![]() ![]() The above screenshot shows the data of the above my_capture.pcap file. pcap file for analysis with Wireshark or any other graphical network protocol analyzer. Keep the packet count to a smaller value otherwise, you may have to stop the process manually. sudo tcpdump -i eth0 -c 10 -w my_capture.pcap If you want to store the captured data in a file, you can do it like this. This is because the rule contains the filter for ports 80 and 443, the common networking ports. You can see this rule is filtering http and https traffic. Here, I have merged different filter rules into a single rule. Interestingly, you can apply multiple filters to your host to target a specific type of packet traffic.įor example: sudo tcpdump -i eth1 -c 50 “(host 192.168.56.11) and (port 443 or port 80)" This will capture all the traffic and out from this host. The host argument simply filters the traffic from a specific host using its IP: sudo tcpdump -i eth0 host 192.168.56.10 It is because 6 is the protocol number for TCP. To your surprise, the two commands above are equivalent. Here, you can either use the protocol name or the protocol number as the argument value: sudo tcpdump -i eth0 proto tcp sudo tcpdump -i eth0 proto 6 Similar to the port directive, the proto directive filters the packet capture based on particular traffic. ![]() This command will capture both the TCP and UDP traffic. In case you want to filter traffic based on port number, say port 22, then execute the tcpdump command as: sudo tcpdump -i eth0 port 22 You can do this by filtering the traffic based on host, ports, protocols, and other criteria. This will eliminate unnecessary traffic and simplify your job. It's a good approach to narrow down your captured data for inspection. Let’s take the destination IP as 192.168.56.11 and see the details of the traffic: sudo tcpdump -i eth1 -c 5 dst 192.168.56.11 8. In case you want to inspect the traffic sent to a specific destination IP address, use the command: sudo tcpdump -i eth0 dst Capturing Packets Sent to a Specific Destination IP This will help us to clearly and easily understand the output of a command.ħ. In the following article, I'll add the -c flag with other flags wherever required. If you do not specify a count, the capture operation is to be manually interrupted using the key combination ctrl+c or ctrl+z. The command, in this case, will be: sudo tcpdump -c 4 -i eth0 ![]() The -c flag can be used to preset the number of packets to be captured.Īs an example, let’s set this value to 4 for capturing four packets. So wherever you see the -i flag, it will be accompanied by either the interface eth0 or eth1. Note: From now on, I'll use the eth0 or eth1 as the target interface. For example, in the case of the interface eth0, this command will be as: sudo tcpdump -i eth0 Replace the target-interface with the name of the interface you want to scan. The -i flag captures traffic from a specific interface: tcpdump -i Without using any option, Tcpdump will scan all the interfaces. Capturing Packets for a Specific interface The same functionality can also be gained with the -list-interfaces flag: sudo tcpdump -list-interfaces 2. This will list all the interfaces on the system including wireless and wired interfaces and others. To check all the available interfaces to capture on, use the ‘-D’ flag as: sudo tcpdump -D In this article, I'll focus on core options that are frequently used. Tcpdump has a long list of options available for use. Windows has a ‘Microolap TCPDUMP for Windows’ variant with an associated price tag. ![]() It is freely available on Unix and Linux systems. Thanks to the developers, who have kept the Tcpdump as an open source project. The output from the command displays on the STDOUT and can also be stored in a file. It captures packets as they go by and shows you what’s going on and coming in on your network. Tcpdump is a great tool for analyzing networks and hunting down associated network problems. ![]()
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