Network Working Group Yakov Rekhter INTERNET-DRAFT Mark Stapp Cisco Systems
FebruaryJune 1999 Expires AugustDecember 1999 Interaction between DHCP and DNS <draft-ietf-dhc-dhcp-dns-09.txt><draft-ietf-dhc-dhcp-dns-10.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- DraftsInternet-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. Copyright Notice Copyright (C) The Internet Society (1999). All Rights Reserved. Abstract DHCP provides a powerful mechanism for IP host autoconfiguration.configuration. However, the autoconfigurationconfiguration capability provided by DHCP does not include updating DNS, and specifically updating the name to address and address to name mappings maintained byin the DNS. This document specifies how DHCP clients and servers should use the Dynamic DNS Updates mechanism in [RFC2136] to update the DNS name to address and address to name mapping,mappings so that the mappings for DHCP clients wouldwill be consistent with the IP addresses that the clients acquire via DHCP. 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 [RFC2119]. 2. Interaction between DHCP and DNS DNS [RFC1034, RFC1035] maintains (among other things) the information about mapping between hosts' Fully Qualified Domain Names (FQDNs) [RFC1594] 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 a mapping from aan FQDN to an IP address; the PTR RR containscon- tains a mapping from an IP address to a FQDN. The Dynamic DNS Updates specification [RFC2136] describes a mechanism that enables DNS information to be updated over a network. DHCP [RFC1541][RFC2131] provides a mechanism by which a host (a DHCP client) could acquire certain configuration information, and specifically 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. Dynamic DNS Updates [RFC2136] is a mechanism that enables DNS infor- mation to be updated over a network.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 gets 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 should 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, both the A RR (for the client's FQDN) and the PTR RR (for the acquired address) have to be updated. Therefore, we havetwo separate Dynamic DNS Update transac- tions.transactions 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, upon some reflectionreflec- tion one could realize that not all permutations make sense. This document covers the 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 One could observe that 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 RRs.domain names. A 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 arbi- trary addresses or associating its address with arbitrary addresses.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 them. 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 will support some or all of these possibilities. 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 should do in cases where clients do not supply domain names. 3.1. 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 code for this option is 81. Its minimum length is 4. Code LenFQDN Option also contains Flags RCODE1 RCODE2 Domain Name +------+------+------+------+------+------+-- | 81and RCode fields which DHCP servers can use to convey information about DNS updates to clients. 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 sub- sequent REQUEST messages. The code for this option is 81. Its minimum length is 4. Code Len Flags RCODE1 RCODE2 Domain Name +------+------+------+------+------+------+-- | 81 | n | | | | ... +------+------+------+------+------+------+-- 3.1.1. The Flags field allowsField This figure presents the format of the Flags field, which is one byte long: 0 1 2 3 4 5 6 7 +-+-+-+-+-+-+-+-+ | MBZ |E|O|S| +-+-+-+-+-+-+-+-+ When a DHCPclient 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 perform its own FQDN-to-IP mapping update. If a DHCP server intends to take responsibility for the A RR update whether (a)or not the client wants to be responsible for updatingsending the FQDN to IP address mapping (if Flags isoption has set to 0), or (b)the client wants"S" bit, it sets both the server to be responsible for updating"O" bit and the "S" bit, and sends the FQDN to IP address map- ping (if Flags is set to 1).option in its corresponding DHCPOFFER and/or DHCPACK messages. The Flagsdata in the Domain Name field also allows a DHCP servermay appear in one of two formats: ASCII, or DNS-style binary encoding (without compression, of course). A client which sends the FQDN option sets the "E" bit to indicate to a DHCP clientthat the server assumesdata in the responsibilityDomain Name field is DNS-encoded, as described in [RFC1035]. The remaining bits in the Flags field are reserved for updatingfuture assign- ment. DHCP clients and servers which send the FQDN to IP address mapping, even if the client wants to be responsible for this update (if Flags isoption MUST set the MBZ bits to 3).0, and they MUST ignore values in the part of the field labelled "MBZ". 3.1.2. The RCODE Fields The RCODE1 and RCODE2 fields are used by a DHCP server to indicate to a DHCP client the Response Code from the an A RR Dynamic DNS Updates.Update performed on the client's behalf. 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. 3.1.3. The Domain Name Field The Domain Name part of the option carries FQDNthe 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]. 