script for updating DNS records dynamically via RFC 2136

rfc2136_dynDNS.py

from json import load
from sys import argv
import subprocess
from time import sleep, monotonic


with open(argv[1],"r") as file:
	config=load(file)

def send_nsupdate(ip,domain,keyfile,server,zone,ttl=60):
	cmd=f"""server {server}
zone {zone}
update delete {domain} AAAA
send
update add {domain} {ttl} AAAA {ip[1]}
send
update delete {domain} A
send
update add {domain} {ttl} A {ip[0]}
send
quit"""
	#print(cmd)
	sp = subprocess.run(['nsupdate', '-k', keyfile], input=cmd, text=True)
	if sp.returncode != 0:
		print("ERROR, return code",sp.returncode)
	else:
		print(domain,"OK")
def update_ip(conf,ip):
	for host in config["hosts"]:
		send_nsupdate(ip,host["domain"],host["keyfile"],host["server"],host["zone"],host["ttl"])

def get_ip(conf):
	v4=subprocess.check_output(config["ipv4_cmd"]).decode('utf-8').strip()
	v6=subprocess.check_output(config["ipv6_cmd"]).decode('utf-8').strip()
	return (v4,v6)

prev_ip=(None,None)
last_update_time=0
while True:
	ip=get_ip(config)
	seconds_since_last_update = monotonic() - last_update_time
	if ip != prev_ip or seconds_since_last_update > config['max_interval']:
		print("Updating to",ip)
		update_ip(config,ip)
		prev_ip=ip
		last_update_time=monotonic()
	else:
		print("IP has not changed")
	print()
	sleep(int(config["interval"]))

rfc2136_dynDNS.service

[Unit]
Description=RFC 2136 Dynamic DNS Update Client
Wants=network-online.target
After=network-online.target nss-lookup.target

[Service]
Type=exec
User=dyndns
ExecStart=python3 /opt/rfc2136_updater.py /etc/rfc2136/conf.json
Restart=on-failure

[Install]
WantedBy=multi-user.target

zzzz.config.example.json

{
"interval": 10,
"max_interval": 3600,
"ipv4_cmd": ["bash","/opt/fritzbox_ipv4.sh"],
"ipv6_cmd": ["bash","/opt/interface_ipv6.sh"],

"hosts": [
{
"server": "45.129.95.255",
"keyfile": "/etc/rfc2136/glauca-aaaa.private",
"ttl": "120",
"zone": "example.com",
"domain": "aaaa.example.com"
},
{
"server": "45.129.95.255",
"keyfile": "/etc/rfc2136/glauca-bbbb.private",
"ttl": "120",
"zone": "example.com",
"domain": "bbbb.example.com"
}
]
}