Learn how to use tcpdump command with examples

Tcpdump command is a famous network packet analysing tool that is used to display TCP\IP & other network packets being transmitted over the network attached to the system on which tcpdump has been installed. Tcpdump uses libpcap library to capture the network packets & is available on almost all Linux/Unix flavors.

Tcpdump command can read the contents from a network interface or from a previously created packet file or we can also write the packets to a file to be used for later. One must use the tcpdump command as root or as a user with sudo privileges.

In this tutorial, we are going to discuss the uses of tcpdump command along with some examples, but first let’s start with installation of tcpdump on various Linux OS.

(Recommended Read: Monitoring network bandwidth with iftop command)

 

Installation

By default, tcpdump is available on almost all Linux distributions but if that’s not the case for you, install it on your system using the following method.

CentOS/RHEL

Install tcpdump on CentOS & RHEL using the following command ,

$ sudo yum install tcpdump

Fedora

On Fedora, install tcpdump using the following command,

$ dnf install tcpdump

Ubuntu/Debian/Linux Mint

On Ubuntu or Debian or Linux Mint, install tcp dumo using the following command,

$ apt-get install tcpdump

Now that we have install tcpdump on our systems, let’s discuss some examples for tcpdump.

 

Examples

Get packets from all interfaces

To get the network packets from all network interfaces, run the following command,

$ tcpdump -i any

 

Get packets from a single interfaces

To get the network packets from a single interface, use

$ tcpdump -i eth0

 

Writing captured packets to file

To write all the captured packets to a file, use the ‘-w’ option,

$ tcpdump -i eth1 -w packets_file

 

Reading an old tcpdump file

To read an already created, old tcpdump file, use the following command,

$ tcpdump -r packets_file

 

Getting more packets information with readable timestamps

To get more information regarding the packets along with readable timestamp, use

$ tcpdump -ttttnnvvS

 

Check packets of whole network

To get the packets for whole network, execute the following command from terminal

$ tcpdump net 192.168.1.0/24

 

Check packets based on IP address

Get all the packets based on the IP address, whether source or destination or both, using the following command,

$ tcpdump host 192.168.1.100

To get packets based on source or destination of an IP address, use

$ tcpdump src 192.168.1.100

$ tcpdump dst 192.168.1.100

 

Check packets for a protocol or port number

To check all the packets used based on the protocol, run the following command

$ tcpdump ssh

To get packets for a single port ot for a range of ports, use

$ tcpdump port 22

$ tcpdump portrange 22-125

We can also use ‘src’ & ‘dst’ options to get packets for ports based on source & destination.

We can also combine two conditions with AND (and , && ), OR ( or. || ) & EXCEPT (not , ! ). This helps when we have analyze network packets based on the some condtions.

 

Using AND
We can use ‘and’ or symbol ‘&&’ to combine two conditions or mote with tcpdump. An example would be,

$ tcpdump src 192.168.1.100 && port 22 -w ssh_packets

 

Using OR

OR will check the command agtcpdump -i eth0 src port not 22ainst one the mentioned conditions in the command, like

$ tcpdump src 192.168.1.100 or dst 192.168.1.50 && port 22 -w ssh_packets

$ tcpdump port 443 or 80 -w http_packets

 

Using EXCEPT

EXCEPT will be used when we want not fulfill a condition, like

$ tcpdump -i eth0 src port not 22

This will monitor all the traffic on eth0 but will not capture port 22.

This was our tutorial on how to install & use tcpdump command to capture the network packets. Please feel free to send in any queries or suggestions using the comment box below.

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Shusain

Passionate about Linux & open source. Loves to learn, read & write about Linux as well as new technologies.

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