Wrapper to bind Transmission to the IP for a VPN interface on Linux or refuse to start on failure

transmission_vpn.py

#!/usr/bin/env python
"""Simple wrapper to update Transmission's IPv4 binding on startup for use with VPNs.

(For Linux systems. Tested on Ubuntu 14.04 LTS)
"""

import json, os, subprocess

VPN_IF = "tun0"
TRANSMISSION_CFG = os.path.expanduser('~/.config/transmission/settings.json')

def get_iface_ipv4(interface):
    # Call ifconfig for information. Exception if there is no such interface.
    if_ip = subprocess.check_output(['ifconfig', interface])

    # Extract the interface's IPv4 address. Die with an exception if there is
    # none. (ie. if the interface isn't up)
    return [x.split(':')[1].split()[0].strip() for x in if_ip.split('\n')
            if 'inet addr:' in x][0]

def set_transmission_iface(interface):
    try:
        vpn_ip = get_iface_ipv4(interface)
    except Exception:
        subprocess.call(['zenity', '--error', '--text',
            'VPN not found.\nNot starting torrent client.'])
        raise

    try:
        with open(TRANSMISSION_CFG, 'r') as fobj:
            config = json.load(fobj)

        config['bind-address-ipv4'] = vpn_ip

        with open(TRANSMISSION_CFG, 'w') as fobj:
            json.dump(config, fobj, indent=2)
    except Exception:
        subprocess.call(['zenity', '--error', '--text',
            'Could not update Transmission configuration.\n'
            'Not starting torrent client.'])
        raise

if __name__ == '__main__':
    set_transmission_iface(VPN_IF)
    os.execlp('transmission-gtk', 'transmission-gtk')