Simple HTTP Reverse Proxy with ASP.NET and IIS

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Introduction

A reverse proxy is the same as a proxy except instead of delivering pages for internal users, it delivers them for external users. It can be used to take some load off web servers and provide an additional layer of protection. If you have a content server that has sensitive information that must remain secure, you can set up a proxy outside the firewall as a stand-in for your content server. When outside clients try to access the content server, they are sent to the proxy server instead. The real content resides on your content server, safely inside the firewall. The proxy server resides outside the firewall, and appears to the client to be the content server.

Where to use?

If you have an intranet with IP filtered security, you can offer the functionality of external consultation. You must just add the authentication system of your choice.

Functionality

This reverse proxy can run in two modes:

• Mode 0: all web sites can be requested by clients with an URL like http://RevrseProxyURL/http//www.site.com/
• Mode 1: only one web site can be requested. When the user requests the web application address, the proxy returns the content of this web site.

You can setup this mode in the web configuration file of you web application:

<appSettings>
    <!-- PROXY Mode
  0 : all web site can be requested by a client with 
      an url like http://ReverseProxyURL/http//www.site.com/
  1 : Only on web site can be resuested by clients, \
      In this case Web Application uses RemoteWebSite variable 
      to deliver the content of the web site.
    -->
   <add key="ProxyMode" value="1" />
   <add key="RemoteWebSite" value="http://www.codeproject.com/" />
</appSettings>

The code

Create an HttpHandler to intercept all requests:

using System;
using System.Configuration;
using System.Web;
using System.Net;
using System.Text;
using System.IO; 
namespace ReverseProxy
{
  /// <summary>
  /// Handler that intercept Client's request and deliver the web site
  /// </summary>

  public class ReverseProxy: IHttpHandler
  {
    /// <summary>
    /// Method calls when client request the server
    /// </summary>
    /// &;lt;param name="context">HTTP context for client</param>

    public void ProcessRequest(HttpContext context)
    {
      //read values from configuration 
      fileint proxyMode = 
        Convert.ToInt32(ConfigurationSettings.AppSettings["ProxyMode"]);
      string remoteWebSite = 
        ConfigurationSettings.AppSettings["RemoteWebSite"];
      string remoteUrl;
      if (proxyMode==0)
        remoteUrl= ParseURL(context.Request.Url.AbsoluteUri);
      //all site 
      acceptedelseremoteUrl= 
        context.Request.Url.AbsoluteUri.Replace("http://"+
        context.Request.Url.Host+
        context.Request.ApplicationPath,remoteWebSite);
      //only one site accepted
      //create the web request to get the remote stream
      HttpWebRequest request = 
        (HttpWebRequest)WebRequest.Create(remoteUrl);
      //TODO : you can add your own credentials system
      //request.Credentials = CredentialCache.DefaultCredentials;
      HttpWebResponse response;
      try
      {
        response = (HttpWebResponse)request.GetResponse ();
      }
      catch(System.Net.WebException we)
      {
        //remote url not found, send 404 to client 
        context.Response.StatusCode = 404;
        context.Response.StatusDescription = "Not Found";
        context.Response.Write("<h2>Page not found</h2>");
        context.Response.End();
        return;
      }
      Stream receiveStream = response.GetResponseStream();

      if ((response.ContentType.ToLower().IndexOf("html")>=0) 
        ||(response.ContentType.ToLower().IndexOf("javascript")>=0))
      {
        //this response is HTML Content, so we must parse it
        StreamReader readStream = 
          new StreamReader (receiveStream, Encoding.Default);
        Uri test = new Uri(remoteUrl);
        string content;
        if (proxyMode==0)
          content= ParseHtmlResponse(readStream.ReadToEnd(), 
            context.Request.ApplicationPath+"/http//"+test.Host);
        else
          content= ParseHtmlResponse(readStream.ReadToEnd(),
            context.Request.ApplicationPath);
        //write the updated HTML to the client
        context.Response.Write(content);
        //close streamsreadStream.Close();
        response.Close();
        context.Response.End();
      }
      else
      {
        //the response is not HTML 
        Contentbyte[] buff = new byte[1024];
        int bytes = 0;
        while( ( bytes = receiveStream.Read( buff, 0, 1024 ) ) > 0 )
        {
          //Write the stream directly to the client 
          context.Response.OutputStream.Write (buff, 0, bytes );
        }
        //close streams
        response.Close();
        context.Response.End();
      }
    }

    /// <summary>
    /// Get the remote URL to call
    /// </summary>
    /// <param name="url">URL get by client</param>
    /// <returns>Remote URL to return to the client</returns>

    public string ParseURL(string url)
    {
      if (url.IndexOf("http/")>=0)
      {
        string externalUrl=url.Substring(url.IndexOf("http/"));
        return externalUrl.Replace("http/","http://") ;
      }
      else
        return url;
    }

    /// <summary>
    /// Parse HTML response for update links and images sources
    /// </summary>
    /// <param name="html">HTML response</param>
    /// <param name="appPath">Path of application for replacement</param>
    /// <returns>HTML updated</returns>
    public string ParseHtmlResponse(string html,string appPath)
    {
      html=html.Replace("\"/","\""+appPath+"/");
      html=html.Replace("'/","'"+appPath+"/");
      html=html.Replace("=/","="+appPath+"/");
      return html;
    }
    ///
    /// Specifies whether this instance is reusable by other Http requests
    ///
    public bool IsReusable
    {
      get
      {
        return true;
      }
    }
  }
}

Configure the handler in web.config

You must add these lines in web.config file to redirect all user queries to the HTTPHandler:

<httpHandlers>
   <add verb="*" path="*" type="ReverseProxy.ReverseProxy, ReverseProxy" />
</httpHandlers>

Configure IIS

If you want to process a request with any file extension, then you need to change IIS to pass all requests through to the ASP.NET ISAPI extension. Add the HEAD, GET and POST verbs to all files with .* file extension and map those to the ASP.NET ISAPI extension - aspnet_isapi.dll (in your .NET framework directory). The complete range of mappings, includes the new .* mapping.

TODO's

Now, you can develop your own security system, based on form, Windows or passport authentication.