Application control of TCP retransmission on Linux

Question

For the impatient:

How to change the value of /proc/sys/net/ipv4/tcp_retries2 for a single connection in Linux, using setsockopt()

Answer 1

int name[] = {CTL_NET, NET_IPV4, NET_IPV4_TCP_RETRIES2};
long value = 0;
size_t size = sizeof(value);
if(!sysctl(name, sizeof(name)/sizeof(name[0]), &value, &size, NULL, 0) {
  value // It contains current value from /proc/sys/net/ipv4/tcp_retries2
}
value = ... // Change value if it needed
if(!sysctl(name, sizeof(name)/sizeof(name[0]), NULL, NULL, &value, size) {
  // Value in /proc/sys/net/ipv4/tcp_retries2 changed successfully
}

Programmatically way using C. It works at least on Ubuntu. But (according with code and system variables) looks like it influences on all TCP connections in system, not only one single connection.