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DHC Working Group                                               M. Stapp
Internet-Draft                                                Y. Rekhter
Expires: April 2000                                  Cisco Systems, Inc.
                                                           October, 1999

                    Interaction between DHCP and DNS
                    <draft-ietf-dhc-dhcp-dns-11.txt>

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that
   other groups may also distribute working documents as
   Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six
   months and may be updated, replaced, or obsoleted by other documents
   at any time. It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

     The list of current Internet-Drafts can be accessed at
     http://www.ietf.org/ietf/1id-abstracts.txt

     The list of Internet-Draft Shadow Directories can be accessed at
     http://www.ietf.org/shadow.html.

   This Internet-Draft will expire April, 2000.

Copyright Notice

   Copyright (C) The Internet Society (1999). All Rights Reserved.

Abstract

   DHCP provides a powerful mechanism for IP host configuration.
   However, the configuration capability provided by DHCP does not
   include updating DNS, and specifically updating the name to address
   and address to name mappings maintained in the DNS.

   This document specifies how DHCP clients and servers should use the
   Dynamic DNS Updates mechanism in RFC2136[5] to update the DNS name
   to address and address to name mappings so that the mappings for
   DHCP clients will be consistent with the IP addresses that the
   clients acquire via DHCP.

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Table of Contents

   1.    Terminology  . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.    Introduction . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.    Models of Operation  . . . . . . . . . . . . . . . . . . . .  3
   4.    Client FQDN Option . . . . . . . . . . . . . . . . . . . . .  4
   4.1   The Flags Field  . . . . . . . . . . . . . . . . . . . . . .  5
   4.2   The RCODE Fields . . . . . . . . . . . . . . . . . . . . . .  6
   4.3   The Domain Name Field  . . . . . . . . . . . . . . . . . . .  6
   5.    DHCP Client behavior . . . . . . . . . . . . . . . . . . . .  6
   6.    DHCP Server behavior . . . . . . . . . . . . . . . . . . . .  8
   7.    Procedures for performing DNS updates  . . . . . . . . . . . 10
   7.1   Name Collisions  . . . . . . . . . . . . . . . . . . . . . . 10
   7.2   Multiple DHCP servers  . . . . . . . . . . . . . . . . . . . 11
   7.3   Use of the KEY RR  . . . . . . . . . . . . . . . . . . . . . 11
   7.3.1 Format of the KEY RR . . . . . . . . . . . . . . . . . . . . 12
   7.4   DNS RR TTLs  . . . . . . . . . . . . . . . . . . . . . . . . 12
   7.5   Adding A RRs to DNS  . . . . . . . . . . . . . . . . . . . . 13
   7.6   Adding PTR RR Entries to DNS . . . . . . . . . . . . . . . . 14
   7.7   Removing Entries from DNS  . . . . . . . . . . . . . . . . . 14
   7.8   Updating other RRs . . . . . . . . . . . . . . . . . . . . . 14
   8.    Security Considerations  . . . . . . . . . . . . . . . . . . 15
   9.    Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 16
         References . . . . . . . . . . . . . . . . . . . . . . . . . 16
         Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 17
         Full Copyright Statement . . . . . . . . . . . . . . . . . . 18

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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 RFC 2119[6].

2. Introduction

   DNS RFC1034[1], RFC1035[2] maintains (among other things) the
   information about mapping between hosts' Fully Qualified Domain
   Names (FQDNs) RFC1594[4] and IP addresses assigned to the hosts. The
   information is maintained in two types of Resource Records (RRs): A
   and PTR. The A RR contains mapping from a FQDN to an IP address; the
   PTR RR contains mapping from an IP address to a FQDN.  The Dynamic
   DNS Updates specification RFC2136[5] describes a mechanism that
   enables DNS information to be updated over a network.

