findroute

main.c

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <netinet/in.h>
#include <arpa/inet.h>

#define VERSION "0.1"
#define PATH "/proc/net/route"

static int find_route (struct in_addr ia)
{
	struct
	{
		char iface[10];
		int dest;
		int gateway;
		int mask;
	} route;

	struct in_addr gateway;
	struct in_addr netmask;
	struct in_addr destion;

	int is_firstline = 1;
	int is_default = 1;
	char buf[BUFSIZ] = { '\0' };

	FILE *fp = NULL;
	fp = fopen (PATH, "r");
	if (NULL == fp)
	{
		perror ("cannot open route table!\n");
		return -1;
	}

	while (fgets (buf, BUFSIZ, fp) != NULL)
	{
		if (is_firstline)
		{
			is_firstline = 0;
			continue;
		}
		sscanf (buf, "%s%x%x%*x%*x%*x%*x%x%*x%*x%*x", route.iface, &route.dest, &route.gateway, &route.mask);

		destion.s_addr = route.dest;
		gateway.s_addr = route.gateway;
		netmask.s_addr = route.mask;

		if (ia.s_addr == route.dest)
		{
			printf ("via the gateway: %s\n", inet_ntoa (gateway));
			is_default = 0;
			break;
		}
	}

	if (is_default)
	{
		printf ("via default gateway: %s\n", inet_ntoa (gateway));
	}
	fclose (fp);
	return 0;
}

int main (int argc, char **argv)
{
	struct in_addr ia;
	int opt;
	while ((opt = getopt (argc, argv, "ha:")) != -1)
	{
		switch (opt)
		{
		case 'a':
			if (inet_aton (optarg, &ia))
			{
				return find_route (ia) ? EXIT_SUCCESS : EXIT_FAILURE;
			}
			else
			{
				puts ("invalid address");
				return EXIT_FAILURE;
			}
		default:
			printf ("version %s\n" "usage: %s [-h help] [-a address]\n", VERSION, argv[0]);
			return (opt == '?' || opt == 'h') ? EXIT_SUCCESS : EXIT_FAILURE;
		}
	}
}