DNS lookup

Query A records via a specific DNS server

dig +short A example.com @8.8.8.8

More linux commands

Reverse DNS lookup

dig +short -x 8.8.8.8

PTR record for an IP address
Trace DNS delegation

dig +trace example.com

Follow full DNS resolution chain from root nameservers
Show established TCP connections

ss -tnp state established

All established TCP connections with process info
Count connections per state

ss -tan | awk 'NR>1{print $1}' | sort | uniq -c | sort -rn

How many connections are in ESTABLISHED, TIME_WAIT, etc.
Traceroute with MTU discovery

mtr --report --tcp --port 443 example.com

Combine traceroute and ping; TCP mode to bypass ICMP blocks