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[dns-wg] Action Item 48.1: Lame Delegations -- first draft
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Peter Koch
pk at DENIC.DE
Wed May 4 23:55:03 CEST 2005
Dear all, here is a first draft addressing action item 48.1 on lame delegation problems on large scale name servers. It's a -00 kind of document mainly issuing the problem statement. Although there are some hours left, I don't expect anyone to have read it by the WG meeting. An HTML version may be made available later and depending on how the PDP evolves, we might want or need to inject it into the policy developing engine. Comments are welcome! -Peter -------------- next part -------------- RIPE DNS WG 48.1 P. Koch DENIC eG May 4, 2005 DNS lame delegations caused by AXFR source unavailability Abstract This document analyses causes for DNS lame delegations seen on large name servers and investigates and assesses countermeasures. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Signs of Lame Delegations . . . . . . . . . . . . . . . . . . 2 3. Problems on large name servers . . . . . . . . . . . . . . . . 3 4. Ways to deal with missing XFR source . . . . . . . . . . . . . 4 5. Proactive measures . . . . . . . . . . . . . . . . . . . . . . 5 6. Wishlist . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 7. Security Considerations . . . . . . . . . . . . . . . . . . . 5 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 5 9. Disclaimer . . . . . . . . . . . . . . . . . . . . . . . . . . 5 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Author's Address . . . . . . . . . . . . . . . . . . . . . . . 6 A. Copyright Statement . . . . . . . . . . . . . . . . . . . . . 6 Koch [Page 1] RIPE DNS WG DRAFT Large Scale DNS Lame Dels May 2005 1. Introduction This document analyses causes for DNS lame delegations seen on large (thousands of zones) name servers and investigates and assesses countermeasures. First we will define the term "lame delegation" and similar operational problems. In the third section we will address various reasons that lead to lame delegations. The fourth paragraph will summarize mechanisms server administrators currently do or could apply to lower the impact of lame delegations. 2. Signs of Lame Delegations A lame delegation is a DNS delegation where the target of the NS RR does not respond authoritatively to queries for the domain so delegated. Lame delegations show different symptoms, which are sometimes given separate names: 1. The server's responses do not have the AA bit set 2. The server responses with an (upward) referral 3. The server responses SERVFAIL 4. The server responses REFUSED 5. The server refuses the query packet (giving either ICMP port unreachable or TCP RST) 6. The server does not respond at all 7. The server's name does not exist (NXDOMAIN) 8. The server's name does not own any A (or AAAA) RRs Cases (1) and (2) above are classical signs of a zone that has been forgotten by its server, either by expiry or due to syntax errors. Cases (1) through (4) are common lame delegations, cases (5) and (6) often just appear as temporary operational problems and cases (7) and (8) are sometimes called stale delegations. The latter may result in a significant increase of the query volume at the servers serving the domain the non existing name server is expected to reside in. When all delegations for a domain are lame, the domain is delegated Koch [Page 2] RIPE DNS WG DRAFT Large Scale DNS Lame Dels May 2005 perfectly lame. While operational guidelines suggest that the NS RRSet of a zone and the corresponding delegation in the parent zone should match, there are sometimes inconsistencies. Without acknowledging or endorsing this practice the union of both NS RRSets shall be eligible for a lame delegation assessment. In other words, even an NS RR that is only present in the delegated (child) zone may constitute a lame delegation as well. 3. Problems on large name servers RFC 1034 [RFC1034] specifies the way and frequency the slave will check with the master for a zone change. Details are controlled by the "refresh", "retry" and "expire" timer values. A slave is expected to try checking a zone with its master (For the sake of simplicity we assume there is only a single master per slave) until it either succeeds or the "expire" timer is reached, after which the zone should be "discarded". Different software implementations handle this in different ways [[list to be added]]. Software implementations currently either apply hard coded upper and lower bounds to these timer values or allow for their manual configuration. This aids in preventing unjustified resource consumption by unfortunate configurations. While the (primary) master not necessarily needs to be delegated the zone (hidden or stealth master), it may become unresponsive or non- authoritative. As