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netstat


There's a few parameters to netstat that are useful for this :

  • -l or --listening shows only the sockets currently listening for incoming connection.
  • -a or --all shows all sockets currently in use.
  • -t or --tcp shows the tcp sockets.
  • -u or --udp shows the udp sockets.
  • -n or --numeric shows the hosts and ports as numbers, instead of resolving in dns and looking in /etc/services.

You use a mix of these to get what you want. To know which port numbers are currently in use, use one of these:

netstat -atn           # For tcp
netstat -aun           # For udp
netstat -atun          # For both

In the output all port mentioned are in use either listening for incoming connection or connected to a peer** all others are closed. TCP and UDP ports are 16 bits wide (they go from 1-65535)

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