   DHCP RFC2131[3] provides a mechanism by which a host (a DHCP client)
   can acquire certain configuration information, along with its IP
   address(es). However, DHCP does not provide any mechanisms to update
   the DNS RRs that contain the information about mapping between the
   host's FQDN and its IP address(es) (A and PTR RRs). Thus the
   information maintained by DNS for a DHCP client may be incorrect - a
   host (the client) could acquire its address by using DHCP, but the A
   RR for the host's FQDN wouldn't reflect the address that the host
   acquired, and the PTR RR for the acquired address wouldn't reflect
   the host's FQDN.

   The Dynamic DNS Update protocol can be used to maintain consistency
   between the information stored in the A and PTR RRs and the actual
   address assignment done via DHCP. When a host with a particular FQDN
   acquires its IP address via DHCP, the A RR associated with the
   host's FQDN would be updated (by using the Dynamic DNS Updates
   protocol) to reflect the new address. Likewise, when an IP address
   is assigned to a host with a particular FQDN, the PTR RR associated
   with this address would be updated (using the Dynamic DNS Updates
   protocol) to reflect the new FQDN.

   Although this document refers to the A and PTR DNS record types and
   to DHCP assignment of IPv4 addresses, the same procedures and
   requirements apply for updates to the analogous RR types that are
   used when clients are assigned IPv6 addresses via DHCPv6.

3. 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 Dynamic DNS Update transactions occur. Acquiring an address

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   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. This document covers these possible design permutations:

   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 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.

   In any case, whether a site permits all, some, or no DHCP servers
   and clients to perform DNS updates into the zones which 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 which allow the client
   to modify its own domain name. 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 which initiate dynamic DNS updates.

   This document describes a new 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.

4. 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 FQDN Option
   also contains Flags and RCode fields which DHCP servers can use to
   convey information about DNS updates to clients.

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   Clients MAY send the FQDN option, setting appropriate Flags values,
   in both their DISCOVER and REQUEST messages. If a client sends the
   FQDN option in its DISCOVER message, it MUST send the option in
   subsequent REQUEST messages.

   The code for this option is 81. Its minimum length is 4.

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

4.1 The Flags Field

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

   When a DHCP client sends the FQDN option in its DHCPDISCOVER and/or
   DHCPREQUEST messages, it sets the right-most bit (labelled "S") to
   indicate that it will not perform any Dynamic DNS updates, and that
   it expects the DHCP server to perform any FQDN-to-IP (the A RR) DNS
   update on its behalf. If this bit is clear, the client indicates
   that it intends to maintain its own FQDN-to-IP mapping update.

   If a DHCP server intends to take responsibility for the A RR update
   whether or not the client sending the FQDN option has set the "S"
   bit, it sets both the "O" bit and the "S" bit, and sends the FQDN
   option in its DHCPOFFER and/or DHCPACK messages.

   The data in the Domain Name field may appear in one of two formats:
   ASCII, or DNS-style binary encoding (without compression, of
   course), as described in RFC1035[2]. A client which sends the FQDN
   option MUST set the "E" bit to indicate that the data in the Domain
   Name field is DNS-encoded. If a server receives an FQDN option from
   a client, and intends to include an FQDN option in its reply, it
   MUST use the same encoding that the client used. The DNS encoding is
   recommended. The use of ASCII-encoded domain-names is fragile, and
   the use of ASCII encoding in this option should be considered
   deprecated.

   The remaining bits in the Flags field are reserved for future
   assignment. DHCP clients and servers which send the FQDN option MUST
   set the MBZ bits to 0, and they MUST ignore values in the part of

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   the field labelled "MBZ".

4.2 The RCODE Fields

   The RCODE1 and RCODE2 fields are used by a DHCP server to indicate
   to a DHCP client the Response Code from any A or PTR RR Dynamic DNS
   Updates it has performed. The server also uses these fields to
   indicate whether it has attempted such an update before sending the
   DHCPACK message. Each of these fields is one byte long.