a consequence, the slave or slaves depending on this master will subsequently have to discard the zone and become lame themselves. Reasons may be one or more of: o The slave cannot connect to the master o SOA not available at master o SOA not authoritative at master o SOA OK, AXFR refused o AXFR breaks o Zone does not load If the SOA check or the subsequent AXFR between master and slave fails, there are multiple potential causes: Koch [Page 3] RIPE DNS WG DRAFT Large Scale DNS Lame Dels May 2005 o Syntax error in zone at master o CNAME sin (CNAME at zone apex) o SOA serial decremented o error in zone list at master (forgot zone) o Customer changed AXFR source o Customer applied (too strict) access control o Customer vanishes o Address space reassigned ("you are attacking me on port 53") o Software problem at master Name servers serving large numbers of slave zones, may thus face two different problems related to lameness: o Increased traffic, sometimes even query storms, due to lame delegations. This may be worse if the zone is delegated perfectly lame (master and all slaves are delegated lame. o A significant fraction of resources may be spent on the higher frequency retries caused by SOA check failures. This may decrease the servers' performance and degrade service for all other zones on that server. 4. Ways to deal with missing XFR source ignore This may result in a service degradation for all customers. call customer This is tedious, expensive and sometimes impossible, if there is no direct relationship between the name server operator and the zone maintainer. deconfigure zone The delegation will usually remain and queries will continue to arrive. substitute missing zone with last available copy Introduces (other type of) inconsistencies, may be difficult to get hands on "latest version" Koch [Page 4] RIPE DNS WG DRAFT Large Scale DNS Lame Dels May 2005 substitute missing zone with empty zone This is invasive, introduces hard failures and may thus increase load on the hotline. have delegation revoked The name service provider may not necessarily be able to authoritatively (pun intended) communicate with a registrar or a registry. 5. Proactive measures Logs on the slave should be inspected automatically for signs of AXFR source unavailability as an early warning measure. Zone maintainers, name server operators and registries can prevent certain problems by (more) careful tailoring of SOA timer parameters. 6. Wishlist From a name server operator's perspective it would be helpful if the operator could change or delete a delegation to his name server and could change the server's registered IP address, if any. There are two problems with this approach: authentication of the server operator and status of the domain should the number of active delegations fall below a required minimum. In the long run it may be useful to give the name server operator a dedicated role in the current concept of the "triple R trio". Software vendors could add logic to prefer sane zones' SOA checks over those for potentially abandoned ones, maybe by applying some (timer) penalty to zones that could not be verified with the master for a "long" time (for given "refresh", "retry" and "expire" values. 7. Security Considerations DNSSEC may introduce additional problems, when a slave has data with outdated sigantures only. In addition, substituting an expireed zone with an empty or the last available copy will no longer be feasible. 8. Acknowledgements Thanks to Andre Koopal from MCI NL for bringing this up at RIPE 48. 9. Disclaimer The measures are listed here for informational purposes only. None of them is yet recommended and they should be used after a careful risk/benefit assessment only. Koch [Page 5] RIPE DNS WG DRAFT Large Scale DNS Lame Dels May 2005 This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE REPRESENTS OR IS SPONSORED BY (IF ANY), DISCLAIM 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. Any position taken by the author reflects his personal views and does not constitute a policy statement of the sponsoring institution. 10. References [RFC1034] Mockapetris, P., "Domain names - concepts and facilities", STD 13, RFC 1034, November 1987. [RFC1035] Mockapetris, P., "Domain names - implementation and specification", STD 13, RFC 1035, November 1987. [RFC1912] Barr, D., "Common DNS Operational and Configuration Errors", RFC 1912, February 1996. [RFC1996] Vixie, P., "A Mechanism for Prompt Notification of Zone Changes (DNS NOTIFY)", RFC 1996, August 1996. [RIPE203] Koch, P., "Recommendations for DNS SOA Values", RIPE 203, June 1999. Author's Address Peter Koch DENIC eG Wiesenhuttenplatz 26 Frankfurt 60329 DE Phone: +49 69 27235 0 Email: pk at DENIC.DE Appendix A. Copyright Statement Copyright (C) Peter Koch, DENIC eG (2005) Koch [Page 6]
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