DNS queries can be an issue on Cloud infrastructure, where SaaS are generally configured using FQDN instead of IP. This means that every time your application needs MySQL or Redis or any of the Platform/Software as a service, a DNS query will be issued.
Sometimes this could hammer the local DNS and service providers might block your DNS traffic.
To minimise this, you could use nscd to cache the DNS queries and reduce the traffic to the local DNS servers.
>> Test if port 53 has been blocked
tcpdump -vvv -s 0 -l -n port 53
>> Check what is currently cached
nscd -g (and check hosts cache)
>> Configuration file
grep -v "^#" /etc/nscd.conf (leaving only 'hosts' details)
enable-cache hosts yes
positive-time-to-live hosts 3600
negative-time-to-live hosts 20
suggested-size hosts 211
check-files hosts yes
persistent hosts yes
shared hosts yes
max-db-size hosts 33554432