The general syntax for the tcpdump command is as follows:
tcpdump [options] [expression]
- The command options allow you to control the behavior of the command.
- The filter expression defines which packets will be captured.
Use the -D option to print a list of all available network interfaces that tcpdump can collect packets from:
sudo tcpdump -D
For each interface, the command prints the interface name, a short description, and an associated index (number)
To specify the interface on which you want to capture traffic, invoke the command with the -i option followed by the interface name or the associated index. For example, to capture all packets from all interfaces, you would specify the any interface:
sudo tcpdump -i any
By default, tcpdump performs reverse DNS resolution on IP addresses and translates port numbers into names. Use the -n option to disable the translation:
sudo tcpdump -n
Instead of displaying the output on the screen, you can redirect it to a file. Two options and its important you use the correct one depending on how you plan on reading the output.
OPTION 1: text file
This is great if you just want what would be displayed on the screen to be captured in a text file. NOTE: this will more then likely not be readable by any of the software packages designed to analyze captures like the very popular Wireshark
sudo tcpdump -n -i any > file.out
You can also watch the data while saving to a file using the tee command:
sudo tcpdump -n -l | tee file.out
The -l option in the command above tells tcpdump to make the output line buffered. When this option is not used, the output will not be written on the screen when a new line is generated.
OPTION 2: binary file
This is the way you want to go if you plan on sending to someone or even yourself to analyze the capture in a tool such as Wireshark.
sudo tcpdump -w <filename>
sudo tcpdump -n -i any -w file.pcap
or a more intense version of the command
tcpdump -s0 -nnnvi 0.0:nnnp -vw /var/tmp/appname_$(date +%d_%b_%H_%M_%S)_$HOSTNAME.pcap host 10.47.78.103
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