tcpdump

Tcpdump is a command-line packet analyser.

Monitor external traffic (i.e. not originating from subnet) with destination 192.168.1.123

sudo tcpdump -i eth0 "not src net 192.168.1.0/24 and dst host 192.168.1.123"

Monitor traffic from 192.168.1.123 going external (outside of subnet)

sudo tcpdump -i eth0 "dst host 192.168.1.123 and not src net 192.168.1.0/24"

SSH using public and private keys

I have made a guide here: Raspberry Pi: SSH using public and private keys