socket, detect connection is lost

Question

I'm connecting a server process and a client process with a TCP connection, and I have to detect
that physical connection between the two machines is down. I'm trying to do this using the keepalive,
decreasing the default system wide values to:

TCP_KEEPIDLE=5
TCP_KEEPCNT = 5
TCP_KEEPINTVL = 1

When the connection goes down ( I disconnect the cable ) only the server in 10 seconds detect that the connection has been lost, the client just hangs on the send.

This is the client code:

#include <iostream>
#include <string.h>
#include <sys/socket.h>
#include <stdlib.h>
#include <arpa/inet.h>
#include <errno.h>
#include <netinet/tcp.h>

int main(int argc, char** argv) {
  char myVector[1600];

  int mySocket = socket(AF_INET, SOCK_STREAM, IPPROTO_IP);
  if (mySocket < 0 ) {
    std::cout << "error creating the socket" << strerror(errno) << std::endl;
    ::exit(-1);
 }

 struct sockaddr_in sin;
 memset( (char *)&sin, 0, sizeof( sin ) );
 sin.sin_addr.s_addr = inet_addr("192.168.21.27");
 sin.sin_port   = htons(7788);
 sin.sin_family = AF_INET;

 if ( connect( mySocket, (struct sockaddr *)&sin, sizeof( sin )) < 0 ) {
   std::cout << "Error on connection: " << strerror(errno) << std::endl;
   ::exit(-1);
 }

 int optval = 1;
 socklen_t optlen = sizeof(optval);

 /*Enabling keep alive*/
 if(setsockopt(mySocket, SOL_SOCKET, SO_KEEPALIVE, &optval, optlen) < 0) {
   std::cout << "Error setting SO_KEEPALIVE: " << strerror(errno) << std::endl;
 }

 optval = 5;
 optlen = sizeof(optval);
 if(setsockopt(mySocket, SOL_TCP, TCP_KEEPIDLE, &optval, optlen) < 0) {
    std::cout << "Error setting TCP_KEEPIDLE: " << strerror(errno) << std::endl;
 }

 optval = 5;
 optlen = sizeof(optval);
 if(setsockopt(mySocket, SOL_TCP, TCP_KEEPCNT, &optval, optlen) < 0) {
   std::cout << "Error setting TCP_KEEPCNT: " << strerror(errno) << std::endl;
 }

 optval = 1;
 optlen = sizeof(optval);
 if(setsockopt(mySocket, SOL_TCP, TCP_KEEPINTVL, &optval, optlen) < 0) {
   std::cout << "Error setting TCP_KEEPINTVL: " << strerror(errno) << std::endl;
 }

 for (;;) {
   ssize_t myRet= ::send(mySocket,
                                      myVector,
                                      sizeof(myVector),
                                     0);
   if (myRet < 0) {
     std::cout << "Error: " << strerror(errno) << std::endl;
     break;
   }
   std::cout << myRet << "."; std::cout.flush();
   sleep(1);
 }
}

I'm sure I'm missing something, but what ?

Answer 1

TCP Keepalive isn't intended for this use.

If you want to detect outages at the application layer, do what protocols like SSH, IMAP and IRC do - implement an echo/ping type message at the application layer. Send them on a regular basis, and if you don't get a timely reply, the connection can be assumed to be down.