gistfile1.c

#include <sys/types.h>
#include <sys/ioctl.h>
#include <sys/socket.h>
#include <sys/sockio.h>
#include <sys/stat.h>
#include <arpa/inet.h>
#include <net/if.h>
#include <net/if_tap.h>
#include <netinet/in.h>
#include <err.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

int
main(int argc, const char *argv[])
{
    int dev;
    if ((dev = open("/dev/tap", O_RDWR)) == -1)
        err(1, "open()");
    char *name = fdevname(dev);
    printf("opened %s\n", name);
    int sock;
    if ((sock = socket(AF_INET, SOCK_DGRAM, 0)) == -1)
        err(1, "socket()");
    struct ifreq ifr;
    strlcpy(ifr.ifr_name, name, IFNAMSIZ);
    if (ioctl(sock, SIOCGIFMTU, &ifr) == -1)
        err(1, "ioctl(SIOCGIFMTU)");
    printf("mtu: %d\n", ifr.ifr_mtu);
    struct ifaliasreq ifa;
    strlcpy(ifa.ifra_name, name, IFNAMSIZ);

#define SIN(_sa) (*(struct sockaddr_in *)&(_sa))

    SIN(ifa.ifra_addr).sin_len = sizeof(struct sockaddr_in);
    SIN(ifa.ifra_addr).sin_family = AF_INET;
    SIN(ifa.ifra_addr).sin_addr.s_addr = inet_addr("1.2.3.4");

    SIN(ifa.ifra_broadaddr).sin_len = sizeof(struct sockaddr_in);
    SIN(ifa.ifra_broadaddr).sin_family = AF_INET;
    SIN(ifa.ifra_broadaddr).sin_addr.s_addr = inet_addr("255.255.255.0");

    SIN(ifa.ifra_mask).sin_len = sizeof(struct sockaddr_in);
    SIN(ifa.ifra_mask).sin_family = AF_INET;
    SIN(ifa.ifra_mask).sin_addr.s_addr = inet_addr("1.2.3.255");

    printf("trying to set address to 1.2.3.4/24, go check ifconfig %s and press enter\n", name);
    if (ioctl(sock, SIOCAIFADDR, &ifa) == -1)
        err(1, "ioctl(SIOCAIFADDR)");
    getchar();

    if (ioctl(sock, SIOCGIFFLAGS, &ifr) == -1)
        err(1, "ioctl(SIOCGIFFLAGS)");
    int flags = (ifr.ifr_flags & 0xffff) | (ifr.ifr_flagshigh << 16);
    flags |= IFF_UP;
    ifr.ifr_flags = flags & 0xffff;
    ifr.ifr_flagshigh = flags >> 16;
    if (ioctl(sock, SIOCSIFFLAGS, &ifr) == -1)
        err(1, "ioctl(SIOCSIFFLAGS)");

    printf("is it up? go check ifconfig %s and press enter\n", name);
    getchar();

    if (ioctl(sock, SIOCIFDESTROY, &ifr) == -1)
        err(1, "ioctl(SIOCIFDESTROY)");
}