get-ip-netmask-defaultgw.c

#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <sys/ioctl.h>
#include <net/if.h>
#include <arpa/inet.h>
#include <string.h>

#define __sockaddr_in(x) ((struct sockaddr_in *)(x))

struct in_addr get_host_ip(const char *ifname)
{
    int rval;
    int sock;
    struct ifreq ifr;
    struct in_addr error = {
            .s_addr = 0,
    };

    sock = socket(AF_INET, SOCK_DGRAM, 0);
    if (sock == -1)
            return error;

    memset(&ifr, '\0', sizeof(ifr));
    strncpy(ifr.ifr_name, ifname, strlen(ifname));
    rval = ioctl(sock, SIOCGIFADDR, &ifr);
    if (rval)
            return error;

    return __sockaddr_in(&ifr.ifr_addr)->sin_addr;
}

struct in_addr get_host_netmask(const char *ifname)
{
    int rval;
    int sock;
    struct ifreq ifr;
    struct in_addr error = {
            .s_addr = 0,
    };


    sock = socket(AF_INET, SOCK_DGRAM, 0);
    if (sock == -1)
            return error;

    memset(&ifr, '\0', sizeof(ifr));
    strncpy(ifr.ifr_name, ifname, strlen(ifname));
    rval = ioctl(sock, SIOCGIFNETMASK, &ifr);
    if (rval)
            return error;

    return __sockaddr_in(&ifr.ifr_netmask)->sin_addr;
}

struct in_addr get_host_default_gw(void)
{
        FILE *fp;
        char line[BUFSIZ];
        unsigned long gw;
        unsigned long dest;
        struct in_addr rval = {
                .s_addr = 0,
        };

        fp = fopen("/proc/net/route", "r");
        if (!fp)
                return rval;

        while (fgets(line, BUFSIZ, fp)) {
                char *ptr = strchr(line, '\t');

                if (!ptr)
                        return rval;

                sscanf(ptr, "%x", &gw);
                if (gw == 0) {
                        ptr = strchr(++ptr, '\t');
                        if (!ptr)
                                return rval;
                        sscanf(ptr, "%x", &dest);
                        if (dest) {
                                rval.s_addr = dest;
                                return rval;
                        }
                }
        }

        return rval;
}

int main(void)
{
        printf("NETMASK: %s\n", inet_ntoa(get_host_ip("eth0")));
        printf("IP: %s\n",  inet_ntoa(get_host_netmask("eth0")));
        printf("DEFAULT GW: %s\n", inet_ntoa(get_host_default_gw()));

        return 0;
}