Match packets only to/from a particular host, in this case 10.x.x.x.( NOTE: Neither tcpdump itself nor pcap-filter refers to this operator as the slice operator, but wireshark-filter does, so I do as well.) So the filter should: The main idea is to use the slice operator, (see the pcap-filter man page) to compare various bytes of the TCP payload to specific values. OK, but whether you decide to apply a snaplen or not, if you want to filter based on the specific topic name, most likely you can achieve this however there are a couple of caveats that I listed below. This is done using the -s option, and it's the same option for either capture tool. Now, if you want to reduce the size of the capture file, or the number of packets that you see, then you should be able to modify the tcpdump or tshark command-line arguments to accomplish that.įirst off, if you don't need the entire payload, you can apply a snaplen to cut the packets short after some appropriate value. The location of the temporary file varies depending on the platform that tshark is run on, but you should be able to easily locate the directory by running tshark -G folders | grep "^Temp". In the case of tshark, packets are written to a temporary file, which will continue to grow until the capture session is terminated and then ideally it will be deleted, but not always. Just because a capture file name wasn't specified doesn't mean that packets aren't being written to a file they are. This means that you're capturing the same amount of data as you were before. If you're trying to limit the size of the capture file, then the previously accepted answer isn't doing that because it uses the exact same capture filter as was originally provided, namely src 10.x.x.x. The original question stated, "But it results in very big file within minutes, Can i filter tcpdump on base of topic name" I don't think the previously accepted answer necessarily does what you think it does and possibly not even what you want it to do. I am capturing MQTT traffic for troubleshooting using below command tcpdump -i team0 -w mqtt-trace.pcap src 10.x.x.xīut it results in very big file within minutes, Can i filter tcpdump on base of topic nameįollowing is tcp payload, I want it only capture payload which has PKGCTRL/1/status/frequency or if tcpdump can directly support filter on application layer protocol like wireshark mqtt.topic = PKGCTRL/1/status/frequency 0000 00 13 95 36 2e ef 00 10 7e 07 87 3d 08 00 45 00.
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