When a Win XP box tries to get updates from 'windows update' via apache
as proxy, we get requests like this in the access log:

192.168.164.219 - - [27/Sep/2004:15:17:11 +0200] "HEAD http://download.windowsupdate.com/msdownload/update/v5/redir/wuredir.cab?0409271318 HTTP/1.0" 200 - "-" "Windows Update Control"
192.168.164.219 - - [27/Sep/2004:15:17:11 +0200] "HEAD http://download.microsoft.com/windowsupdate/v5/redir/wuredir.cab?0409271318 HTTP/1.0" 200 - "-" "Windows Update Control"
192.168.164.219 - - [27/Sep/2004:15:17:13 +0200] "HEAD http://windowsupdate.microsoft.com/v5/redir/wuredir.cab?0409271318 HTTP/1.0" 200 - "-" "Windows Update Control"

(Each of these lines is repeated four times.)

After that, lines like this appear:

192.168.164.219 - - [27/Sep/2004:15:17:14 +0200] "GET http://v5.windowsupdate.microsoft.com/v5consumer/errorinformation.aspx?error=-2147012746&ln=de HTTP/1.0" 200 6345 "http://v5.windowsupdate.microsoft.com/v5consumer/default.aspx?ln=de" "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1)"

and the user is informed that 'windows update' can't be reached.

If I try to mimic the Win XP box's behaviour, I get the following
(tfkp01l is the proxy server):

$ HEAD -Ue -H "User-Agent: Windows Update Control" "http://download.windowsupdate.com/msdownload/update/v5/redir/wuredir.cab?0409281342"
HEAD http://download.windowsupdate.com/msdownload/update/v5/redir/wuredir.cab?0409281342
User-Agent: Windows Update Control

200 OK
Connection: close
Date: Tue, 28 Sep 2004 13:42:15 GMT
Accept-Ranges: bytes
ETag: "479d6932db2ec41:8037"
Server: Microsoft-IIS/6.0
Content-Length: 7201
Content-Type: application/octet-stream
Last-Modified: Fri, 30 Apr 2004 17:47:25 GMT
Client-Date: Tue, 28 Sep 2004 13:42:15 GMT
Client-Response-Num: 1

$ HEAD -Ue -p http://tfkp01l -H "User-Agent: Windows Update Control" "http://download.windowsupdate.com/msdownload/update/v5/redir/wuredir.cab?0409281342"
HEAD http://download.windowsupdate.com/msdownload/update/v5/redir/wuredir.cab?0409281342
User-Agent: Windows Update Control

200 OK
Connection: close
Date: Tue, 28 Sep 2004 13:42:45 GMT
Accept-Ranges: bytes
ETag: "479d6932db2ec41:8037"
Server: Microsoft-IIS/6.0
Content-Type: application/octet-stream
Last-Modified: Fri, 30 Apr 2004 17:47:25 GMT
Client-Date: Tue, 28 Sep 2004 13:42:45 GMT
Client-Response-Num: 1


The only difference between the direct response and the one via apache
as proxy is the missing C-L header. 

BTW, we are running apache 2.0.49 from SuSE Linux 9.1:

$ rpm -q apache2
apache2-2.0.49-27.14