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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12 13 RFC 4702

DHC                                                             M. Stapp
Internet-Draft                                                   B. Volz
Expires: September 23, 2006                          Cisco Systems, Inc.
                                                              Y. Rekhter
                                                        Juniper Networks
                                                          March 22, 2006


                      The DHCP Client FQDN Option
                  <draft-ietf-dhc-fqdn-option-13.txt>

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
   aware will be disclosed, in accordance with Section 6 of BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on September 23, 2006.

Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   This document describes a Dynamic Host Configuration Protocol for
   IPv4, DHCPv4, option which can be used to exchange information about
   a DHCPv4 client's fully-qualified domain name and about
   responsibility for updating the DNS RR related to the client's
   address assignment.




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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  3
     1.2.  Models of Operation  . . . . . . . . . . . . . . . . . . .  3
   2.  The Client FQDN Option . . . . . . . . . . . . . . . . . . . .  4
     2.1.  The Flags Field  . . . . . . . . . . . . . . . . . . . . .  5
     2.2.  The RCODE Fields . . . . . . . . . . . . . . . . . . . . .  6
     2.3.  The Domain Name Field  . . . . . . . . . . . . . . . . . .  6
       2.3.1.  Deprecated ASCII Encoding  . . . . . . . . . . . . . .  7
   3.  DHCP Client Behavior . . . . . . . . . . . . . . . . . . . . .  7
     3.1.  Interaction With Other Options . . . . . . . . . . . . . .  7
     3.2.  Client Desires to Update A RRs . . . . . . . . . . . . . .  8
     3.3.  Client Desires Server to Do DNS Updates  . . . . . . . . .  8
     3.4.  Client Desires No Server DNS Updates . . . . . . . . . . .  8
     3.5.  Domain Name and DNS Update Issues  . . . . . . . . . . . .  9
   4.  DHCP Server Behavior . . . . . . . . . . . . . . . . . . . . .  9
     4.1.  When to Perform DNS Updates  . . . . . . . . . . . . . . . 10
   5.  DNS RR TTLs  . . . . . . . . . . . . . . . . . . . . . . . . . 11
   6.  DNS Update Conflicts . . . . . . . . . . . . . . . . . . . . . 12
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 12
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 12
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 14
     10.2. Informative References . . . . . . . . . . . . . . . . . . 14
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 16
   Intellectual Property and Copyright Statements . . . . . . . . . . 17























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1.  Introduction

   DNS ([2], [3]) maintains (among other things) the information about
   the mapping between hosts' Fully Qualified Domain Names (FQDNs) [11]
   and IP addresses assigned to the hosts.  The information is
   maintained in two types of Resource Records (RRs): A and PTR.  The
   DNS update specification ([4]) describes a mechanism that enables DNS
   information to be updated over a network.

   The Dynamic Host Configuration Protocol for IPv4 (DHCPv4 or just DHCP
   in this document) [5] provides a mechanism by which a host (a DHCP
   client) can acquire certain configuration information, along with its
   address.  This document specifies a DHCP option, the Client FQDN
   option, which can be used by DHCP clients and servers to exchange
   information about the client's fully-qualified domain name for an
   address and who has the responsibility for updating the DNS with the
   associated A and PTR RRs.

1.1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [1].

1.2.  Models of Operation

   When a DHCP client acquires a new address, a site's administrator may
   desire that one or both of the A RR for the client's FQDN and the PTR
   RR for the acquired address be updated.  Therefore, two separate DNS
   update transactions may occur.  Acquiring an address via DHCP
   involves two entities: a DHCP client and a DHCP server.  In principle
   each of these entities could perform none, one, or both of the
   transactions.  However, in practice not all permutations make sense.
   The DHCP Client FQDN option is primarily intended to operate in the
   following two cases:

   1.  DHCP client updates the A RR, DHCP server updates the PTR RR
   2.  DHCP server updates both the A and the PTR RRs

   The only difference between these two cases is whether the FQDN to IP
   address mapping is updated by a DHCP client or by a DHCP server.  The
   IP address to FQDN mapping is updated by a DHCP server in both cases.

   The reason these two are important, while others are unlikely, has to
   do with authority over the respective DNS domain names.  A DHCP
   client may be given authority over mapping its own A RRs, or that
   authority may be restricted to a server to prevent the client from
   listing arbitrary addresses or associating its address with arbitrary



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   domain names.  In all cases, the only reasonable place for the
   authority over the PTR RRs associated with the address is in the DHCP
   server that allocates the address.

