pf DIOCNATLOOK example - taken from spamd/grey.c

pf_nat_reverse.c

/* Stolen from ftp-proxy */
int
server_lookup(struct sockaddr *client, struct sockaddr *proxy,
    struct sockaddr *server)
{
	if (client->sa_family == AF_INET)
		return (server_lookup4(satosin(client), satosin(proxy),
		    satosin(server)));

	if (client->sa_family == AF_INET6)
		return (server_lookup6(satosin6(client), satosin6(proxy),
		    satosin6(server)));

	errno = EPROTONOSUPPORT;
	return (-1);
}

int
server_lookup4(struct sockaddr_in *client, struct sockaddr_in *proxy,
    struct sockaddr_in *server)
{
	struct pfioc_natlook pnl;

#ifdef __FreeBSD__
	if(!use_pf){
		memset(server, 0, sizeof(struct sockaddr_in));
		server->sin_family = AF_INET;
		server->sin_len = sizeof(struct sockaddr_in);
		memcpy(&server->sin_addr.s_addr, &proxy->sin_addr.s_addr, sizeof proxy->sin_addr.s_addr);
		memcpy(&server->sin_port, &proxy->sin_port, sizeof proxy->sin_port);
		return (0);
	}
#endif

	memset(&pnl, 0, sizeof pnl);
	pnl.direction = PF_OUT;
	pnl.af = AF_INET;
	pnl.proto = IPPROTO_TCP;
	memcpy(&pnl.saddr.v4, &client->sin_addr.s_addr, sizeof pnl.saddr.v4);
	memcpy(&pnl.daddr.v4, &proxy->sin_addr.s_addr, sizeof pnl.daddr.v4);
	pnl.sport = client->sin_port;
	pnl.dport = proxy->sin_port;

	if (ioctl(pfdev, DIOCNATLOOK, &pnl) == -1)
		return (-1);

	memset(server, 0, sizeof(struct sockaddr_in));
	server->sin_len = sizeof(struct sockaddr_in);
	server->sin_family = AF_INET;
	memcpy(&server->sin_addr.s_addr, &pnl.rdaddr.v4,
	    sizeof server->sin_addr.s_addr);
	server->sin_port = pnl.rdport;

	return (0);
}

int
server_lookup6(struct sockaddr_in6 *client, struct sockaddr_in6 *proxy,
    struct sockaddr_in6 *server)
{
	struct pfioc_natlook pnl;

#ifdef __FreeBSD__
	if(!use_pf){
		memset(server, 0, sizeof(struct sockaddr_in6));
		server->sin6_family = AF_INET6;
		server->sin6_len = sizeof(struct sockaddr_in6);
		memcpy(&server->sin6_addr.s6_addr, &proxy->sin6_addr.s6_addr, sizeof proxy->sin6_addr.s6_addr);
		memcpy(&server->sin6_port, &proxy->sin6_port, sizeof proxy->sin6_port);
		return (0);
	}
#endif

	memset(&pnl, 0, sizeof pnl);
	pnl.direction = PF_OUT;
	pnl.af = AF_INET6;
	pnl.proto = IPPROTO_TCP;
	memcpy(&pnl.saddr.v6, &client->sin6_addr.s6_addr, sizeof pnl.saddr.v6);
	memcpy(&pnl.daddr.v6, &proxy->sin6_addr.s6_addr, sizeof pnl.daddr.v6);
	pnl.sport = client->sin6_port;
	pnl.dport = proxy->sin6_port;

	if (ioctl(pfdev, DIOCNATLOOK, &pnl) == -1)
		return (-1);

	memset(server, 0, sizeof(struct sockaddr_in6));
	server->sin6_len = sizeof(struct sockaddr_in6);
	server->sin6_family = AF_INET6;
	memcpy(&server->sin6_addr.s6_addr, &pnl.rdaddr.v6,
	    sizeof server->sin6_addr);
	server->sin6_port = pnl.rdport;

	return (0);
}