Python: get default gateway for a local interface/ip address in linux

Question

On Linux, how can I find the default gateway for a local ip address/interface using python?

I saw the question "How to get internal IP, external IP and default gateway for UPnP", but the accepted solution only shows how to get the local IP address for a network interface on windows.

Thanks.

Answer 1

For those people who don't want an extra dependency and don't like calling subprocesses, here's how you do it yourself by reading /proc/net/route directly:

import socket, struct

def get_default_gateway_linux():
    """Read the default gateway directly from /proc."""
    with open("/proc/net/route") as fh:
        for line in fh:
            fields = line.strip().split()
            if fields[1] != '00000000' or not int(fields[3], 16) & 2:
                # If not default route or not RTF_GATEWAY, skip it
                continue

            return socket.inet_ntoa(struct.pack("<L", int(fields[2], 16)))

I don't have a big-endian machine to test on, so I'm not sure whether the endianness is dependent on your processor architecture, but if it is, replace the < in struct.pack('<L', ... with = so the code will use the machine's native endianness.

Answer 2

For completeness (and to expand on alastair's answer), here is an example that uses "netifaces" (tested under Ubuntu 10.04, but this should be portable):

$ sudo easy_install netifaces
Python 2.6.5 (r265:79063, Oct  1 2012, 22:04:36)
...
$ ipython
...
In [8]: import netifaces
In [9]: gws=netifaces.gateways()
In [10]: gws
Out[10]:
{2: [('192.168.0.254', 'eth0', True)],
 'default': {2: ('192.168.0.254', 'eth0')}}
In [11]: gws['default'][netifaces.AF_INET][0]
Out[11]: '192.168.0.254'

Documentation for 'netifaces': https://pypi.python.org/pypi/netifaces/

Answer 3

It seems http://pypi.python.org/pypi/pynetinfo/0.1.9 can do this, but I haven't tested it.