getifaddr.c

#include <arpa/inet.h>
#include <sys/socket.h>
#include <netdb.h>
#include <ifaddrs.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>

int Usage () {
    printf("Usage: getifaddr [ifaddr] [inet|inet6]\nDefaults to eth0/inet.\n");
    return 8;
}

int main(int argc, char *argv[]) {
    struct ifaddrs *ifaddr, *ifa;
    int family, s, inet;
    char host[NI_MAXHOST], *iface;

    iface = NULL; inet = AF_INET;
    if ( argc >= 2 ) {
        if ( !strcmp(argv[1], "-h") || !strcmp(argv[1], "--help") ) {
            return Usage();
        }
    } else if ( argc <= 2 ) {
        return Usage();
    }

    if ( argc >= 2 ) {
        iface = strndup(argv[1], strlen(argv[1]));
    }

    if ( argc >= 3 ) {
        if ( !strcmp(argv[2], "inet6") ) {
            inet = AF_INET6;
        }
    }

    if ( iface == NULL ) {
        iface = malloc(4);
        if ( iface == NULL ) return 4;
        strncpy(iface, "eth0", 4);
    }

    if (getifaddrs(&ifaddr) == -1) {
        perror("getifaddrs");
        exit(EXIT_FAILURE);
    }

    memset(host, 0, sizeof host);

    /* Walk through linked list, maintaining head pointer so we can free list later */

    for (ifa = ifaddr; ifa != NULL; ifa = ifa->ifa_next) {
        if (ifa->ifa_addr == NULL) continue;
        family = ifa->ifa_addr->sa_family;

        if ( family == AF_PACKET ) continue;

        /* Display interface name and family (including symbolic
            form of the latter for the common families) */
        if ( !strcmp(ifa->ifa_name, iface) ) {
            if ( family == inet ) {
                s = getnameinfo(ifa->ifa_addr, ( (family == AF_INET) ? sizeof(struct sockaddr_in) : sizeof(struct sockaddr_in6) ), host, NI_MAXHOST, NULL, 0, NI_NUMERICHOST);
                if (s != 0) {
                    fprintf(stderr, "getnameinfo() failed: %s\n", gai_strerror(s));
                    exit(EXIT_FAILURE);
                }
            }

            printf( "%s\n", host );
            freeifaddrs(ifaddr);
            free(iface);
            return 0;
        }
    }

    fprintf(stderr, "Could not find devivce.\n");
    freeifaddrs(ifaddr);
    free(iface);
    exit(EXIT_SUCCESS);
}