max tcp sockets for bind 9.4.2-P1

[Jason Bratton]

Hello all,

Like many of you, I recently upgraded all of our caching nameservers. 
Since we were already running BIND 9.4.2, I chose to upgrade to 9.4.2-P1. 
After the upgrade, I started receiving complaints of DNS queries that were
truncated and retried over TCP failing.

It appears that BIND is limiting the number of open TCP connections to ~
100 per IP address it listens on.  For example, on one of our caching
nameservers:

cachens-4:~# netstat -an | grep tcp | grep 72.3.128.240 | wc -l
99
cachens-4:~# netstat -an | grep tcp | grep 72.3.128.241 | wc -l
105