Ifconfig netstat nc - NetCat. A very powerful tool for testing connectivity iftop - kind of like top, but for network connections traceroute ping telnet - not used in the normal way, but instead as a way to see if a port is open on a particular system "telnet hostname.foo.comm 53" tcpdump wireshark lsof - useful to see which process has a socket open (netstat -p will do that as well). ip - great tool for working with networking on a linux box nmap - nice tool for seeing what's out on the network. It's also a great way to upset the network security team if you're not one of them. scp - swiss army knife of connectivity. You can tunnel TCP connections with it, you can forward X sessions, you can even tar from machine to machine with an SSH tunnel.
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