SOA record

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A start of authority record (abbreviated as SOA record) is a type of resource record in the Domain Name System (DNS) containing administrative information about the zone, especially regarding zone transfers. The SOA record format is specified in RFC 1035. [1]

Contents

Background

Normally DNS name servers are set up in clusters. The database within each cluster is synchronized through zone transfers. The SOA record for a zone contains data to control the zone transfer. This is the serial number and different timespans.

It also contains the email address of the responsible person for this zone, as well as the name of the primary master name server. Usually the SOA record is located at the top of the zone. A zone without a SOA record does not conform to the standard required by RFC 1035.

Structure

NAME
Name of the zone. @ is a shortcut to match previous record in BIND syntax.
CLASS
Zone class (all but universally IN for internet)
TYPE
SOA, abbreviation for start of authority
TTL
Time-to-live
MNAME
Primary master name server for this zone
RNAME
Email address of the administrator responsible for this zone. (As usual, the email address is encoded as a name. The part of the email address before the @ becomes the first label of the name; the domain name after the @ becomes the rest of the name. In zone-file format, dots in labels are escaped with backslashes; thus the email address john.doe@example.com would be represented in a zone file as john\.doe.example.com.)
SERIAL
Serial number for this zone. If a secondary name server slaved to this one observes an increase in this number, the slave will assume that the zone has been updated and initiate a zone transfer.
REFRESH
Number of seconds after which secondary name servers should query the master for the SOA record, to detect zone changes. Recommendation for small and stable zones: [4] 86400 seconds (24 hours).
RETRY
Number of seconds after which secondary name servers should retry to request the serial number from the master if the master does not respond. It must be less than Refresh. Recommendation for small and stable zones: [4] 7200 seconds (2 hours).
EXPIRE
Number of seconds after which secondary name servers should stop answering request for this zone if the master does not respond. This value must be bigger than the sum of Refresh and Retry. Recommendation for small and stable zones: [4] 3600000 seconds (1000 hours).
MINIMUM
Used in calculating the time to live for purposes of negative caching. Authoritative name servers take the smaller of the SOA TTL and the SOA MINIMUM to send as the SOA TTL in negative responses. Resolvers use the resulting SOA TTL to understand for how long they are allowed to cache a negative response. Recommendation for small and stable zones: [4] 172800 seconds (2 days). Originally this field had the meaning of a minimum TTL value for resource records in this zone; it was changed to its current meaning by RFC 2308. [5]

Sample SOA record

Sample SOA record for example.org, in BIND syntax.

$TTL86400@INSOAns.icann.org.noc.dns.icann.org.(        2020080302;Serial        7200;Refresh        3600;Retry        1209600;Expire        3600;Negative response caching TTL)

Serial number changes

Several methods have been established for updates to the SERIAL field of a zone's SOA record:

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References

  1. ↑ Mockapetris, P.V. (November 1987). "RFC 1035 — Domain names - implementation and specification". Network Working Group. doi:10.17487/RFC1035 . Retrieved 2017-12-28.
  2. ↑ Thomson, S.; Rekhter, Y.; Bound, J.; Bound, J. (April 1997). Vixie, P (ed.). "RFC 2136 — Dynamic Updates in the Domain Name System (DNS UPDATE)". Network Working Group. doi:10.17487/RFC2136 . Retrieved 2017-12-28.
  3. ↑ Vixie, P. (August 1996). "RFC 1996 — A Mechanism for Prompt Notification of Zone Changes (DNS NOTIFY)". Network Working Group. doi:10.17487/RFC1996 . Retrieved 2017-12-28.
  4. 1 2 3 4 "RIPE 203 — Recommendations for DNS SOA Values". Network Working Group. 1999-06-07. Retrieved 2017-12-28. These recommendations are aimed at small and stable DNS zones.
  5. ↑ Andrews, M. (March 1998). "RFC 2308 — Negative Caching of DNS Queries (DNS NCACHE)". Network Working Group. doi: 10.17487/RFC2308 . Retrieved 2017-12-28.
  6. ↑ Barr, D. (February 1996). "RFC 1912 — Common DNS Operational and Configuration Errors". Network Working Group. doi:10.17487/RFC1912 . Retrieved 2017-12-28.
  7. ↑ Bernstein, D.J. "How to run a DNS server in place of an existing BIND server" . Retrieved 2023-03-13.
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