How to check Ethernet in Linux?

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.

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

#include <stdio.h>        // printf
#include <string.h>       // strncpy
//#include <sys/socket.h> // AF_INET
#include <sys/ioctl.h>    // SIOCGIFFLAGS
#include <errno.h>        // errno
#include <netinet/in.h>   // IPPROTO_IP
#include <net/if.h>       // 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 <asm/types.h>     // __u32
#include <linux/ethtool.h> // ETHTOOL_GLINK
#include <linux/sockios.h> // 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;
}

The network interfaces can be seen in sysfs: /sys/class/net/eth[x]. There you can check the link, interface status, and more.

You may want to take advantage of libudev to get around in /sys:

http://www.signal11.us/oss/udev/

I simply check if an Ip address is assigned to the network card.

You can use something like this to check if lan is up in the given network card (say eth0) :

/sbin/ifconfig eth0| grep 'inet addr:' | wc -l

This should simply return 0 or 1 based on whether or not an ip address is assigned to the nic.

Also you may use Yann Ramin's method to list out all the nic's & perform the check.

I missed out noticing you are looking for a c code. Maybe adding a tag would be good.

Either ways, I think you can look at the same file (ifconfig) manually in c by reading it for an ip.

As this question is somewhat important, I'll add another answer despite its extreme age. You can read the contents of /sys/class/net/eth0/operstate which will simply contain the string "up\n" or "down\n".

How to check Ethernet in Linux?

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