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Simple HTTP Reverse Proxy with ASP.NET and IIS

, 22 May 2004
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Learn how easy it is to create HTTP Reverse Proxy in .NET using IIS.


Sample Image - reverse-proxy.png

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.


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//
  • 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:

    <!-- PROXY Mode
  0 : all web site can be requested by a client with 
      an url like <A href="http://reverseproxyurl/http//">http://ReverseProxyURL/http//</A>
  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="<A href=""></A>" />

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 = 
      string remoteWebSite = 
      string remoteUrl;
      if (proxyMode==0)
        remoteUrl= ParseURL(context.Request.Url.AbsoluteUri);
      //all site 
      //only one site accepted
      //create the web request to get the remote stream
      HttpWebRequest request = 
      //TODO : you can add your own credentials system
      //request.Credentials = CredentialCache.DefaultCredentials;
      HttpWebResponse response;
        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>");
      Stream receiveStream = response.GetResponseStream();

      if ((response.ContentType.ToLower().IndexOf("html")>=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(), 
          content= ParseHtmlResponse(readStream.ReadToEnd(),
        //write the updated HTML to the client
        //close streamsreadStream.Close();
        //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

    /// <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://") ;
        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)
      return html;
    /// Specifies whether this instance is reusable by other Http requests
    public bool IsReusable
        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:

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

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.


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


This article has no explicit license attached to it but may contain usage terms in the article text or the download files themselves. If in doubt please contact the author via the discussion board below.

A list of licenses authors might use can be found here

About the Author

Vincent Brossier
Web Developer
France France
Vincent Brossier is a software engineer from Paris, FRANCE, specializing in .Net.
Vincent is in charge of development of the Parisian university's Intranet, I take part in a nationnal project of numerical campus.
His role is to seek best technicals solutions to satisfy the 35000 users of this Intranet. Specialized in the development of distributed applications, vincent work primarily with technologies microsoft .NET (C#, ASP.NET, Webservices, SQLServer) even if he have also work with java technologies (Peer-to-peer sharing documents tool).
Vincent have also set up Open-Source solutions such as SPIP in nursery schools.
When he's not programming, he enjoys playing Guitar.

Comments and Discussions

QuestionNot Work in internet PinmemberDothanhhai2-Nov-12 9:57 
QuestionWhen size of page to display too big, hangs briefly, renders incomplete page PinmemberJim Cross23-Nov-11 8:53 
QuestionSpurious null character added to end of stream? PinmemberSteven Hirschorn25-Aug-11 1:46 
Questionreverse proxy and WCF-Service Pinmemberwalter.anna24-Jan-10 1:56 
Questionhow can i bypass the processrequest method? Pinmembervitiris4-Jan-10 18:11 
QuestionHow to set it up? Pinmembermaria_mir7-Jul-09 20:48 
AnswerRe: How to set it up? Pinmembermaria_mir12-Jul-09 22:18 
AnswerRe: How to set it up? Pinmembereric_ruck4-Aug-11 5:44 
GeneralSome changes Pinmemberrickleb21-Mar-09 6:16 
I used this code to set up another IIS 6 website in parallel with my public site. This is a secure (SSL) site. So I wanted to use https externally to access internal http servers (Nagios running on Apache on Linux). Here is a summary of the changes that I made. First, note that I did not test the "all sites" proxy mode, just the individual site mode.
So my IIS 6 setup looks like this:
Default site (my primary HTTPS site)
nagios1 (my first nagios server)
nagios2 (my second nagios server)
nagios1 and nagios2 each have the ReverseProxy application installed in the root. On IIS 5 & 6, if you want to map all mime types to the ihttphandler, you must do it for the entire site. That is why I didn't just use a virtual directory. IIS 7 removes this restraint.
The first change was to this section:
 string remoteUrl;
 if (proxyMode==0)
    remoteUrl= ParseURL(context.Request.Url.AbsoluteUri); //all site accepted
   remoteUrl= context.Request.Url.AbsoluteUri.Replace("http://"+

There were 2 issues. The first was that "http://" was hard coded. Wouldn't work for https. In my case, the external site was https and internally it was http. After the following changes, this wasn't an issue. The second issue was that sometimes "proxy.aspx" was in the URL, sometimes not. I put some code in to check for this as well:
 string remoteUrl;
 string sProtocol = context.Request.Url.Scheme + "://" + 
           context.Request.Url.Host +  context.Request.ApplicationPath;
 remoteUrl = context.Request.Url.AbsoluteUri.Replace(sProtocol, remoteWebSite); //only one site accepted
 int iLength = remoteUrl.LastIndexOf("proxy.aspx");
 if (iLength > 0)
    remoteUrl = remoteUrl.Substring(0, iLength);
A second issue was that on any error, "404" was returned, which made it very hard to determine what the real back end error was. I changed the exception block to pass along the backend error with a description:
Original code:
 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>");
New code:
 catch(System.Net.WebException we)
	int		iStatus;
	string	sStatus;
	//remote url not found, send status to client 
	sStatus = we.ToString();
	int iIndex = we.ToString().IndexOf('(');
	iStatus = 491; // Our own error code if theirs is invalid - unlikely
	if (iIndex != -1)
            iStatus = Convert.ToInt32(we.ToString().Substring(++iIndex, 3));
	context.Response.StatusCode = iStatus;
	context.Response.StatusDescription = we.Message;
	context.Response.Write("<h2>From ReverseProxy - " + we.Message + 
" + we.Data + "
URL: " +
                   we.Response.ResponseUri + "</br></br></h3></center>");

