TCPDUMP - A MUST !

tcpdump is one of the most powerful linux/unix commands that helps us to analyze the network packet mainly for troubleshooting purpose. It mainly captures packets received or transferred over a network on a specific interface .

tcpdump is mainly used on linux/unix distro and can be easily installed.

INSTALLING TCPDUMP

$ sudo apt-get install tcpdump [On Debian, Ubuntu and Mint]

$ sudo yum install tcpdump [On RHEL/CentOS/Fedora and Rocky Linux/AlmaLinux]

$ sudo emerge -a sys-apps/tcpdump [On Gentoo Linux]

$ sudo pacman -S tcpdump [On Arch Linux]

$ sudo zypper install tcpdump [On OpenSUSE]

TCPDUMP COMMANDS

1. CAPTURE HOST

# tcpdump -i < interface name > host <host ip >

# tcpdump -i eth0 host 10.1.1.1

2. NO DOMAIN LOOKUP

# tcpdump -i eth0 -n host 8.8.8.8

3. NO DOMAIN AND PROTOCOL LOOKUP

# tcpdump -i eth0 -nn host 8.8.8.8

4. CAPTURE SERVICE

# tcpdump -i eth0 -nn port 443

5. CAPTURE HOST AND SERVICE

# tcpdump -i eth0 -nn host 118.91.171.10 and port 80

6. CAPTURE HOST AND PROTOCOL

# tcpdump -i eth0 -nn host 118.91.171.10 and udp

7. NEGATE FILTER

# tcpdump -i eth0 -nn host 118.91.171.10 and not icmp -c 5

8. COMPLEX COMBINATION

# tcpdump -i eth0 -nn "dst 8.8.8.8 or dst 1.1.1.1" and port 443 -c 10

9. CAPTURE MAC ADDRESS

#tcpdump -i eth0 -nn -e host 8.8.8.8 and icmp -c 100

10. VPN TRAFFIC

#tcpdump -i eth0 -nn host 118.91.171.10 and esp -c 50 ( FOR PHASE II )

#tcpdump -i eth0 -nn host 118.91.171.10 and udp and port 500 -c 100 ( IKE PHASE I )

11. VERBOSE ( PROVIDES HEADER LEVEL INFO )

#tcpdump -i eth0 -nn -vv host 118.91.171.10 and tcp -c 100

12. SAVE THE CAPTURE FILES

#tcpdump -i eth0 -nn -vv host 118.91.171.10 and port 80 -c 100 > /var/log/capture.txt

#tcpdump -i eth0 -nn -vv host 118.91.171.10 and port 80 -c 100 > /var/log/capture.cap

13 . CAPTURE NETWORK

#tcpdump -i eth0 -nn net 118.91.171.0/24

Note : Capturing for network can only be for classful network and we cannot define classless network. tcpdump will simply not capture the packet

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