Check if a port is open

Question

I can't seem to find anything that tells me if a port in my router is open or not. Is this even possible?

The code I have right now doesn't really seem to work...

private void ScanPort() {     string hostname = "localhost";     int portno = 9081;     IPAddress ipa = (IPAddress) Dns.GetHostAddresses(hostname)[0];     try     {         System.Net.Sockets.Socket sock =                 new System.Net.Sockets.Socket(System.Net.Sockets.AddressFamily.InterNetwork,                                               System.Net.Sockets.SocketType.Stream,                                               System.Net.Sockets.ProtocolType.Tcp);         sock.Connect(ipa, portno);         if (sock.Connected == true) // Port is in use and connection is successful             MessageBox.Show("Port is Closed");         sock.Close();     }     catch (System.Net.Sockets.SocketException ex)     {         if (ex.ErrorCode == 10061) // Port is unused and could not establish connection              MessageBox.Show("Port is Open!");         else             MessageBox.Show(ex.Message);     } } 

Answer 1

Try this:

using(TcpClient tcpClient = new TcpClient()) {     try {         tcpClient.Connect("127.0.0.1", 9081);         Console.WriteLine("Port open");     } catch (Exception) {         Console.WriteLine("Port closed");     } } 

You should probably change 127.0.0.1 to something like 192.168.0.1 or whatever your router's IP address is.

Answer 2

A better solution where you can even specify a timeout:

using System; using System.Net.Sockets;  // ...  bool IsPortOpen(string host, int port, TimeSpan timeout) {     try     {         using(var client = new TcpClient())         {             var result = client.BeginConnect(host, port, null, null);             var success = result.AsyncWaitHandle.WaitOne(timeout);             client.EndConnect(result);             return success;         }     }     catch     {         return false;     } } 

And, in F#:

open System open System.Net.Sockets  let isPortOpen (host: string) (port: int) (timeout: TimeSpan): bool =     try         use client = new TcpClient()         let result = client.BeginConnect(host, port, null, null)         let success = result.AsyncWaitHandle.WaitOne timeout         client.EndConnect result         success     with     | _ -> false  let available = isPortOpen "stackoverflow.com" 80 (TimeSpan.FromSeconds 10.) printf "Is stackoverflow available? %b" available