How to check if interface is up

Question

Title pretty much says it all. If I run ifconfig, I get this:

eth0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
    inet -snip-  netmask 255.255.255.0  broadcast -snip-
    ...

Using this, I can know if it's up or not (<UP,...), but I want to be able to do this in C (or C++, if there is a simpler solution there) without relying on parsing external processes.

---

Here is what I've got so far (doesn't work):

bool is_interface_online(std::string interface) {
    struct ifreq ifr;
    int sock = socket(PF_INET6, SOCK_DGRAM, IPPROTO_IP);
    memset(&ifr, 0, sizeof(ifr));
    strcpy(ifr.ifr_name, interface.c_str());
    if (ioctl(sock, SIOCGIFFLAGS, &ifr) < 0) {
            perror("SIOCGIFFLAGS");
    }
    close(sock);
    return !!(ifr.ifr_flags | IFF_UP);
}

Can anyone point me in the correct direction for this?

Solution 1

Answer was simple: I used the bitwise OR (|) operator instead of the AND (&) operator. Fixed code is:

bool is_interface_online(std::string interface) {
    struct ifreq ifr;
    int sock = socket(PF_INET6, SOCK_DGRAM, IPPROTO_IP);
    memset(&ifr, 0, sizeof(ifr));
    strcpy(ifr.ifr_name, interface.c_str());
    if (ioctl(sock, SIOCGIFFLAGS, &ifr) < 0) {
            perror("SIOCGIFFLAGS");
    }
    close(sock);
    return !!(ifr.ifr_flags & IFF_UP);
}

Solution 2

If you care about the up/down state of the interface you might want to use the "IFF_RUNNING" flag instead of the "IFF_UP" flag provided by the current answer.

Solution 3

Have you considered using the strace command to see how ifconfig works? you can even see what parameters are passed to functions and other interesting details of how ifconfig works ..

Comments

[MiJyn]

Yes I have, in fact, that's what I used to write the function :D

[David Pickup]

I know this is an extremely old post, but what's the purpose of the !! in the return statement? I'm not aware of a !! operator, so is this simply "not not"? If so, then the !! can be removed.

[Aodren BARY]

a little late answer but the purpose of two !! is simple. It's use to return 0 or 1 from large return exit code. Exemple !! 0 will return 0 because !0 = 1 and !1 = 0. But for any other values n != 0, it will return 1. Exemple with !!10: !10 = 0 and !0 = 1

[Brijesh Valera]

Can I use the same in virtual environment? How can I set interface down in virtual environment? I am running test program which checks IFF_RUNNING flag at every 500 milliseconds. I am not receiving interface down, it always finds IFF_RUNNING flag set. Need help in this matter.