/* Lookup a route for a specified destination */

#include <stdio.h>
#include <netdb.h>
#include <string.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <net/if.h>
#include <sys/ioctl.h>
#include <unistd.h>

struct in_addr *lookup_iface(char *);
struct my_ifreq {
	char ifrn_name[IFNAMSIZ];	/* Interface name, e.g. "en0".  */
	struct sockaddr_in ifru_addr;
} ifreq;

int main(int argc, char **argv) {
	struct hostent *remote_host;
	unsigned int remote_ip;
	FILE *PROC;
	char line[512];

	if (argc > 1) {
		remote_host = gethostbyname(argv[1]);
		memcpy((char *) &remote_ip, remote_host->h_addr_list[0],sizeof(remote_host->h_addr_list[0]));
	} else {
		fprintf(stderr, "Usage: %s remote_ip\n",argv[0]);
		exit(1);
	}

	PROC = fopen("/proc/net/route","r");
	/* First line contains headers */
	fgets(line,sizeof(line),PROC);

	while (!feof(PROC)) {
		char iface[8];
		unsigned int dest, gateway, mask;
		int i,field = 0;
		char *fields[10];

		fgets(line,sizeof(line),PROC);

		for (i=0;i<sizeof(line);i++) {
			char *byteoffset;

			fields[field++] = line + i;
			if ((byteoffset = strchr(line + i,'\t')) == NULL)
				/* No more fields to parse */
				i = sizeof(line);
			else if (field > 9)
				/* Don't need any more fields */
				/* Why 9 and not 8?  Well, because during the 9th loop, we figure the end of the 8th field */
				i = sizeof(line);
			else {
				*byteoffset = '\0';
				i = byteoffset - line;
			}
		}

		sscanf(fields[0],"%s",iface);
		sscanf(fields[1],"%x",&dest);
		sscanf(fields[2],"%x",&gateway);
		sscanf(fields[7],"%x",&mask);

		if (((remote_ip & mask) ^ dest) == 0) {
			struct in_addr *temp;

			printf("Interface is %s\n",iface);
			temp = lookup_iface(iface);
			printf("IP Address is %s\n",inet_ntoa(*temp));
			exit(0);
		}
	}
	fclose(PROC);
	exit(0);
}

struct in_addr *lookup_iface(char *iface) {
	int mysock;

	strncpy(ifreq.ifrn_name,iface,sizeof(ifreq.ifrn_name));

	mysock = socket(PF_INET,SOCK_DGRAM,IPPROTO_IP);
	ioctl(mysock,SIOCGIFADDR,&ifreq);
	close(mysock);

	return( (struct in_addr *) &ifreq.ifru_addr.sin_addr );
}