   Note: A third case is supported - the client requests that the server
   perform no updates.  However, this case is presumed to be rare
   because of the authority issues.

   It is considered local policy to permit DHCP clients and servers to
   perform DNS updates to zones.  This document does not require any
   specific administrative policy, and does not propose one.
   Furthermore, this specification applies only to DHCP client and
   server processes: it does not apply to other processes which initiate
   DNS updates.

   This document describes a DHCP option which a client can use to
   convey all or part of its domain name to a DHCP server.  Site-
   specific policy determines whether DHCP servers use the names that
   clients offer or not, and what DHCP servers may do in cases where
   clients do not supply domain names.


2.  The Client FQDN Option

   To update the IP address to FQDN mapping a DHCP server needs to know
   the FQDN of the client to which the server leases the address.  To
   allow the client to convey its FQDN to the server this document
   defines a new DHCP option, called "Client FQDN".  The Client FQDN
   option also contains Flags, which DHCP servers can use to convey
   information about DNS updates to clients, and two deprecated RCODEs.

   Clients MAY send the Client FQDN option, setting appropriate Flags
   values, in both their DHCPDISCOVER and DHCPREQUEST messages.  If a
   client sends the Client FQDN option in its DHCPDISCOVER message, it
   MUST send the option in subsequent DHCPREQUEST messages though the
   contents of the option MAY change.

   Only one Client FQDN option MAY appear in a message, though it may be
   instantiated in a message as multiple options [9].  DHCP clients and
   servers supporting this option, MUST implement DHCP option
   concatenation [9].  In the terminology of [9], the Client FQDN option
   is a concatenation-requiring option.

   The code for this option is 81.  Len contains the number of octets
   that follow the Len field, and the minimum value is 3 (octets).






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   The format of the Client FQDN option is:

        Code   Len    Flags  RCODE1 RCODE2   Domain Name
       +------+------+------+------+------+------+--
       |  81  |   n  |      |      |      |       ...
       +------+------+------+------+------+------+--


   The above figure follows the conventions of [12].

2.1.  The Flags Field

   The format of the 1-octet Flags field is:

        0 1 2 3 4 5 6 7
       +-+-+-+-+-+-+-+-+
       |  MBZ  |N|E|O|S|
       +-+-+-+-+-+-+-+-+


   The "S" bit indicates whether the server SHOULD or SHOULD NOT perform
   the A RR (FQDN to address) DNS updates.  A client sets the bit to 0
   to indicate the server SHOULD NOT perform the updates and 1 to
   indicate the server SHOULD perform the updates.  The state of the bit
   in the reply from the server indicates the action to be taken by the
   server; if 1, the server has taken responsibility for A RR updates
   for the FQDN.

   The "O" bit indicates whether the server has overridden the client's
   preference for the "S" bit.  A client MUST set this bit to 0.  A
   server MUST set this bit to 1 if the "S" bit in its reply to the
   client does not match the "S" bit received from the client.

   The "N" bit indicates whether the server SHOULD NOT perform any DNS
   updates.  A client sets this bit to 0 to request that the server
   SHOULD perform updates (the PTR RR and possibly the A RR based on the
   "S" bit) or to 1 to request that the server SHOULD NOT perform any
   DNS updates.  A server sets the "N" bit to indicate whether the
   server SHALL (0) or SHALL NOT (1) perform DNS updates.  If the "N"
   bit is 1, the "S" bit MUST be 0.

   The "E" bit indicates the encoding of the Domain Name field. 1
   indicates canonical wire format, without compression, as described in
   [3], section 3.1.  This encoding SHOULD be used by clients and MUST
   be supported by servers. 0 indicates a now deprecated ASCII encoding
   (see Section 2.3.1).  A server MUST use the same encoding as that
   used by the client.  A server that does not support the deprecated
   ASCII encoding MUST ignore Client FQDN options that use that



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   encoding.

   The remaining bits in the Flags field are reserved for future
   assignment.  DHCP clients and servers which send the Client FQDN
   option MUST clear the MBZ bits, and they MUST ignore these bits.

2.2.  The RCODE Fields

   The two 1-octet RCODE1 and RCODE2 fields are deprecated.  A client
   SHOULD set these to 0 when sending the option and SHOULD ignore them
   on receipt.  A server SHOULD set these to 255 when sending the option
   and MUST ignore them on receipt.