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 that Flags 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] and [RFC1035]. A client using DNS encoding sends a single label as a single byte count, fol- lowed by that number of bytes of data, without a client.terminal reference to the root. 3.2. DHCP Client behavior The following describes the behavior of a DHCP client that implements the Client FQDN option. If a client that owns/maintains isits 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 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 completedcompletes a final check on the parameters passed in the message), the client MUSTSHOULD originate an update for the A RR (associated with the client's FQDN). The update MUST be originated following the procedures described in [RFC2136]. Asection 3.4. If the DHCP server from which the client that owns/maintains its ownis requesting a lease includes the FQDN option in its ACK message, and if the server sets both the "S" and the "O" bits in the option's Flags field, the 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 MUSTSHOULD include the Client FQDN option in theits DHCPREQUEST message originated by the client.message. The rightmost (or "S") bit in the Flags field in the option MUST be setMUST 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 1. In this case,its server, and the configuration of the client MAY supply an FQDN inchanges so that its notion of its domain name changes, it should send the Clientnew data in an FQDN option, oroption when it MAY leave that field empty as a signalcommunicates 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 generatedelete that record and attempt an FQDNupdate for the client in any manner the server chooses.client's current domain name. A client thatwhich delegates the responsibility for updating the FQDN to IP address mapping to a server MAYmight not receive any indicationsindication (either positive or negative) from the server whetherabout the server was able to performstatus of the update. In this caseupdate from the server. 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 addresslease prior to the lease expiration time and the client is responsible for updating its A RR(s), the client SHOULD delete the A RR (following the procedures described in [RFC2136]) associated with the leased address before sending a DHCP RELEASE message.DHCPRELEASE mes- sage. 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 client which has not been able to delete an A RR which it added (because it has lost its IP address) SHOULD add an entry to its logfile and/or notify its administrator. 3.3. 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 SHOULD originate an update for the PTR RR (associatedassociated with the address leased to the client).client if the server is configured to perform DNS updates. The update MUST be originated following the procedures described in Section 3.4. The server MAY complete 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 RCODE1 field of the Client FQDN option in the DHCPACK message and the RCODE2 field MUST be set to 0.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, and the RCODE2 field MUST be set to 0.255. The choice between the two alternatives is entirely up todetermined by the configura- tion of the DHCP server. Servers SHOULD support both configuration options. In addition, if the Client FQDN option carried in the DHCPREQUEST message has the "S" bit in its Flags field set to 1,set, then the server MUSTMAY originate an update for the A RR (associated with the FQDN carried inFQDN 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 by the client, or it MAY be configured to modify the option).supplied name, or substitute a different name. The update MUST be originated following the procedures described in Section 3.4. 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. AlternativelyAlterna- tively 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 DHCKACKDHCPACK 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 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 its Flags field set to 0clear (indicating that the client wants to update the A RR), the server MAY (at the determination ofMAY, be configured byt the local adminis- trator)administrator to update the A RR.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 3.4. 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, and the Flags field in the Client FQND option MUST be set to 3.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 DHCKACKDHCPACK message MUST be set to 255, and the Flags field in the Client FQDN option MUST be set to 3.255. Whether the DNS update occurs before or after the DHCPACK is sent is entirely up to the DHCP server.server's configuration. When a server receives a DHCPREQUEST message from a client, and the message contains the Client FQDN option, the server MUST ignore the valuevalues carried in the RCODE1 and RCODE2 fields of the option. When a DHCP server sends the Client FQDN option to a client in the DHCPACK message, the server MUST copy the Domain Name fieldsfield from the Client FQDN option that the client sent to the server in the DHCPRE- QUEST message. If, however, the client sent only a single label, or if the DHCP server has been configured to assign the client a name different from the one the client has sent, the server SHOULD send its notion of the complete FQDN for the client. 