    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: 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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    Full Copyright Statement

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