[jmcnulty]

The first nameserver listed in resolv.conf is kind of a primary as it will always be consulted first, unless you add "options rotate". The next nameserver only come into play if the first doesn't respond (default 5 seconds, also tunable with options). They're not named primary/secondary in the file but could be considered that way.

[nwmcsween]

Don't rely on this behaviour, many DNS libraries, will query all or n to save on latency.