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rfc:rfc4703

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.

Stapp & Volz Standards Track [Page 1] RFC 4703 Resolution of FQDN Conflicts October 2006

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

Stapp & Volz Standards Track [Page 2] RFC 4703 Resolution of FQDN Conflicts October 2006

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.

Stapp & Volz Standards Track [Page 3] RFC 4703 Resolution of FQDN Conflicts October 2006

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.

Stapp & Volz Standards Track [Page 4] RFC 4703 Resolution of FQDN Conflicts October 2006

 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.

Stapp & Volz Standards Track [Page 5] RFC 4703 Resolution of FQDN Conflicts October 2006

 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

Stapp & Volz Standards Track [Page 6] RFC 4703 Resolution of FQDN Conflicts October 2006

 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.

Stapp & Volz Standards Track [Page 7] RFC 4703 Resolution of FQDN Conflicts October 2006

 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

Stapp & Volz Standards Track [Page 8] RFC 4703 Resolution of FQDN Conflicts October 2006

 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.

Stapp & Volz Standards Track [Page 9] RFC 4703 Resolution of FQDN Conflicts October 2006

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

Stapp & Volz Standards Track [Page 10] RFC 4703 Resolution of FQDN Conflicts October 2006

 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.

Stapp & Volz Standards Track [Page 11] RFC 4703 Resolution of FQDN Conflicts 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

Stapp & Volz Standards Track [Page 12] RFC 4703 Resolution of FQDN Conflicts October 2006

Full Copyright Statement

 Copyright (C) The Internet Society (2006).
 This document is subject to the rights, licenses and restrictions
 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
 This document and the information contained herein are provided on an
 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
 ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
 INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
 INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

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Acknowledgement

 Funding for the RFC Editor function is provided by the IETF
 Administrative Support Activity (IASA).

Stapp & Volz Standards Track [Page 13]

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