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

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.

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

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.

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.

   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.

   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
   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.

   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
   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.

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
   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.

   [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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