nsd sendto failure - how to debug?

[Adam Thompson]

I'm seeing lots of "nsd[11026]: error: sendto failed: No route to host" 
errors in my logs on both authoritative nameservers.
Even running nsd in debug mode (using '-d') fails to produce any useful 
information about what reply failed.
Running tcpdump against port 53 doesn't appear to show anything useful 
or interesting (every query I can see has a reply).

How might I find out what's causing these errors, short of recompiling 
nsd with additional logging output?

--
-Adam Thompson
 athom...@athompso.net