INTERNET-DRAFT Peter Koch Expires: August 1999 Universitaet Bielefeld February 1999 Recommendations for DNS SOA Values draft-koch-dns-soa-values-00.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-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. Editorial comments should be sent to the author, technical discussion is taking place on the RIPE DNS WG mailing list. See for details. Abstract The configuration and maintainance of DNS zones offer many degrees of freedom and thus several opportunities of making mistakes. Most DNS zones today are small and have to be set up and maintained by non- experts. This document gives recommendations on which values to use for the SOA resource record of small, stable DNS zones to aid novice administartors and to contribute to DNS stability end efficiency. 1. Background Various DNS surveying activities show that the vast majority of today's DNS zones is populated by very few hosts. In most cases there is only an HTTP server announced under the common name "www", sometimes accompanied by distinct mail or DNS servers or a bastion Koch Expires August 1999 [Page 1] INTERNET-DRAFT SOA Values February 1999 host. For many of these zones the configuration is touched once when it is set up and then left alone without modification for a long time. These recommendations are aimed at small and stable DNS zones. There are many legitimate reasons to use different values, e.g. proposed changes or special purpose applications. Administrators of those zones should consult one of the various more detailed DNS guidelines or books. ISPs and DNS server vendors are encouraged to use this information for their customers [draft: once it has settled]. For more information about initial name server setup and zone configuration see [DNSGUIDE1], [DNSGUIDE2]. 2. Recommended SOA Values example.com. 3600 SOA dns.example.com. hostmaster.example.com. ( 1999022301 ; serial YYYYMMDDnn 86400 ; refresh (24 hours) 7200 ; retry (2 hours) 3600000 ; expire (1000 hours) 172800 ) ; minimum (2 days) 3. Remarks and Explanation 3.1. The MNAME Value The DNS specification explicitly states that the primary master server be named here. The value must be determined and used. Especially it is a mistake to repeat the zone name here, unless this also leads to a valid address of the primary master. 3.2. The RNAME Value The RNAME is to publish a mail address of a person or role account dealing with this zone with the "@" converted to a ".". The best practice is to define (and maintain) a dedicated mail alias "hostmaster" [RFC2142] for DNS operations. 3.3. The Serial Number The most imporatnt issue is that this value be incremented after any modification to the zone data. For debugging purposes it has shown to be helpful to encode the modification date into the serial number. The value "1999022301" so is an example of the YYYYMMDDnn scheme and Koch Expires August 1999 [Page 2] INTERNET-DRAFT SOA Values February 1999 must be replaced by proper values for the year (YYYY, four digits), month (MM, two digits), day of month (DD, two digits) and version per day (nn, two digits). The first version of the day should have the value "01". It is important to preserve the order year - month - day. 3.4. The Refresh and Retry Values The refresh and retry values primarily affect the zone maintainer and the secondary service providers and may be negotiated between them. The values chosen here are aimed at scalability. Modern DNS software implements NOTIFY and reduces the need for frequent SOA checks, as does the assumption of stability of the zone. While lower values would only slightly increase the bandwidth usage, they would increase the load on the secondary servers serving thousands of zones. 3.5. The Expire Value The primary goal is to ensure stability of the zone data, even if a mistake invalidating (non-authorizing) the zone or network outage last for several days. A value of a week or two has proven to be way too short, so a longer time must be used. The specific value was chosen for aesthetic and historic reasons and to disambiguate between the different proposed values of "long". 3.6. The Minimum TTL Value There are two meanings for this value with practical relevance. First, it serves as a default value for the TTL of all RRs without a given value. To be cache-friendly this value was chosen to be two days, which also follows the stability assumption. The second meaning is the default negative TTL [RFC2309], which would call for a lower value. We are in a transition phase now with software implementing either of both meanings, so the TTL of one hour is recommended for the SOA itself, which will lead to nearly the same effect. 4. Security Considerations Filling in the recommended values will not directly influence security of the name servers for the particular zone, any system with a name in that zone or any other system in the Internet. However, following these guidelines will likely contribute to DNS stability and thus to reachability. Maintaining proper contact information in the SOA RNAME field helps people in reporting problems, although the address distributed there is not recommended as a primary security contact. Koch Expires August 1999 [Page 3] INTERNET-DRAFT SOA Values February 1999 5. Acknowledgements This work is a product of the RIPE DNS working group. 6. References [RFC1034] Mockapetris,P., "Domain Names - Concepts and Facilities", RFC 1034, STD 13, November 1987 [RFC1035] Mockapetris,P., "Domain Names - Implementation and Specif- ication", RFC 1035, STD 13, November 1987 [RFC1123] Braden,R., "Requirements for Internet Hosts -- Application and Support", RFC 1123, STD 3, October 1989 [RFC2142] Crocker,D., "MAILBOX NAMES FOR COMMON SERVICES, ROLES AND FUNCTIONS", RFC 2142, May 1997 [RFC2309] Andrews,M., "Negative Caching of DNS Queries (DNS NCACHE)", RFC 2309, March 1998 [DNSGUIDE1] Liman,L., "SIMPLE DNS CONFIGURATION EXAMPLE", work in pro- gress [DNSGUIDE2] Koch,P., "RIPE Guide To Setting Up a DNS Server", work in progress 7. Author's Address Peter Koch Universitaet Bielefeld Technische Fakultaet Postfach 10 01 31 D-33501 Bielefeld Germany +49 521 106 2902 Koch Expires August 1999 [Page 4]