Dynamic Host Configuration                                      M. Stapp
Internet-Draft                                                   B. Volz
Expires: January 14, March 22, 2005                              Cisco Systems, Inc.
                                                           July 16,
                                                      September 21, 2004

          Resolution of DNS Name Conflicts among DHCP Clients

Status of this Memo

   This document is an Internet-Draft and is subject to all provisions
   of section 3 of RFC 3667.  By submitting this Internet-Draft, each
   author represents that any applicable patent or other IPR claims of
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   which he or she become aware will be disclosed, in accordance with
   RFC 3668.

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Copyright Notice

   Copyright (C) The Internet Society (2004).  All Rights Reserved.


   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.

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 or AAAA RRs to DNS  . . . . . . . . . . . . . . . . . . .  7
       6.3.1   Initial DHCID RR Query . . . . . . . . . . . . . . . .  7
       6.3.2   DNS UPDATE When Name Not in Use  . . . . . . . . . . .  7  8
       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 10
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
   9.1   Normative References . . . . . . . . . . . . . . . . . . . . 11
   9.2   Informative References . . . . . . . . . . . . . . . . . . . 11 12
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 12 13
       Intellectual Property and Copyright Statements . . . . . . . . 13 14

1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   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 DHCPv4 [7] clients and servers that use the DHCPv4 client FQDN
   option.  And, "The DHCPv6 Client FQDN Option" [5] includes a
   description of the operation of DHCPv6 [9] 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
   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]) [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 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.

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] [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.

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.

   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.


   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]) [14]) and
   NOTIFY (RFC 1996 [11]) [15]) 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.

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 will likely wish to permit a single name to contain A and
   AAAA RRs from different clients.  Other deployments may the same client.

   Sites that wish to restrict
   the use of permit a DNS single name to a single DHCP client, and allow only contain both A and AAAA
   RRs reflecting MUST make use of DHCPv4 clients and servers that client's support using
   the DHCP leases. Unique Identifier for DHCPv4 client identifiers, see
   Node-Specific Client Identifiers for DHCPv4 [6].  Otherwise, a
   dual-stack client that uses older-style DHCPv4 client identifiers
   (see [7] and [8]) will only be able to have either its A or AAAA
   record in DNS under a name because of the DHCID RR conflicts that

6.3  Adding A or AAAA RRs to DNS

6.3.1  Initial DHCID RR Query

   When a DHCP client or server intends to update an A or AAAA RR, it
   performs a DNS query with QNAME of the target name and with QTYPE of

   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, NOERROR but without an answer, 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

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 or AAAA RR), and the DHCID RR with its unique

   If the update operation succeeds, the A 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 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 UPDATE query contains:
   1.  A delete of any existing A RRs on the name if this is an A update
       or an AAAA update and the updater does not desire A records on
       the name, name.
   2.  A delete of the existing AAAA RRs on the name if the updater does
       not desire AAAA records on the name or this update is adding an
       AAAA and
   adds the updater only desires a single address on the name.
   3.  An add of the A record RR that matches the DHCP binding and if this is an A
   4.  An add of the DHCID AAAA RR with that matches the client identity. DHCP binding if this is an
       AAAA update.

   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 or AAAA 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.

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 all RRs for the existing A RR name and adds the existing DHCID RR, adding A or AAAA and DHCID RRs
   that represent the IP address and client-identity of the new client.

      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. 4, in which case it would
   include a delete of all of the DHCID RRs associated with the lease IP
   address, and adds a DHCID RR for the client.

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

   When a lease expires or a DHCP client issues a DHCPRELEASE [7] or
   Release [9] 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.
   released and should delete all RRs for the name.

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

   In order to perform this A or AAAA RR delete, the updater prepares an
   UPDATE query that contains two prerequisites. 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 query succeeds, the updater prepares a second UPDATE query
   that contains three prerequisites and deletes all RRs for the name.
   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 there are no A RR
   contains the IP address of the lease RRs.  The third prerequisite asserts that has expired or been
   there are no AAAA RRs.

   If the either 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 or AAAA 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
   A, AAAA, PTR, and PTR DHCID 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 [8]) [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
   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

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

9.2  Informative References

   [4]   Stapp, M. and Y. Rekhter, "The DHCP Client FQDN Option
         (draft-ietf-dhc-fqdn-option-*.txt)", July 2004.

   [5]   Volz, B., "The DHCPv6 Client FQDN Option
         (draft-ietf-dhc-dhcpv6-fqdn-*.txt)", September 2004.

   [6]   Lemon, T. and B. Sommerfeld, "Node-Specific Client Identifiers
         for DHCPv4 (draft-ietf-dhc-3315id-for-v4-*txt)", July 2004.

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


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

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

   [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 in DNS", RFC 1995, August


   [15]  Vixie, P., "A Mechanism for Prompt Notification of Zone Changes
         (DNS NOTIFY)", RFC 1996, August 1996.

Authors' Addresses

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

   Phone: 978.936.1535
   EMail: mjs@cisco.com

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

   Phone: 978.936.0382
   EMail: volz@cisco.com

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