Fetch all Google mail netblocks

google_dns_thing.py

#!/usr/bin/env python3
import dns.resolver
from ipaddress import IPv4Network, IPv6Network, ip_address

GOOGLE_NETBLOCKS = [
    '_netblocks.google.com',
    '_netblocks2.google.com',
    '_netblocks3.google.com',
]

def query_netblocks_txt(name):
    a = dns.resolver.query(name, 'TXT')

    assert len(a) == 1
    txt = a[0]

    # This thing is reall an SPF record, but we'll grab just the IP networks
    parts = str(txt).split()
    for p in parts:
        parts2 = p.split(':', maxsplit=1)
        if len(parts2) != 2:
            continue
        atype, net = parts2
        if atype == 'ip4':
            yield IPv4Network(net)
        elif atype == 'ip6':
            yield IPv6Network(net)
        else:
            raise Exception("WTF:" + str(net))


def load_google_netblocks():
    for name in GOOGLE_NETBLOCKS:
        yield from query_netblocks_txt(name)

def cmd_list(args):
    for net in load_google_netblocks():
        print(net)

def cmd_check(args):
    netblocks = list(load_google_netblocks())
    def in_netblocks(ip):
        for net in netblocks:
            if ip in net:
                return net

    for ip in args.ip:
        net = in_netblocks(ip)
        if net:
            print(f"{ip} in {net}")
        else:
            print(f"{ip} not in Google netblocks")


def parse_args():
    import argparse
    ap = argparse.ArgumentParser()

    sub = ap.add_subparsers(dest='command_name', metavar='COMMAND')
    sub.required = True     # https://stackoverflow.com/a/18283730/119527

    p = sub.add_parser('list',
            help="List Google netblocks")
    p.set_defaults(func=cmd_list)

    p = sub.add_parser('check',
            help="Check an IP address to see if it's Google's")
    p.set_defaults(func=cmd_check)
    p.add_argument("ip", nargs="+", type=ip_address,
            help="IP address to check")

    return ap.parse_args()


def main():
    args = parse_args()
    args.func(args)

if __name__ == '__main__':
    main()