DNSSEC Working Group Brian Wellington (TISLabs) INTERNET-DRAFT Olafur Gudmundsson (TISLabs) February 1999 Updates: RFC 2136 Deferred Dynamic Domain Name System (DNS) Delete Operations 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 Abstract This document proposes a mechanism for notifying a dynamic DNS server that a delete operation should be performed at a certain point in the future. This works within the framework of the current DNS dynamic update protocol, and provides needed functionality for clients with leased dynamic addresses. Expires August 1999 [Page 1] INTERNET-DRAFT Deferred Dynamic DNS Deletes February 1999 1 - Introduction Dynamic update operations for the Domain Name System [RFC1034, RFC1035] are defined in [RFC2136], but there is no automated method of specifying that records should have a fixed lifetime, or lease. 1.1 - Overview of DNS Dynamic Update DNS dynamic update defines a new DNS opcode and a new interpretation of the DNS message if that opcode is used. An update can specify insertions or deletions of data, along with prerequisites necessary for the updates to occur. All tests and changes for a DNS update request are restricted to a single zone, and are performed at the primary server for the zone. The primary server for a dynamic zone must increment the zone SOA serial number when an update occurs or before the next retrieval of the SOA. 1.2 - Overview of DHCP leases DHCP [RFC2131] provides a means for a host to obtain a network address from a DHCP server. The server may ``lease'' this address to the host, meaning that it is valid only for the period of time specified in the lease. The host may may extend its lease with subsequent requests, or may issue a message to release the address back to the server when it is no longer needed. 2 - Background When a host receives dynamic addresses with associated dynamic DNS records, the records can be updated by either the host or the DHCP server. If possible, update by the server is recommended, since the server maintains lease information for each address. In some cases, though, the server cannot update some or all of the DNS records. This happens when the DNS and DHCP server are under different administration, for example. A host can easily update its own DNS records when receiving information from the DHCP server. It can also delete its records when shutting down. If the host unexpectedly goes down, though, it cannot delete the records. When the DHCP lease on the address expires and is not renewed, the DHCP server may reassign the address. The DNS records now point to an assigned address, but not the correct address. Until the host updates its records again, DNS will contain bad information. Since the DHCP and DNS servers are often not co-located with the clients, the possibility of a host unexpectedly going down and not communicating with the servers is non-trivial. Expires August 1999 [Page 2] INTERNET-DRAFT Deferred Dynamic DNS Deletes February 1999 If the host could set a lease on the DNS records similar to that on its address, the DNS records would lose validity at the same time as the address. This would prevent bad information from remaining in DNS. DNS has no such provision for leases, though, since this would require storing a lease time along with each record (or each record in a dynamic zone). An alternative method is suggested. A ``delete'' update is sent along with the ``add'' update, but the delete is marked in such a way that it will not be executed immediately. Instead, it will be stored for the specified amount of time before being applied. If the host wishes to extend or shorten the lifetime of the DNS record(s), it can replace the ``deferred delete'' record, which will reset the lease time of the record(s). The ``deferred delete'' record would, of course, also be removed if a normal delete update was received. This functionality is useful for DHCP leases and maintainance of IPv6 routing prefixes. DNSSEC compliant server can also use deferred dynamic delete information to compute signatures of records affected by the delete operation prior to the delete operation, using spare cycles. 3 - Protocol changes When doing a delete update operation as defined in [RFC2136] (deleting an RR, an RRset, or all RRset from a name), the TTL field MUST be specified as 0. An [RFC2136] compliant server will reject an update record with a non-zero TTL. This document overloads the TTL field. If TTL is non-zero, the value represents the number of seconds (a 32 bit unsigned integer) before which the delete will be applied to the zone. Thus, the delete operation will be deferred for that number of seconds, where the number of seconds indicates the lease time. A 32 bit integer provides for a lease time of over 136 years, which should be long enough for most uses. 3.1 - Storage and execution Deferred delete records are stored, persistently, by the name server. The name server SHOULD attempt to evaluate the deletes in a timely manner. 