The 'SOA' Resource Record (RR) defines the key characteristics and attributes for the zone or domain and is standardized in RFC 1035. This RR is considered the most important RR in the zone file and it is among the most complex and takes a significant number of parameters.
Most of the SOA RDATA fields are pertinent only for name server maintenance operations. However, MINIMUM is used in all query operations that retrieve RRs from a zone. Whenever a RR is sent in a response to a query, the TTL field is set to the maximum of the TTL field from the RR and the MINIMUM field in the appropriate SOA. Thus MINIMUM is a lower bound on the TTL field for all RRs in a zone. Note that this use of MINIMUM should occur when the RRs are copied into the response and not when the zone is loaded from a master file or via a zone transfer. The reason for this provison is to allow future dynamic update facilities to change the SOA RR with known semantics.
Specifies authoritative information about a DNS zone, including the primary name server, the email of the domain administrator, the domain serial number, and several timers relating to refreshing the zone.
Specifies the RDATA field will contain data in the SOA RDATA format.
Specifies the primary name server for the zone. The name server referenced here also needs to be defined using the NS RR.
Specifies the email address who is responsible for the zone. Because the @ symbol has speical meaning, you must use a . in place of the @ in the address. Thus, if 'email@example.com' is the email for your admin, it would to be converted to 'dnsadmin.yourdomain.com.' when placed in the SOA RR.
The unsigned 32 bit version number of the original copy of the zone. Zone transfers preserve this value. This value wraps and should be compared using sequence space arithmetic.
A 32 bit time interval (in seconds) before the zone should be refreshed.
A 32 bit time interval (in seconds) that should elapse before a failed refresh should be retried.
A 32 bit time value that specifies the upper limit on the time interval (in seconds) that can elapse before the zone is no longer authoritative.
The unsigned 32 bit minimum TTL (in seconds) that should be exported with any RR from this zone.
Snippets from a fictitious forward lookup 'yourdomain.com' zone file
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