If you want a persistent (=on-disk) cache for some domains you're using a lot, have you considered making your DNS server a "slave" for that zone?
You can be a "stealth-slave": that is, your DNS server does not need to be announced to the world in NS records, so you don't get random queries to that zone from the outside world.
To be a "stealth-slave" for a zone, you first ensure that your server is permitted to "suck" a copy of the desired zone's data from some DNS server that already is authoritative for the zone.
Then you configure something like this in you r named.conf:
zone "some.domain.example" {
type slave;
masters { authoritative.server.ip.addr; };
file { "/etc/bind/slave/db.some.domain"; };
};
Ensure that the bind process can write to the file mentioned in "file" directive, and you're all set. Restart BIND (or say "rndc reconfig" if you've set up rndc properly) and it will automatically fetch the current data for the zone, store it into the file and keep the file up to date.
If the domain's content is changing often, it would be good to have the domain's master server set up an "also-notify" for you: normally a slave server checks periodically on its master for changes to the duplicated zone data, but the master can also send NOTIFY messages to the slaves when the data changes. This is done automatically for official slaves (those mentioned in the zone's NS records), but for stealth-slaves the master server must be configured explicitly to send the notify message, because the master has no other way to know about the slave's existence.
MK
