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Capturing Packets with Tcpdump


Tcpdump is a powerful network debugging tool that can be used for intercepting and displaying packets on a network interface. An important feature of tcpdump is a filter that allows you to display only the packets you want to see.

Installation

In this example we are using Ubuntu 8.10; however, the installation steps are similar for other Linux distributions. The following command will install tcpdump under Ubuntu:

sudo apt-get install tcpdump

Usage

sudo tcpdump [options] [filter expression]

By default, tcpdump will capture packets on eth0. We can specify a different interface using the '-i' command line flag. This command will capture all packets on the eth1 interface:

sudo tcpdump -i eth1

In the following example we will listen to all UDP connections:

sudo tcpdump udp

Use this command to capture packets for a specific port:

sudo tcpdump port 80

Our command is returning all packets which have port 80 as their destination or source port.

Now let's be more specific and capture only packets with destination port 80. If you have a web server on your cloud, then you can use the command below to see incoming packets.

sudo tcpdump dst port 80

You can also capture packets for a specific host. This command will catch packets coming only from IP 1.2.3.4:

sudo tcpdump src host 1.2.3.4

Tcpdump can take logical arguments such as 'and,' as well as 'or.' You can use logical statements in a tcpdump command. For example, to catch all the SSH packets going from an SSH server to a client with IP 1.2.3.4:

sudo  tcpdump "src port 22" and "dst host 1.2.3.4"

Raw packets can be conveniently saved to a file using the '-w' option:

tcpdump host 1.2.3.4 -w /home/users/demo/demo.dump

Let's read the saved file:

tcpdump -r /home/users/demo/demo.dump

Summary

Tcpdump is a powerful packet sniffer and a common tool used by system administrators to solve network problems and investigate traffic. It can be used with Boolean expressions to capture only those packets you're interested.







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