DNS HOWTO : A simple domain. : Slave servers
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5.6. Slave servers

Once you have set up your zones correctly on the master servers you need to set up at least one slave server. Slave servers are needed for robustness. If your master goes down the people out there on the net will still be able to get information about your domain from the slave. A slave should be as long away from you as possible. Your master and slave should share as few as possible of these: Power supply, LAN, ISP, city and country. If all of these things are different for your master and slave you've found a really good slave.

A slave is simply a nameserver that copies zone files from a master. You set it up like this:


zone "freebsd.bogus" {
	type slave;
	file "sz/freebsd.bogus";
	masters { 192.168.196.2; };
};

A mechanism called zone-transfer is used to copy the data. The zone transfer is controlled by your SOA record:


@	IN	SOA	ns.freebsd.bogus. hostmaster.freebsd.bogus. (
			199802151	; serial, todays date + todays serial #
			8H		; refresh, seconds
			2H		; retry, seconds
			4W		; expire, seconds
			1D )		; minimum, seconds

A zone is only transferred if the serial number on the master is larger than on the slave. Every refresh interval the slave will check if the master has been updated. If the check fails (because the master is unavailable) it will retry the check every retry interval. If it continues to fail as long as the expire interval the slave will remove the zone from it's filesystem and no longer be a server for it.


DNS HOWTO : A simple domain. : Slave servers
Previous: You've got a classless subnet
Next: Basic security options.