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

#include <netdb.h>
#include <ifaddrs.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

static void die(const char *str)
{
	fprintf(stderr, "%s\n", str);
	exit(1);
}

static int is_same_subnet(struct sockaddr_in *a, struct sockaddr_in *b,
		      struct sockaddr_in *mask)
{
	if (a->sin_family != b->sin_family)
		return 0;
	return ((a->sin_addr.s_addr & mask->sin_addr.s_addr)
		== (b->sin_addr.s_addr & mask->sin_addr.s_addr));
}

static int get_bcast_addr(const char *iface, struct sockaddr_in *src,
			  struct sockaddr_in *bcast)
{
	struct ifaddrs *ifaddr, *ifa;
	int ret = 0;

	if (getifaddrs(&ifaddr) == -1)
		die("getifaddrs");

	for (ifa = ifaddr; ifa != NULL; ifa = ifa->ifa_next) {
		if (ifa->ifa_addr == NULL
		    || ifa->ifa_addr->sa_family != AF_INET
		    || (strcmp(iface, ifa->ifa_name) != 0))
			continue;

		/* dont forward broadcast from our own subnet(s) */
		if (is_same_subnet((struct sockaddr_in *)ifa->ifa_addr, src,
				   (struct sockaddr_in *)ifa->ifa_netmask))
			continue;

		memcpy(bcast, ifa->ifa_ifu.ifu_broadaddr,
			sizeof(struct sockaddr_in));
		ret = sizeof(struct sockaddr_in);
		break;
	}
	freeifaddrs(ifaddr);
	return ret;
}

static int run_proxy(const char *iface, int port)
{
	struct sockaddr_in src, dst;
	char buf[256];
	int fd, n = 1;
	socklen_t addrlen;

	fd = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);
	if (fd < 0)
		die("socket");

	memset(&src, 0, sizeof(src));
	src.sin_family = AF_INET;
	src.sin_port = htons(port);
	src.sin_addr.s_addr = htonl(INADDR_ANY);

	if (bind(fd, (struct sockaddr *)&src, sizeof(src)) < 0)
		die("bind");
	setsockopt(fd, SOL_SOCKET, SO_BROADCAST, &n, sizeof(n));

	while (1) {
		n = recvfrom(fd, buf, sizeof(buf), 0, (struct sockaddr *)&src,
			     &addrlen);
		if (n < 0)
			die("recvfrom");

		if (get_bcast_addr(iface, &src, &dst)) {
			dst.sin_port = htons(port);
			sendto(fd, buf, n, 0, (struct sockaddr *) &dst,
			       sizeof(dst));
		}
	}
	return 0;
}

int main(int argc, char *argv[])
{
	int port = 9;
	const char *iface = NULL;
	if (argc > 1)
		iface = argv[1];

	return run_proxy(iface, port);
}