4.3 The Domain Name Field

   The Domain Name part of the option carries the FQDN of a DHCP
   client. 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 single label,
   indicating that it knows part of the name but does not necessarily
   know the zone in which the name is to be embedded. The data in the
   Domain Name field may appear in one of two formats: ASCII (with no
   terminating NULL), or DNS encoding as specified in RFC1035[2]. If
   the DHCP client wishes to use DNS encoding, it MUST set the
   third-from-rightmost bit in the Flags field (the "E" bit); if it
   uses ASCII encoding, it must clear the "E" bit.

   A DHCP client that can only send a single label using ASCII encoding
   includes a series of ASCII characters in the Domain Name field,
   excluding the "." (dot) character. The client SHOULD follow the
   character-set recommendations of RFC1034[1] and RFC1035[2]. A client
   using DNS encoding which wants to suggest part of its FQDN MAY send
   a non-terminal sequence of labels in the Domain Name part of the
   option.

5. DHCP Client behavior

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

   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, then 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 DISCOVER messages as well as its REQUEST messages. The
   rightmost ("S") bit in the Flags field in the option MUST be set to
   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). The
   update MUST be originated following the procedures described in

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   RFC2136[5], and Section 7. If the DHCP server from which the client
   is requesting a lease includes the FQDN option in its ACK message,
   and if the server sets both the "S" and the "O" (the two rightmost)
   bits in the option's flags field, the DHCP client MUST NOT initiate
   an update for the name in the Domain Name field.

   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. The rightmost (or "S") bit in the Flags field in the option
   MUST be set to 1. A client which delegates this responsibility MUST
   NOT attempt to perform a Dynamic DNS update for the name in the
   Domain Name field of the FQDN option. The client MAY supply an FQDN
   in the Client FQDN option, or it MAY supply a single label (the
   most-specific label), or it MAY leave that field empty as a signal
   to the server to generate an FQDN for the client in any manner the
   server chooses.

   Since there is a possibility that the DHCP server may be configured
   to complete or replace a domain name that the client was configured
   to send, the client might find it useful to send the FQDN option in
   its DISCOVER messages. If the DHCP server returns different Domain
   Name data in its OFFER message, the client could use that data in
   performing its own eventual A RR update, or in forming the FQDN
   option that it sends in its REQUEST message. There is no requirement
   that the client send identical FQDN option data in its DISCOVER and
   REQUEST messages. In particular, if a client has sent the 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 an FQDN option when it communicates with the server again.
   This may allow 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 might not receive any indication
   (either positive or negative) from the server whether the server was
   able to perform the update. In this case the client MAY use a DNS
   query to check whether the mapping is updated.

   A client MUST set the RCODE1 and RCODE2 fields in the Client FQDN
   option to 0 when sending the option.

   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 (following the procedures described in
   Section 7) associated with the leased address before sending a DHCP

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   RELEASE 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.

6. DHCP Server behavior

   When a server receives a DHCPREQUEST message from a client, if the
   message contains the Client FQDN option, and the server replies to
   the message with a DHCPACK message, the server may be configured to
   originate an update for the PTR RR (associated with the address
   leased to the client). Any such update MUST be originated following
   the procedures described in Section 7. The server MAY complete the
   update before the server sends the DHCPACK message to the client. In
   this case the RCODE from the update MUST be carried to the client in
   the RCODE1 field of the Client FQDN option in the DHCPACK message.
   Alternatively, the server MAY send the DHCPACK message to the client
   without waiting for the update to be completed. In this case the
   RCODE1 field of the Client FQDN option in the DHCPACK message MUST
   be set to 255.  The choice between the two alternatives is entirely
   determined by the configuration of the DHCP server. Servers SHOULD
   support both configuration options.

   When a server receives a DHCPREQUEST message containing the Client
   FQDN option, the server MUST ignore the values carried in the RCODE1
   and RCODE2 fields of the option.

   In addition, if the Client FQDN option carried in the DHCPREQUEST
   message has the "S" bit in its Flags field set, then the server MAY
   originate an update for the A RR (associated with the FQDN carried
   in the option) if it is configured to do so by the site's
   administrator, and if it has the necessary credentials. The server
   MAY be configured to use the name supplied in the client's FQDN
   option, or it MAY be configured to modify the supplied name, or
   substitute a different name.