   As this option with these fields is already in wide use, the fields
   are retained.  These fields were originally defined for use by a DHCP
   server to indicate to a DHCP client the Response Code from any A
   (RCODE1) or PTR (RCODE2) RR DNS updates it has performed or a value
   of 255 was used to indicate that an update had been initiated but had
   not yet completed.  Each of these fields is one octet long.  These
   fields were defined before EDNS0 [13], which describes a mechanism
   for extending the length of a DNS RCODE to 12 bits, which is another
   reason to deprecate them.

   If the client needs to confirm the DNS update has been done, it MAY
   use a DNS query to check whether the mapping is up to date.  However,
   depending on the load on the DHCP and DNS servers and the DNS
   propagation delays, the client can only infer success.  If the
   information is not found to be up to date in DNS, the authoritative
   servers might not have completed the updates or zone transfers, or
   caching resolvers may yet have updated their caches.

2.3.  The Domain Name Field

   The Domain Name part of the option carries all or part of the FQDN of
   a DHCP client.  The data in the Domain Name field SHOULD appear in
   canonical wire format as specified in [3], section 3.1.  If the DHCP
   client uses the canonical wire format, it MUST set the "E" bit in the
   Flags field to 1.  In order to determine whether the FQDN has changed
   between message exchanges, the client and server MUST NOT alter the
   Domain Name field contents unless the FQDN has actually changed.

   A client MAY be configured with a fully-qualified domain name or with
   a partial name that is not fully-qualified.  If a client knows only
   part of its name, it MAY send a name that is not fully-qualified,
   indicating that it knows part of the name but does not necessarily
   know the zone in which the name is to be embedded.

   To send a fully-qualified domain name, the Domain Name field is set



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   to the DNS encoded domain name including the terminating zero-length
   label.  To send a partial name, the Domain Name field is set to the
   DNS encoded domain name without the terminating zero-length label.

   A client MAY also leave the Domain Name field empty if it desires the
   server to provide a name.

2.3.1.  Deprecated ASCII Encoding

   A substantial population of clients implemented an earlier draft
   version of this specification, which permitted an ASCII encoding of
   the Domain Name field.  Server implementations SHOULD be aware that
   clients which send the Client FQDN option with the "E" bit set to 0
   are using an ASCII encoding of the Domain Name field.  Servers MAY be
   prepared to return an ASCII encoded version of the Domain Name field
   to such clients.  Servers that are not prepared to return an ASCII
   encoded version MUST ignore the Client FQDN option if the "E" bit is
   0.  The use of ASCII encoding in this option SHOULD be considered
   deprecated.

   A DHCP client which used ASCII encoding was permitted to suggest a
   single label if it was not configured with a fully-qualified name.
   Such clients send a single label as a series of ASCII characters in
   the Domain Name field, excluding the "." (dot) character.

   Clients and servers SHOULD follow the character set rules of [6]RFC
   952, fourth section ("Assumptions"), first 5 sentences, as modified
   by [7] section 2.1.  However, implementers SHOULD also be aware that
   some client software may send data intended to be in other character
   sets.  This specification does not require support for other
   character sets.


3.  DHCP Client Behavior

   The following describes the behavior of a DHCP client that implements
   the Client FQDN option.

3.1.  Interaction With Other Options

   Other DHCP options MAY carry data that is related to the Domain Name
   field of the Client FQDN option.  The Host Name option [12], for
   example, contains an ASCII string representation of the client's host
   name.  In general, a client does not need to send redundant data, and
   therefore clients which send the Client FQDN option in their messages
   MUST NOT also send the Host Name option.  Clients which receive both
   the Host Name option and the Client FQDN option from a server SHOULD
   prefer Client FQDN option data.  Section 4 instructs servers to



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   ignore the Host Name option in client messages which include the
   Client FQDN option.

3.2.  Client Desires to Update A RRs

   If a client that owns/maintains its own FQDN wants to be responsible
   for updating the FQDN to IP address mapping for the FQDN and
   address(es) used by the client, the client MUST include the Client
   FQDN option in the DHCPREQUEST message originated by the client.  A
   DHCP client MAY choose to include the Client FQDN option in its
   DHCPDISCOVER messages as well as its DHCPREQUEST messages.  The "S",
   "O", and "N" bits in the Flags field in the option MUST be 0.

