tcpdump, how to capture actual data only？

Question

For example I have a server listening on port 8001, a client programe open a tcp socket connect to that port, send some binary data, I want to capture the actual data only without any TCP/IP header such as tcp handshake stuff..

Is that possible with tcpdump?

Answer 1

You should run tcpflow.

https://linux.die.net/man/1/tcpflow

$ sudo tcpflow port 8001

Answer 2

You can obtain something similar using tcpdump -i any <your_filter> -A. From man tcpdump:

-A Print each packet (minus its link level header) in ASCII. Handy for capturing web pages.

To obtain a more focused dump (ie: remove some handshake packet) you can play with the filter rules, for example removing tcp-syn packet from filtering.

Answer 3

Yes, you can capture only TCP PUSH messages by using the following command:

sudo tcpdump "tcp[tcpflags] & (tcp-push) != 0" 

Answer 4

Nope, the tcpdump haven't features to exctract payload (cut off the headers). Also the aren't any built-in features about deep analysis of application layer data. But you can write the traffic into the file, and extract the actual data with wireshark features.