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 the DHCPREQUSTDHCPREQUEST message received by a DHCP server from a DHCP client doesn't carry the Client FQDN option (e.g., the client doesn't implement the Client FQDN option), and the DHCP client acquires its FQDN from a DHCP server (as part of a normal DHCP transaction), then the server MAY be configured to update both A and PTR RRs. TheAny updates MUST be originated following the procedures described in Sec- tion 3.4.Sec- tion 3.4. In this case, the server MAY NOT wish to return the FQDN option to a client which may not be able to understand it. If it can, the DHCP server MAY (optionally) return the host part of the domain name that it will use for the client in the host-name DHCP option (defined in [RFC2132]). Note that it may not be possible to con- sistently encode some domain name data in the format specified by the host-name option. If a server detects that a lease on an address that the server leases to a client expires,has expired or has been released by the client, the server SHOULD delete the PTR RR which it associated with the address.address via DNS Dynamic Update. In addition, if the server added an A RR (of the client) was ini- tially updated byon behalf of the server,client, the server SHOULD also delete the A RR. The deletion MUST follow the procedures described in [RFC2136].Section 3.4. If a server terminates a lease on an address prior to the leaselease's expiration time, for instance by sending a DHCPNAK to a client, the server SHOULD delete the PTR RR which it associated with the address.address via DNS Dynamic Update. In addition, if the server (that leased the address) initially updatedtook responsibil- ity for the client's A RR (of the client),, the server SHOULD also delete thethat A RR. The deletion MUST follow the procedures described in [RFC2136].Section 3.4. 3.4. Procedures for performing DNS updates There are two principal issues that need to be addressed concerning A RR DNS updates: o Name Collisions If the entity updating the A RR (either the DHCP client or DHCP server) attempts to perform a DNS update withto a DNSdomain name that has an A RR which is already in use,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 implementations make this behavior the default. In some scenarios these name collisions are unlikely, in some scenarios they are very likely: 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 will add.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 collisionscolli- sions in this scenario are possible, since the credentials necessary for the client to update DNS are not name specific.necessarily name-specific. Thus, for the client to be attempting to update a unique name requires the existence of some administrativeadminis- trative 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'server's name configuration procedures. See Section 75 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 75 for security issues with this type of deployment. Scenarios 3 and 4 are much more attractive given some form of DHCP Authentication, but the difficulties remain. 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 pro- cedures 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 updat- ing entity for a name gets the name). o 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 population of DHCP clients, the DHCP servers should be able to provide reasonable DNS name update behavior for DHCP clients. The solution to both of these problems is for the updating entities (both(either DHCP clients or DHCP servers) to be able to cooperate when updating DNS A RRs. Specifically, a KEY RR, described in [RFC2065][RFC2535] 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 iden- tity (based on a "client specifier" created from the client's client-id or MAC address: see Appendix A). 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 config- ured administrative policy), and DHCP clients which currently have a host name may move from one DHCP server to another without losing their DNS name. See Appendix A for the details of the use of the KEY RR for this pur- pose. The specific algorithms utilizing the KEY RR to signal client owner- ship 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. 3.4.1. 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 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 would then 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 (see Appendix A). 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 user of this 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 confi- guration 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 'take-over' the name on behalf of the current client. DISCUSSION: The updating entity may be configured to allow the existing owner to keep the name, and to perform 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 such a similar name has been generated, the updating entity should res- tart the process of adding an A RR as specified in this section. 3.4.2. 2Adding 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 whose data is the client's client-identity as described in Appendix A. 3.4.3. 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 thatfor which it added.is responsible. When a lease expires or a DHCP client issues a DHCPRELEASE request, the DHCP server SHOULD delete the PTR RR that matches the DHCP bind- ing, 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 recordand KEY records that waswere 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 Appendix A. 