#!/usr/bin/env python3

import argparse
import dns.resolver
import netifaces

from subprocess import call

def get_interface_by_ip(ip):
    """Given an IP will return the interface associated. Doesn't support the same IP bound to multiple interfaces.

    Args:
        ip: The IP that belongs to a given interface

    Returns:
        A string that represents a given interface
    """
    for interface in netifaces.interfaces():
        # Get the addresses of all interfaces
        addresses = netifaces.ifaddresses(interface)

        # Check if the interface has an ipv4 address assigned to it
        if netifaces.AF_INET in addresses:

            # Iterate through the ipv4s checking if peer matches next hop
            for ipv4 in addresses[netifaces.AF_INET]:

                if 'peer' in ipv4 and ipv4['peer'] == ip:
                    return interface

    raise FileNotFoundError('Could not identify an interface with the IP "' + ip + '"')

def get_ips(domain):
    """ Given a domain, looks up the IP
    Args:
        domain: The domain that should be queried

    Returns:
        A string IP value for the domain
    """
    result = dns.resolver.query(domain, 'A')
    ips = []
    for answer in result:
        ips.append(answer.to_text())

    return ips

def get_args():
    """Renders and returns the arguments from the CLI

    Returns:
        The namespace object containing arguments of the form:

        {
            'argument_name': 'argument_value'
        }
    """
    parser = argparse.ArgumentParser(description='Add a route for a given domain to a given interface. Only supports IPv4')

    # Add the arguments
    parser.add_argument('--domain', dest='domain', action='store', required=True,
                        help='The domain that you would like to route to via a given interface')

    parser.add_argument('--next-hop', dest='next_hop', action='store', required=True,
                        help='The next hop to route over')

    return parser.parse_args()

def main():

    args      = get_args()
    ips       = get_ips(args.domain)
    interface = get_interface_by_ip(args.next_hop)

    for ip in ips:
        result = call(['ip', 'route', 'add',ip + '/32', 'via',args.next_hop,'dev',interface])

        if result == 2:
            raise PermissionError('Unable to add the route: insufficient permissions')
        elif result > 0:
            raise Exception('Unable to add the route: exit code from "ip" nonzero (' + result + ')')
        else:
            print('Added route for "' + ip + '" via "' + args.next_hop + ' (' + interface + ')"')

if __name__ == "__main__":
    main()