Returns the IPv4 address used for outbound connections by means of a "dummy" UDP socket. Tested on Linux and OS X (it should be POSIX compliant).

getoutipv4.cpp

#include <iostream>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <cstring>


int getoutipv4(std::string *outip) {

  if (!outip)
    return -1;

  int fd;
  struct sockaddr_in bind_addr, out_addr, det_addr;
  socklen_t det_addr_len;

  memset( &bind_addr, 0, sizeof(bind_addr) );
  memset( &out_addr,  0, sizeof(out_addr)  );
  memset( &det_addr,  0, sizeof(det_addr)  );

  if ( (fd = socket(AF_INET, SOCK_DGRAM, 0)) < 0 )
    return -1;

  bind_addr.sin_family = AF_INET;
  bind_addr.sin_addr.s_addr = htonl(INADDR_ANY);
  bind_addr.sin_port = htons(0);
  if ( bind(fd, (struct sockaddr *)&bind_addr, sizeof(bind_addr)) < 0 ) {
    close(fd);
    return -1;
  }

  out_addr.sin_family = AF_INET;
  out_addr.sin_addr.s_addr = inet_addr("1.2.3.4");
  out_addr.sin_port = htons(1);
  if ( connect(fd, (struct sockaddr *)&out_addr, sizeof(out_addr) ) < 0) {
    close(fd);
    return -1;
  }

  det_addr_len = sizeof(det_addr);
  if ( getsockname(fd, (struct sockaddr *)&det_addr, &det_addr_len) < 0 ) {
    close(fd);
    return -1;
  }

  *outip = inet_ntoa(det_addr.sin_addr);
  close(fd);

  return 0;
}


int main(int argn, char *argv[]) {
  std::string outip;
  if ( getoutipv4(&outip) == 0 )
    std::cout << outip << std::endl;
  else
    std::cout << "An error occurred" << std::endl;
  return 0;
}