I'm trying to get the QGIS OpenLayers plug-in to work from behind a proxy.
I've set up all my proxy details and this has had the effect of allowing QGIS's Manage & Install Plug-Ins tool to work with no problem. One oddity here is that although my log-in details are stored in Settings->Options->Network->Use Proxy for Web Access, QGIS asks me for my details again when I access Manage & Install Plug-Ins.
The problem is that when I try to add an OpenLayers layer, I just get blank tiles returned or in some cases white tiles with a blue box containing a question mark. I've spoken to our IT department who have monitored what happens when I try to add an OpenLayers layer and they say that the issue appears to be that my credentials aren't being passed to the proxy, it sees it as an anonymous request.
We've tried HttpProxy and DefaultProxy settings with no luck.
I was wondering if anyone had any suggestions how to resolve this?
Below is an example of some of the details that our proxy is returning:
Forefront TMG requires authorization to fulfill the request. Access to the Web Proxy filter is denied.
Client Username: anonymous
URL: http://tile.openstreetmap.org/2/3/2.png
Action: Denied Connection
UPDATE
As advised, I've installed fiddler2. This gives me some unusual results!
When running QGIS and fiddler2 together, with QGIS set up to use the same proxy settings as my browser, Fiddler2 doesn't 'see' any activity coming from QGIS. Therefore I change QGIS proxy settings to point at localhost, which I understand means Fiddler2 acts as a proxy.
Immediately Fiddler2 is able to identify QGIS traffic and OpenLayers begins to work partially. OpenStreetMap layers still fail, Stamen and Google maps load some tiles but not all (Google returns 'Sorry, we have no imagery here' messages for missing tiles).
I'm going to have a dig around in the Fiddler2 logs to see if I can spot what's causing the problem.
Best Answer
We are also behind a Forefront TMG, and I use Cntlm ( http://cntlm.sourceforge.net/ ) as a local proxy. So QGis connects to my local CNTLM on localhost without a password, cntlm is configured with the login credentials for the forefront proxy and connects to that. This is an ugly hack, but works for me.