BIND slave server ignoring responses to all UDP-based SOA queries (zone refresh) for hours at a time

Irwin Tillman irwin at
Mon Jul 13 19:17:09 UTC 2015

At 07 Jul 2015 13:47:45 +0100, Cathy Almond <cathya at> wrote:

>What can happen (and this is really really subtle) is that if there are
>some source ports that named could randomly select, but where
>intermediate firewalls or filters are just dropping, either the SOA
>refresh queries, or the responses, then named can 'get stuck' on using
>and re-using the same refresh source port.

Thank you, that was exactly the cause, and the fix.

Some years ago I'd updated a host-based firewall running on my BIND slave server 
to block traffic to an additional inbound UDP port that falls
into the range BIND may use for ephemeral ports. At that time I neglected to
add that port to BIND's config (avoid-v4-udp-ports and avoid-v6-udp-ports).

When BIND picked that src port for its UDP SOA queries, the incoming SOA replies
were blocked by that firewall.  

For some reason BIND wasn't picking that port often (or wasn't getting stuck on
that port for long enough for me to notice) until I recently made an apparently
unrelated config change (expanding the use of request-ixfr) to my BIND slave
server.  Once I made that change, BIND got stuck on that port 
(for all the SOA queries all the zones it pulled from various unrelated
masters) for hours at a time every 1-3 days (until picking another port),
exposing my latent configuration problem.

Irwin Tillman
OIT Networking & Monitoring Systems, Princeton University

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