TMG “502 Proxy Error. The data is invalid” while downloading Windows 8.1/2012R2 Update KB2919355

Recently I ran into an odd issue when trying to download the KB2919355 Update bundle from Microsoft. The problem affected WSUS and local Windows Update clients that used TMG as their explicit proxy and was reproducible with browsers as well:
The TMG proxy threw 502 Proxy Error ( The data is invalid.  ) when trying to access various different hosts and URLs serving the same update file such as these:
http://au.v4.download.windowsupdate.com/d/msdownload/update/software/crup/2014/02/windows8.1-kb2919355-x64_66955196a82751d1c8d9806d321487562b159f41.psf
http://fg.v4.download.windowsupdate.com/d/msdownload/update/software/crup/2014/02/windows8.1-kb2919355-x64_66955196a82751d1c8d9806d321487562b159f41.psf
http://wsus.ds.download.windowsupdate.com/d/msdownload/update/software/crup/2014/02/windows8.1-kb2919355-x64_66955196a82751d1c8d9806d321487562b159f41.psf

Strange enough the problem did not occur in transparent proxy mode.

I first suspected the problem was related to the issue described here, but in my case neither HTTP compression nor chunked transfer-encoding are used, thus this article and its explanation do not apply.

First let’s have a look at a normal response (bypassing the Proxy/transparent mode). We can see the file is approximately 3.8GiB in size, quite big but I’ve downloaded larger files without issues. There are no strange HTTP headers or anything sent by the servers (goes for GETs as well) and everything looks just fine:

 $ curl -I 'http://wsus.ds.download.windowsupdate.com/d/msdownload/update/software/crup/2014/02/windows8.1-kb2919355-x64_66955196a82751d1c8d9806d321487562b159f41.psf'
HTTP/1.1 200 OK
Via: 1.1 TMGPROXY01
Connection: Keep-Alive
Proxy-Connection: Keep-Alive
Content-Length: 4052160113
Date: Mon, 13 Oct 2014 11:12:31 GMT
Content-Type: application/octet-stream
ETag: "0cabe2eb931cf1:0"
Server: Microsoft-IIS/7.5
Accept-Ranges: bytes
Last-Modified: Mon, 24 Feb 2014 23:36:04 GMT
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X-CCC: IT
X-CID: 2

Forefront TMG Log Export with MSDEToText.vbs messing up IPs

Logging Firewall or Web Proxy traffic on a Forefront TMG/ISA node into the local SQL Express-based database (which is the default setting) has a few advantages, like being able to query past logs through the TMG console. But sometimes it’s better to have logs stored in a plain text format as well for a 3rd party tool or your own log analysis scripts.

For this purpose, Microsoft provides the MSDEToText.vbs tool to export logs from a TMG/ISA SQL database into text files.

why

However, the MSDEToText script is producing some weird results for my TMG environments, namely it fails to convert the source and destination IP-addresses properly:
For example, what should be exported as “192.168.1.11” ends up as “-63.-87.-254.-245”, with negative numbers per octet in the text log. Notice something? Yeah, subtracting each value from 255 yields us the correct IP (well, almost except for the last octet which is off by 1). This happens only for IPs that don’t have an existing computer object defined in the TMG policy.

There is obviously something wrong with the logic inside the MSDEToText VB script. Being completely clueless about VBS (I can’t even remember ever seriously coding/editing something longer than two lines), I dug into the script to see what makes it go bonkers and found the following function to be responsible: