route_lookup.c

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


int main(int argc, char* argv[])
{
    int s;
    /* struct in_addr addr; */
    struct sockaddr_in dest = { 0 };
    dest.sin_family = AF_INET;
    dest.sin_len = sizeof(struct sockaddr_in);
    dest.sin_port = 80;

    if(argc < 2)
    {
        fprintf(stderr, "not enough arguments: must supply destination ip address\n");
        return 1;
    }

    if(0 == inet_aton(argv[1], &dest.sin_addr))
    {
        fprintf(stderr, "invalid ip address\n");
        return 2;
    }

    s = socket(AF_INET, SOCK_DGRAM, 0);
    if(s)
    {
        struct sockaddr_in sin = { 0 };
        int result;
        socklen_t socklen = sizeof(struct sockaddr_in);

        sin.sin_family = AF_INET;


        result = bind(s, (struct sockaddr*) &sin, sizeof(struct sockaddr_in));
        if(0 != result)
        {
            perror("bind");
            goto exit;
        }

        result = connect(s, (struct sockaddr*) &dest, sizeof(struct sockaddr_in));
        if(0 != result)
        {
            perror("connect");
            goto exit;
        }
        

        result = getsockname(s, (struct sockaddr*) &sin, &socklen);
        if(0 == result)
        {
            // printf("sin_family=%d, sin_addr.s_addr=%08x, sin_port=%d\n", sin.sin_family, sin.sin_addr.s_addr, sin.sin_port);
            printf("Source address: %s\n", inet_ntoa(sin.sin_addr));
        }
        else
        {
            perror("getsockname");
        }


exit:
        close(s);
    }

    return 0;
}