Thread: TCP server not detecting broken connection

I am attempting to write a server application in C on a linux machine which listens for TCP connections and transfers data. I am trying to detect on the server side when the connection is broken. The closest thing that I got to work was looking at the return value from sending data. For example the server’s job is to mainly read data from the socket but also sends data back to the client periodically to test if the connection is still up. I look at the return value from send() to determine if the connection is broken e.g.

Code:

int ret = send(session->clientSocket, &data[sentCnt], count - sentCnt, MSG_NOSIGNAL);

I found that this does not immediately return an error when the connection is broken. The reason for this is because even though the connection is broken send() is still successful because it is able to put it on the network buffer. To fix the issue I did the following things;

Code:

 //set send timeout
    struct timeval timeout;
    timeout.tv_sec = 4;
    timeout.tv_usec = 0;

    //apply send timeout socket options
    setsockopt(tcp->socket, SOL_SOCKET, SO_SNDTIMEO, &timeout, sizeof(struct timeval));

    //set output buffer size so that send blocks
    int buffersize = 2;
    setsockopt(tcp->socket, SOL_SOCKET, SO_SNDBUF, &buffersize, 4);

Setting the timeout and the buffer should ensure that the socket will timeout if the data sent is more than 2 bytes and the connection is broken.

I have tested this and it worked i.e. when i pull the plug from the modem the server detects that the conneciton is broken fairly quickly.
The server runs as a virtual machine somewhere in the US and the client which im using to test is in Sydney Australia. I tested the client on my Telstra cable connection and it worked. I also tested on my Optus 3G wireless broadband connection and it also worked. The problem is however when I tested on my Telstra 3G wireless broadband and it DID NOT work. The server was happily thinking that it was sending data to the client when the client was well and truly disconnected from the internet. I had swaped modems and simcards around isolated the problem down to the network provider i.e. works on Telstra cable and Optus 3G but not on Telstra 3G.

How can the server think that it’s successfully sent data to the client which is not connected to the internet, I thought TCP was in end to end protocol.

Does anyone have any idea why this occurs or how else i can go about doing this?

Hi MK27,
Thanks for you input, im not sure what you mean exactly when you say

it seems to me these can be prevented by using signal() to ignore SIGPIPE and SIGBUS, and always catching errors returned by recv/read/send/write.

but as you can see from send() im using NO_SIGNAL because i found that if it tries to send when the connection is broken the application exits (probably because i don't handle the signals properly).

I have also found that using read() and looking for -1 and errno is probably not the best because i have set a read timeout and it always returns -1 with errno EAGAIN when there is no data to read i.e. the client has not sent anything. it is possible in my application for the client to not send anything for some time and i don't want to break the connection in this case.

I think its strange that i need to implement my own protocol because TCP does not do what its supposed to do. I know that in TCP for any data that is sent there should be an ACK which should come from the other end. If it does not receive it then it tries again until it times out. I expected that this is what was occurring with the ISP's that were working but i dont know whats happening with the one that does not. I think ill do some local testing and look at whats happening in wireshark.

Originally Posted by MK27

it seems to me these can be prevented by using signal() to ignore SIGPIPE and SIGBUS

It's good to catch SIGPIPE, but SIGBUS should never be thrown during networking. If your getting SIGBUS then something is wrong with your code.

Originally Posted by MK27

Code:

        int errs = 0;
        [ ... ]
        while ((t < sz) && (r = read(sock, &data[t], sz-t))) {
                if (r == -1) {
                        if (errno == EAGAIN) {
                                if (++errs == 10) {
                                        error("takecall() EAGAIN repeated 10 times on read()", NULL);
                                        sendMsg(sock, "ERROR");
                                } else continue;
                        } else error("takecall()->read() failed: ", strerror(errno), NULL);
                        close(sock);
                        return;
                }
                t += r;
        }

From the looks of this, if the socket reports no data (EAGAIN), you just call read() again 10 consecutive times. This is wrong: What if the network is slow, and can't get more data to the recv buffer within the time it takes to call read() 10 more times? Your code would incorrectly assume the connection is broken. You should poll() or select() for the data.