QTcpSocket state always connected, even unplugging ethernet wire

Question

I have a QTcpSocket and I am reading into a loop. Each time a full packet has been read, or there has been an error, I manually check the status of the socket inside the loo

Answer 1

Finally found the solution in this Qt forum:

If no data is exchanged for a certain while, TCP will start sending keep-alive segments (basically, ACK segments with the acknowledgement number set to the current sequence number less one). The other peer then replies with another acknowledgement. If this acknowledgment is not received within a certain number of probe segments, the connection is automatically dropped. The little problem is that the kernel starts sending keep-alive segments after 2 hours since when the connection becomes idle! Therefore, you need to change this value (if your OS allows that) or implement your own keep-alive mechanism in your protocol (like many protocols do, e.g. SSH). Linux allows you to change it using setsockopt:

int enableKeepAlive = 1;
int fd = socket->socketDescriptor();
setsockopt(fd, SOL_SOCKET, SO_KEEPALIVE, &enableKeepAlive, sizeof(enableKeepAlive));

int maxIdle = 10; /* seconds */
setsockopt(fd, IPPROTO_TCP, TCP_KEEPIDLE, &maxIdle, sizeof(maxIdle));

int count = 3;  // send up to 3 keepalive packets out, then disconnect if no response
setsockopt(fd, SOL_TCP, TCP_KEEPCNT, &count, sizeof(count));

int interval = 2;   // send a keepalive packet out every 2 seconds (after the 5 second idle period)
setsockopt(fd, SOL_TCP, TCP_KEEPINTVL, &interval, sizeof(interval));