Threat Vector : Capture packets in Ascii and grep on traffic

There is so many different features to tcpdump for packet captures which can be used in many situations. Recently I wanted to capture traffic and grep on a certain string in the packet.

Installation

On Linux Mint or Ubuntu :

$ sudo apt-get -y install tcpdump

Example

To capture all http web traffic (port 80) containing 'www.wikipedia.org'

$ sudo tcpdump -i eth0 -A -nn -vvv -s 0 port 80 | grep -e 'www.wikipedia.org'

Either open a browser or use 'wget'

$ wget http://www.wikipedia.org

The capture will show for example

$ sudo tcpdump -i eth0 -A -nn -vvv -s 0 port 80 | grep -e 'www.wikipedia.org'
tcpdump: listening on eth0, link-type EN10MB (Ethernet), capture size 65535 bytes
Host: www.wikipedia.org
<body id="www-wikipedia-org">
<form class="search-form" action="//www.wikipedia.org/search-redirect.php">

It can be more useful to direct to a file :

$ sudo tcpdump -i eth0 -A -nn -vvv -s 0 port 80 | grep -e 'www.wikipedia.org' > $HOME/tmp/wikipedia.tmp

Note on some Linux systems you may need to direct all ASCII traffic to a file as the redirect after the grep may not work

$ sudo tcpdump -l -i eth0 -A -nn -vvv -s 0 port 80 > $HOME/tmp/wikipedia.tmp

To sort by uniq values and count the amount of occurrences in columns

$ grep "wikipedia.org" wikipedia.tmp | grep href | sort | uniq -c | sort -nr | head -n 2

1 <span lang="no">Norsk (<a href="//no.wikipedia.org/" lang="nb">bokm..l</a> ... <a href="//nn.wikipedia.org/" lang="nn">nynorsk</a>)</span> ...
1 ..&..O.. href="//chy.wikipedia.org/" lang="chy">Ts..hesen..stsestotse</a> ...

Threat Vector

Sunday 13 April 2014

Capture packets in Ascii and grep on traffic

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