Currently, I am working on client-side Javascript which interacts with JSON web services from a different domain. I've learned that certain browsers don't permit cross-domain scripting, so it's recommended to set up a proxy on my local server to handle the data.
Could someone direct me to a straightforward example of how to achieve this using ASP.Net?
Typically, the proxy operates on your server - often using IIS in this scenario - and forwards requests to a different domain server.
For instance, here's a C# .NET example:
One way to bypass a proxy is by utilizing a method like JSONP. If the web service you're interacting with supports JSONP (such as Flickr or Twitter) or if you have the ability to control the data returned by the web service, you can transmit JSON data across different domains using a library that includes JSONP support.
For instance, with jQuery, you can execute a JSON call to an external source:
jQuery.getJSON("http://www.someothersite.com/webservice?callback=?", function(result)
{
processResult(result);
});
Since the call is to a different domain, jQuery employs certain techniques to enable cross-domain communication. jQuery automatically replaces the ? in the URL with a callback function name that the web service can utilize to structure the JSON data being sent back.
If you have control over the web service, you can manage the JSONP request by accessing the "callback" request parameter, which will contain the callback function name you should use. The callback function accepts one parameter, representing the JSON data to be returned. Therefore, if the callback parameter is "jsonp2342342", the web service should respond like this:
jsonp2342342({key: value, key2: value});
If the web service you're utilizing already supports JSONP, you won't need to worry about formatting the data on your own.
If you need to fetch a remote page and present it on your website, you can create a basic .NET page to accomplish this task:
using System;
using System.Collections.Generic;
using System.Linq;
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());
}
}
}
}
}
To learn more about this method, check out my article on the topic:
No web browsers currently support cross-domain scripting, despite the W3C leaving room for it in their xmlHTTPRequest-object recommendation. We will likely have to wait a while before it is implemented in a secure manner...
For those looking for a generalized solution, here is a pseudocode snippet:
SomeWebAbstraction.Fetch('proxyPage', {
params: {
url: 'http://example.com/api?query=something'
}
});
Next, in the proxyPage:
var url = GET['url'];
if (ValidURL(url) && AllowConnection(url)) {
// Validating URL and approving connections is left to the reader
var result = SomeHTTPRequestFunction(url);
output XssAndMaliciousContentFilter(result);
} else {
// Error handling logic goes here
}
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