   Any such update MUST be originated following the procedures
   described in Section 7. The server MAY originate the update before
   the server sends the DHCPACK message to the client. In this case the
   RCODE from the update [RFC2136] MUST be carried to the client in the
   RCODE2 field of the Client FQDN option in the DHCPACK message.
   Alternatively the server MAY send the DHCPACK message to the client
   without waiting for the update to be completed. In this case the
   RCODE2 field of the Client FQDN option in the DHCPACK message MUST
   be set to 255. The choice between the two alternatives is entirely
   up to the DHCP server. In either case, if the server intends to
   perform the DNS update and the client's REQUEST message included the

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   FQDN option, the server SHOULD include the FQDN option in its ACK
   message, and MUST set the "S" bit in the option's Flags field.

   Even if the Client FQDN option carried in the DHCPREQUEST message
   has the "S" bit in its Flags field clear (indicating that the client
   wants to update the A RR), the server MAY be configured by the local
   administrator to update the A RR on the client's behalf. A server
   which is configured to override the client's preference SHOULD
   include an FQDN option in its ACK message, and MUST set both the "O"
   and "S" bits in the FQDN option's Flags field. The update MUST be
   originated following the procedures described in Section 7. The
   server MAY originate the update before the server sends the DHCPACK
   message to the client. In this case the RCODE from the update
   [RFC2136] MUST be carried to the client in the RCODE2 field of the
   Client FQDN option in the DHCPACK message. Alternatively, the server
   MAY send the DHCPACK message to the client without waiting for the
   update to be completed. In this case the RCODE2 field of the Client
   FQDN option in the DHCPACK message MUST be set to 255. Whether the
   DNS update occurs before or after the DHCPACK is sent is entirely up
   to the DHCP server's configuration.

   When a DHCP server sends the Client FQDN option to a client in the
   DHCPACK message, the DHCP 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 in the DHCPREQUEST message. The
   DHCP server MAY be configured to complete or modify the domain name
   which a client sent, or it MAY be configured to substitute a
   different name. If the server initiates a DDNS update which is not
   complete until after the server has replied to the DHCP client, the
   server's The server MUST use the same encoding format (ASCII or
   DNS-encoding) that the client used in the FQDN option in its
   DHCPREQUEST, and MUST set the "E" bit in the option's Flags field
   accordingly.

   If a client's DHCPREQUEST message doesn't carry the Client FQDN
   option (e.g., the client doesn't implement the Client FQDN option),
   the server MAY be configured to update either or both of the A and
   PTR RRs. The updates MUST be originated following the procedures
   described in Section 7.

   If 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 dynamic update. In addition, if the server added
   an A RR on the client's behalf, the server SHOULD also delete the A
   RR. The deletion MUST follow the procedures described in Section 7.

   If 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

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   server SHOULD delete any PTR RR which it associated with the address
   via DNS Dynamic Update. In addition, if the server took
   responsibility for an A RR, the server SHOULD also delete that A RR.
   The deletion MUST follow the procedures described in Section 7.

7. Procedures for performing DNS updates

   There are two principal issues that need to be addressed concerning
   A RR DNS updates:

7.1 Name Collisions

   If the entity updating the A RR (either the DHCP client or DHCP
   server) attempts to perform a DNS update to a domain name that has
   an A RR which is already in use by a different DHCP client, what
   should be done?  Similarly, should a DHCP client or server update a
   domain name which has an A RR that has been configured by an
   administrator? In either of these cases, the domain name in question
   would either have an additional A RR, or would have its original A
   RR replaced by the new record. Either of these effects may be
   considered undesirable in some sites. This specification describes
   behavior designed to prevent these undesirable effects, and requires
   that DHCP implementations make this behavior the default.

   1.  Client updates A RR, uses DNSSEC. Name collisions in this
       scenario are unlikely (though not impossible), since for the
       client to use DNSSEC, it has already received credentials
       specific to the name it desires to use.  This implies that the
       name has already been allocated (through some implementation- or
       organization-specific procedure, and presumably uniquely) to
       that client.

