Simple DNS server intended for responding ACME challenges. Assumes ownership of /etc/authbind/byport/53.

dnsserver.py

#/usr/bin/env authbind python3

from dnslib.label import DNSLabel
from dnslib.server import DNSServer, DNSLogger, DNSRecord, RR
from dnslib.dns import TXT
import re
import sys
import os
from os import listdir

def find_challenge(s_un):
	s = s_un.lower()
	PREFIX = "_acme-challenge."
	SUFFIX = "."
	if re.match("^[A-Za-z0-9_.-]*$", s) and (s.replace(".", "") != '') and s.startswith(PREFIX) and s.endswith(SUFFIX):
		name = s[len(PREFIX):-len(SUFFIX)]
		dirname = path+os.sep+name
		try:
			files = listdir(dirname)
		except FileNotFoundError:
			return []
		responses = []
		for file in files:
			with open(dirname+os.sep+file, 'r') as f:
				responses.append(f.read().replace('\n', ''))
		return responses
	else:
		return []

class TestResolver:
	def resolve(self,request,handler):
		reply = request.reply()
		for q in request.questions:
			name = str(q.qname)
			challenges = find_challenge(name)
			if (q.qtype == 16):
				for challenge in challenges:
					reply.add_answer(RR(rname=DNSLabel(name), rtype=16, rdata=TXT(challenge)))
		return reply

path = sys.argv[1]

logger = DNSLogger(prefix=False)
resolver = TestResolver()
server = DNSServer(resolver, port=53, address="0.0.0.0", logger=logger, tcp=False)
server.start()

dnsserver.sh

#!/bin/bash
# safety settings
set -u
set -e
set -o pipefail

cd "$(dirname "$(realpath "$0")")"
pip3 install --user -r requirements.txt
authbind python3 dnsserver.py ~letsencrypt/.challenges-per-domain

requirements.txt

dnslib>=0.9.7