We have a customer who has their own cache server, but in the afternoons
before they close up for the day, they commit off-site backups, this
process takes them about 90 mins, anyone trying to use the internet in this
time fails 99.9% of the time due to DNS lookup errors, but if they use an
external DNS server, such as ours, it works - albeit slow but it does get a
response. The local DNS cache server operates fine and instant for their
private LAN, and pinging around their LAN is sub 1ms so the problem exists
when bind tries to go out to get answers for real hostnames. When  their
internet link is not fully utilized there is no problems.

The problem arose again today before the off-site backups when just one PC
got its message from Microsoft to grab the anniversary update, at 11
o'clock in the morning, strangely it did not fill their link, but the pps
must have been rampant because the DNS errors again failed when using their
local cache resolver server.

Is there a named.conf setting we can suggest they use on their cache server
that perseveres and waits a little longer for answers to send to their
client machines?
They are using bind 9.10.4-p2 with default settings from source package
along with options of -

        directory "/opt/named";
        allow-query { xxxxx; };
        allow-query-cache { xxxxx; };
        allow-transfer { xxxxxx; };

Thanks for any advice.
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