Tcpdump
From charlesreid1
Installing
Linux
tcpdump should come with your distro, but if it doesn't, use aptitude or your package manager to install:
apt-get install tcpdump
Once you've done that, you can list your network devices:
iwconfig
Pick out which ones you want to listen to.
Mac
tcpdump comes with Mac. Man page for tcpdump: https://developer.apple.com/library/mac/documentation/Darwin/Reference/ManPages/man1/tcpdump.1.html
List your network devices:
ifconfig
Pick out which ones you want to listen to.
Usage
You will need to run tcpdump as sudo.
Unfiltered Packet Capture
The simplest way to use tcpdump is to do an unfiltered packet capture - no filters on packets, so everything is captured.
To do this, specify which device you want to listen to with the -i flag. Also specify an output file with the -w flag:
tcpdump -i en0 -w output_file.pcap
-w prevents your computer from having a meltdown trying to print every single packet in a busy place.
You can also monitor multiple interfaces by specifying a list: -i en0,en1