   Once the client's DHCP configuration is completed (the client
   receives a DHCPACK message and successfully completes a final check
   on the parameters passed in the message), the client MAY originate an
   update for the A RR (associated with the client's FQDN) unless the
   server has set the "S" bit to 1.  If the "S" is 1, the DHCP client
   SHOULD NOT initiate an update for the name in the server's returned
   Client FQDN option Domain Name field.  However, a DHCP client that is
   explicitly configured with a FQDN MAY ignore the state of the "S" bit
   if the server's returned name matches the client's configured name.

3.3.  Client Desires Server to Do DNS Updates

   A client can choose to delegate the responsibility for updating the
   FQDN to IP address mapping for the FQDN and address(es) used by the
   client to the server.  In order to inform the server of this choice,
   the client SHOULD include the Client FQDN option in its DHCPREQUEST
   message and MAY include the Client FQDN option in its DHCPDISCOVER.
   The "S" bit in the Flags field in the option MUST be 1 and the "O"
   and "N" bits MUST be 0.

3.4.  Client Desires No Server DNS Updates

   A client can choose to request that the server perform no DNS updates
   on its behalf.  In order to inform the server of this choice, the
   client SHOULD include the Client FQDN option in its DHCPREQUEST
   message and MAY include the Client FQDN option in its DHCPDISCOVER.
   The "N" bit in the Flags field in the option MUST be 1 and the "S"
   and "O" bits MUST be 0.

   Once the client's DHCP configuration is completed (the client
   receives a DHCPACK message and successfully completes a final check
   on the parameters passed in the message), the client MAY originate
   its DNS updates provided the server's "N" bit is 1.  If the server's
   "N" bit is 0, the server MAY perform the PTR RR updates; and, MAY
   also perform the A RR updates if the "S" bit is 1.



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3.5.  Domain Name and DNS Update Issues

   As there is a possibility that the DHCP server is configured to
   complete or replace a domain name that the client sends, the client
   MAY find it useful to send the Client FQDN option in its DHCPDISCOVER
   messages.  If the DHCP server returns different Domain Name data in
   its DHCPOFFER message, the client could use that data in performing
   its own eventual A RR update, or in forming the Client FQDN option
   that it sends in its DHCPREQUEST message.  There is no requirement
   that the client send identical Client FQDN option data in its
   DHCPDISCOVER and DHCPREQUEST messages.  In particular, if a client
   has sent the Client FQDN option to its server, and the configuration
   of the client changes so that its notion of its domain name changes,
   it MAY send the new name data in a Client FQDN option when it
   communicates with the server again.  This MAY cause the DHCP server
   to update the name associated with the PTR record, and, if the server
   updated the A record representing the client, to delete that record
   and attempt an update for the client's current domain name.

   A client that delegates the responsibility for updating the FQDN to
   IP address mapping to a server will not receive any indication
   (either positive or negative) from the server whether the server was
   able to perform the update.  The client MAY use a DNS query to check
   whether the mapping is up to date (see Section 2.2).

   If a client releases its lease prior to the lease expiration time and
   the client is responsible for updating its A RR, the client SHOULD
   delete the A RR associated with the leased address before sending a
   DHCPRELEASE message.  Similarly, if a client was responsible for
   updating its A RR, but is unable to renew its lease, the client
   SHOULD attempt to delete the A RR before its lease expires.  A DHCP
   client which has not been able to delete an A RR which it added
   (because it has lost the use of its DHCP IP address) SHOULD attempt
   to notify its administrator, perhaps by emitting a log message.

   A client that desires to perform DNS updates to A RRs SHOULD NOT do
   so if the client's address is a private address [8].


4.  DHCP Server Behavior

   The following describes the behavior of a DHCP server that implements
   the Client FQDN option when the client's message includes the Client
   FQDN option.

   The server examines its configuration and the Flag bits in the
   client's Client FQDN option to determine how to respond:




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   o  If the client's "E" bit is 0 and the server does not support ASCII
      encoding (Section 2.3.1), the server SHOULD ignore the Client FQDN
      option.
   o  The server sets to 0 the "S", "O", and "N" bits in its copy of the
      option it will return to the client.  The server copies the
      client's "E" bit.
   o  If the client's "N" bit is 1 and the server's configuration allows
      it to honor the client's request for no server initiated DNS
      updates, the server sets the "N" bit to 1.
   o  Otherwise, if the client's "S" bit is 1 and the server's
      configuration allows it to honor the client's request for the
      server to initiate A RR DNS updates, the server sets the "S" to 1.
      If the server's "S" bit does not match the client's "S" bit, the
      server sets the "O" bit to 1.

