In C#, how to check if a TCP port is available?

Question

In C# to use a TcpClient or generally to connect to a socket how can I first check if a certain port is free on my machine?

more info: This is the code I use:

TcpClient c;
//I want to check here if port is free.
c = new TcpClient(ip, port);

Answer 1

netstat! That's a network command line utility which ships with windows. It shows all current established connections and all ports currently being listened to. You can use this program to check, but if you want to do this from code look into the System.Net.NetworkInformation namespace? It's a new namespace as of 2.0. There's some goodies there. But eventually if you wanna get the same kind of information that's available through the command netstat you'll need to result to P/Invoke...

Update: System.Net.NetworkInformation

That namespace contains a bunch of classes you can use for figuring out things about the network.

I wasn't able to find that old pice of code but I think you can write something similar yourself. A good start is to check out the IP Helper API. Google MSDN for the GetTcpTable WINAPI function and use P/Invoke to enumerate until you have the information you need.

Answer 2

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 3

You say

I mean that it is not in use by any other application. If an application is using a port others can't use it until it becomes free.

But you can always connect to a port while others are using it if something's listening there. Otherwise, http port 80 would be a mess.

If your

   c = new TcpClient(ip, port);

fails, then nothing's listening there. Otherwise, it will connect, even if some other machine/application has a socket open to that ip and port.

Answer 4

Since you're using a TcpClient, that means you're checking open TCP ports. There are lots of good objects available in the System.Net.NetworkInformation namespace.

Use the IPGlobalProperties object to get to an array of TcpConnectionInformation objects, which you can then interrogate about endpoint IP and port.

---

 int port = 456; //<--- This is your value
 bool isAvailable = true;

 // Evaluate current system tcp connections. This is the same information provided
 // by the netstat command line application, just in .Net strongly-typed object
 // form.  We will look through the list, and if our port we would like to use
 // in our TcpClient is occupied, we will set isAvailable to false.
 IPGlobalProperties ipGlobalProperties = IPGlobalProperties.GetIPGlobalProperties();
 TcpConnectionInformation[] tcpConnInfoArray = ipGlobalProperties.GetActiveTcpConnections();

 foreach (TcpConnectionInformation tcpi in tcpConnInfoArray)
 {
   if (tcpi.LocalEndPoint.Port==port)
   {
     isAvailable = false;
     break;
   }
 }

 // At this point, if isAvailable is true, we can proceed accordingly.

Answer 5

You're on the wrong end of the Intertube. It is the server that can have only one particular port open. Some code:

  IPAddress ipAddress = Dns.GetHostEntry("localhost").AddressList[0];
  try {
    TcpListener tcpListener = new TcpListener(ipAddress, 666);
    tcpListener.Start();
  }
  catch (SocketException ex) {
    MessageBox.Show(ex.Message, "kaboom");
  }

Message: Only one usage of each socket address (protocol/network address/port) is normally permitted.

Answer 6

ipGlobalProperties.GetActiveTcpConnections() doesnt return connections in Listen State

Port can be used for listening but no one connected to it, and the method described above will not work

Answer 7

TcpClient c;

//I want to check here if port is free.
c = new TcpClient(ip, port);

...how can I first check if a certain port is free on my machine?

I mean that it is not in use by any other application. If an application is using a port others can't use it until it becomes free. – Ali

You have misunderstood what's happening here.

TcpClient(...) parameters are of server ip and server port you wish to connect to.

The TcpClient selects a transient local port from the available pool to communicate to the server. There's no need to check for the availability of the local port as it is automatically handled by the winsock layer.

In case you can't connect to the server using the above code fragment, the problem could be one or more of several. (i.e. server ip and/or port is wrong, remote server not available, etc..)

Answer 8

Some many answers and none with the correct answer. :\

Socket.Select is what you are looking for.

Answer 9

When you set up a TCP connection, the 4-tuple (source-ip, source-port, dest-ip, dest-port) has to be unique - this is to ensure packets are delivered to the right place.

There is a further restriction on the server side that only one server program can bind to an incoming port number (assuming one IP address; multi-NIC servers have other powers but we don't need to discuss them here).

So, at the server end, you:

• create a socket.
• bind that socket to a port.
• listen on that port.
• accept connections on that port. and there can be multiple connections coming in (one per client).

On the client end, it's usually a little simpler:

• create a socket.
• open the connection. When a client opens the connection, it specifies the ip address and port of the server. It can specify its source port but usually uses zero which results in the system assigning it a free port automatically.

There is no requirement that the destination IP/port be unique since that would result in only one person at a time being able to use Google, and that would pretty well destroy their business model.

This means you can even do such wondrous things as multi-session FTP since you set up multiple sessions where the only difference is your source port, allowing you to download chunks in parallel. Torrents are a little different in that the destination of each session is usually different.

And, after all that waffling (sorry), the answer to your specific question is that you don't need to specify a free port. If you're connecting to a server with a call that doesn't specify your source port, it'll almost certainly be using zero under the covers and the system will give you an unused one.

Answer 10

Be aware the time window between you make check and the moment you try to make connection some process may take the port - classical TOCTOU. Why don't you just try to connect? If it fails then you know the port is not available.

Answer 11

You don't have to know what ports are open on your local machine to connect to some remote TCP service (unless you want to use a specific local port, but usually that is not the case).

Every TCP/IP connection is identified by 4 values: remote IP, remote port number, local IP, local port number, but you only need to know remote IP and remote port number to establish a connection.

When you create tcp connection using

TcpClient c;
c = new TcpClient(remote_ip, remote_port);

Your system will automatically assign one of many free local port numbers to your connection. You don't need to do anything. You might also want to check if a remote port is open. but there is no better way to do that than just trying to connect to it.

Answer 12

If I'm not very much mistaken, you can use System.Network.whatever to check.

However, this will always incur a race condition.

The canonical way of checking is try to listen on that port. If you get an error that port wasn't open.

I think this is part of why bind() and listen() are two separate system calls.

Answer 13

"portno" sounds good

Answer 14

thanks for the answer jro. I had to tweak it for my usage. I needed to check if a port was being listened on, and not neccessarily active. For this I replaced

TcpConnectionInformation[] tcpConnInfoArray = ipGlobalProperties.GetActiveTcpConnections();

with

IPEndPoint[] objEndPoints = ipGlobalProperties.GetActiveTcpListeners();.  

I iterated the array of endpoints checking that my port value was not found.