INTERNET DRAFT M. Ohta draft-ietf-dnsext-ixfr-00.txt Tokyo Institute of Technology March 2000 Incremental Zone Transfer in DNS 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. Copyright (C) The Internet Society (March/5/2000). All Rights Reserved. Abstract This document proposes extensions to the DNS protocols to provide an incremental zone transfer (IXFR) mechanism. 1. Introduction For rapid propagation of changes to a DNS database [STD13], it is necessary to reduce latency by actively notifying servers of the change. This is accomplished by the NOTIFY extension of the DNS [NOTIFY]. The current full zone transfer mechanism (AXFR) is not an efficient means to propagate changes to a small part of a zone, as it transfers the entire zone file. Incremental transfer (IXFR) as proposed is a more efficient mechanism, as it transfers only the changed portion(s) of a zone. M. Ohta Expires on September 5, 2000 [Page 1]
INTERNET DRAFT Incremental Zone Transfer in DNS March 2000 In this document, a secondary name server which requests IXFR is called an IXFR client and a primary or secondary name server which responds to the request is called an IXFR server. The AXFR specification is very terse, and in practice it does not contain sufficient information to construct interoperable implementations. [XFRCLARIFY] gives the clarification on the AXFR specification based on existing practice, which is, unless otherwise noted, also applicable to IXFR. 2. Brief Description of the Protocol If an IXFR client, which likely has an older version of a zone, thinks it needs new information about the zone (typically through SOA refresh timeout or the NOTIFY mechanism), it sends an IXFR message containing the SOA serial number of its, presumably outdated, copy of the zone. An IXFR server should keep record of the newest version of the zone and the differences between that copy and several older versions. When an IXFR request with an older version number is received, the IXFR server needs to send only the differences required to make that version current. Alternatively, the server may choose to transfer the entire zone just as in a normal full zone transfer. When a zone has been updated, it should be saved in stable storage before the new version is used to respond to IXFR (or AXFR) queries. Otherwise, if the server crashes, data which is no longer available may have been distributed to secondary servers, which can cause persistent database inconsistencies. If an IXFR query with the same or newer version number than that of the server is received, it is replied to with a single SOA record of the server's current version, just as a SOA query before TCP AXFR. Transport of a query may be by either UDP or TCP. If an IXFR query is via UDP, the IXFR server may attempt to reply using UDP if the entire response can be contained in a single UDP packet. If the UDP reply does not fit, the query is responded to with a single SOA record of the server's current version to inform the client that a TCP query should be initiated. Thus, a client should first make an IXFR query using UDP. If the query type or other part of the query is not recognized by the server, an AXFR (preceded by a UDP SOA query) should be tried, ensuring backward compatibility. If the query response is a single packet with the entire new zone, or if the server does not have a newer version than the client, everything is done. Otherwise, a TCP M. Ohta Expires on September 5, 2000 [Page 2]
INTERNET DRAFT Incremental Zone Transfer in DNS March 2000 IXFR query should be tried. To ensure integrity, servers should use UDP checksums for all UDP responses. A cautious client which receives a UDP packet with a checksum value of zero should ignore the result and try a TCP IXFR instead. The query type value of IXFR assigned by IANA is 251. 3. Query Format The IXFR query packet format is the same as that of a normal DNS query, but with the query type being IXFR and the authority section containing the SOA record of client's version of the zone. 4. Response Format If incremental zone transfer is not available, the entire zone is returned. The first and the last RR of the response is the SOA record of the zone. I.e. the behavior is the same as an AXFR response except the query type is IXFR. If incremental zone transfer is available, one or more difference sequences is returned. The list of difference sequences is preceded and followed by a copy of the server's current version of the SOA. Each difference sequence represents one update to the zone (one SOA serial change) consisting of deleted RRs and added RRs. The first RR of the deleted RRs is the older SOA RR and the first RR of the added RRs is the newer SOA RR. Modification of an RR is performed first by removing the original RR and then adding the modified one. A difference sequence which indicates the removal of a non-existent RR is an indication of an error that the IXFR client is out-of-sync with the IXFR server. The IXFR SHOULD be aborted, and an AXFR requested from the same server. A difference sequence which indicates the addition of a seemingly duplicate (through a node may have multiple TXT RRs of the duplicated content) or conflicting RR may just be a result of malformed zone ando should be treated just as a AXFR with malformed zone content. The sequences of differential information are ordered oldest first newest last. Thus, the differential sequences are the history of changes made since the version known by the IXFR client up to the server's current version. M. Ohta Expires on September 5, 2000 [Page 3]
