adds a PTR RR corresponding to its IP address lease.
When the second client, client B, boots, it is also configured via
DHCP, and it also begins to update "foo.org.nil".
At this point, the "org.nil" administrators may wish to establish
some policy about DHCP clients' DNS names. If the policy is that
each client that boots should replace any existing A RR that matches
its name, Client B can proceed, though Client A may encounter
problems. In this example, Client B replaces the A RR associated
with "foo.org.nil". Client A must have some way to recognize that
the RR associated with "foo.org.nil" now contains information for
Client B, so that it can avoid modifying the RR. When Client A's
lease expires, for example, it should not remove an RR that reflects
Client B's DHCP lease.
If the policy is that the first DHCP client with a given name should
be the only client associated with that name, Client B needs to be
able to determine that it is not the client associated with
"foo.org.nil". It could be that Client A booted first, and that
Client B should choose another name. Or it could be that B has
booted on a new subnet, and received a new lease. It must either
retain persistent state about the last lease it held (in addition to
its current lease) or it must have some other way to detect that it
was the last updater of "foo.org.nil" in order to implement the
site's policy.
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3.2 Multiple DHCP Servers
At many sites, the difficulties with distributing DNS update
credentials to all of the DHCP clients lead to the desire for the
DHCP servers to perform A RR updates on behalf of their clients. If
a single DHCP server managed all of the DHCP clients at a site, it
could maintain some database of the DNS names that it was managing,
and check that database before initiating a DNS update for a client.
Such a database is necessarily proprietary, however, and that
approach does not work once more than one DHCP server is deployed.
Consider an example in which DHCP Client A boots, obtains a DHCP
lease from Server S1, presenting the hostname "foo" in a Client FQDN
option[6] in its DHCPREQUEST message. Server S1 updates its domain
name, "foo.org.nil", adding an A RR that matches Client A's lease.
The client then moves to another subnet, served by Server S2. When
Client A boots on the new subnet, Server S2 will issue it a new
lease, and will attempt to add an A RR matching the new lease to
"foo.org.nil". At this point, without some communication mechanism
which 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 domain name, or whether A is a
different client configured with the same hostname. 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 a DNS name, in order to offer administrators
the opportunity to configure updater behavior.
For this purpose, a DHCID RR, specified in [4], is used to associate
client identification information with a DNS name and the A or PTR
RR associated with that name. When either a client or server adds an
A or PTR RR for a client, it also adds a DHCID RR that specifies a
unique client identity, based on data from the client's DHCPREQUEST
message. In this model, only one A RR is associated with a given DNS
name at a time.
By associating this ownership information with each DNS name,
cooperating DNS updaters may determine whether their client is
currently associated with a particular DNS name and implement the
appropriately configured administrative policy. In addition, DHCP
clients which currently have domain names may move from one DHCP
server to another without losing their DNS names.
The specific algorithms utilizing the DHCID RR to signal client
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ownership are explained below. The algorithms only work 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. DNS RR TTLs
RRs associated with DHCP clients may be more volatile than
statically configured RRs. DHCP clients and servers that perform
dynamic updates should attempt to specify resource record TTLs which
reflect this volatility, in order to minimize the possibility that
answers to DNS queries will return records that refer to DHCP lease
bindings that have expired.
The coupling among primary, secondary, and caching DNS servers is
'loose'; that is a fundamental part of the design of the DNS. This
looseness makes it impossible to prevent all possible situations in
which a resolver may return a record reflecting a DHCP lease binding
that has expired. In deployment, this rarely if ever represents a
significant problem. Most DHCP-managed hosts are rarely looked-up by
name in the DNS, and the deployment of IXFR (RFC 1995[14]) and
NOTIFY (RFC 1996[15]) can reduce the latency between updates and
their visibility at secondary servers.
We suggest these basic guidelines for implementors. In general, the
TTLs for RRs added as a result of DHCP lease activity SHOULD be less
than the initial lease time. The RR TTL on a DNS record added for a
DHCP lease SHOULD NOT exceed 1/3 of the lease time, and SHOULD be at
least 10 minutes. We recognize that individual administrators will
have varying requirements: DHCP servers and clients SHOULD allow
administrators to configure TTLs, either as an absolute time
interval or as a percentage of the lease time.