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,query's prerequisites fail, the updater MUST conclude that it cannotNOT 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 dif- ferent 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 policyadministrative pol- icy at the site 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- identityclient-identity of the host pointed to by the DNS name. 4. 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 this document. 5. Security Considerations Whether the client wants tomay be responsible for updating the FQDN to IP address mapping, or whether the client wants to delegatethis responsibility to alies with the DHCP server is a local to the clientsite-local matter. The choice between the two alternativesalter- natives may be based on a particular security model that is used with the Dynamic DNS Update protocol (e.g., only a client may have sufficientsuffi- cient credentials to perform updates to the FQDN to IP address mappingmap- ping 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), regarlessregardless of the wishes of a DHCP client, is also a local to the serversite-local matter. The choice between the two alternatives may be based on a particular security model. The client SHOULD use some form of data origin authentication pro- cedures (e.g., DNSSEC(e.g. [TSIG], [DNSSEC]) when performing DNS updates. While the DHCP client SHOULDMAY be the one to update the DNS A record, in certain specialized cases 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 not satisfied -- one needs to be mindful of the possibility of MAC address spoofing in a DHCP packet. It is not difficult for a DHCP server to know unambi- guously the DNS name to use for a client, but only in certain rela- tively unusual circumstancescir- cumstances will the DHCP server know for sure the identity of the client. OneIf DHCP authentication [DHCPAUTH] becomes widely deployed this may become more customary. An example of sucha circumstancesituation which offers some extra assurances is one where the DHCP client is connectedcon- nected to a network through an MCNS cable modem, and the CMTS (head-end)(head- end) of the cable modem ensures that MAC address spoofing simply does not occur. Another example where the DHCP server would know the identity of the client would be in a case where it was interacting with a remote access server which encoded a client identification into the DHCP client-id option. In this case, the remote access server as well as the DHCP server would be operating within a trusted environment, and the DHCP server could trust that the user authentication and authori- zation procedure of the remote access server was sufficient, and would therefore trust the client identification encoded within the DHCP client-id. In either of these cases, a DHCP server would be able to correctly enter the DNS A record on behalf of a client, since it would know the name associated with a client (through some administrative procedure outside the scope of this protocol), and it would also know the client's identity in a secure manner. 6. Appendix A - Use 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 0x42 - that is, the 1 bit and the 6 bit are set. The protocol field is set to 0.[TBD]. The algorithm field is set to 254.[TBD]. The first byte in the key field contains the length of the client- identity, and is followed by that number of bytes.bytes of client-identity data. If a DHCP client sent the client-id option in its request,request mes- sage, 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. References [RFC1034] P. Mockapetris, "Domain names - concepts and facilities", RFC1034, 11/01/1987 [RFC1035] P. Mockapetris, "Domain names - implementation and specifi- cation",specif- ication", RFC1035, 11/01/1987 [RFC2131] R. Droms, "Dynamic Host Configuration Protocol", RFC2131, March 19971997. [RFC2132] S. Alexander, R. Droms, "DHCP Options and BOOTP Vendor Extensions", RFC2132, March 1997. [RFC1594] A. Marine, J. Reynolds, G. Malkin, "FYI on Questions and Answer Answers to Commonly asked ``New Internet User'' Questions", RFC1594, 03/11/1994 [DNSSEC][RFC2136] P. Vixie, S. Thomson, Y. Rekhter, J. Bound, "Dynamic Updates in the Domain Name System (DNS UPDATE)", RFC2136, April 1997 [RFC2119] Bradner, S. "Key words for use in RFCs to Indicate Require- mentRequirement Levels", RFC 2119. [RFC2065]RFC2119. [DNSSEC] D. Eastlake, C. Kaufman,"Domain Name System Security Extensions", RFC 2065, January 1997.RFC2535, March 1999. [TSIG] P. Vixie, O. Gudmundsson, D. Eastlake, B. Wellington, "Secret Key Transaction Signatures for DNS", draft-ietf- dnsind-tsig-*, Work in Progress. [DHCPAUTH] R. Droms, W. Arbaugh, "Authentication for DHCP Messages", draft-ietf-dhc-authentication-*, Work in Progress. 8. Acknowledgements Many thanks to Mark Beyer, Jim Bound, Ralph Droms, Robert Elz, Peter Ford, Edie Gunter, R. Barr Hibbs, Kim Kinnear, Stuart Kwan, Ted Lemon, Michael Lewis, Michael Patton, and Glenn Stump for their review and comments. 9. Author information Yakov Rekhter Cisco Systems, Inc. 170 Tasman Dr. San Jose, CA 95134 Phone: (914) 235-2128 email: firstname.lastname@example.org Mark Stapp Cisco SystemsSystems, Inc. 250 Apollo Drive Chelmsford, MA 01824 Phone: (978) 244-8498 email: email@example.com 10. Full Copyright Statement Copyright (C) The Internet Society (1999). All Rights Reserved. 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