   The server MAY be configured to use the name supplied in the client's
   Client FQDN option, or it MAY be configured to modify the supplied
   name, or substitute a different name.  The server SHOULD send its
   notion of the complete FQDN for the client in the Domain Name field.
   The server MAY simply copy the Domain Name field from the Client FQDN
   option that the client sent to the server.  The server MUST use the
   same encoding format (ASCII or DNS binary encoding) that the client
   used in the Client FQDN option in its DHCPDISCOVER or DHCPREQUEST,
   and MUST set the "E" bit in the option's Flags field accordingly.

   If a client sends both the Client FQDN and Host Name option, the
   server SHOULD ignore the Host Name option.

   The server SHOULD set the RCODE1 and RCODE2 fields to 255 before
   sending the Client FQDN message to the client in a DHCPOFFER or
   DHCPACK.

4.1.  When to Perform DNS Updates

   The server SHOULD NOT perform any DNS updates if the "N" bit is 1 in
   the Flags field of the Client FQDN option in the DHCPACK messages (to
   be) sent to the client.  However, the server SHOULD delete any RRs
   which it previously added via DNS updates for the client.

   The server MAY perform the PTR RR DNS update (unless the "N" bit is
   1).

   The server MAY perform the A RR DNS update if the "S" bit is 1 in the
   Flags field of the Client FQDN option in the DHCPACK message (to be)
   sent to the client.

   The server MAY perform these updates even if the client's DHCPREQUEST
   did not carry the Client FQDN option.  The server MUST NOT initiate



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   DNS updates when responding to DHCPDISCOVER messages from a client.

   The server MAY perform its DNS updates (PTR RR or PTR and A RR)
   before or after sending the DHCPACK message to the client.

   If the server's A RR DNS update does not complete until after the
   server has replied to the DHCP client, the server's interaction with
   the DNS server MAY cause the DHCP server to change the domain name
   that it associates with the client.  This can occur, for example, if
   the server detects and resolves a domain-name conflict [10].  In such
   cases, the domain name that the server returns to the DHCP client
   would change between two DHCP exchanges.

   If the server previously performed DNS updates for the client and the
   client's information has not changed, the server MAY skip performing
   additional DNS updates.

   When a server detects that a lease on an address that the server
   leases to a client has expired, the server SHOULD delete any PTR RR
   which it added via DNS update.  In addition, if the server added an A
   RR on the client's behalf, the server SHOULD also delete the A RR.

   When a server terminates a lease on an address prior to the lease's
   expiration time, for instance by sending a DHCPNAK to a client, the
   server SHOULD delete any PTR RR which it associated with the address
   via DNS update.  In addition, if the server took responsibility for
   an A RR, the server SHOULD also delete that A RR.


5.  DNS RR TTLs

   RRs associated with DHCP clients may be more volatile than statically
   configured RRs.  DHCP clients and servers that perform dynamic
   updates should attempt to specify resource record TTLs which reflect
   this volatility, in order to minimize the possibility that answers to
   DNS queries will return records that refer to DHCP IP address
   assignments that have expired or been released.

   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 assigned IP
   address that has expired or been released.  In deployment, this
   rarely, if ever, represents a significant problem.  Most DHCP-managed
   clients are infrequently looked-up by name in the DNS, and the
   deployment of IXFR ([16]) and NOTIFY ([17]) can reduce the latency
   between updates and their visibility at secondary servers.




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   We suggest these basic guidelines for implementers.  In general, the
   TTLs for RRs added as a result of DHCP IP address assignment activity
   SHOULD be less than the initial lease time.  The RR TTL on a DNS
   record added SHOULD NOT exceed 1/3 of the lease time, but SHOULD NOT
   be less than 10 minutes.  We recognize that individual administrators
   will have varying requirements: DHCP servers and clients SHOULD allow
   administrators to configure TTLs and upper and lower bounds on the
   TTL values, either as an absolute time interval or as a percentage of
   the lease time.

   While clients and servers MAY update the TTL of the records as the
   lease is about to expire, there is no requirement that they do so as
   this puts additional load on the DNS system with likely little
   benefit.


