ASP.NET application exhibits strange behaviour through firewall

This problem has been solved thanks to your suggestions. See the bottom for details. Thanks very much for your help!

Our ASP.NET website is accessed from several specific and highly secure international locations. It has been operating fine, but we have added another client location which is exhibiting very strange behaviour.

In particular, when the user enters search criteria and clicks the search button the result list returns empty. It doesn't even show the '0 results returned' text, so it is as if the Repeater control did not bind at all. Similar behaviour appears in some, but not all, other parts of the site. The user is able to log in to the site fine and their profile information is displayed.

I have logged in to the site locally using exactly the same credentials as them and the site works well from here. We have gone through the steps carefully so I am confident it is not a user issue.

I bind the search results in the Page_Load of the search results page the first time it is loaded (the criteria is in the query string). i.e.

if (!IsPostBack) {

I can replicate exactly the same behaviour locally by commenting out the BindResults() method call.

Does anybody know how the value of IsPostBack is calculated? Is it possible that their highly-secure firewall setup would cause IsPostBack to always return true, even when it is a redirect from another page? That could be a red herring as the problem might be elsewhere. It does exactly replicate the result though.

I have no access to the site, so troubleshooting is restricted to giving them instructions and asking for them to tell me the result.

Thanks for your time!

Appended info: Client is behind a Microsoft ISA 2006 firewall running default rules. The site has been added to the Internet Explorer trusted sites list and tried in FireFox and Google Chrome, all with the same result.

SOLUTION: The winner for me was the suggestion to use Fiddler. What an excellent tool that no web developer should be without. Using this I was able to strip various headers from the request until I reproduced the problem. There were actually two factors that caused this bug, as is so often the case with such confusing issues.

Factor one – Where possible the web application uses GZIP compression as supported by all major browsers. The firewall was stripping off the header that specifies GZIP decompression support (Accept-Encoding: gzip, deflate).

Factor two – A bug in my code meant that some processing was bypassed when the content was being sent uncompressed. This problem was not noticed before because the application is used by a limited audience, all of which supported GZIP decompression.


If they're at all tech-savvy, I would have them download Fiddler or something similar, capture the entire HTTP session, and then send you the saved session. Maybe something in there will stick out.

Meanwhile, see if you can get an install of ISA Server (an evaluation install, if you have to, or one from MSDN if you have or know anyone with a sub) and see if you can replicate it locally.

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