Specifications

Line 1: $TTL defines the standard TTL that applies for all the entries in this
file, here 2 days. TTL means “time to live”.
Line 2: The SOA control record begins here:
The name of the domain to administer is world.cosmos in the first
position. This ends with a ‘.’, because otherwise the zone would
be appended a second time. Alternatively, a ‘@’ can be entered
here. Then, the zone would be extracted from the corresponding
entry in /etc/named.conf.
After IN SOA is the name of the name server in charge as mas-
ter for this zone. The name is extended from gateway to gate-
way.world.cosmos, because it does not end with a ‘.’.
Afterwards, an e-mail address of the person in charge of this name
server will follow. Since the ‘@’ sign already has a special signif-
icance, ‘.’ is to be entered here instead, for [e-mail],
consequently root.world.cosmos..The ‘.’ sign at the end cannot
be neglected, otherwise, the zone will still be added here.
A ‘(’ follows at the end here, including the following lines up
until ‘)’ into the SOA record.
Line 3: The serial number is an arbitrary number that is increased each time
this file is changed. It is needed to inform the secondary name servers
(slave servers) of changes. For this, a ten-digit number of the date and
run number, written as YYYYMMDDNN, has become the customary
format.
Line 4: The refresh rate specifies the time interval at which the secondary
name servers verify the zone serial number. In this case, 1 day.
Line 5: The retry rate specifies the time interval at which a secondary name
server, in case of error, attempts to contact the primary server again.
Here, 2 hours.
Line 6: The expiration time specifies the time frame after which a secondary
name server discards the cached data if it has not regained contact to
the primary server. Here, it is a week.
Line 7: The minimum time to live states how long the results of the DNS
requests from other servers can be cached before they become invalid
and have to be requested again.
164 The Configuration File /etc/named.conf