Looking for Tools to debug DNS Lookup and Replication Failures

[Russell Findley]

I have two physical locations separated by a IPSec tunnel.  For simplicity,
I'll refer to them as point A and point B.
Point A can do a zone transfer from B and all nslookup's and dig's work from
A to B.

However, doing any type of zone transfer, nslookup or dig from point B to A
fails with timeouts.

To the best of my knowledge, nothing on the firewalls are blocking port 53,
but I'm not convinced.  Are there any tools that would be useful to help me
debug this problem?

Russ