You're probably aware that it's good practice (and sometimes required) to have at least two DNS servers for every domain (or LAN, or organizational unit, or whatever). There's a common misconception that one of those two servers must be the "master" and the others must be "slaves." If you have multiple LANs (or subdomains) and each has its own pair of DNS servers, you'll have a nightmare trying to manage all the master servers scattered around your organization.
So here's the tip: it's not true that one of the two DNS servers has to be a master. The real requirement is that both should be authoritative. That's different.
There's nothing wrong with both authoritative servers being slaves. In fact, I do that quite frequently, even for small domains. Having authoritative slave servers frees me up to do some interesting things.
Let's say I have two existing DNS servers (A and B) and want to migrate A's data over to a new server, C, and get rid of A. Here are the steps:
- Set up C as a slave to A. It copies over all the zone files. (Or you can copy them manually if you want to preserve your nice formatting.)
- Update DHCP so that clients query B and C. Now you have two slaves servers (B and C) that are being authoritative for your domain.
- Re-configure C as the master.
- Re-configure B to slave from C.
- Let things settle down for a day or so.
- Disable DNS on A.
Another trick is to use only your slaves as the authoritative servers. The master server isn't used by clients and exists only to hold the master zone files. The slaves get their zone files from the master, which might be tucked away behind a firewall to help prevent tampering. You could have one single master that all slaves (across your whole organization) pull from, eliminating the administrative overhead of managing multiple masters.