Network Working Group Yakov Rekhter Internet Draft Cisco Systems Expiration Date: August 1996 February 1996 Interaction between DHCP and DNS draft-ietf-dhc-dhcp-dns-00.txt 1. Status of this Memo This document is an Internet-Draft. 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.'' To learn the current status of any Internet-Draft, please check the ``1id-abstracts.txt'' listing contained in the Internet-Drafts Shadow Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe), munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or ftp.isi.edu (US West Coast). 2. Abstract DHCP provides a powerful mechanism for IP host autoconfiguration. However, the autoconfiguration provided by DHCP does not include updating DNS, and specifically updating the name to address and address to name mappings maintained by DNS. This document specifies how DHCP clients and servers should use the Dynamic DNS Updates mechanism to update the DNS name to address and address to name mapping, so that the mappings for DHCP clients would be consistent with the IP addresses that the clients acquire via DHCP. Yakov Rekhter [Page 1] Internet Draft draft-ietf-dhc-dhcp-dns-00.txt February 1996 3. 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 mapping from a FQDN to an IP address; the PTR RR contains mapping from an IP address to a FQDN. DHCP [RFC1541] 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 [DynDNS] is a mechanism that enables to update DNS information over a network. Use of the Dynamic DNS Updates protocol enables 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. 4. Models of operations 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 have two separate Dynamic DNS Update transactions. 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 introspection one could realize that not all permutations make sense. This document restricts the possible design permutations to the following cases: (1) DHCP client updates the A RR, DHCP server updates the PTR RR Yakov Rekhter [Page 2] Internet Draft draft-ietf-dhc-dhcp-dns-00.txt February 1996 (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. 4.1. Client FQDN Option To update the IP address to FQDN mapping a DHCP server needs to know 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 option, called "Client FQDN". The code for this option is TBD. Its minimum length is 2. Code Len Flags Domain Name +-----+-----+-----+-----+-----+-----+-- | TBD | n | 0/1 | d1 | d2 | d3 | ... +-----+-----+-----+-----+-----+-----+-- The Flags field allows a DHCP client to indicate to a DHCP server whether the client wants the server to be responsible for updating the FQDN to IP address mapping (if Flags is set to 1), or whether the client wants to take this responsibility (if Flags is set to 0). The Domain Name part of the option carries FQDN of a client. 4.2. DHCP Client behavior If a client 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 shall include the Client FQDN option in the DHCPREQUEST message originated by the client. The Flags field in the option shall be set to 0. Once the client's DHCP configuration is completed (the client receives a DHCPACK message, and successfully completed a final check on the parameters passed in the message), the client shall originate an update for the A RR (associated with the client's FQDN). The update shall be originated following the procedures described in [DynDNS]. Yakov Rekhter [Page 3] Internet Draft draft-ietf-dhc-dhcp-dns-00.txt February 1996 If a client does not want to be responsible for updating the FQDN to IP address mapping for the FQDN and address(es) used by the client, then the client shall include the Client FQDN option in the DHCPREQUEST message originated by the client. The Flags field in the option shall be set to 1. A client that delegates the responsibility for updating the FQDN to IP address mapping to a server does not receive any indications (either positive or negative) from the server whether the server was able to perform the update. The client may use DNS query to check whether the mapping is updated. Whether the client wants to be responsible for updating the FQDN to IP address mapping, or whether the client wants to delegate this responsibility to a server is a local to the client matter. 4.3. 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 shall originate an update for the PTR RR (associated with the address leased to the client). The server shall originate the update only after the server sends the DHCPACK message to the client. The update shall be originated following the procedures described in [DynDNS]. In addition, if the Client FQDN option carried in the DHCPREQUEST message has its Flags field set to 1, then the server shall originate an update for the A RR (associated with the FQDN carried in the option). The server shall originate the update only after the server sends the DHCPACK message to the client. The update shall be originated following the procedures described in [DynDNS]. If a server originates updates for both the A and PTR RRs, then the order in which the updates are generated is not significant. [Discussion: should it be possible to configure a server to perform updates for the FQDN to IP address mapping, even when a client indicates to the server that the client wants to update this mapping ?] [Discussion: how should the duration of the lease be reflected in the DNS updates ? At the minimum we can set TTL on the A and PTR RRs to the value of the lease time. What else ?] Yakov Rekhter [Page 4] Internet Draft draft-ietf-dhc-dhcp-dns-00.txt February 1996 [Discussion: when a server detects that a lease on an address that the server leases to a client expires, should the server delete the PTR RR associated with the address ?] [Discussion: if a server terminates a lease prior to the lease expiration time, should the server update the associated PTR RR ? Should the A RR be updated, and if yes, then by whom ? ] 5. Updating other RRs The procedures described in this document cover updates only to the A and PTR RRs. Updating other types of RRs is outside the scope of this document. 6. Applicability to IPv6 The procedures described above are directly applicable to an IPv6 client. The only difference is that instead of updating its A RR(s) the client has to update its AAAA RR(s). 7. Security Considerations Security issues are not discussed in this document. 8. References [RFC1034] P. Mockapetris, "Domain names - concepts and facilities", RFC1034, 11/01/1987 [RFC1035] P. Mockapetris, "Domain names - implementation and specification", RFC1035, 11/01/1987 [RFC1541] R. Droms, "Dynamic Host Configuration Protocol", RFC1541, 10/27/1993 [RFC1594] A. Marine, J. Reynolds, G. Malkin, "FYI on Questions and Answer Answers to Commonly asked ``New Internet User'' Questions", RFC1594, 03/11/1994 [DynDNS] P. Vixie, S. Thomson, Y. Rekhter, J. Bound, "Dynamic Updates in the Domain Name System (DNS UPDATE)", draft-ietf-dnsind-dynDNS- 06.txt, Feb 1996 Yakov Rekhter [Page 5] Internet Draft draft-ietf-dhc-dhcp-dns-00.txt February 1996 9. Acknowledgements Many thanks to Ralph Droms for his review and comments. 10. Author Information Yakov Rekhter cisco Systems, Inc. 170 Tasman Dr. San Jose, CA 95134 Phone: (914) 528-0090 email: yakov@cisco.com Yakov Rekhter [Page 6]