Lastly, the remapping of the links didn't work if the application was in the web site's root directory. This was because the replacement string was basically a single '/'. In this case, the browser would not have the correct relative path or the absolute path.
The solution was easy. Pass the absolute path. So
content= ParseHtmlResponse(readStream.ReadToEnd(),context.Request.ApplicationPath);
content = ParseHtmlResponse(readStream.ReadToEnd(), sProtocol);
Note that "sProtocol" was calculated above.
And finally, change ParseHtmlResponse from
 public string ParseHtmlResponse(string html,string appPath)
	return html;
 public string ParseHtmlResponse(string html,string appPath)
	html = html.Replace("\"/", "\"" + appPath);
	html = html.Replace("'/", "'" + appPath);
	html = html.Replace("=/", "=" + appPath); 
	return html;
Since the trailing backslash is already included in sProtocol.
These changes did not break the way the code worked originally, but they do make it more robust and work in more environments.
Finally, to save others a lot of heartache, here is a link to an article on how to create a server certificate that can be shared by different sites on the same machine using SSL host headers. This is non-obvious, and you can't have multiple SSL sites without it.[^]
Also, remember that the "common name" for each site must be in DNS so IIS knows which site to direct the request to.
Now you can host multiple servers from a single IIS servers, using https externally and http internally ig you desire.
Hope this helps others.
Rick Bross
Hope this helps others.
GeneralRe: Some changes PinmemberMunirS30-Aug-09 2:04 
GeneralPour Infos PinmemberMember 210144124-Sep-08 3:34 
GeneralInstallation Instructions PinmemberChris Spencer15-Jul-08 3:17 
GeneralRegarding Host Header Pinmemberpmselvan_20005-Jul-08 8:29 
GeneralRe: Regarding Host Header PinmemberMunirS30-Aug-09 1:56 
GeneralRe: Regarding Host Header PinmemberStimphy21-Dec-09 15:03 
GeneralError PinmemberVJLaks16-Jun-08 20:39 
GeneralDerivative Work PinmemberPaul Johnston4-May-08 4:35 
GeneralErrors Pinmemberhanisacsouka13-Apr-08 2:32 
General Pinmemberhanisacsouka12-Apr-08 8:05 
GeneralIIS 7.0 Pinmemberbrice2nice13-Nov-07 10:50 
Generalenabling file download Pinmembertarak4v5-Sep-07 19:49 
GeneralVery usefull for me. View my project for download files from based on this sample [modified] PinmemberW.M. Spon29-Jun-07 22:54 
GeneralI couldnt get it work Pinmemberjeanmoura12-Apr-07 5:01 
Questionimage path doesnt work Pinmembernolovelust5-Apr-07 10:45 
AnswerRe: image path doesnt work PinmemberMySuperName11-Jan-09 21:51 
GeneralBisuits Pinmembersafari7501223-Mar-07 4:52 
GeneralHTTPS support PinmemberDeason21-Nov-06 18:11 
GeneralReverse proxy apache and IIS Pinmemberharinath28-Oct-06 2:29 
General401 error Pinmembermredlon14-Sep-06 2:18 
GeneralReverse proxy with .css and .aspx files. Pinmemberyashchawhan3-Aug-06 19:40 
GeneralRe: Reverse proxy with .css and .aspx files. PinmemberMunirS30-Aug-09 1:39 
GeneralRe: Reverse proxy with .css and .aspx files. PinmemberStimphy21-Dec-09 15:06 
QuestionReverse Proxy Pinmemberyashchawhan24-Jul-06 2:45 
GeneralTrailing slash Pinmembereggsovereasy17-Apr-06 3:49 
GeneralRe: Trailing slash PinmemberRitchie Carroll13-Jun-06 10:11 
AnswerRe: Trailing slash PinmemberUL-Tomten16-Jan-08 7:54 
Generaljavascript and postback Pinmembertomanyquestions19-Mar-06 23:30 
GeneralCaching images Pinmemberpratz1722-Feb-06 19:03 
Question..what about conversion to VS 2005?? Pinmembernikolair16-Nov-05 4:27 
GeneralPass values from .aspx page to remote URL that contains an asp page PinsussAnonymous18-Oct-05 20:10 
GeneralW2k3 / IIS 6 PinmemberDavid Llamas15-Jul-05 17:44 
AnswerRe: W2k3 / IIS 6 Pinmembertarak4v9-Sep-07 22:10 
GeneralAccess Denied... PinmemberRitchie Carroll28-Jun-05 4:03 
GeneralRe: Access Denied... PinmemberRitchie Carroll29-Jun-05 10:59 
GeneralUnit Testing ReverseProxy Pinmemberbtakita25-May-05 11:49 
GeneralCant see images Pinmemberpratz1728-Apr-05 3:08 
GeneralRe: Cant see images PinmemberRitchie Carroll28-Jun-05 3:57 
JokeRe: Cant see images PinmemberUL-Tomten16-Jan-08 7:56 
GeneralAdding Session Handling PinmemberDavid Osbourn10-Mar-05 6:01 
GeneralRe: Adding Session Handling PinmemberVincent Brossier10-Mar-05 8:51 

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