3.2 - Processing of deferred deletes When a deferred delete is received, the server must check to see if it matches an existing deferred delete records, where matching indicates the same name, type, class, and rdata. If a match is found, the new deferred delete MUST replace the old one. If the deferred delete does not refer to any record in the server, it should fail as a normal delete would. Expires August 1999 [Page 3] INTERNET-DRAFT Deferred Dynamic DNS Deletes February 1999 3.3 - Processing of normal deletes When a normal delete is received and accepted, the server MUST purge any deferred delete that no longer refers to any records. 3.4 - Processing of cancellations The value 0xFFFFFFFF (the largest unsigned 32 bit integer) in the TTL field has a special meaning. If a delete containing this lease time is received, the server will unconditionally remove any matching deferred deletes. 3.5 - Processing of adds When data is added through a dynamic update which matches a deferred delete, there is no additional processing done. 3.6 - Detecting server support of Deferred Delete Client can determine if server supports deferred delete by the return code of a deferred delete request by sending a ADD + Delete request to the server, where the delete has low TTL. RFC2136 compliant server MUST return either rcode FORMERR or NOTIMPL. Server compliant with this document will return NOERROR. 4 - TTL Modification 4.1 - Initial TTL Limits The TTL of all added or updated RRs in the Update Section SHOULD be maximized silently to one half of the lease time. This will cause downstream caching name servers to purge RRsets containing this RR at least once before expiry. 4.2 - TTL Half Life Each time an RR's expiry reaches half of its previous value, that RR's TTL will be reduced to half of its previous value, until the TTL reaches a value less than or equal to sixty (60), i.e., one minute of real time, at which time the TTL will not be automatically reduced further by the primary master. It should be noted that RRs held in a server's memory need not be swept for half life processing, as long as the TTL changes appear when the RR next becomes externally visible, and as long as the ``zone has changed'' processing (see below) is done at the time of the half life expiration. Expires August 1999 [Page 4] INTERNET-DRAFT Deferred Dynamic DNS Deletes February 1999 Whenever the TTL is automatically reduced by this process, the zone will be considered ``changed'' for the purpose of automatic SOA SERIAL increment (as in [RFC2136]) and real time zone slave notification [RFC1996]. 5 - Usage Normally, a deferred delete update will initially be sent along with an add, although this is not required. Further updates to the deferred delete will be sent independently. If the deferred delete is associated with a leased address, the lease time of the update SHOULD be approximately equal to the lease time of the address. 6 - Security considerations This addition to the dynamic DNS protocol does not affect the security of the protocol. If security is desired, TSIG [TSIG] and/or DNSSEC [secext2] authentication should be used, as specified in [simple-update] or [RFC2137, update2]. The authors strongly recommend using security along with this protocol. If a DNSSEC signed-zone is modified with deferred deletes, the server must resign any affected records when the delete is executed. No special processing is required when the delete is received. 7 - IANA Considerations None. 8 - References [RFC1034] P. Mockapetris, ``Domain Names - Concepts and Facilities,'' RFC 1034, ISI, November 1987. [RFC1035] P. Mockapetris, ``Domain Names - Implementation and Specification,'' RFC 1035, ISI, November 1987. [RFC1996] P. Vixie ``A Mechanism for Prompt Notification of Zone Changes (DNS NOTIFY),'' RFC 1996, ISC, August 1996. [RFC2136] P. Vixie (Ed.), S. Thomson, Y. Rekhter, J. Bound ``Dynamic Updates in the Domain Name System,'' RFC 2136, ISC & Bellcore & Cisco & DEC, April 1997. [RFC2137] D. Eastlake ``Secure Domain Name System Dynamic Update,'' RFC 2137, CyberCash, April 1997. Expires August 1999 [Page 5] INTERNET-DRAFT Deferred Dynamic DNS Deletes February 1999 [secext2] D. Eastlake ``Domain Name System Security Extensions,'' draft-ietf-dnssec-secext2-07.txt, IBM, December 1998. [TSIG] P. Vixie (ed), O. Gudmundsson, D. Eastlake, B. Wellington ``Secret Key Transaction Signatures for DNS (TSIG),'' draft- ietf-dnsind-tsig-08.txt, ISC & TIS & IBM, February 1999. [simple-update] B. Wellington ``Simple Secure Domain Name System (DNS) Dynamic Update,'' draft-ietf-dnsind-simple-update-00.txt, TISLabs, November 1998. [update2] D. Eastlake ``Secure Domain Name System (DNS) Dynamic Update,'' draft-ietf-dnssec-update2-00.txt, Transfinite Systems Company, August 1998. 8 - Author's Address Brian Wellington Olafur Gudmundsson TISLabs at Network Associates TISLabs at Network Associates 3060 Washington Road, Route 97 3060 Washington Road, Route 97 Glenwood, MD 21738 Glenwood, MD 21738 +1 443 259 2369 +1 443 259 2389 +1 301 854 6889 (main number) +1 301 854 6889 Expires August 1999 [Page 6]