Determine IP address of CONNECTED interface (linux) in python

Question

On my linux machine, 1 of 3 network interfaces may be actually connected to the internet. I need to get the IP address of the currently connected interface, keeping in mind that my other 2 interfaces may be assigned IP addresses, just not be connected.

I can just ping a website through each of my interfaces to determine which one has connectivity, but I'd like to get this faster than waiting for a ping time out. And I'd like to not have to rely on an external website being up.

Update:

All my interfaces may have ip addresses and gateways. This is for an embedded device. So we allow the user to choose between say eth0 and eth1. But if there's no connection on the interface that the user tells us to use, we fall back to say eth2 which (in theory) will always work.

So what I need to do is first check if the user's selection is connected and if so return that IP. Otherwise I need to get the ip of eth2. I can get the IPs of the interfaces just fine, it's just determining which one is actually connected.

Answer 1

If the default gateway for the system is reliable, then grab that from the output from route -n the line that contains " UG " (note the spaces) will also contain the IP of the gateway and interface name of the active interface.

Answer 2

the solution is here : http://code.activestate.com/recipes/439093-get-names-of-all-up-network-interfaces-linux-only/

import fcntl
import array
import struct
import socket
import platform
"""
global constants.  If you don't like 'em here,
move 'em inside the function definition.
"""
SIOCGIFCONF = 0x8912
MAXBYTES = 8096

def localifs():
    """
    Used to get a list of the up interfaces and associated IP addresses
    on this machine (linux only).

    Returns:
    List of interface tuples.  Each tuple consists of
    (interface name, interface IP)
    """
    global SIOCGIFCONF
    global MAXBYTES

    arch = platform.architecture()[0]

    # I really don't know what to call these right now
    var1 = -1
    var2 = -1
    if arch == '32bit':
        var1 = 32
        var2 = 32
    elif arch == '64bit':
        var1 = 16
        var2 = 40
    else:
        raise OSError("Unknown architecture: %s" % arch)

    sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
    names = array.array('B', '\0' * MAXBYTES)
    outbytes = struct.unpack('iL', fcntl.ioctl(
        sock.fileno(),
        SIOCGIFCONF,
        struct.pack('iL', MAXBYTES, names.buffer_info()[0])
        ))[0]

    namestr = names.tostring()
    return [(namestr[i:i+var1].split('\0', 1)[0], socket.inet_ntoa(namestr[i+20:i+24])) \
            for i in xrange(0, outbytes, var2)]


print localifs()