   2.  Client updates A RR, uses some form of TSIG. Name collisions in
       this scenario are possible, since the credentials necessary for
       the client to update DNS are not necessarily name-specific.
       Thus, for the client to be attempting to update a unique name
       requires the existence of some administrative procedure to
       ensure client configuration with unique names.

   3.  Server updates the A RR, uses a name for the client which is
       known to the server. Name collisions in this scenario are likely
       unless prevented by the server's name configuration procedures.
       See Section 8 for security issues with this form of deployment.

   4.  Server updates the A RR, uses a name supplied by the client.
       Name collisions in this scenario are highly likely, even with
       administrative procedures designed to prevent them.  (This
       scenario is a popular one in real-world deployments in many
       types of organizations.)  See Section 8 for security issues with
       this type of deployment.

   Scenarios 3 and 4 are much more attractive given some form of DHCP
   Authentication, but difficulties remain.

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   Scenarios 2, 3, and 4 rely on administrative procedures to ensure
   name uniqueness for DNS updates, and these procedures may break
   down. Experience has shown that, in fact, these procedures will
   break down at least occasionally.  The question is what to do when
   these procedures break down or, for example in scenario #4, may not
   even exist.

   In all cases of name collisions, the desire is to offer two modes of
   operation to the administrator of the combined DHCP-DNS capability:
   first-update-wins (i.e., the first updating entity gets the name) or
   most-recent-update-wins (i.e., the last updating entity for a name
   gets the name).

7.2 Multiple DHCP servers

   If multiple DHCP servers are able to update the same DNS zones, or
   if DHCP servers are performing A RR updates on behalf of DHCP
   clients, and more than one DHCP server may be able to serve
   addresses to the same DHCP clients, the DHCP servers should be able
   to provide reasonable and consistent DNS name update behavior for
   DHCP clients.

7.3 Use of the KEY RR

   A solution to both of these problems is for the updating entities
   (both DHCP clients and DHCP servers) to be able to cooperate when
   updating DNS A RRs.

   Specifically, a KEY RR, described in RFC2535[7] is used to associate
   client ownership information with a DNS name and the A RR associated
   with that name.  When either a client or server adds an A RR for a
   client, it also adds a KEY RR which specifies a unique client
   identity (based on a "client specifier" created from the client's
   client-id or MAC address).  In this model, only one A RR is
   associated with a given DNS name at a time.

   By associating this ownership information with each A RR,
   cooperating DNS updating entities may determine whether their client
   is the first or last updater of the name (and implement the
   appropriately configured administrative policy), and DHCP clients
   which currently have a host name may move from one DHCP server to
   another without losing their DNS name.

   The specific algorithms utilizing the KEY RR to signal client
   ownership are explained below.  The algorithms only work in the case
   where the updating entities all cooperate -- this approach is
   advisory only and does not substitute for DNS security, nor is it
   replaced by DNS security.

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7.3.1 Format of the KEY RR

   The KEY RR used to hold the DHCP client's identity is formatted as
   follows:

   The name of the KEY RR is the name of the A or PTR RR which refers
   to the client.

   The flags field is set to 0x4200 - that is, the 1 bit and the 6 bit
   are set.

   The protocol field is set to [TBD].

   The algorithm field is set to [TBD].

        0                            15                               31
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |             Version           |       Identity-length         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       /                        Client-identity...                     /
       /                                                               /
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Version field indicates the version of the data used in this RR.
   The Version field is a 2-byte integer in network byte-order. Its
   value MUST be 1.

   The remainder of the Key field contains the length of the
   client-identity, followed by that number of bytes of client-identity
   data. The data length is represented as a 2-byte integer in network
   byte order. If a DHCP client sent the client-id option in its
   request message, the client-identity MUST be identical to the data
   in the client-id option. If a client did not send the client-id
   option, the client-identity is constructed from the htype byte, the
   hlen byte, and hlen bytes of the client's chaddr from its request
   message.