INTERNET DRAFT Incremental Zone Transfer in DNS March 2000 RR sets (RRs of the same RR types) in the incremental transfer messages may be partial. For examle, if a single RR of multiple RRs of the same RR type changes, only the changed RR needs to be transferred. An IXFR client, should only replace an older version with a newer version after all the differences have been successfully processed. An incremental response is different from that of a non-incremental response in that it begins with two SOA RRs, the server's current SOA followed by the SOA of the client's version which is about to be replaced. To determine whether an IXFR response received over TCP is incremental or not, it is necessary to try to receive the first two answer RRs in the answer stream (which may or may not involve receiving two TCP DNS messages). The first RR is always an SOA. If the second RR does not exist, the client copy of the zone is up to date and no zone transfer is necessary. If the second RR is an SOA with a serial number different from the first SOA, the response is incremental, in IXFR format. If it is a non-SOA RR or a SOA RR with the same serial number as the initial SOA RR, it is a nonincremental response in AXFR format. The last case cannot arise unless the zone is malformed containing only the SOA record without NS records. This is the only way to identify an incremental response. A slave cannot reliably identify an incremental response based on the presence or absence of a question section, the QTYPE field of a possible question section, or the number of response RRs per message, 5. Purging Strategy An IXFR server can not be required to hold all previous versions forever and may delete them anytime. In general, there is a trade-off between the size of storage space and the possibility of using IXFR. Information about older versions should be purged if the total length of an IXFR response would be longer than that of an AXFR response. Given that the purpose of IXFR is to reduce AXFR overhead, this strategy is quite reasonable. The strategy assures that the amount of storage required is at most twice that of the current zone information. Information older than the SOA expire period should also be purged. 6. Optional Condensation of Multiple Versions An IXFR server may optionally condense multiple difference sequences M. Ohta Expires on September 5, 2000 [Page 4]
INTERNET DRAFT Incremental Zone Transfer in DNS March 2000 into a single difference sequence, thus, dropping information on intermediate versions. This may be beneficial if a lot of versions, not all of which are useful, are generated. For example, if multiple ftp servers share a single DNS name and the IP address associated with the name is changed once a minute to balance load between the ftp servers, it is not so important to keep track of all the history of changes. But, this feature may not be so useful if an IXFR client has access to two IXFR servers: A and B, with inconsistent condensation results. The current version of the IXFR client, received from server A, may be unknown to server B. In such a case, server B can not provide incremental data from the unknown version and a full zone transfer is necessary. Condensation is completely optional. Clients can't detect from the response whether the server has condensed the reply or not. For interoperability, IXFR servers, including those without the condensation feature, should not flag an error even if it receives a client's IXFR request with a version number known not to exist (which means that the server has versions with version numbers newer and older than, but not equal to, the version number) and should, instead, attempt to perform a full zone transfer by replying with a single SOA record of the server's current version (UDP case) or with a full zone content (UDP or TCP case). 7. Example Given the following three generations of data with the current serial number of 3, JAIN.AD.JP. IN SOA NS.JAIN.AD.JP. mohta.jain.ad.jp. ( 1 600 600 3600000 604800) IN NS NS.JAIN.AD.JP. NS.JAIN.AD.JP. IN A 133.69.136.1 NEZU.JAIN.AD.JP. IN A 133.69.136.5 NEZU.JAIN.AD.JP. is removed and JAIN-BB.JAIN.AD.JP. is added. jain.ad.jp. IN SOA ns.jain.ad.jp. mohta.jain.ad.jp. ( 2 600 600 3600000 604800) IN NS NS.JAIN.AD.JP. NS.JAIN.AD.JP. IN A 133.69.136.1 JAIN-BB.JAIN.AD.JP. IN A 133.69.136.4 IN A 192.41.197.2 M. Ohta Expires on September 5, 2000 [Page 5]
