TCP Dump commands and basic tutorial

After installing, check if tcpdump is installed successfully executing the below command #>which tcpdump Should return the binary location of the package installed /usr/sbin/tcpdump After confirming you can run tcpdump with various options passing to the command to check for the options [root@ashwin ~]# tcpdump –help tcpdump version 3.9.8 libpcap version 0.9.8 Usage: tcpdump [-aAdDeflLnNOpqRStuUvxX] [-c count] [ -C file_size ] [ -E algo:secret ] [ -F file ] [ -i interface ] [ -M secret ] [ -r file ] [ -s snaplen ] [ -T type ] [ -w file ] [ -W filecount ] [ -y datalinktype ] [ -Z user ] [ expression ] We will be capturing packets for 2 minutes and analyze for any issues with hosts connecting to and from. Running the below command will capture all the packets transferring between the hosts through the Network Interface Cards and will be redirected to a file tcpdump -n -i bond0 -e -vvv > output.log   Tcpdump commands So back to tcpdump, to look at for example web traffic Always remember that if you want to see the traffic as ASCII, just apply the argument ‘-A’ to tcpdump I am assuming you are using eth0, -n turns off DNS. tcpdump -i eth0 -n port 80 Now a little more fancy, using egrep – this will show all your web requests in real time! tcpdump -i eth0 -A -n port 80 | egrep -i \(GET.\/\|POST.\/\|Host:\) Did you know you can tcpdump for a subnet by just excluding the last octet? tcpdump -i eth0 -n port 80 and host 10.0.5 You can see I used ‘and’ here to specify more filter, you can also use or For example port 80 or port 81 If you forgot your pop3 password, but have it stored in the client tcpdump -i eth0 -n port 110 -A | egrep -i \(user\|pass\) This also applies to passwords for the web, I have used this a lot instead of the ‘forgot password’ mechanism. http://www.alexonlinux.com/tcpdump-for-dummies

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