Add support for systemd style socket activation.

If the LISTEN_FDS environment variable is set and tinc is run in the
foreground, tinc will use filedescriptors 3 to 3 + LISTEN_FDS for its listening
TCP sockets. For now, tinc will create matching listening UDP sockets itself.

There is no dependency on systemd or on libsystemd-daemon.

src/net_setup.c

@@ -581,6 +581,46 @@ static bool setup_myself(void) {
 
 	/* Open sockets */
 
+	if(!do_detach && getenv("LISTEN_FDS")) {
+		sockaddr_t sa;
+		socklen_t salen;
+
+		listen_sockets = atoi(getenv("LISTEN_FDS"));
+#ifdef HAVE_UNSETENV
+		unsetenv("LISTEN_FDS");
+#endif
+
+		if(listen_sockets > MAXSOCKETS) {
+			logger(LOG_ERR, "Too many listening sockets");
+			return false;
+		}
+
+		for(i = 0; i < listen_sockets; i++) {
+			salen = sizeof sa;
+			if(getsockname(i + 3, &sa.sa, &salen) < 0) {
+				logger(LOG_ERR, "Could not get address of listen fd %d: %s", i + 3, sockstrerror(errno));
+				return false;
+			}
+
+			listen_socket[i].tcp = i + 3;
+
+#ifdef FD_CLOEXEC
+		        fcntl(i + 3, F_SETFD, FD_CLOEXEC);
+#endif
+
+			listen_socket[i].udp = setup_vpn_in_socket(&sa);
+			if(listen_socket[i].udp < 0)
+				return false;
+
+			ifdebug(CONNECTIONS) {
+				hostname = sockaddr2hostname(&sa);
+				logger(LOG_NOTICE, "Listening on %s", hostname);
+				free(hostname);
+			}
+
+			memcpy(&listen_socket[i].sa, &sa, salen);
+		}
+	} else {
 		listen_sockets = 0;
 		cfg = lookup_config(config_tree, "BindToAddress");
 
@@ -646,6 +686,7 @@ static bool setup_myself(void) {
 
 			freeaddrinfo(ai);
 		} while(cfg);
+	}
 
 	if(listen_sockets)
 		logger(LOG_NOTICE, "Ready");

src/tincd.c

@@ -539,6 +539,12 @@ int main(int argc, char **argv) {
 
 	g_argv = argv;
 
+	if(getenv("LISTEN_PID") && atoi(getenv("LISTEN_PID")) == getpid())
+		do_detach = false;
+#ifdef HAVE_UNSETENV
+	unsetenv("LISTEN_PID");
+#endif
+
 	init_configuration(&config_tree);
 
 	/* Slllluuuuuuurrrrp! */