#!/usr/bin/env python3

import argparse
import ipaddress
import json
import re
import subprocess

from typing import List

DEFAULT_ROUTE = "default"
NS_IP_RE = re.compile(r'^nameserver\s+(\S+)$')
RESOLV_CONF_PATH = '/etc/resolv.conf'

DHCP_CONF_TEMPLATE = """
start {host_addr}
end   {host_addr}

# avoid dhcpd complaining that we have
# too many addresses
maxleases 1

interface {dhcp_intf}

option dns    {dns}
option router {gateway}
option subnet {subnet}
"""

def nameservers() -> List[str]:
    """Returns the list of nameserver IPs in resolv.conf"""
    result = []
    with open(RESOLV_CONF_PATH) as resolv_f:
        for line in resolv_f:
            match = NS_IP_RE.match(line)
            if match:
                result.append(match.group(1))
    return result

def default_route(routes):
    """Returns the host's default route"""
    for route in routes:
        if route['dst'] == DEFAULT_ROUTE:
            return route
    raise ValueError('no default route')

def addr_of(addrs, dev : str) -> ipaddress.IPv4Interface:
    """Finds and returns the IP address of `dev`"""
    for addr in addrs:
        if addr['ifname'] != dev:
            continue
        if len(addr['addr_info']) != 1:
            raise ValueError('only exactly one address on dev is supported')
        info = addr['addr_info'][0]
        return ipaddress.IPv4Interface((info['local'], info['prefixlen']))
    raise ValueError('dev {0} not found'.format(dev))

def generate_conf(intf_name : str) -> str:
    """Generates a dhcpd config. `intf_name` is the interface to listen on."""
    with subprocess.Popen(['ip', '-json', 'route'],
                          stdout=subprocess.PIPE) as proc:
        routes = json.load(proc.stdout) 
    with subprocess.Popen(['ip', '-json', 'addr'],
                          stdout=subprocess.PIPE) as proc:
        addrs = json.load(proc.stdout)
    
    droute = default_route(routes)
    host_addr = addr_of(addrs, droute['dev'])

    return DHCP_CONF_TEMPLATE.format(
        dhcp_intf = intf_name,
        dns = ' '.join(nameservers()),
        host_addr = host_addr.ip,
        gateway = droute['gateway'],
        subnet = host_addr.network.netmask,
    )

if __name__ == '__main__':
    parser = argparse.ArgumentParser()
    parser.add_argument('intf_name')
    args = parser.parse_args()

    print(generate_conf(args.intf_name))