ASP.NET Proxy Page – Used for Cross Domain Requests from AJAX and JavaScript

One of the pain points with developing AJAX, JavaScript, JQuery, and other client-side behaviors is that JavaScript doesn’t allow for cross domain request for pulling content. For example, JavaScript code on www.johnchapman.name could not pull content or data from www.bing.com.  I have found this particularly painful when doing SharePoint development.

One way to overcome this issue is by using a server-side proxy on the site running the JavaScript code. There is already are well documented PHP solutions on the web, however I couldn’t find very many .NET-based solutions. This simple C# code takes the URL passed to it through the URL encoded query string, retrieves the content of the URL using an HttpWebRequest and outputs in the original form it as if it were content on the site.

The following code can be added to an ASP.NET application or to SharePoint.  For SharePoint, you can place the two files (Proxy.aspx and Proxy.aspx.cs) into the "_layouts" folder (c:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\12\TEMPLATE\LAYOUTS).

Please Note: To keep this example simple and to the point it does not contain any error exception handling and such.  You can modify this code and add error handling as you see fit.  Also, allowing all websites to be proxied through your site could allow for malicious site content to appear as if they are coming from your site.  The Query String method below is not recommended for use on publicly accessible websites, instead use an ID in the Query String and lookup the actual URL from a database.  This will put you in complete control of the sites allowed to be proxied.


 Complete Proxy.aspx.cs

using System;
                      using System.Collections.Generic;
                      using System.Web;
                      using System.Web.UI;
                      using System.Web.UI.WebControls;
                      using System.Net;
                      using System.IO;
                      
                      namespace Proxy {
                          public partial class _Proxy : System.Web.UI.Page {
                              protected void Page_Load(object sender, EventArgs e) {
                                  string proxyURL = string.Empty;
                                  try {
                                      proxyURL = HttpUtility.UrlDecode(Request.QueryString["u"].ToString());
                                  }
                                  catch { }
                      
                                  if (proxyURL != string.Empty) {
                                      HttpWebRequest request = (HttpWebRequest)WebRequest.Create(proxyURL);
                                      request.Method = "GET";
                                      HttpWebResponse response = (HttpWebResponse)request.GetResponse();
                      
                                      if (response.StatusCode.ToString().ToLower() == "ok") {
                                          string contentType = response.ContentType;
                                          Stream content = response.GetResponseStream();
                                          StreamReader contentReader = new StreamReader(content);
                                          Response.ContentType = contentType;
                                          Response.Write(contentReader.ReadToEnd());
                                      }
                                  }
                              }
                          }
                      }
                      

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Proxy.aspx.cs Breakdown


Note: After the "if (response.StatusCode.ToString().ToLower() == "ok") { … }" snippet, you could add an else statement to output the StatusCode to an error log so that you can troubleshoot if necessary.  

This tutorial shows a simplified method for retrieving remote content and displaying it on your site so that AJAX, JQuery, JavaScript, etc can access to remote content.  When using this code, it is important to consider security concerns and error handling to ensure the best implementation of a proxy page.


Proxy.aspx

<%@ Page Language="C#" 
                               AutoEventWireup="true" 
                               CodeBehind="Proxy.aspx.cs" 
                               Inherits="Proxy._Proxy" %>
                      

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The Proxy.aspx page is simply blank except for the Page tag. When passing the URL to the query string, it is important that it is URL encoded. This helps to prevent query strings of the remote site URL from interfering with the Proxy page.


Example Usage

http://www.yourdotnetapplication.com/proxy.aspx?u=http%3a%2f%2fwww.google.com
                      http://www.yoursharepointsite.com/_layouts/proxy.aspx?u=http%3a%2f%2fwww.google.com
                      

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The original article as well as source code can be found at:
http://www.johnchapman.name/aspnet-proxy-page-cross-domain-requests-from-ajax-and-javascript/