7.4 DNS RR TTLs

   RRs associated with DHCP clients may be more volatile than
   statically configured RRs. DHCP clients and servers which perform
   dynamic updates should attempt to specify resource record TTLs which
   reflect this volatility, in order to minimize the possibility that
   there will be stale records in resolvers' caches. A reasonable basis
   for RR TTLs is the lease duration itself: TTLs of 1/2 or 1/3 the
   expected lease duration might be reasonable defaults. Because
   configured DHCP lease times vary widely from site to site, it may

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   also be desirable to establish a fixed TTL ceiling. DHCP clients and
   servers MAY allow administrators to configure the TTLs they will
   supply, possibly as a fraction of the actual lease time, or as a
   fixed value.

7.5 Adding A RRs to DNS

   When a DHCP client or server intends to update an A RR, it first
   prepares a DNS UPDATE query which includes as a prerequisite the
   assertion that the name does not exist.  The update section of the
   query attempts to add the new name and its IP address mapping (an A
   RR), and the KEY RR with its unique client-identity.

   If this update operation succeeds, the updater can conclude that it
   has added a new name whose only RRs are the A and KEY RR records.
   The A RR update is now complete (and a client updater is finished,
   while a server might proceed to perform a PTR RR update).

   If the first update operation fails with YXDOMAIN, the updater can
   conclude that the intended name is in use.  The updater then
   attempts to confirm that the DNS name is not being used by some
   other host. The updater prepares a second UPDATE query in which the
   prerequisite is that the desired name has attached to it a KEY RR
   whose contents match the client identity.  The update section of
   this query deletes the existing A records on the name, and adds the
   A record that matches the DHCP binding and the KEY RR with the
   client identity.

   If this query succeeds, the updater can conclude that the current
   client was the last client associated with the domain name, and that
   the name now contains the updated A RR. The A 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 second query fails with NXRRSET, the updater must conclude
   that the client's desired name is in use by another host.  At this
   juncture, the updater can decide (based on some administrative
   configuration outside of the scope of this document) whether to let
   the existing owner of the name keep that name, and to (possibly)
   perform some name disambiguation operation on behalf of the current
   client, or to replace the RRs on the name with RRs that represent
   the current client. If the configured policy allows replacement of
   existing records, the updater submits a query that deletes the
   existing A RR and the existing KEY RR, adding A and KEY RRs that
   represent the IP address and client-identity of the new client.

   DISCUSSION:

      The updating entity may be configured to allow the existing DNS
      records on the domain name to remain unchanged, and to perform

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      disambiguation on the name of the current client in order to
      attempt to generate a similar but unique name for the current
      client. In this case, once another candidate name has been
      generated, the updater should restart the process of adding an A
      RR as specified in this section.

7.6 Adding PTR RR Entries to DNS

   The DHCP server submits a DNS query which deletes all of the PTR RRs
   associated with the lease IP address, and adds a PTR RR whose data
   is the client's (possibly disambiguated) host name. The server also
   adds a KEY RR specified in Section 7.3.

7.7 Removing Entries from DNS

   The first rule in removing DNS entries is be sure that an entity
   removing a DNS entry is only removing an entry that it added.

   When a lease expires or a DHCP client issues a DHCPRELEASE request,
   the DHCP server SHOULD delete the PTR RR that matches the DHCP
   binding, if one was successfully added. The server's update query
   SHOULD assert that the name in the PTR record matches the name of
   the client whose lease has expired or been released.

   The entity chosen to handle the A record for this client (either the
   client or the server) SHOULD delete the A record that was added when
   the lease was made to the client.

   In order to perform this delete, the updater prepares an UPDATE
   query which contains two prerequisites.  The first prerequisite
   asserts that the KEY RR exists whose data is the client identity
   described in Section 7.3. The second prerequisite asserts that the
   data in the A RR contains the IP address of the lease that has
   expired or been released.

