Dynamic Host Configuration M. Stapp
Internet-Draft B. Volz
Expires: January 14, 2005 Cisco Systems, Inc.
July 16, 2004
Resolution of DNS Name Conflicts among DHCP Clients
<draft-ietf-dhc-ddns-resolution-07.txt>
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Copyright Notice
Copyright (C) The Internet Society (2004). 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 Misconfiguration . . . . . . . . . . . . . . . . . 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 . . . . . . . . . . . 7
6.3 Adding A RRs to DNS . . . . . . . . . . . . . . . . . . . 7
6.3.1 Initial DHCID RR Query . . . . . . . . . . . . . . . . 7
6.3.2 DNS UPDATE When Name Not in Use . . . . . . . . . . . 7
6.3.3 DNS UPDATE When Name in Use . . . . . . . . . . . . . 8
6.3.4 Name in Use by another Client . . . . . . . . . . . . 8
6.4 Adding PTR RR Entries to DNS . . . . . . . . . . . . . . . 9
6.5 Removing Entries from DNS . . . . . . . . . . . . . . . . 9
6.6 Updating Other RRs . . . . . . . . . . . . . . . . . . . . 9
7. Security Considerations . . . . . . . . . . . . . . . . . . . 10
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
9.1 Normative References . . . . . . . . . . . . . . . . . . . . 11
9.2 Informative References . . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 12
Intellectual Property and Copyright Statements . . . . . . . . 13
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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" [4] includes a description of the operation
of DHCP [5] 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 [2] 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 [3], RFC 3007 [7]) 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 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. 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 Misconfiguration
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 [6] 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 [4] 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 [2], 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.
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The specific algorithms utilizing the DHCID RR to signal client
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 [10]) and
NOTIFY (RFC 1996 [11]) can reduce the latency between updates and
their visibility at secondary servers.
We suggest these basic guidelines for implementers. 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 [3] 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.
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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, 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 Initial DHCID RR Query
When a DHCP client or server intends to update an A RR, it performs a
DNS query with QNAME of the target name and with QTYPE of DHCID.
If the query returns NXDOMAIN, the updater can conclude that the name
is not in use and proceeds to Section 6.3.2.
If the query returns 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 to Section 6.3.3. Otherwise
the updater must conclude that the client's desired name is in use by
another host and proceeds to Section 6.3.4.
If any other status is returned, the updater MUST NOT attempt an
update.
6.3.2 DNS UPDATE When Name Not in Use
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 update operation succeeds, 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).
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If the update returns YXDOMAIN, the updater can now conclude that the
intended name is in use and proceeds to Section 6.3.3.
6.3.3 DNS UPDATE When Name in Use
The updater next attempts to confirm that the DNS name is not being
used by some other host. The updater prepares a 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 the update succeeds, the updater can conclude that the current
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).
If the update returns NXRRSET, the updater must conclude that the
client's desired name is in use by another host and proceeds to
Section 6.3.3.
6.3.4 Name in Use by another Client
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.
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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.
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.
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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 [8]) 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 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
[9] 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.
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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, and Glenn Stump for their review
and comments.
9. References
9.1 Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Stapp, M., Gustafsson, A. and T. Lemon, "A DNS RR for Encoding
DHCP Information (draft-ietf-dnsext-dhcid-rr-*)", July 2004.
[3] Vixie, P., Thomson, S., Rekhter, Y. and J. Bound, "Dynamic
Updates in the Domain Name System (DNS UPDATE)", RFC 2136, April
1997.
9.2 Informative References
[4] Stapp, M. and Y. Rekhter, "The DHCP Client FQDN Option
(draft-ietf-dhc-fqdn-option-*.txt)", July 2004.
[5] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
March 1997.
[6] Eastlake, D., "Domain Name System Security Extensions", RFC
2535, March 1999.
[7] Wellington, B., "Secure Domain Name System (DNS) Dynamic
Update", RFC 3007, November 2000.
[8] Vixie, P., Gudmundsson, O., Eastlake, D. and B. Wellington,
"Secret Key Transaction Authentication for DNS (TSIG)", RFC
2845, May 2000.
[9] Droms, R. and W. Arbaugh, "Authentication for DHCP Messages",
RFC 3118, June 2001.
[10] Ohta, M., "Incremental Zone Transfer in DNS", RFC 1995, August
1996.
[11] Vixie, P., "A Mechanism for Prompt Notification of Zone Changes
(DNS NOTIFY)", RFC 1996, August 1996.
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Authors' Addresses
Mark Stapp
Cisco Systems, Inc.
1414 Massachusetts Ave.
Boxborough, MA 01719
USA
Phone: 978.936.1535
EMail: mjs@cisco.com
Bernie Volz
Cisco Systems, Inc.
1414 Massachusetts Ave.
Boxborough, MA 01719
USA
Phone: 978.936.0382
EMail: volz@cisco.com
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