How to create a simple proxy in ASP.NET

A common obstruction when developing Silverlight web applications is cross domain access. Silverlight has built-in security that prevents communication to servers that do not explicitly granted access using with either a client access policy or cross domain file. Even though a server has services intended for public use, if the server does not have a cross domain file then Silverlight apps will be denied access.

This post presents a convenient and legitimate way to overcome this restriction with a simple proxy service in three easy steps!

Step 1 – In Visual Studio 2010, create a new ASP.NET Web Application project called WebApplication1

Step 2 – Add a new generic handler called proxy.ashx

Step 3 – Copy and paste this code into proxy.ashx.cs.

using System;
using System.IO;
using System.Net;
using System.Web;
 
namespace WebApplication1 {
    public class proxy : IHttpHandler {
        public void ProcessRequest(HttpContext context) {
            HttpResponse response = context.Response;
 
            // Check for query string
            string uri = Uri.UnescapeDataString(context.Request.QueryString.ToString());
            if (string.IsNullOrWhiteSpace(uri)) {
                response.StatusCode = 403;
                response.End();
                return;
            }
 
            // Filter requests
            if (!uri.ToLowerInvariant().Contains("wikimedia.org")) {
                response.StatusCode = 403;
                response.End();
                return;
            }
 
            // Create web request
            WebRequest webRequest = WebRequest.Create(new Uri(uri));
            webRequest.Method = context.Request.HttpMethod;
 
            // Send the request to the server
            WebResponse serverResponse = null;
            try {
                serverResponse = webRequest.GetResponse();
            }
            catch (WebException webExc) {
                response.StatusCode = 500;
                response.StatusDescription = webExc.Status.ToString();
                response.Write(webExc.Response);
                response.End();
                return;
            }
 
            // Exit if invalid response
            if (serverResponse == null) {
                response.End();
                return;
            }
 
            // Configure reponse
            response.ContentType = serverResponse.ContentType;
            Stream stream = serverResponse.GetResponseStream();
 
            byte[] buffer = new byte[32768];
            int read = 0;
 
            int chunk;
            while ((chunk = stream.Read(buffer, read, buffer.Length - read)) > 0) {
                read += chunk;
                if (read != buffer.Length) { continue; }
                int nextByte = stream.ReadByte();
                if (nextByte == -1) { break; }
 
                // Resize the buffer
                byte[] newBuffer = new byte[buffer.Length * 2];
                Array.Copy(buffer, newBuffer, buffer.Length);
                newBuffer[read] = (byte)nextByte;
                buffer = newBuffer;
                read++;
            }
 
            // Buffer is now too big. Shrink it.
            byte[] ret = new byte[read];
            Array.Copy(buffer, ret, read);
 
            response.OutputStream.Write(ret, 0, ret.Length);
            serverResponse.Close();
            stream.Close();
            response.End();
        }
        public bool IsReusable {
            get { return false; }
        }
    }
}

You are done! To test, press F5 to run the web application in debug mode. Your default web browser will display the Default.aspx page from your project.

Without closing the web browser, append the name of the proxy service and the web resource (highlighted below) that you need access to. For example:
http://localhost:51220/proxy.ashx?http://upload.wikimedia.org/wikipedia/en/f/f0/New-esri-logo.jpg

This is the result.

In summary, this post described how to crate a simple proxy web service. The proxy can be used by Silverlight web application to access resources that are restricted due to a missing cross domain file. To prevent malicious use of the proxy it is advisable to add some sort of access restriction, for example, in this exercise the proxy was configured to only accept requests for content from the wikimedia.org domain.