   If the query fails, the updater MUST NOT delete the DNS name.  It
   may be that the host whose lease on the server has expired has moved
   to another network and obtained a lease from a different server,
   which has caused the client's A RR to be replaced. It may also be
   that some other client has been configured with a name that matches
   the name of the DHCP client, and the policy was that the last client
   to specify the name would get the name.  In this case, the KEY RR
   will no longer match the updater's notion of the client-identity of
   the host pointed to by the DNS name.

7.8 Updating other RRs

   The procedures described in this document only cover updates to the
   A and PTR RRs. Updating other types of RRs is outside the scope of

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   this document.

8. Security Considerations

   Unauthenticated updates to the DNS can lead to tremendous confusion,
   through malicious attack or through inadvertent misconfiguration.
   Administrators should be wary of unsecured DNS updates to zones
   which are exposed to the global Internet.

   Whether the client may be responsible for updating the FQDN to IP
   address mapping, or whether the this responsibility lies with the
   DHCP server is a site-local matter. The choice between the two
   alternatives may be based on a particular 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 a DHCP client, is a site-local matter.
   The choice between the two alternatives may be based on a particular
   security model. Both DHCP clients and servers SHOULD use some form
   of update request origin authentication procedure (e.g., TSIG[8],
   Simple Secure DNS Update[10]) when performing DNS updates.

   While the DHCP client MAY be the one to update the DNS A record, in
   certain configurations a DHCP server MAY do so instead.  In this
   case, the DHCP server MUST be sure of both the name to use for the
   client, as well as the identity of the client.

   In the general case, both of these conditions are difficult to
   satisfy, given the absence of security 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[9] 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) of the cable modem 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
   trust that the user authentication and authorization procedure of

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   the remote access server was sufficient, and would therefore trust
   the client identification encoded within the DHCP client-id.

9. Acknowledgements

   Many thanks to Mark Beyer, Jim Bound, Ralph Droms, Robert Elz, Peter
   Ford, Edie Gunter, Andreas Gustafsson, R. Barr Hibbs, Kim Kinnear,
   Stuart Kwan, Ted Lemon, Ed Lewis, Michael Lewis, Josh Littlefield,
   Michael Patton, and Glenn Stump for their review and comments.

References

   [1]  Mockapetris, P., "Domain names - Concepts and Facilities", RFC
        1034, Nov 1987.

   [2]  Mockapetris, P., "Domain names - Implementation and
        Specification", RFC 1035, Nov 1987.

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

   [4]  Marine, A., Reynolds, J. and G. Malkin, "FYI on Questions and
        Answers to Commonly asked ``New Internet User'' Questions", RFC
        1594, March 1994.

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

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

   [7]  Eastlake, D., "Domain Name System Security Extensions", RFC
        2535, March 1999.

   [8]  Vixie, P., Gudmundsson, O., Eastlake, D. and B. Wellington,
        "Secret Key Transaction Signatures for DNS (TSIG)
        (draft-ietf-dnsind-tsig-*)", July 1999.

   [9]  Droms, R. and W. Arbaugh, "Authentication for DHCP Messages
        (draft-ietf-dhc-authentication-*)", June 1999.

   [10]  Wellington, B., "Simple Secure DNS Dynamic Updates
         (draft-ietf-dnsind-simple-secure-update-*)", June 1999.

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Authors' Addresses

   Mark Stapp
   Cisco Systems, Inc.
   250 Apollo Dr.
   Chelmsford, MA  01824
   US

   Phone: 978.244.8498
   EMail: mjs@cisco.com

   Yakov Rekhter
   Cisco Systems, Inc.
   170 Tasman Dr.
   San Jose, CA  95134
   US

   Phone: 914.235.2128
   EMail: yakov@cisco.com

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Full Copyright Statement

   Copyright (C) The Internet Society (1999). All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implmentation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph
   are included on all such copies and derivative works. However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
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   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
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Acknowledgement

   Funding for the RFC editor function is currently provided by the
   Internet Society.

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