Build a host file from a dhcp.leases file

build_hosts.py

# This is a simple little script for parsing dhcpd.leases files and
# returning a hosts file for them.

# on DNS server: */5 * * * * curl http://10.0.0.1:4400/ -o /etc/dhcp_hosts

from http.server import BaseHTTPRequestHandler, HTTPServer
import re
from ipaddress import ip_network, ip_address

class S(BaseHTTPRequestHandler):
    def do_GET(self):
        output = build_hosts()

        self.send_response(200)
        self.send_header('Content-type', 'text/plain')
        self.end_headers()

        self.wfile.write(output)

def build_hosts():
    matcher = re.compile(r'lease (10\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}) {.+?(?:client-hostname "((?:[^\"]|\\")*)";)?.}',
                         re.S)

    output = b''

    # TODO: see if we can split-horizon this and return the correct
    # gateway for the subnet we're being queried from
    output += b'10.0.0.1 gaff.doyl.net\n'

    with open('/config/dhcpd.leases') as f:
        data = f.read()

        matches = re.findall(matcher, data)
        for (ip, host) in matches:
            hostname = map_host(ip, host)

            if hostname:
                output += b' '.join([ip.encode('utf-8'), map_host(ip, host)]) + b'\n'

    return output


def map_host(ip, client_hostname):
    if not client_hostname:
        return

    client_hostname = client_hostname.encode('utf-8')

    subnets = [
        (b'vm.gaff.doyl.net', ip_network('10.0.0.0/24')),
        (b'tv.gaff.doyl.net', ip_network('10.1.0.0/24')),
    ]

    ip = ip_address(ip)

    for hostname, subnet in subnets:
        if ip in subnet:
            return b'.'.join([client_hostname, hostname])


def main():
    httpd = HTTPServer(('', 4400), S)
    print('Serving on port 4400')
    httpd.serve_forever()

main()