Bind 9 Administrator Reference Manual

Chapter 6. BIND 9 Configuration Reference
domain name. The priority controls the order in which email delivery is attempted, with the lowest
number first. If two priorities are the same, a server is chosen randomly. If no servers at a given priority
are responding, the mail transport agent will fall back to the next largest priority. Priority numbers do not
have any absolute meaning — they are relevant only respective to other MX records for that domain
name. The domain name given is the machine to which the mail will be delivered. It must have an
associated A record — CNAME is not sufficient.
For a given domain, if there is both a CNAME record and an MX record, the MX record is in error, and
will be ignored. Instead, the mail will be delivered to the server specified in the MX record pointed to by
the CNAME.
example.com. IN MX 10 mail.example.com.
IN MX 10 mail2.example.com.
IN MX 20 mail.backup.org.
mail.example.com. IN A 10.0.0.1
mail2.example.com. IN A 10.0.0.2
For example:
Mail delivery will be attempted to mail.example.com and mail2.example.com (in any order), and
if neither of those succeed, delivery to mail.backup.org will be attempted.
6.3.3. Setting TTLs
The time to live of the RR field is a 32 bit integer represented in units of seconds, and is primarily used
by resolvers when they cache RRs. The TTL describes how long a RR can be cached before it should be
discarded. The following three types of TTL are currently used in a zone file.
SOA The last field in the SOA is the negative caching TTL. This controls how
long other servers will cache no-such-domain (NXDOMAIN) responses from
you.
The maximum time for negative caching is 3 hours (3h).
$TTL The $TTL directive at the top of the zone file (before the SOA) gives a
default TTL for every RR without a specific TTL set.
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