CODEWITHSUNDEEP
c#.netsocketsexceptionnetwork-programming
User
User

Reputation: 154

C# SocketException: Address already in use

I am trying to establish a connection through Sockets to send data (for testing purposes to the local IP address, meaning the computer is sending data to "itself").

Previously, everything worked perfectly fine for weeks throughout the development, meaning the connection could be established, the data could be sent and received and the connection could finally be closed, but a few days later, also after restarting my router, I am getting a SocketException (98) when trying to bind the "listener" Socket to an end point with IPAddress.Any (0.0.0.0) with the message: Address already in use.

The source code has not changed in between. The Socket is supposed to accept any connection from any IP-Address, as it is the "listener" Socket for receiving data. I am using .NET 6.0.301.

This is a simplified version of the relevant source code:

// In the constructor of the base-class:
IpEndPoint = new IPEndPoint(IPAddress.Any, Port); // Random unused port that also hasn't changed
// ...

// Gets called in the constructor of the derived class
private async Task ReceiveDataAsync()
{
    using Socket listener = new(IpEndPoint.AddressFamily, SocketType.Stream, ProtocolType.Tcp);
    listener.SetSocketOption(SocketOptionLevel.Socket, SocketOptionName.ReuseAddress, true);
    try
    {
        listener.Bind(IpEndPoint); // Fails here!!!
        listener.Listen(100);
    }
    catch(Exception e)
    {
        Console.WriteLine(e);
        throw;
    }

    while(true)
    {
        using Socket handler = await listener.AcceptAsync();
        // Receive data...
    }
}

This method is called only once and the exception throws at the very first cycle of the loop. It is called, of course, long before the attempt to establish a connection to this IP. It might also be important noting that I didn't close the connection after receiving the data with the Shutdown, DisconnectAsync and Close methods, but this has always been done on the client side after the data had been sent.

And here is the exception:

System.Net.Sockets.SocketException (98): Address already in use
   at System.Net.Sockets.Socket.UpdateStatusAfterSocketErrorAndThrowException(SocketError error, String callerName)
   at System.Net.Sockets.Socket.DoBind(EndPoint endPointSnapshot, SocketAddress socketAddress)
   at System.Net.Sockets.Socket.Bind(EndPoint localEP)
   at ....ReceiveDataAsync() ...

EDIT: This is the output of netstat -ntpl | grep 0.0.0.0 while my IDE, JetBrains Rider is running with the relevant project being opened (but it isn't executing):

tcp        0      0 0.0.0.0:31415           0.0.0.0:*               LISTEN      
10064/dotnet        
tcp        0      0 127.0.0.1:42103         0.0.0.0:*               LISTEN      
9560/Rider.Backend  
tcp        0      0 127.0.0.1:33747         0.0.0.0:*               LISTEN      
9560/Rider.Backend  
tcp        0      0 127.0.0.1:631           0.0.0.0:*               LISTEN      
-                   
tcp        0      0 127.0.0.53:53           0.0.0.0:*               LISTEN
-

Killing the process, which occupies the port, doesn't have much of an effect, as a new process gets created when I launch the application again.

Upvotes: 1

Views: 1631

Answers (1)

Marc Gravell
Marc Gravell

Reputation: 1062745

A Bind on a fixed endpoint inside a while (true) is hugely suspicious. You only need to bind once - a single listener socket can accept any number of client connections. Refactor the code so you only bind and listen once, and then just have the accept in the while loop.

If it can't bind even the first time, then that suggests a rogue process is holding the port, or possibly a firewall problem. But my money would be on a rogue process. If you can't find it, try rebooting - if that fixes it: it was a rogue process (probably your own program started from a debugger, or similar).

Upvotes: 2

Related Questions