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WIRESHARK CAPTURE FILTER PROBLEM

Posted on 2009-02-23
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Why can't I get my Wireshark capture filter to return only HTTP GET requests?  I've reviewed many web pages with ideas, but they don't work, I can nail it down to just tcp on port 80 with the http get, but can't filter out those tcps.

I've checked the following websites and successfully implemented other capture filters, just not what I need!  Here's a couple out of the many resources I looked at.

http://openmaniak.com/wireshark_filters.php
http://74.125.47.132/search?q=cache:qPTCArYYJcoJ:wiki.wireshark.org/CaptureFilters+%22capture+http+filter+syntax%22&hl=en&gl=us&strip=1

Anyone help?
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Question by:Gillat
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by:rowansmith
ID: 23718513
Are you trying to build a capture filter - which applies on the interface when you are listening to packets, or a display filter - which is applied to the packet capture after you have captured the packets?
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by:Gillat
ID: 23718539
A capture filter.  I can do this with a display filter, but want to do it with the capture filter.  Thanks.
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Accepted Solution

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rowansmith earned 500 total points
ID: 23718553
Applying the following to the Capture Filter of the interface will only capture packets which are HTTP GET Requests:

tcp[20:4] = 0x47455420

By Capture Filter I mean: Capture -> Interfaces -> Options -> "Enter the above in the Capture Filter TextBox" -> Start

This is not a display filter.

NB: This will not capture packets where the TCP session is established using TCP-Options - such as window scaling as this increases the offset in the packet.

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Author Comment

by:Gillat
ID: 23718563
That worked!  Thank you so much.  I think I understand how you arrived at the tcp [20:4], but what are the figures following the =?
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Expert Comment

by:rowansmith
ID: 23718586
0x47455420 = "GET "

including the space, you have to match on 4 byte boundaries, would be a real pain if the GET was acutally "GP" for Get Page :-) lucky they thought of that when developing the HTTP Protocol :-) NOT!!!  I guess some things are just meant to be.

0x47 = G
0x45 = E
0x54 = T
0x20 = SPACE



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Author Comment

by:Gillat
ID: 23718594
Again, thanks for explaining - I now get it that its a character string.  Appreciate your help.  Have a good one!
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