Tcpdump

tcpdump -X <- capture everything.

tcpdump -i <iface> -vvv -nn host <hostname>

• -i the interface to listen
• -vvv very very verbose
• -nn do not resolv ipaddr or ports.
• host the BFP filter to ignore all other hosts.

When reading output: [S] for SYN, [S.] for SYN/ACK -- the port is open! [R] for Reset, [R.] for RESET/ACK -- the port is closed!