How to get interface index from interface ip addres

Question

Can anyone tell me how to get interface index from interface ip address? e.g. If interface ip address is 192.168.23.25 then what is it's interface index.

I want to add on that i need to use it in one code written in c so if any function with some option can give me the interface index number on the base of the interface ip address.

Answer 1

Why dont you use netstat command?

Answer 2

You could use this:. It will list the network interfaces on your Linux box.

    #include <sys/ioctl.h>
#include <net/if.h>
#include <netinet/in.h>
#include <stdio.h>
#include <arpa/inet.h>

int main(void)
{
    char          buf[1024];
    struct ifconf ifc;
    struct ifreq *ifr;
    int           sck;
    int           nInterfaces;
    int           i;

/* Get a socket handle. */
    sck = socket(AF_INET, SOCK_DGRAM, 0);
    if(sck < 0)
    {
        perror("socket");
        return 1;
    }

/* Query available interfaces. */
    ifc.ifc_len = sizeof(buf);
    ifc.ifc_buf = buf;
    if(ioctl(sck, SIOCGIFCONF, &ifc) < 0)
    {
        perror("ioctl(SIOCGIFCONF)");
        return 1;
    }

/* Iterate through the list of interfaces. */
    ifr         = ifc.ifc_req;
    nInterfaces = ifc.ifc_len / sizeof(struct ifreq);
    for(i = 0; i < nInterfaces; i++)
    {
        struct ifreq *item = &ifr[i];

    /* Show the device name and IP address */
        printf("%s: IP %s",
               item->ifr_name,
               inet_ntoa(((struct sockaddr_in *)&item->ifr_addr)->sin_addr));

    /* Get the MAC address */
        if(ioctl(sck, SIOCGIFHWADDR, item) < 0)
        {
            perror("ioctl(SIOCGIFHWADDR)");
            return 1;
        }

    /* Get the broadcast address (added by Eric) */
        if(ioctl(sck, SIOCGIFBRDADDR, item) >= 0)
            printf(", BROADCAST %s", inet_ntoa(((struct sockaddr_in *)&item->ifr_broadaddr)->sin_addr));
        printf("\n");
    }

        return 0;
}

I got it from here.

Answer 3

You can't do that, you have to look at all interfaces, then loop through all IP addresses until you find the one you want. I think this code does what you want.

#include <sys/ioctl.h>
#include <net/if.h>
#include <netinet/in.h>
#include <stdio.h>
#include <arpa/inet.h>

int main(int argc, char *argv[])
{
    in_addr_t ia;
    int id;

    ia = inet_addr(argv[1]);

    id = do_lookup(ia);
}

int do_lookup(in_addr_t ia) {
    char          buf[1024];
    struct ifconf ifc;
    struct ifreq *ifr;
    int           sck;
    int           nInterfaces;
    int           i;

/* Get a socket handle. */
    sck = socket(AF_INET, SOCK_DGRAM, 0);
    if(sck < 0)
    {
        perror("socket");
        return -1;
    }

/* Query available interfaces. */
    ifc.ifc_len = sizeof(buf);
    ifc.ifc_buf = buf;
    if(ioctl(sck, SIOCGIFCONF, &ifc) < 0)
    {
        perror("ioctl(SIOCGIFCONF)");
        return -1;
    }

/* Iterate through the list of interfaces. */
    ifr         = ifc.ifc_req;
    nInterfaces = ifc.ifc_len / sizeof(struct ifreq);
    for(i = 0; i < nInterfaces; i++)
    {
        struct ifreq *item = &ifr[i];
        if(((struct sockaddr_in *)&item->ifr_addr)->sin_addr == ia) {
            return i;
        }
    }

    return -1;
}

Answer 4

SIOCGIFCONF won't cut it for secondary IP addresses added using

ip addr add 192.168.25.23/24 dev eth1

If you really need to do this, then look at whatever "ip addr sh" uses - probably a netlink socket operation (which involves really strange macros)

Answer 5

You should be able to do this with getifaddrs(). It should account for MarkR's concern about secondary addresses. As a test,

After adding something like this:

ip addr add 192.168.25.23/24 dev eth0

compiling and running the example program on the man page should show something like:

lo  address family: 17 (AF_PACKET)
eth0  address family: 17 (AF_PACKET)
lo  address family: 2 (AF_INET)
        address: <127.0.0.1>
eth0  address family: 2 (AF_INET)
        address: <192.168.1.105>
eth0  address family: 2 (AF_INET)
        address: <192.168.25.23>
lo  address family: 10 (AF_INET6)
        address: <::1>
eth0  address family: 10 (AF_INET6)
        address: <fe84::82d6:baaf:fe14:4c22%eth0>

You should be able to get the index as you traverse the list but you can also additionally look at the if_nametoindex(), if_indextoname(), and if_nameindex() functions. Since you will be able to associate an address with an interface name you could just call these as appropriate.