wsl_dns.py

#!/usr/bin/env python

import argparse
import re
import datetime
import ipaddress
import os
import sys

def get_iface_networks(dev):
    # IPv6 is cool, but Nokia doesn't use it
    return re.search(re.compile(r'(?<=inet )(.*)(?=\ brd)', re.M), os.popen(f'ip addr show {dev}').read()).groups()


def get_first_network_address(net):
    # Expects IP/Netmask (172.16.20.164/28)
    return ipaddress.IPv4Interface(net).network[1]


def get_default_gateway():
    return re.search(re.compile(r'via ([0-9.]+)', re.M), os.popen(f'ip route list default').read()).groups()[0]

def generate_resolv(servers, search_list):
    out = []
    out.append(f"# Generated by {sys.argv[0]} at {str(datetime.datetime.now())}")
    for x in servers:
        out.append(f"nameserver {x}")

    if search_list:
        search_str = ','.join(search_list)
        out.append(f"search {search_str}")

    out.append("options timeout:1 retries:1")
    return '\n'.join(out)


def write_resolv(data):
    with open('/etc/resolv.conf', 'w') as fd:
        fd.write(data)
        fd.write('\n')

def main():
    parser = argparse.ArgumentParser()
    parser.add_argument('--servers', type=str, help="dns servers")
    parser.add_argument('--search', type=str, help="dns search suffix")
    args = parser.parse_args()

    # Assumes we only have one IP configured on the Linux eth0 interface
    eth0_network = get_iface_networks('eth0')[0]
    eth0_gateway = str(get_first_network_address(eth0_network))
    default_gateway = get_default_gateway()

    # Sanity Checks
    if eth0_gateway != default_gateway:
        print("Warning: default gateway does not match guessed gateway of eth0")
        print("Warning: script makes assumptions that may not be true on your system!")

    servers = []
    search = []
    if args.servers:
        servers = args.servers.split(',')
    servers += [eth0_gateway]

    if args.search:
        search = args.search.split(',')
    

    config = generate_resolv(servers, search)
    print("Generated resolv.conf")
    print(config)
    write_resolv(config)


if __name__ == "__main__":
    main()