Self-Coded Proxy cannot retrieve image from wikipedia - c#-4.0

Question

I'm trying to write a small proxy server in c#. It is working nicely for many webpages I tested (including google.com and microsoft.com). For testing I started my proxy server and configured IE 10 on Windows 8 to use it.
But when I try wikipedia.org it does only load the main page but no pictures. I tried to load a single picture (http://upload.wikimedia.org/wikipedia/commons/6/63/Wikipedia-logo.png). When I use IE without proxy it works, but with the proxy I get a 404 response.
This is the GET Request which IE (my proxy just forwards it) issues:
GET http://upload.wikimedia.org/wikipedia/commons/6/63/Wikipedia-logo.png HTTP/1.1
Accept: text/html, application/xhtml+xml, */*\
Accept-Language: de-CH\
User-Agent: Mozilla/5.0 (compatible; MSIE 10.0; Windows NT 6.2; WOW64; Trident/6.0)
Accept-Encoding: gzip, deflate
Host: upload.wikimedia.org
DNT: 1
Proxy-Connection: Keep-Alive
IMHO it looks correct. This is the response I get (omited some html tags):
HTTP/1.1 404 Not Found
Content-Type: text/html; charset=UTF-8
X-Varnish: 1427845074 1427806476, 274786836, 3671934588
Via: 1.1 varnish, 1.1 varnish, 1.1 varnish
Content-Length: 262
Accept-Ranges: bytes
Date: Mon, 01 Jul 2013 21:30:54 GMT
Age: 28
Connection: keep-alive
X-Cache: cp1063 hit (1), cp3004 miss (0), cp3003 frontend miss (0)
Access-Control-Allow-Origin: *
...404 Not Found\n The resource could not be found.\nRegexp failed to match URI: "http:/upload.wikimedia.org/wikipedia/commons/6/63/Wikipedia-logo.png"
The strange part is here:
Regexp failed to match URI: "http:/upload.wikimedia.org/wikipedia/commons/6/63/Wikipedia-logo.png"
-> the URL starts with a http:/
In the code I connect to uploads.wikimedia.org like this:
// connect to uploads.wikimedia.org
ServerSocket.Connect(RemoteHost, 80);
byte[] SendBuffer = Request.ToArray();
// send the clients request to the server
ServerSocket.Send(SendBuffer);
I have no idea why it doesn't work. Any help is appreciated. My full code is located on Github: Proxy_C_Sharp

Answer 1

I just found out why.
According to the HTTP/1.1 specification (http://www.w3.org/Protocols/rfc2616/rfc2616-sec5.html#sec5) in Chapter 5.2.1:
"To allow for transition to absoluteURIs in all requests in future versions of HTTP, all HTTP/1.1 servers MUST accept the absoluteURI form in requests, even though HTTP/1.1 clients will only generate them in requests to proxies."
I tried it out with a small tool. if I make a request like this:
GET /wikipedia/commons/6/63/Wikipedia-logo.png HTTP/1.1
Host: upload.wikimedia.org
It works. So the reason is that Wikipedia is not conform to the standard. It should accept absolute urls. But it works if I visit the site without a proxy because the browser uses absolute URIs only with proxies. If there is no proxy configured it uses a relative one.