Linux programmatically up/down an interface kernel

Question

What is the programmatic way of enabling or disabling an interface in kernel space? What should be done?

Answer 1

...by using IOCTL's...

ioctl(skfd, SIOCSIFFLAGS, &ifr);

... with the IFF_UP bit set or unset depending on whether you want bring the interface up or down accordingly, i.e.:

static int set_if_up(char *ifname, short flags)
{
    return set_if_flags(ifname, flags | IFF_UP);
}

static int set_if_down(char *ifname, short flags)
{
    return set_if_flags(ifname, flags & ~IFF_UP);
}

Code copy-pasted from Linux networking documentation.

Answer 2

Code to bring eth0 up:

int sockfd;
struct ifreq ifr;

sockfd = socket(AF_INET, SOCK_DGRAM, 0);

if (sockfd < 0)
    return;

memset(&ifr, 0, sizeof ifr);

strncpy(ifr.ifr_name, "eth0", IFNAMSIZ);

ifr.ifr_flags |= IFF_UP;
ioctl(sockfd, SIOCSIFFLAGS, &ifr);