INTERNET DRAFT Incremental Zone Transfer in DNS March 2000 One of the IP addresses of JAIN-BB.JAIN.AD.JP. is changed. JAIN.AD.JP. IN SOA ns.jain.ad.jp. mohta.jain.ad.jp. ( 3 600 600 3600000 604800) IN NS NS.JAIN.AD.JP. NS.JAIN.AD.JP. IN A 133.69.136.1 JAIN-BB.JAIN.AD.JP. IN A 133.69.136.3 IN A 192.41.197.2 The following IXFR query +---------------------------------------------------+ Header | OPCODE=SQUERY | +---------------------------------------------------+ Question | QNAME=JAIN.AD.JP., QCLASS=IN, QTYPE=IXFR | +---------------------------------------------------+ Answer | <empty> | +---------------------------------------------------+ Authority | JAIN.AD.JP. IN SOA serial=1 | +---------------------------------------------------+ Additional | <empty> | +---------------------------------------------------+ could be replied to with the following full zone transfer message: +---------------------------------------------------+ Header | OPCODE=SQUERY, RESPONSE | +---------------------------------------------------+ Question | QNAME=JAIN.AD.JP., QCLASS=IN, QTYPE=IXFR | +---------------------------------------------------+ Answer | JAIN.AD.JP. IN SOA serial=3 | | JAIN.AD.JP. IN NS NS.JAIN.AD.JP. | | NS.JAIN.AD.JP. IN A 133.69.136.1 | | JAIN-BB.JAIN.AD.JP. IN A 133.69.136.3 | | JAIN-BB.JAIN.AD.JP. IN A 192.41.197.2 | | JAIN.AD.JP. IN SOA serial=3 | +---------------------------------------------------+ Authority | <empty> | +---------------------------------------------------+ Additional | <empty> | +---------------------------------------------------+ or with the following incremental message: +---------------------------------------------------+ Header | OPCODE=SQUERY, RESPONSE | +---------------------------------------------------+ Question | QNAME=JAIN.AD.JP., QCLASS=IN, QTYPE=IXFR | M. Ohta Expires on September 5, 2000 [Page 6]
INTERNET DRAFT Incremental Zone Transfer in DNS March 2000 +---------------------------------------------------+ Answer | JAIN.AD.JP. IN SOA serial=3 | | JAIN.AD.JP. IN SOA serial=1 | | NEZU.JAIN.AD.JP. IN A 133.69.136.5 | | JAIN.AD.JP. IN SOA serial=2 | | JAIN-BB.JAIN.AD.JP. IN A 133.69.136.4 | | JAIN-BB.JAIN.AD.JP. IN A 192.41.197.2 | | JAIN.AD.JP. IN SOA serial=2 | | JAIN-BB.JAIN.AD.JP. IN A 133.69.136.4 | | JAIN.AD.JP. IN SOA serial=3 | | JAIN-BB.JAIN.AD.JP. IN A 133.69.136.3 | | JAIN.AD.JP. IN SOA serial=3 | +---------------------------------------------------+ Authority | <empty> | +---------------------------------------------------+ Additional | <empty> | +---------------------------------------------------+ or with the following condensed incremental message: +---------------------------------------------------+ Header | OPCODE=SQUERY, RESPONSE | +---------------------------------------------------+ Question | QNAME=JAIN.AD.JP., QCLASS=IN, QTYPE=IXFR | +---------------------------------------------------+ Answer | JAIN.AD.JP. IN SOA serial=3 | | JAIN.AD.JP. IN SOA serial=1 | | NEZU.JAIN.AD.JP. IN A 133.69.136.5 | | JAIN.AD.JP. IN SOA serial=3 | | JAIN-BB.JAIN.AD.JP. IN A 133.69.136.3 | | JAIN-BB.JAIN.AD.JP. IN A 192.41.197.2 | | JAIN.AD.JP. IN SOA serial=3 | +---------------------------------------------------+ Authority | <empty> | +---------------------------------------------------+ Additional | <empty> | +---------------------------------------------------+ or, if UDP packet overflow occurs, with the following message: M. Ohta Expires on September 5, 2000 [Page 7]
INTERNET DRAFT Incremental Zone Transfer in DNS March 2000 +---------------------------------------------------+ Header | OPCODE=SQUERY, RESPONSE | +---------------------------------------------------+ Question | QNAME=JAIN.AD.JP., QCLASS=IN, QTYPE=IXFR | +---------------------------------------------------+ Answer | JAIN.AD.JP. IN SOA serial=3 | +---------------------------------------------------+ Authority | <empty> | +---------------------------------------------------+ Additional | <empty> | +---------------------------------------------------+ 8. Acknowledgements The original idea of IXFR was conceived by Anant Kumar, Steve Hotz and Jon Postel. For the refinement of the protocol and documentation, many people have contributed including, but not limited to, Anant Kumar, Robert Austein, Paul Vixie, Randy Bush, Mark Andrews, Robert Elz, Andreas Gustafsson, Josh Littlefield, Olafur Gudmundsson, William King and the members of the IETF DNSEXT working group. 9. References [NOTIFY] Vixie, P., "A Mechanism for Prompt Notification of Zone Changes (DNS NOTIFY)", RFC1996, August 1996. [STD13] Mockapetris, P., "Domain Name System" (RFC1034 and RFC1035), November 1987. [XFRCLARIFY] 10. Security Considerations Though DNS is related to several security problems, no attempt is made to fix them in this document. This document is believed to introduce no additional security problems to the current DNS protocol. 11. Author's Address Masataka Ohta Computer Center, Tokyo Institute of Technology 2-12-1, O-okayama, Meguro-ku, Tokyo 152-8550, JAPAN Phone: +81-3-5734-3299, Fax: +81-3-5734-3415 M. Ohta Expires on September 5, 2000 [Page 8]
INTERNET DRAFT Incremental Zone Transfer in DNS March 2000 EMail: mohta@necom830.hpcl.titech.ac.jp Comments should be directed to DNSEXT WG <namedroppers@ops.ietf.org>. M. Ohta Expires on September 5, 2000 [Page 9]
INTERNET DRAFT Incremental Zone Transfer in DNS March 2000 12. Full Copyright Statement "Copyright (C) The Internet Society (March/5/2000). 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 implementation 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 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. M. Ohta Expires on September 5, 2000 [Page 10]
