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:
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 X-Powered-By: ASP.NET X-CCC: IT X-CID: 2