WinDbg symbol proxies

One of the cool things included with WinDbg is an ISAPI DLL that allows you to host what is called a “symbol proxy” on any Windows Server 2003 computer running IIS.

What is a symbol proxy, you might ask, and why would I want one?

A symbol proxy allows you to serve up multiple symbol paths using a single HTTP symbol server path.  Additionally, the symbol proxy will cache everything that it downloads locally, so that the next time you request the same symbol, it won’t need to hit the Internet (or a slow WAN link) to download the symbols again.  You can configure one symbol proxy to cover your internal symbols as well as the public Microsoft symbols available at the Microsoft symbol server.  This can be really handy if you are debugging in many different VMs and don’t want to wait for every VM to download a bunch of MS symbols over the Internet each time you use one – instead, they’ll be locally cached at your site and will come up much faster.  Additionally, you won’t have to set a multitude of symbol paths to talk to the MS symbol server and your internal one, as you can have the symbol proxy talk to both “behind the scenes”.

There is some documentation relating to setting up the symbol proxy included with the DTW distribution.  I would highly recommend taking a look and considering setting up a symbol proxy yourself if you do Windows debugging, as it can significantly reduce the hassle of finding the right symbols for both MS binaries and your binaries.

N.B. There is a known problem with the previously current version ( of the symbol proxy DLL included with the DTW distribution that causes it to be unable to talk to HTTP-based symbol repositories if you do not configure a WinHttp HTTP proxy with proxycfg.  You might send a mail to the WinDbg feedback alias (windbgfb at and ask for a version of symproxy.dll that has this bug fixed if you cannot use a WinHttp-compatible HTTP proxy).  [Note: I have not tested to see if this bug is fixed in the release that came out two days ago.  It should be fixed, however, from what I know.]

One Response to “WinDbg symbol proxies”

  1. […] One small snag here was that I happened to be running a symbol proxy in native x64 mode on this system already. Since the 32-bit vs 64-bit IIS worker process flag is an all-or-nothing option, I had to go install the 32-bit WinDbg distribution on this system and copy over the 32-bit symproxy.dll and symsrv.dll into %systemroot%system32inetsrv. Additionally, the registry settings used by the 64-bit symproxy weren’t directly accessible to the 32-bit version (due to a compatiblity feature in 64-bit versions of Windows known as Registry Reflection), so I had to manually copy over the registry settings describing which symbol paths the symbol proxy used to the Wow64 version of HKLMSoftware. No big deal so far, just a minor annoyance. […]

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