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CONTINUATION WIRESHARK HTTP TCP INSTALLExecute ls -la my_capture.pcap to verify that the file has been created.ĭownload the file to your local machine by executing scp ~/Desktop (on Mac or Linux, replace with something equivalent or use a GUI on Windows).ĭownload and install Wireshark and open the file you just downloaded. Then stop the tcpdump recording with ctrl-c. This does almost the same as the previous command, but writes the captured packages to a file named my_capture.pcap. Then execute the following command: sudo tcpdump port 80 -w my_capture.pcap. If your previous capture is still running, cancel it with ctrl-c. Most EC2 instances run a shell-only version of Linux, which is why we’re capturing data with Tcpdump. CONTINUATION WIRESHARK HTTP TCP WINDOWSHowever, it comes with a GUI so you need a Windows or Linux desktop to run it. Note: Wireshark is perfectly capable of capturing packets itself. CONTINUATION WIRESHARK HTTP TCP DOWNLOADInstead we will dump the output to a PCAP file and download that to our local machine. To make the output of Tcpdump more usable, we will not use stdout to analyse the traffic. If the output would be much more verbose (which it easily could!) it would very quickly become impossible to find relevant info. The point of this part of the exercise is to show you that Tcpdump can very easily show you some usable information, but it’s hard to drill down into the details. This can help to quickly localize issues on the network. For example, if a NACL would be blocking return traffic we would see outbound traffic, but no responses from the remote server. The most important takeaway from the output above is that bidirectional communication is functioning correctly. Not entirely coincidently, these are the two first steps of the TCP Three-Way Handshake which we’ll talk more about later. 209.85.203.113.80 > 10.0.0.179.40364: Flags means remote IP address is responding on the same ports, and the SYN and ACK flags are set.As soon as you press enter, the other screen should explode with information.Įxperienced engineers will be able to tell a lot from this output. While the tcpdump command is running in one session, run curl -I in the other. ![]() For more expression options, see the PCAP filter manpage. In this case, only traffic to and from port 80.įor more tcpdump options, see the Tcpdump manpage.
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