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rfc:rfc4702

Network Working Group M. Stapp Request for Comments: 4702 B. Volz Category: Standards Track Cisco Systems, Inc.

                                                            Y. Rekhter
                                                      Juniper Networks
                                                          October 2006
       The Dynamic Host Configuration Protocol (DHCP) Client
             Fully Qualified Domain Name (FQDN) Option

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

 This document describes a Dynamic Host Configuration Protocol for
 IPv4 (DHCPv4) option that 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.

Stapp, et al. Standards Track [Page 1] RFC 4702 The DHCP Client FQDN Option October 2006

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 ...........................................10
    4.1. When to Perform DNS Updates ...............................11
 5. DNS RR TTLs ....................................................12
 6. DNS Update Conflicts ...........................................12
 7. IANA Considerations ............................................13
 8. Security Considerations ........................................13
 9. Acknowledgements ...............................................14
 10. References ....................................................14
    10.1. Normative References .....................................14
    10.2. Informative References ...................................15

Stapp, et al. Standards Track [Page 2] RFC 4702 The DHCP Client FQDN Option October 2006

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

Stapp, et al. Standards Track [Page 3] RFC 4702 The DHCP Client FQDN Option October 2006

 authority may be restricted to a server to prevent the client from
 listing arbitrary addresses or associating its address with arbitrary
 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 that 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).

Stapp, et al. Standards Track [Page 4] RFC 4702 The DHCP Client FQDN Option October 2006

 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

Stapp, et al. Standards Track [Page 5] RFC 4702 The DHCP Client FQDN Option October 2006

 used by the client.  A server that does not support the deprecated
 ASCII encoding MUST ignore Client FQDN options that use that
 encoding.
 The remaining bits in the Flags field are reserved for future
 assignment.  DHCP clients and servers that 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 that 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.

Stapp, et al. Standards Track [Page 6] RFC 4702 The DHCP Client FQDN Option October 2006

 To send a fully qualified domain name, the Domain Name field is set
 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 of
 this specification, which permitted an ASCII encoding of the Domain
 Name field.  Server implementations SHOULD be aware that clients that
 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 that 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],
 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 that send the Client FQDN option in their messages
 MUST NOT also send the Host Name option.  Clients that receive both
 the Host Name option and the Client FQDN option from a server SHOULD

Stapp, et al. Standards Track [Page 7] RFC 4702 The DHCP Client FQDN Option October 2006

 prefer Client FQDN option data.  Section 4 instructs servers to
 ignore the Host Name option in client messages that 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

Stapp, et al. Standards Track [Page 8] RFC 4702 The DHCP Client FQDN Option October 2006

 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; it MAY also
 perform the A RR updates if the "S" bit is 1.

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 as to 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
 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 that has not
 been able to delete an A RR that 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].

Stapp, et al. Standards Track [Page 9] RFC 4702 The DHCP Client FQDN Option October 2006

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:
 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 to 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 it 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.

Stapp, et al. Standards Track [Page 10] RFC 4702 The DHCP Client FQDN Option October 2006

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
 that 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
 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
 that 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 that 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.

Stapp, et al. Standards Track [Page 11] RFC 4702 The DHCP Client FQDN Option October 2006

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 that 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.
 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 prevents
 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

Stapp, et al. Standards Track [Page 12] RFC 4702 The DHCP Client FQDN Option October 2006

 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 that 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 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 that offers some extra assurances is when
 the DHCP client is connected to a network through an Multimedia Cable
 Network System (MCNS) cable modem, and the cable modem termination

Stapp, et al. Standards Track [Page 13] RFC 4702 The DHCP Client FQDN Option October 2006

 system (CMTS), i.e., 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
 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.

Stapp, et al. Standards Track [Page 14] RFC 4702 The DHCP Client FQDN Option October 2006

 [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 Fully Qualified Domain
       Name (FQDN) Conflicts among Dynamic Host Configuration Protocol
       (DHCP) Clients", RFC 4703, October 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"
       Questions", FYI 4, 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.

Stapp, et al. Standards Track [Page 15] RFC 4702 The DHCP Client FQDN Option October 2006

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

Stapp, et al. Standards Track [Page 16] RFC 4702 The DHCP Client FQDN Option October 2006

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Stapp, et al. Standards Track [Page 17]

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