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DNS administrators often speak of master/slave servers, primary/secondary servers, and authoritative/non-authoritative servers. These do not all mean the same thing but are often confused, both due to ignorance and because the official usage has changed over time. So the person you're speaking with may match any term with any of the meanings below, and you'll have to figure out from context what he means. This also means you should explain the term with anybody you're speaking with, or at least put a few words of context so they know which meaning you intend. Note that all these terms are domain-specific. A server can be master for one domain while simultaneously being slave for another domain.
When I originally set up a domain for a nonprofit organization, I thought the first IP listed at the registrar had to be a master, and the others had to be slaves or the zone transfers wouldn't work properly. This turned out to be hogwash. A "hidden master" is actually quite common. That's where the real records are kept at a private or unadvertised server, and all the authoritative servers are slaves. This protects you from attacks: the cracker can get the money but he can't get the family silverware.
A question that comes up in those cases is "what value do I put in the SOA record?" (the item at the top of a DNS zone that tells which computers have the original configuration data). Traditional practice is to list the masters, but that is what you would not do if you really wanted to hide the masters. No DNS program actually uses the SOA value for anything as far as we know; it's more a note to humans than anything else, so you can use it to cue yourself, or your fellow system administrators, in whatever way you prefer.
Mike is a Contributing Editor at Linux Gazette. He has been a
Linux enthusiast since 1991, a Debian user since 1995, and now Gentoo.
His favorite tool for programming is Python. Non-computer interests include
martial arts, wrestling, ska and oi! and ambient music, and the international
language Esperanto. He's been known to listen to Dvorak, Schubert,
Mendelssohn, and Khachaturian too.