Socket.Close doesn't really close tcp socket?

问题:

It seems that using socket.Close() for a tcp socket, doesn't fully close the socket. In the following example I'm trying to connect to example.com at port 9999, which is not opened, and after a short-timeout, I'm trying to close the socket.

  for (int i = 0; i < 200; i++)
  {
    Socket sock = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
    sock.LingerState = new LingerOption(false, 0);
    sock.BeginConnect("www.example.com", 9999, OnSocketConnected, sock);
    System.Threading.Thread.Sleep(50);
    sock.Close();
  }

But when I take a look at netstat after the loop completes, I find that there are many half-opened sockets:

  TCP    israel-xp:6506         www.example.com:9999   SYN_SENT
  TCP    israel-xp:6507         www.example.com:9999   SYN_SENT
  TCP    israel-xp:6508         www.example.com:9999   SYN_SENT
  TCP    israel-xp:6509         www.example.com:9999   SYN_SENT

EDIT . Ok, some context was missing. I'm using beginconnect because I expect the socket connection to fail (9999 is not opened), and in my real code, I call the socket.Close() once a timer is set. On OnSocketConnected I call EndConnect, which throws an exception (trying to call a method of a disposed object). My goal is having a short timeout for the socket connection stage.

Any clue what I'm doing wrong? Thanks!

回答1:

It will close the .NET part of the socket. However according to the TCP specification the OS have to keep the lower level tidbits of the socket open for a certain amount of time in order to detect retransmission, and similar. In this particular case it's likely keeping the socket around for a bit in order to detect a reply to the SYN packet sent so it can reply more sensibly and not mix up the reply with further packets sent.

回答2:

By design, you should always call Shutdown before closing the socket.

mySocket.Shutdown(SocketShutdown.Both);
mySocket.Close();

Doing so effectively disables send and receive on the socket, so it will not be accepting incoming data after you've closed it, even if the OS still has control of it.

Jon Skeet also has a point, that since you're opening the connection asynchronously, it may actually be connecting while you're trying to close it. However, if you call Shutdown on it, it will not allow information to be received as you are experiencing.

Edit: You can only Shutdown a socket that is already connected, so bear this in mind as you write your code.

回答3:

You're calling *Begin*Connect - so doing it asynchronously. You're quite possibly trying to close the socket before it's even connected - so when it then connects, it remains open.

Try connecting synchronously - or closing it in OnSocketConnected so you can see the effect of closing a genuinely connected socket.

回答4:

You are initialising a new Socket in every Loop... with *.close() you close the old and at the begin you are creating a new one with the same parameters as the Socket before.