Yes, a secondary DNS server. Mirroring is quite easy and automatic.
For example, take these chapters:
http://docstore.mik.ua/orelly/networ...ns/ch04_08.htm
Beware that locations of the zone files in the master and slave could be different. In the master they are stored in /etc/bind9, but on the slave in /var/something. This is because the synchronisation process is not allowed to write in /etc. If follows from the configuration, but once I spent 2 days troubleshooting on this.
jlinkels