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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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