Capture HTTP GET requests

Question

In the Wireshark wiki is an example for filtering HTTP GET requests:

Capture HTTP GET requests. This looks for the bytes 'G', 'E', 'T', and ' ' (hex values 47, 45, 54, and 20) just after the TCP header. "tcp[12:1] & 0xf0) >> 2" figures out the TCP header length. From Jefferson Ogata via the tcpdump-workers mailing list.

with this filter:

port 80 and tcp[((tcp[12:1] & 0xf0) >> 2):4] = 0x47455420

Unfortunately this does not work. How is the correct filter for HTTP GET requests?

Answer 1

It does work, make sure you are surrounding your filter in double quotes so the shell doesn't try and parse the filter arguments.

e.g. a curl of google.com for me:

$ sudo tshark -i eth0 "port 80 and tcp[((tcp[12:1] & 0xf0) >> 2):4] = 0x47455420"
Running as user "root" and group "root". This could be dangerous.
Capturing on eth0
  0.000000   10.53.0.66 -> 209.85.143.104 HTTP GET / HTTP/1.1 

This is a rather complicated way of doing it though. tshark does allow you the concept of applying read filters. Now these may not be as useful if you've got a large volume of data (filtering happens after capturing) but they're certainly more intuitive and readable.

$ sudo tshark -i eth0 -R 'http.request.method == "GET"' "port 80"
Running as user "root" and group "root". This could be dangerous.
Capturing on eth0
  5.641015   10.53.0.66 -> 209.85.143.104 HTTP GET / HTTP/1.1