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Wireshark-users: Re: [Wireshark-users] Hex Offset Needed

From: "Sheahan, John" <John.Sheahan@xxxxxxxxxxxxx>
Date: Tue, 2 Mar 2010 07:09:23 -0500

Good points Martin.


You’re right about there being no HTTP response code of 0.


The software that the web guys use to analyze the front end web traffic will put a “0” in if it finds a packet that has an http accept and for some reason the HTTP response code is missing or unreadable and these are the packets that I’m trying to capture however there is so much HTTP traffic on the web segment that my buffer fills up in seconds so I need to try and narrow it down with a filter.


The only things I have to go by are:


1.       Sometimes the HTTP Response code can’t be read.

2.       The problem seems to come from Safari browsers on MAC machines


Since the User Agent data comes after a variable length Accept field as you point out, wouldn’t be easier for me at this point to filter on just Accept messages? I think if I do it this way, it will take a good amount of time to fill up the buffer and I can look to the web admins to tell me when they see the error in their logs and match it up that way?


Thanks for the help




From: wireshark-users-bounces@xxxxxxxxxxxxx [mailto:wireshark-users-bounces@xxxxxxxxxxxxx] On Behalf Of Martin Visser
Sent: Monday, March 01, 2010 9:53 PM
To: Community support list for Wireshark
Subject: Re: [Wireshark-users] Hex Offset Needed




This is a bit tricky. Firstly I don't believe that there is a HTTP response code (or status code) with a value of "0"



Also the HTTP "User-Agent" is going to go out in the request, and is not seen in the response. So whatever you do needs to be "stateful" knowing that the response is associated with a particular requests.


Also I don't think there is a guarantee and on the "offset" in a packet where the response code will be and almost certainly not for the "User-Agent"  string as it usually preceded by the "Accept" string which is quite variable amongst browsers. 


However you can use the Wireshark "Packet Bytes" pane (usually at the bottom of the window) to see if you cand devise something that is a "good enough" filter to limit what you capture and then refine it further with Wireshark to do it properly.



Regards, Martin


On Tue, Mar 2, 2010 at 11:36 AM, Sheahan, John <John.Sheahan@xxxxxxxxxxxxx> wrote:

Another way for me  to track this problem down is for me to sniff all Safari browsers on MAC’s using HTTP coming into our webservers.


I will need to create a filter using the offset values for:


HTTP_USER_AGENT=Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10_4_11; en)


Can anyone help me this together?








From: wireshark-users-bounces@xxxxxxxxxxxxx [mailto:wireshark-users-bounces@xxxxxxxxxxxxx] On Behalf Of Sheahan, John
Sent: Monday, March 01, 2010 5:38 PM
To: 'Community support list for Wireshark'
Subject: [Wireshark-users] Hex Offset Needed


I am trying to troubleshoot an HTTP problem where the StatusCode=0 in the HTTP header.

I need to capture packets containing this parameter but since I am doing it on a Netscout probe, I have no way to figure out the offset of this in a packet.


Can anyone tell me what hex offset I would need to put in as a filter to capture these packets?





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