DNS not working on online server

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ASP.NET

Hi,

I want client machine name where the ASP.Net web application is running.

I am using below code:

var hostEntry = Dns.GetHostEntry(Request.UserHostAddress);
          Response.Write("machinename=" + hostEntry.HostName);

Its working Offline but not working on online server.

It shows this error:

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code. 

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

Source Error: 

An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.

Stack Trace: 


[SocketException (0x2af9): No such host is known]
   System.Net.Dns.GetAddrInfo(String name) +471
   System.Net.Dns.InternalGetHostByAddress(IPAddress address, Boolean includeIPv6, Boolean throwOnFailure) +125
   System.Net.Dns.GetHostEntry(String hostNameOrAddress) +5390027
   Web.TestMachineName.GetMachineName() +282
   Web.TestMachineName.Page_Load(Object sender, EventArgs e) +5
   System.Web.Util.CalliHelper.EventArgFunctionCaller(IntPtr fp, Object o, Object t, EventArgs e) +14
   System.Web.Util.CalliEventHandlerDelegateProxy.Callback(Object sender, EventArgs e) +35
   System.Web.UI.Control.OnLoad(EventArgs e) +99
   System.Web.UI.Control.LoadRecursive() +50
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +627