how to check if a slave zone is expired

[John Wobus]

I try to catch zones that are not updating on the slaves
to which I have access.  I compare the modtime of the zone
file with the current time and the refresh interval
for the zone.  Typically I allow a failure or two
before alerting, e.g. wait 1 refresh + 2 retry intervals.
If the expire interval is very short, this could
be too late.

Depending upon the expire interval and refresh interval,
the window in which you can be alerted and troubleshoot
a problem might be short.  If you're slaving zones
for another site, you might not have control of that.

If you find out refreshes aren't happening long before
the expiration, and if the zone is pretty static (e.g. a single
www.example.com address), you don't have to jump very fast to
address things if the expire interval is weeks.  If folks are
depending upon records that are dynamic, you want to respond
pretty quickly.

John Wobus