brew install iftop
For some reason, iftop does not get symlinked to the usual homebrew bin directory, so either symlink it yourself, or move to installation directory.
iftop -i en0
This will start monitoring en0 and display a nice graph with network usage.
If you want to see more traffic information for a specific host, use tcpdump.
sudo tcpdump -i en0 "192.168.1.101"
Apple has modified tcpdump to give it the ability to display process information alongside the packets.
sudo tcpdump -i en0 -k N
Here is the list of
-k modifiers you can use.
I interface name (or interface ID) N process name P process ID S service class D direction C comment C flags U process UUID (not shown by default) A display all types of metadata
To see a list of processes and their connections, use
h for the help menu.