This is an example of the tcpdump command to see DNS traffic in your network.
Read answer to: "HowTo: Find Out DNS Server IP Address Used By My Router?"
tcpdump: Monitor ALL eth1 Traffic Except My Own SSH Session
FreeBSD dnstop: Monitor and Display DNS Server Traffic On Your Network 
Find Out What Ports Are Listening / Open On My Linux & FreeBSD Server
Monitor or sniff Apache http packets
tc: Linux HTTP Outgoing Traffic Shaping (Port 80 Traffic Shaping)
Monit: Monitor BIND 9 (named) Name Server and Auto Restart SSH If It Does Not Respond IP Trace Command In Bash
Linux iptables: Port Redirection Example
Debian / Ubuntu: Set Port Knocking With Knockd and Iptables
Linux: Iptables Forward Multiple Ports
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