Is there a way to detect that TCP socket has been

问题:

First, a little background to explain the motivation: I'm working on a very simple select()-based TCP "mirror proxy", that allows two firewalled clients to talk to each other indirectly. Both clients connect to this server, and as soon as both clients are connected, any TCP bytes sent to the server by client A is forwarded to client B, and vice-versa.

This more or less works, with one slight gotcha: if client A connects to the server and starts sending data before client B has connected, the server doesn't have anywhere to put the data. I don't want to buffer it up in RAM, since that could end up using a lot of RAM; and I don't want to just drop the data either, as client B might need it. So I go for the third option, which is to not select()-for-read-ready on client A's socket until client B has also connected. That way client A just blocks until everything is ready to go.

That more or less works too, but the side effect of not selecting-for-read-ready on client A's socket is that if client A decides to close his TCP connection to the server, the server doesn't get notified about that fact -- at least, not until client B comes along and the server finally selects-for-read-ready on client A's socket, reads any pending data, and then gets the socket-closed notification (i.e. recv() returning 0).

I'd prefer it if the server had some way of knowing (in a timely manner) when client A closed his TCP connection. Is there a way to know this? Polling would be acceptable in this case (e.g. I could have select() wake up once a minute and call IsSocketStillConnected(sock) on all sockets, if such a function existed).

回答1:

If you want to check if the socket is actually closed instead of data, you can add the MSG_PEEK flag on recv() to see if data arrived or if you get 0 or an error.

/* handle readable on A */
if (B_is_not_connected) {
    char c;
    ssize_t x = recv(A_sock, &c, 1, MSG_PEEK);
    if (x > 0) {
        /* ...have data, leave it in socket buffer until B connects */
    } else if (x == 0) {
        /* ...handle FIN from A */
    } else {
        /* ...handle errors */
    }
}

Even if A closes after sending some data, your proxy probably wants to forward that data to B first before forwarding the FIN to B, so there is no point in knowing that A has sent FIN on the connection sooner than after having read all the data it has sent.

A TCP connection isn't considered closed until after both sides send FIN. However, if A has forcibly shutdown its endpoint, you will not know that until after you attempt to send data on it, and receive an EPIPE (assuming you have suppressed SIGPIPE).

---

After reading your mirror proxy application a bit more, since this is a firewall traversal application, it seems that you actually need a small control protocol to allow to you verify that these peers are actually allowed to talk to each other. If you have a control protocol, then you have many solutions available to you, but the one I would advocate would be to have one of the connections describe itself as the server, and the other connection describe itself as the client. Then, you can reset the connection the client if there is no server present to take its connection. You can let servers wait for a client connection up to some timeout. A server should not initiate any data, and if it does without a connected client, you can reset the server connection. This eliminates the issue of buffering data for a dead connection.

回答2:

It appears the answer to my question is "no, not unless you are willing and able to modify your TCP stack to get access to the necessary private socket-state information".

Since I'm not able to do that, my solution was to redesign the proxy server to always read data from all clients, and throw away any data that arrives from a client whose partner hasn't connected yet. This is non-optimal, since it means that the TCP streams going through the proxy no longer have the stream-like property of reliable in-order delivery that TCP-using programs expect, but it will suffice for my purpose.

回答3:

I don't see the problem as you see it. Let's say A connects to the server sends some data and close, it does not need any message back. Server won't read its data until B connects, once it does server read socket A and send the data to B. The first read will return the data A had sent and the second return either 0 or -1 in either case the socket is closed, server close B. Let's suppose A send a big chunk of data, the A's send() method will block until server starts reading and consumes the buffer.

I would use a function with a select which returns 0, 1, 2, 11, 22 or -1, where;

• 0=No data in either socket (timeout)
• 1=A has data to read
• 2=B has data to read
• 11=A socket has an error (disconnected)
• 22=B socket has an error (disconnected)

• -1: One/both socket is/are not valid

  int WhichSocket(int sd1, int sd2, int seconds, int microsecs) { 
     fd_set sfds, efds;
     struct timeval timeout={0, 0};
     int bigger;
     int ret;
  
  
     FD_ZERO(&sfds);
     FD_SET(sd1, &sfds);
     FD_SET(sd2, &sfds);
     FD_SET(sd1, &efds);
     FD_SET(sd2, &efds);
     timeout.tv_sec=seconds;
     timeout.tv_usec=microsecs;
     if (sd1 > sd2) bigger=sd1;
     else bigger=sd2;
     // bigger is necessary to be Berkeley compatible, Microsoft ignore this param.
     ret = select(bigger+1, &sfds, NULL, &efds, &timeout);
     if (ret > 0) {
         if (FD_ISSET(sd1, &sfds)) return(1); // sd1 has data
         if (FD_ISSET(sd2, &sfds)) return(2); // sd2 has data
         if (FD_ISSET(sd1, &efds)) return(11);    // sd1 has an error
         if (FD_ISSET(sd2, &efds)) return(22);    // sd2 has an error
     }
     else if (ret < 0) return -1; // one of the socket is not valid
     return(0); // timeout
  }
  

回答4:

For me the solution was to poll the socket status.

On Windows 10, the following code seemed to work (but equivalent implementations seem to exist for other systems):

WSAPOLLFD polledSocket;
polledSocket.fd = socketItf;
polledSocket.events = POLLRDNORM | POLLWRNORM;

if (WSAPoll(&polledSocket, 1, 0) > 0)
{
    if (polledSocket.revents &= (POLLERR | POLLHUP))
    {
        // socket closed
        return FALSE;
    }
}

回答5:

If your proxy must be a general purpose proxy for any protocol, then you should handle also those clients which sends data and immediately calls close after the send (one way data transfer only).

So if client A sends a data and closes the connection before the connection is opened to B, don't worry, just forward the data to B normally (when connection to B is opened).

There is no need to implement special handling for this scenario.

Your proxy will detect the closed connection when:

• read returns zero after connection to B is opened and all pending data from A is read. or
• your programs try to send data (from B) to A.

回答6:

You could check if the socket is still connected by trying to write to the file descriptor for each socket. Then if the return value of the write is -1 or if errno = EPIPE, you know that socket has been closed.
for example

int isSockStillConnected(int *fileDescriptors, int numFDs){
     int i,n;
     for (i=0;i<numFDs;i++){
          n = write(fileDescriptors+i,"heartbeat",9);
          if (n < 0) return -1;
          if (errno == EPIPE) return -1;
     }
     //made it here, must be okay
     return 0;
}