small demo program for tunneling over UDP with tun/tap

/*
 * tunproxy.c --- small demo program for tunneling over UDP with tun/tap
 *
 * Copyright (C) 2003  Philippe Biondi <phil@secdev.org>
 *
 * This program is free software; you can redistribute it and/or modify it
 * under the terms of the GNU Lesser General Public License as published by
 * the Free Software Foundation.
 *
 * This program is distributed in the hope that it will be useful, but
 * WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * Lesser General Public License for more details.
 */



#include <stdio.h>
#include <unistd.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <string.h>
#include <net/if.h>
#include <linux/if_tun.h>
#include <getopt.h>
#include <sys/ioctl.h>

#define PERROR(x) do { perror(x); exit(1); } while (0)
#define ERROR(x, args ...) do { fprintf(stderr,"ERROR:" x, ## args); exit(1); } while (0)

char MAGIC_WORD[] = "Wazaaaaaaaaaaahhhh !";


void usage()
{
	fprintf(stderr, "Usage: tunproxy [-s port|-c targetip:port] [-e]/n");
	exit(0);
}

int main(int argc, char *argv[])
{
	struct sockaddr_in sin, sout, from;
	struct ifreq ifr;
	int fd, s, fromlen, soutlen, port, PORT, l;
	char c, *p, *ip;
	char buf[1500];
	fd_set fdset;
	
	

	int MODE = 0, TUNMODE = IFF_TUN, DEBUG = 0;

	while ((c = getopt(argc, argv, "s:c:ehd")) != -1) {
		switch (c) {
		case 'h':
			usage();
		case 'd':
			DEBUG++;
			break;
		case 's':
			MODE = 1;
			PORT = atoi(optarg);
			break;
		case 'c':
			MODE = 2;
			p = memchr(optarg,':',16);
			if (!p) ERROR("invalid argument : [%s]/n",optarg);
			*p = 0;
			ip = optarg;
			port = atoi(p+1);
			PORT = 0;
			break;
		case 'e':
			TUNMODE = IFF_TAP;
			break;
		default:
			usage();
		}
	}
	if (MODE == 0) usage();

	if ( (fd = open("/dev/net/tun",O_RDWR)) < 0) PERROR("open");

	memset(&ifr, 0, sizeof(ifr));
	ifr.ifr_flags = TUNMODE;
	strncpy(ifr.ifr_name, "toto%d", IFNAMSIZ);
	if (ioctl(fd, TUNSETIFF, (void *)&ifr) < 0) PERROR("ioctl");

	printf("Allocated interface %s. Configure and use it/n", ifr.ifr_name);
	
	s = socket(PF_INET, SOCK_DGRAM, 0);
	sin.sin_family = AF_INET;
	sin.sin_addr.s_addr = htonl(INADDR_ANY);
	sin.sin_port = htons(PORT);
	if ( bind(s,(struct sockaddr *)&sin, sizeof(sin)) < 0) PERROR("bind");

	fromlen = sizeof(from);

	if (MODE == 1) {
		while(1) {
			l = recvfrom(s, buf, sizeof(buf), 0, (struct sockaddr *)&from, &fromlen);
			if (l < 0) PERROR("recvfrom");
			if (strncmp(MAGIC_WORD, buf, sizeof(MAGIC_WORD)) == 0)
				break;
			printf("Bad magic word from %s:%i/n", 
			       inet_ntoa(from.sin_addr.s_addr), ntohs(from.sin_port));
		} 
		l = sendto(s, MAGIC_WORD, sizeof(MAGIC_WORD), 0, (struct sockaddr *)&from, fromlen);
		if (l < 0) PERROR("sendto");
	} else {
		from.sin_family = AF_INET;
		from.sin_port = htons(port);
		inet_aton(ip, &from.sin_addr);
		l =sendto(s, MAGIC_WORD, sizeof(MAGIC_WORD), 0, (struct sockaddr *)&from, sizeof(from));
		if (l < 0) PERROR("sendto");
		l = recvfrom(s,buf, sizeof(buf), 0, (struct sockaddr *)&from, &fromlen);
		if (l < 0) PERROR("recvfrom");
		if (strncmp(MAGIC_WORD, buf, sizeof(MAGIC_WORD) != 0))
			ERROR("Bad magic word for peer/n");
	}
	printf("Connection with %s:%i established/n", 
	       inet_ntoa(from.sin_addr.s_addr), ntohs(from.sin_port));
	while (1) {
		FD_ZERO(&fdset);
		FD_SET(fd, &fdset);
		FD_SET(s, &fdset);
		if (select(fd+s+1, &fdset,NULL,NULL,NULL) < 0) PERROR("select");
		if (FD_ISSET(fd, &fdset)) {
			if (DEBUG) write(1,">", 1);
			l = read(fd, buf, sizeof(buf));
			if (l < 0) PERROR("read");
			if (sendto(s, buf, l, 0, (struct sockaddr *)&from, fromlen) < 0) PERROR("sendto");
		} else {
			if (DEBUG) write(1,"<", 1);
			l = recvfrom(s, buf, sizeof(buf), 0, (struct sockaddr *)&sout, &soutlen);
			if ((sout.sin_addr.s_addr != from.sin_addr.s_addr) || (sout.sin_port != from.sin_port))
				printf("Got packet from  %s:%i instead of %s:%i/n", 
				       inet_ntoa(sout.sin_addr.s_addr), ntohs(sout.sin_port),
				       inet_ntoa(from.sin_addr.s_addr), ntohs(from.sin_port));
			if (write(fd, buf, l) < 0) PERROR("write");
		}
	}
}