How to specify server IP in HttpWebRequest

Junxiao Shi

2009-01-01

I was writing an HTTP proxy during last weeks. I need to address the origin server with a custom IP address, but I should keep the "Host" header.

I googled about "HttpWebRequest set server IP", "WebRequest modify Host", etc. I read through several posts, and realized:

When HttpWebRequest is created, the "Host" header is automatically set to the "host:port" portion of the Uri.
Attempt to modify HttpWebRequest.Headers["Host"] will throw an exception.
The server IP address of the HTTP Request is determined by the host portion of the Uri, and is automatically resolved against the DNS server.
When HttpWebRequest.Proxy is set to use a proxy, the server IP address is determined by resolving the host of Proxy property. I can set the origin server as the proxy, this is called "Proxy hack".
When using a proxy, the HTTP Request-Line looks like "GET http://host/ HTTP/1.1" rather than "GET / HTTP/1.1". Every HTTP/1.1 compliant origin server should accept this, but firewalls may think it's a "proxy request" and block it, and some non-compliant servers does not accept it.

SOLUTION

My purpose is: make HttpWebRequest send its request to a custom IP address, but leave the Request-Line as "GET / HTTP/1.1".

After hours of Reflector, I found the solution:

HttpWebRequest req = (HttpWebRequest)WebRequest.Create("http://blogs.msdn.com");//url and Host header
FieldInfo field_ServicePoint_ProxyServicePoint = (typeof(ServicePoint))
    .GetField("m_ProxyServicePoint", BindingFlags.NonPublic | BindingFlags.Instance);
req.Proxy = new WebProxy("www.google.com:80");//server IP and port
field_ServicePoint_ProxyServicePoint.SetValue(req.ServicePoint, false);
HttpWebResponse resp = (HttpWebResponse)req.GetResponse();

When the above code is executed, I can see the following request in Wireshark:

GET / HTTP/1.1
Host: blogs.msdn.com
Connection: Keep-Alive

And, the request is sent to 64.233.189.99:80, a www.google.com server.

DOWNLOAD: C# code & Wireshark capture

BEHIND THE SCENES

ServicePoint class provides connection management for HTTP connections. A ServicePoint instance is created for each "host:port" combination, and reused across several requests. ServicePoint.m_ProxyServicePoint determines whether this ServicePoint connects to a proxy. When m_ProxyServicePoint is set to false, HttpWebRequest don't think it's a proxy, so the Request-Line becomes "GET / HTTP/1.1". However, m_ProxyServicePoint is a private property of ServicePoint, so I have to use Reflection to change it.

HttpWebRequest.Host in .Net Framework 4.0

2010-05-31 update: As of .Net Framework 4.0, you can use HttpWebRequest.Host property to set Host header independent from the request URI.

HttpWebRequest req = (HttpWebRequest)WebRequest.Create("http://www.google.com");//url path and actual server
req.Host = "blogs.msdn.com");//Host header
HttpWebResponse resp = (HttpWebResponse)req.GetResponse();

Tags: .Net