6. Procedures for performing DNS updates
6.1 Error Return Codes
Certain RCODEs defined in RFC 2136[5] indicate that the destination
DNS server cannot perform an update: FORMERR, SERVFAIL, REFUSED,
NOTIMP. If one of these RCODEs is returned, the updater MUST
terminate its update attempt. Because these errors may indicate a
misconfiguration of the updater or of the DNS server, the updater
MAY attempt to signal to its administrator that an error has
occurred, e.g. through a log message.
6.2 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,
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that a transition will take place over a period of time, and more
sites will have dual-stack clients present. IPv6 clients will be
represented by AAAA RRs; IPv4 clients by A RRs. The administrators
of some mixed deployments may wish to permit a single name to
contain A and AAAA RRs from different clients. Other deployments may
wish to restrict the use of a DNS name to a single DHCP client, and
allow only A and AAAA RRs reflecting that client's DHCP leases.
6.3 Adding A RRs to DNS
6.3.1
When a DHCP client or server intends to update an A RR, it performs
a DNS query with QNAME of the target name, and QTYPE is DHCID. If
the result is NXDOMAIN, the updater can conclude that the name is
not in use. In that case, the updater prepares a DNS UPDATE query
that includes as a prerequisite the assertion that the name does not
exist. The update section of the query attempts to add the new name
and its IP address mapping (an A RR), and the DHCID RR with its
unique client-identity.
If the query returns with NODATA, the updater can conclude that the
target name is in use, but that no DHCID RR is present. This
indicates that some records have been configured by an
administrator. Whether the updater proceeds with an update is a
matter of local administrative policy.
If the DHCID rrset is returned, the updater uses the hash
calculation defined in the DHCID RR specification[4] to determine
whether the client associated with the name matches the current
client's identity. If so, the updater proceeds following the steps
in [subsection xxx].
If any other status is returned, the updater MUST NOT attempt an
update.
6.3.2
If the update operation in Section 6.3.1 fails with YXDOMAIN, the
updater can conclude that the intended name is in use. The updater
then attempts to confirm that the DNS name is not being used by some
other host. The updater prepares a second UPDATE query in which the
prerequisite is that the desired name has attached to it a DHCID RR
whose contents match the client identity. The update section of
this query deletes the existing A records on the name, and adds the
A record that matches the DHCP binding and the DHCID RR with the
client identity.
If this query succeeds, the updater can conclude that the current
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client was the last client associated with the domain name, and that
the name now contains the updated A RR. The A RR update is now
complete (and a client updater is finished, while a server would
then proceed to perform a PTR RR update).
6.3.3
If the second query fails with NXRRSET, the updater must conclude
that the client's desired name is in use by another host. At this
juncture, the updater can decide (based on some administrative
configuration outside of the scope of this document) whether to let
the existing owner of the name keep that name, and to (possibly)
perform some name disambiguation operation on behalf of the current
client, or to replace the RRs on the name with RRs that represent
the current client. If the configured policy allows replacement of
existing records, the updater submits a query that deletes the
existing A RR and the existing DHCID RR, adding A and DHCID RRs that
represent the IP address and client-identity of the new client.
DISCUSSION:
The updating entity may be configured to allow the existing DNS
records on the domain name to remain unchanged, and to perform
disambiguation on the name of the current client in order to
attempt to generate a similar but unique name for the current
client. In this case, once another candidate name has been
generated, the updater should restart the process of adding an A
RR as specified in this section.
6.4 Adding PTR RR Entries to DNS
The DHCP server submits a DNS query that deletes all of the PTR RRs
associated with the lease IP address, and adds a PTR RR whose data
is the client's (possibly disambiguated) host name. The server MAY
also add a DHCID RR as specified in Section 4.
6.5 Removing Entries from DNS
The most important consideration in removing DNS entries is 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 a lease expires or a DHCP client issues a DHCPRELEASE request,
the DHCP server SHOULD delete the PTR RR that matches the DHCP
binding, if one was successfully added. The server's update query
SHOULD assert that the name in the PTR record matches the name of
the client whose lease has expired or been released.
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The entity chosen to handle the A record for this client (either the
client or the server) SHOULD delete the A record that was added when
the lease was made to the client.
In order to perform this delete, the updater prepares an UPDATE
query that contains two prerequisites. The first prerequisite
asserts that the DHCID RR exists whose data is the client identity
described in Section 4. The second prerequisite asserts that the
data in the A RR contains the IP address of the lease that has
expired or been released.
