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) server-user nscd debug-level 0 paranoia no 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