| SOA
(Start of Authority) Record |
There are several record types for data
in the domain name system (DNS). At the top level of a domain, the
name database must contain a Start of Authority (SOA) record,
which identifies who has authoritative responsibility for this domain,
and which identifies the current version of the database.
There must be exactly one SOA record for each separately nameserved domain
(each subdomain). This goes for subdomains of IN-ADDR.ARPA (reverse domains.)
A region of namespace that has a separate SOA is known as a zone.
The format for this record is as follows:
DOMAIN.NAME. IN SOA Hostname.Domain.Name. Mailbox.Domain.Name. (
serno ; serial
28800 ; refresh - 8 hours
7200 ; retry - 2 hours
604800 ; expire - 1 week
86400 ) ; default TTL - 1 day
For example, the SOA record for Arizona.EDU would look something like this:
Arizona.EDU. IN SOA Arizona.EDU. Hostmaster.Arizona.EDU. (
910612 ; serial
28800 ; refresh - 8 hours
7200 ; retry - 2 hours
604800 ; expire - 1 week
86400 ) ; default TTL - 1 day
Here is an explanation of the data fields in the SOA record:
- DOMAIN.NAME.
- The name of the domain to which the SOA record pertains. Note the trailing
dot (.). This signifies that no suffix is to be appended to the name.
- IN
- The class of the DNS record. IN stands for "Internet."
- SOA
- The type of DNS record–Start of Authority in this example.
- Hostname.Domain.Name.
- The "origin field"; it should contain the host name of the primary name
server for this zone–the host where the authoritative data resides.
- Mailbox.Domain.Name.
- The mailbox of the individual responsible for (name service for) this
domain. To translate this field into a usable e-mail address, replace the
first . (dot) with an @ (at-sign). In this example, if there are problems
with arizona.edu, send mail to hostmaster@arizona.edu.
- serno
- The serial number of the current version of the dbs database for this
domain. The serial number is the means by which other nameservers realize
that your database has been updated. This serial number must be a monotonically
increasing integer–do not put a decimal point into the serial number,
as this can yield confusing and unpleasant results. Some DNS administrators
use the date last modified as the serial number, in the format YYMMDDHHMM;
others simply increment the serno by a small number every time the
database is updated.
The other numeric fields are known as the time-to-live (TTL) fields. These
control the frequency with which nameservers poll each other to get information
updates, how long the data will be cached, and so forth. Use these values unless
you have a strong reason to use something else here.
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