example of finding an unreserved port in /etc/services

bind_unreserved_port.c (text/plain), 1.32 KB, created by SpanKY on 2007-03-25 10:13:21 UTC

/* Allocate a privileged port that is not reserved according to
 * the /etc/services database.  This allows the user to reserve
 * mark ports as "taken" by updating that file rather than having
 * a ton of packages install a ton of little files that somehow
 * magically get accumulated to reserve ports ...
 * http://bugs.gentoo.org/77810
 *
 * We do this recursively so that we dont have to worry about the
 * libc rebinding the same privileged port over and over again.
 * We keep all the sockets open until we find one we're happy
 * with and we keep it.
 */
int bind_unreserved_port(int domain, int type, int protocol)
{
	int s;
	struct servent *serv;
	struct sockaddr_in saddr;
	socklen_t slen;

	/* allocate a socket */
	s = socket(domain, type, protocol);
	if (s < 0)
		return s;

	/* attempt to bind to port */
	if (bindresvport(s, (struct sockaddr_in *) 0) != 0)
		goto err_close;

	/* make sure we only bind to non-reserved ports */
	if (getsockname(s, (struct sockaddr *)&saddr, &slen) != 0)
		goto err_close;

	serv = getservbyport(saddr.sin_port, "udp");
	if (serv != NULL) {
		/* this port is listed in /etc/services, so let's try again */
		int punt_socket = s;
		s = bind_unreserved_port(domain, type, protocol);
		close(punt_socket);
	}

	/* awesome, got ourselves a decent port */
	return s;

err_close:
	close(s);
	return -1;
}