SocketException: No such host is known

Visual Studio provides all-in-one experience for software developers. From coding to testing, it makes IT professional’s lives easier. Error messages like “Socket Exception: No such host is known” refers to certain issues in the code or configuration of the application. In this post, I will explain the root cause and solution for this error message.

This error message may appear in may forms. Here is one of them:

System.Net.Sockets.SocketException: No such host is known

Error "No such host is known"

In some cases, you may see the exception code 0x80004005 along with the error message above. In addition, if you are receiving ThreadAbortException, check this post out.

The root cause of the “No such host is known” error

In the scenario I tested and in the online posts I reviewed, the root cause was the same: DNS name resolution failure. Basically, your application tries to call an API URL or to connect to a service but it is not able to find the hostname or your server is not able to do DNS lookup for that hostname.

Since it is an issue with DNS name resolution, nothing you do with Visual Studio, IIS, TFS or Windows will help fixing this issue. However, a simple code update may do the magic!

Similar issues on external sites: CodeProject, GitHub, Microsoft Support.

Another Visual Studio error: Remote Debugging Monitor (MSVSMON.EXE)

Solution for “No such host is known” error

The solution is fairly simple. Make sure your server is able to access to the DNS server and your DNS server is able to lookup the hostname in your code. Follow the steps below to troubleshoot and solve this issue.

  • In Command Prompt, run ipconfig /all. This will list the IP addresses of the DNS server you use. Try to ping them. Additionally, check with a colleague to see he or she has the same configuration
  • Run nslookup. Enter the hostname in your code. Make sure the DNS server is able to lookup this hostname.
  • Make sure there is no typo or white spaces in the hostname. If you are getting this input from a JSON call or configuration file, it is possible that the source string and the variable you have are different.

If this is an ASP.NET WebForms application, you may also come across Validation of viewstate MAC failed error. Follow the link for the specific solution for it.

Dynamic scripts with CSP (Content Security Policy)

An ASP.NET WebForms project adds several scripts to the page on the fly. Since these scripts don’t exist in the compile time, how to whitelist these dynamic scripts with CSP?

The title and the first paragraph may sound pretty abstract. Let’s look at the fundamentals first.

Are you receiving “Custom JavaScript is not allowed” error? Check this post out.

What is CSP (Content Security Policy)?

CSP is an HTTP header that we use to prevent cross site scripting (XSS) and packet sniffing attacks. Long story short: By using CSP header, we tell the browser which scripts or other resources we trust. The browser executes these resources and ignores the rest.

Here is an example CSP header:

object-src 'none';  script-src 'nonce-{random}' 'unsafe-inline'

As mentioned in this article, this header means:

object-src 'none' Prevents fetching and executing plugin resources embedded using <object>, <embed> or <applet> tags. The most common example is Flash.

script-src nonce-{random} 'unsafe-inline' The nonce directive means that <script> elements will be allowed to execute only if they contain a nonce attribute matching the randomly-generated value which appears in the policy.

Note: In the presence of a CSP nonce the unsafe-inline directive will be ignored by modern browsers. Older browsers, which don’t support nonces, will see unsafe-inline and allow inline scripts to execute.

For more information about CSP:

If the browser blocks a resource because it doesn’t comply with CSP, you will see this error in browser logs:

Refused to execute inline script because it violates the following Content Security Policy directive: “script-src ‘self'”. Either the ‘unsafe-inline’ keyword, a hash, or a nonce is required to enable inline execution.

Dynamic scripts with CSP

How to prevent executing dynamic scripts with CSP?

Using unsafe-inline removes the error above (“Refuse to execute inline script“). However, it is not a secure way of whitelisting dynamically created scripts.

script-src 'self' 'unsafe-inline'

Other than using unsafe-inline, it doesn’t seem like there is another way to whitelist dynamic scripts with CSP. There are a few open questions below if you want to keep posted about future updates on this topic.

PageMethods is not working (Solved)

PageMethods is a JavaScript class that you can use to call a server-side function. It’s basically an AJAX call which works asynchronously. In .NET world, you can use PageMethods to call a C# code-behind function. If you think that PageMethods is not working no matter what you do, keep reading.

For PageMethods to work, you need a JavaScript function on the client-side and a WebMethod function on the server-side.

Sample JavaScript code (Put it in asp:content section of Default.aspx):

<script type="text/jscript">

   function callCodeBehind() {
		PageMethods.GetMessage("test string", onSuccess, onFailure);
	}

   function onSuccess(result, usercontext, methodname) {
	   alert(result)
   }

   function onFailure(error, usercontext, methodname) { alert("failed: "+ error.get_message()); }

</script>

Sample C# code (code-behind file – Default.aspx.cs):

public partial class Welcome : System.Web.UI.Page
{
	protected void Page_Load(object sender, EventArgs e)
	{

	}

	[WebMethod]
	protected static void GetMessage(string val)
	{
		return "doingit";
	}
}

A button to call the JavaScript function (in Default.aspx)

<button onclick="callCodeBehind()">Click me</button> 

ScriptManager (in Site.Master – note that EnablePageMethods is set to True):

<asp:ScriptManager ID="ScriptManager1" runat="server" EnablePageMethods="True">
</asp:ScriptManager>

Having issues with your AJAX call as part of Google reCAPTCHA implementation? Check this post out.

What to do if PageMethods is not working

If you have the code above, your project should be working. In exceptional cases, you may not receive the return value. In my case, the issue was the post-back that was happening after clicking the button.

The reason behind the post-back was that friendy URL setting. As soon as I disabled it, my PageMethods function started working.

In order to disable friendly URL feature, remove or comment out the line below in your RouteConfig file.

routes.EnableFriendlyUrls(settings)
Successfully returned value

Are you losing data after a post-back? Here is what to do.

References: