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.

Basic Usage

You may need to run tcpdump as sudo to access certain information from the hardware.

Tcpdump options can vary from platform to platform (e.g. mac vs linux) but this guide will cover some universal usage.

The simplest way to use tcpdump is to do an unfiltered packet capture - no filters on packets, so everything is captured.

The bare minimum you'll have to specify is a network interface. You may want to specify a file, too.

The -i and -w flags

To specify a network device you want to listen to, use the -i flag (for interface). 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 monitor multiple interfaces by specifying a list: -i en0,en1

If you are using wireless, you'll need to use additional commands to control the channel your wireless card is listening to.

Controlling Output

To control output, you can have tcpdump create a new pcap file every N seconds, or every N megabytes.

G flag

Use the G flag to create a new pcap file every N seconds:

-G [seconds]

If you use the G flag without the C flag (see below), you specify new filenames with strftime date/time format when you pass the filename to the -w flag. Like this:

tcpdump -G 100 -w filename_%H-%M-%S.pcap

C flag

The C flag sets the maximum pcap file size, in millions of bytes. New files will have a common name with an incrementing number at the end. From the man page:

-C

Before writing a raw packet to a savefile, check whether the file is currently larger than file_size and, if so, close the current savefile and open a new one. Savefiles after the first savefile will have the name specified with the -w flag, with a number after it, starting at 1 and continuing upward. The units of file_size are millions of bytes (1,000,000 bytes, not 1,048,576 bytes).

W flag

-W     Used  in  conjunction  with the -C option, this will limit the
          number of files created to the  specified  number,  and  begin
          overwriting  files from the beginning, thus creating a 'rotat-
          ing' buffer.  In addition, it will name the files with  enough
          leading  0s  to  support the maximum number of files, allowing
          them to sort correctly.

          Used in conjunction with the -G option, this  will  limit  the
          number  of  rotated  dump files that get created, exiting with
          status 0 when reaching the limit. If used with -C as well, the
          behavior will result in cyclical files per timeslice.

Wireless Tcpdump

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 capital I flag -I with tcpdump.

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

You can also use the -n flag to make things more readable.

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

More information over at Unix Stack Exhange.

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. Once airodump is channel-hopping, you can run tcpdump.

Alternatively, you can set airodump to listen on a single channel.

Setting Channel from Shell Script

Another way to set the channel of the wireless card uses iwconfig, which means you can control the channel from a shell script:

iwconfig en1 channel 3

You can also use iwlist to view available channels and see the current channel of the wireless card:

root@kali:~# iwlist frequency
wlan0     14 channels in total; available frequencies :
          Channel 01 : 2.412 GHz
          Channel 02 : 2.417 GHz
          Channel 03 : 2.422 GHz
          Channel 04 : 2.427 GHz
          Channel 05 : 2.432 GHz
          Channel 06 : 2.437 GHz
          Channel 07 : 2.442 GHz
          Channel 08 : 2.447 GHz
          Channel 09 : 2.452 GHz
          Channel 10 : 2.457 GHz
          Channel 11 : 2.462 GHz
          Channel 12 : 2.467 GHz
          Channel 13 : 2.472 GHz
          Channel 14 : 2.484 GHz
lo        no frequency information.

eth0      no frequency information.

Faster Packet Capture

To minimize overhead processing packets and maximize the number of packets captured, you can turn off host name resolution with the -n flag. This also makes things slightly more readable.

tcpdump -I -n -i wlan1 -w output_pcap_file.pcap

Further, if you're running tcpdump for a long period of time, you can use the -G flag to create a new .pcap file every N seconds (e.g., 3600 seconds or 1 file hourly)

tcpdump -G 3600 -I -n -i wlan1 -w output_pcap_file_%H.pcap

Link with more info:

http://stackoverflow.com/questions/16084699/scapy-how-to-get-the-statistics

Analysis

You can also use tcpdump to analyze a pcap file.

Counting Packets

$ tcpdump -nn -r output.pcap | wc -l 

This will give you a count of the total number of packets in the pcap file.

Parsing Information

You can parse information by column using the cut utility.

$ tcpdump -n -r output.pcap

The output has the fields:

[timestamp] [network protocol] [source IP] . [source port] > [destination IP] . [destination port]

[1]