bug#20816: Excessive negative cache time for DNS lookups

[Mark H Weaver <mhw@netris.org>]

On my GuixSD system, DNS lookup failures (due to lack of network) are
cached for at least 4 minutes, and possibly much longer (I've not had
the patience to wait until it expires).

Even after restarting nscd, the negative cache entry persists for about
another 20 seconds.

I run into this problem quite frequently after waking my system up from
suspend-to-ram.  It typically takes several seconds for wicd to
establish a network connection, and if I make the mistake of triggering
a name lookup of my server (world.peace.net), then I must do the
following:

(1) deco restart nscd
(2) wait about 20 seconds

Looking at %nscd-default-caches in (gnu services base), I see that the
intent is to cache host lookup failures for 20 seconds, and service
lookup failures for an hour.  The fact that the negative cache entry
persists for 20 seconds after restarting nscd would seem to indicate
that nscd considers this to be a host lookup.  So perhaps this is a bug
in nscd.

Is anyone else able to reproduce this problem?

      Mark