How to check Ethernet in Linux?

Question

I need the test case for Ethernet in Linux using C code to check eth0. If eth0 is down, we enable the net then check if up and the test is passed.<

Answer 1

To check if the link is up, try something like this. It works without root privileges.

#include         // printf
#include        // strncpy
//#include  // AF_INET
#include     // SIOCGIFFLAGS
#include         // errno
#include    // IPPROTO_IP
#include        // IFF_*, ifreq

#define ERROR(fmt, ...) do { printf(fmt, __VA_ARGS__); return -1; } while(0)

int CheckLink(char *ifname) {
    int state = -1;
    int socId = socket(AF_INET, SOCK_DGRAM, IPPROTO_IP);
    if (socId < 0) ERROR("Socket failed. Errno = %d\n", errno);

    struct ifreq if_req;
    (void) strncpy(if_req.ifr_name, ifname, sizeof(if_req.ifr_name));
    int rv = ioctl(socId, SIOCGIFFLAGS, &if_req);
    close(socId);

    if ( rv == -1) ERROR("Ioctl failed. Errno = %d\n", errno);

    return (if_req.ifr_flags & IFF_UP) && (if_req.ifr_flags & IFF_RUNNING);
}

int main() {
    printf("%d\n", CheckLink("eth0"));
}

If IFF_UP is set, it means interface is up (see ifup). If IFF_RUNNING is set then interface is plugged. I also tried using the ethtool ioctl call, but it failed when the gid was not root. But just for the log:

...
#include      // __u32
#include  // ETHTOOL_GLINK
#include  // SIOCETHTOOL
...
int CheckLink(char *ifname) {
    ...
    struct ifreq if_req;
    (void) strncpy( if_req.ifr_name, ifname, sizeof(if_req.ifr_name) );

    struct ethtool_value edata;
    edata.cmd = ETHTOOL_GLINK;
    if_req.ifr_data = (char*) &edata;

    int rv = ioctl(socId, SIOCETHTOOL, &if_req);
    ...

    return !!edata.data;
}