6.  DNS Update Conflicts

   This document does not resolve how a DHCP client or server prevent
   name conflicts.  This document addresses only how a DHCP client and
   server negotiate who will perform the DNS updates and the fully
   qualified domain name requested or used.

   Implementers of this work will need to consider how name conflicts
   will be prevented.  If a DNS updater needs a security token in order
   to successfully perform DNS updates on a specific name, name
   conflicts can only occur if multiple updaters are given a security
   token for that name.  Or, if the fully qualified domains are based on
   the specific address bound to a client, conflicts will not occur.
   Or, a name conflict resolution technique as described in "Resolving
   Name Conflicts" [10]) SHOULD be used.


7.  IANA Considerations

   IANA has already assigned DHCP option 81 to the Client FQDN option.
   As this document describes the option's use, IANA is requested to
   reference this document for option 81.


8.  Security Considerations

   Unauthenticated updates to the DNS can lead to tremendous confusion,
   through malicious attack or through inadvertent misconfiguration.
   Administrators need to be wary of permitting unsecured DNS updates to
   zones which are exposed to the global Internet.  Both DHCP clients
   and servers should use some form of update request origin
   authentication procedure (e.g., Secure DNS Dynamic Update [14]) when



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   performing DNS updates.

   Whether a DHCP client is 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 is likely based on the security model that is used with
   the 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 is likely
   based on the security model that is being used with 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 its 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 unusual circumstances will the DHCP server
   know for certain the identity of the client.  If DHCP Authentication
   [15] becomes widely deployed this may become more customary.

   One example of a situation which 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) 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 procedure of the remote access
   server was sufficient, and would therefore trust the client
   identification encoded within the DHCP client-id.

   It is critical to implement proper conflict resolution, and the
   security considerations of conflict resolution apply [10].


9.  Acknowledgements

   Many thanks to Mark Beyer, Jim Bound, Ralph Droms, Robert Elz, Peter



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   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,
   Jyrki Soini, and Glenn Stump for their review and comments.


10.  References

10.1.  Normative References

   [1]   Bradner, S., "Key words for use in RFCs to Indicate Requirement
         Levels", BCP 14, RFC 2119, March 1997.

   [2]   Mockapetris, P., "Domain names - concepts and facilities",
         STD 13, RFC 1034, November 1987.

   [3]   Mockapetris, P., "Domain names - implementation and
         specification", STD 13, RFC 1035, November 1987.

   [4]   Vixie, P., Thomson, S., Rekhter, Y., and J. Bound, "Dynamic
         Updates in the Domain Name System (DNS UPDATE)", RFC 2136,
         April 1997.

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

   [6]   Harrenstien, K., Stahl, M., and E. Feinler, "DoD Internet host
         table specification", RFC 952, October 1985.

   [7]   Braden, R., "Requirements for Internet Hosts - Application and
         Support", STD 3, RFC 1123, October 1989.

   [8]   Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and E.
         Lear, "Address Allocation for Private Internets", BCP 5,
         RFC 1918, February 1996.

   [9]   Lemon, T. and S. Cheshire, "Encoding Long Options in the
         Dynamic Host Configuration Protocol (DHCPv4)", RFC 3396,
         November 2002.

   [10]  Stapp, M. and B. Volz, "Resolution of DNS Name Conflicts Among
         DHCP Clients (draft-ietf-dhc-ddns-resolution-*.txt)",
         February 2006.

10.2.  Informative References

   [11]  Marine, A., Reynolds, J., and G. Malkin, "FYI on Questions and
         Answers - Answers to Commonly asked "New Internet User"



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         Questions", RFC 1594, March 1994.

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

   [13]  Vixie, P., "Extension Mechanisms for DNS (EDNS0)", RFC 2671,
         August 1999.

   [14]  Wellington, B., "Secure Domain Name System (DNS) Dynamic
         Update", RFC 3007, November 2000.

   [15]  Droms, R. and W. Arbaugh, "Authentication for DHCP Messages",
         RFC 3118, June 2001.

   [16]  Ohta, M., "Incremental Zone Transfer in DNS", RFC 1995,
         August 1996.

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
































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Authors' Addresses

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

   Phone: 978.936.1535
   Email: mjs@cisco.com


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

   Phone: 978.936.0382
   Email: volz@cisco.com


   Yakov Rekhter
   Juniper Networks
   1194 North Mathilda Avenue
   Sunnyvale, CA  94089
   USA

   Phone: 408.745.2000
   Email: yakov@juniper.net





















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