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Jeremy Jameson - Founder and Principal



Fiddler + WPAD - VPN = SlowPerformance

This post originally appeared on my MSDN blog:

Since I no longer work for Microsoft, I have copied it here in case that blog ever goes away.

I needed to look at some low-level HTTP traffic this morning, so I fired up Fiddler -- my tool of choice for this kind of thing. Unfortunately, I found that as soon as I enabled Fiddler, my browsing experience slowed to a crawl. Page requests that previously completed in 1-2 seconds were subsequently taking 30-40 seconds. Ugh!

Since I typically work inside a virtual machine running Windows Server 2003, I thought that maybe I was hitting a problem specific to Fiddler on Vista. I then did a quick Windows Live Search using the terms Fidder Vista slow and quickly found a couple of people -- apparently including Eric Lawrence, himself -- referring to disabling IPv6 within Fiddler when using Vista. I assumed that this must be the problem that I was encountering, but no matter what I tried (e.g. restarting Fiddler after disabling the option, repeatedly enabling and disabling IPv6, etc.) I couldn't get the performance to improve.

I then fired up Network Monitor 3.1 and performed a quick trace while I requested the page through Fiddler again. [By the way, I absolutely love the new filtering capabilities in NetMon 3.1 versus the old version. The interface is much more intuitive than the old 2.x version and it couldn't be any easier than right-clicking a frame in the capture and selecting Add Cell to Display Filter. NetMon Team, you guys rock! I haven't quite got around to downloading the 3.2 beta, but hopefully someday soon.]

Also note that I had to right-click Microsoft Network Monitor 3.1 and click Run as administrator in order to avoid the dreaded "Unknown Error" that occurs when no networks are detected to capture.

So, a couple of minutes later, sure enough, there it was right in my capture:

DNS: QueryId = 0x18FC, QUERY (Standard query), Query for of type Host Addr on class Internet

As soon as I saw the old "Web Proxy Auto Detect" I immediately became suspicious that this was the culprit. I then closed Fiddler and modified my Internet Explorer options to clear the Automatically detect settings checkbox.

Shazam! Problem solved.

Of course, had I actually been connected to CorpNet (which would have enabled DNS requests for "" to resolve), I suppose I wouldn't have encountered the problem in the first place.

Perhaps it is the fact that I am frequently connecting to different networks (e.g. wireless, VPN, etc.) -- whereas many others don't -- but I'd really like to think most people don't encounter little gotchas like this. The mere thought of having to guide my mother through capturing a NetMon trace is, quite frankly, horrifying ;-)


  1. # Always Enable Disk-Based Caching in MOSS 2007

    March 26, 2009 8:37 PM

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