If the query fails, the updater MUST NOT delete the DNS name. It
may be that the client whose lease on has expired has moved to
another network and obtained a lease from a different server, which
has caused the client's A RR to be replaced. It may also be that
some other client has been configured with a name that matches the
name of the DHCP client, and the policy was that the last client to
specify the name would get the name. In these cases, the DHCID RR
will no longer match the updater's notion of the client-identity of
the host pointed to by the DNS name.
6.6 Updating Other RRs
The procedures described in this document only cover updates to the
A and PTR RRs. Updating other types of RRs is outside the scope of
this document.
7. Security Considerations
Unauthenticated updates to the DNS can lead to tremendous confusion,
through malicious attack or through inadvertent misconfiguration.
Administrators should be wary of permitting unsecured DNS updates to
zones that are exposed to the global Internet. Both DHCP clients and
servers SHOULD use some form of update request authentication (e.g.,
TSIG[12]) 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
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updates. In cases where a DHCP server is performing DNS updates on
behalf of a client, the DHCP server should be sure of the DNS name
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 a DNS name 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 DHCP
Authentication[13] becomes widely deployed this may become more
customary.
One example of a situation that offers some extra assurances is one
where the DHCP client is connected to a network through an MCNS
cable modem, and the CMTS (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 one where clients obtain
network access via a network access server using PPP. The NAS itself
might be obtaining IP addresses via DHCP, encoding a client
identification into the DHCP client-id option. In this case, the
network access server as well as the DHCP server might be operating
within a trusted environment, in which case the DHCP server could be
configured to trust that the user authentication and authorization
processing of the remote access server was sufficient, and would
therefore trust the client identification encoded within the DHCP
client-id.
8. Acknowledgements
Many thanks to Mark Beyer, Jim Bound, Ralph Droms, Robert Elz, Peter
Ford, Olafur Gudmundsson, Edie Gunter, Andreas Gustafsson, R. Barr
Hibbs, Kim Kinnear, Stuart Kwan, Ted Lemon, Ed Lewis, Michael Lewis,
Josh Littlefield, Michael Patton, Glenn Stump, and Bernie Volz for
their review and comments.
Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", RFC 2119, March 1997.
[2] Mockapetris, P., "Domain names - Concepts and Facilities", RFC
1034, Nov 1987.
[3] Mockapetris, P., "Domain names - Implementation and
Specification", RFC 1035, Nov 1987.
[4] Stapp, M., Gustafsson, A. and T. Lemon, "A DNS RR for Encoding
DHCP Information (draft-ietf-dnsext-dhcid-rr-*)", October 2003.
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[5] Vixie, P., Thomson, S., Rekhter, Y. and J. Bound, "Dynamic
Updates in the Domain Name System", RFC 2136, April 1997.
Informative References
[6] Stapp, M. and Y. Rekhter, "The DHCP Client FQDN Option
(draft-ietf-dhc-fqdn-option-*.txt)", October 2003.
[7] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
March 1997.
[8] Rivest, R., "The MD5 Message Digest Algorithm", RFC 1321,
April 1992.
[9] Marine, A., Reynolds, J. and G. Malkin, "FYI on Questions and
Answers to Commonly asked ``New Internet User'' Questions",
RFC 1594, March 1994.
[10] Eastlake, D., "Domain Name System Security Extensions", RFC
2535, March 1999.
[11] Wellington, B., "Secure Domain Name System (DNS) Dynamic
Update", RFC 3007, November 2000.
[12] Vixie, P., Gudmundsson, O., Eastlake, D. and B. Wellington,
"Secret Key Transaction Authentication for DNS (TSIG)", RFC
2845, May 2000.
[13] Droms, R. and W. Arbaugh, "Authentication for DHCP Messages",
RFC 3118, June 2001.
[14] Ohta, M., "Incremental Zone Transfer", RFC 1995, August 1996.
[15] Vixie, P., "A Mechanism for Prompt Notification of Zone
Changes (DNS NOTIFY)", RFC 1996, August 1996.
Author's Address
Mark Stapp
Cisco Systems, Inc.
1414 Massachusetts Ave.
Boxborough, MA 01719
USA
Phone: 978.936.1535
EMail: mjs@cisco.com
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