1.7 KiB
Intro
xxx RFC1982
SOA Records
There is only one SOA that is guaranteed to exist on the internet and that is the one for the root zone (called '.'). As of 2018, it looks like this:
. 86400 IN SOA a.root-servers.net. nstld.verisign-grs.com. 2018032802 1800 900 604800 86400
This says: the authoritative server for the root zone is called 'a.root-servers.net'. This name is however only used for diagnostics. Secondly, nstld@verisign-grs.com is the email address of the zone maintainer. Note that the '@' is replaced by a dot. Specifically, if the email address had been 'nstld.maintainer@verisign-grs.com', this would have been stored as nstld\.maintainer.verisign-grs.com. This name would then still be 3 labels long, but the first one has a dot in it.
The following field, 2018032802, is a serial number. Quite often, but by all means not always, this is a date in proper order (YYYYMMDD), followed by two digits indicating updates over the day. This serial number is used for replication purposes, as are the following 3 numbers.
Zones are hosted on 'masters'. Meanwhile, 'slave' servers poll the master for updates, and pull down a new zone if they see new contents, as noted by an increase in serial number.
The numbers 1800 and 900 describe how often a zone should be checked for updates (twice an hour), and that if an update check fails it should be repeated after 900 seconds. Finally, 604800 says that if a master server was unreachable for over a week, the zone should be deleted from the slave. This is not a popular feature.
The final number, 86400, denotes that if a response says a name or RRSET does not exist, it will continue to not exist for the next day, and that this knowledge may be cached.