Tcpdump

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

Wireless Packet Capture

If you want to capture wireless packets, you need to know a bit more about a few things.

First is channels.

The 802.11 protocol allocates 12 channels for wireless (in the US), and your wireless card can only listen to one channel at a time. To listen to twelve channels, you need twelve wireless cards - or you need to hop from channel to channel with your single wireless card.

If it is critical to capture all traffic, you will want to use multiple wireless cards - if you're hopping from channel 5 to channel 6, and traffic shows up on channel 4, you won't see it.

Second is monitor mode. If your wireless card is not in monitor mode, your wireless card will be throwing away any packets that are not intended for itself, meaning you'll only be creating a pcap file of your own traffic.

Monitor Mode in Mac

To put the wireless card into monitor mode, you can use the -I flag with tcpdump (and the -n flag to make things more readable):

sudo tcpdump -In -i en1 -w save.pcap

More information over at Unix Stack Exhange.

Alternatively, you can use the airport utility, located at:

/System/Library/PrivateFrameworks/Apple80211.framework/Versions/Current/Resources/airport.

conveniently symlinked to /usr/local/bin:

sudo ln -s /System/Library/PrivateFrameworks/Apple80211.framework/Versions/Current/Resources/airport /usr/local/bin/airport

Channel-Hopping on Mac

When you run tcpdump with the -I flag, it will put the card in monitor mode and automatically cycle through all the channels. This will significantly boost the amount of traffic you dump to your pcap file!

Note that you can also use airport to monitor a single channel, e.g., channel 7:

sudo airport en1 sniff 7

Monitor Mode in Linux

Put your card into monitor mode with these steps:

iwconfig # list all devices
ifconfig wlan1 down # assuming wlan1 is wireless
iwconfig wlan1 mode monitor # put into monitor mode
ifconfig wlan1 up # bring wlan1 online

Channel-Hopping on Linux

Once you've put the card into monitor mode, you can run airodump-ng, which will automatically channel-hop unless you specify a specific channel.