Dynamic DNS for NSD

DNS OpenBSD Linux Software


nsd-dyndns is a simple script that adds dynamic DNS dunctionality to NSD (authoritative DNS name server).


The following is required or suggested:

  • OpenBSD (or another BSD or some Linux distro) with HTTPD and NSD installed (pkg_add nsd), configured and running
  • (sub-)domain for your webserver. Needed for updating the NS record of your actual DynDNS domain.
    • In this example: update.example.com
  • (sub-)domain that is updated dynamically.
    • In this example: dyn.example.com
  • A router capable of sending custom GET-requests to your DynDNS server.
    • In this example: A FritzBox


Configure your HTTPD

Add the following new virtual host to your /etc/httpd.conf:

 server "update.example.com" {
 	listen on $ext_addr port 80
 	root "/htdocs/dyndns"
 	log access dyndns.log

Create an empty update.html:

# mkdir /var/www/htdocs/dyndns/ 
# touch /var/www/htdocs/dyndns/update.html 

After reloading HTTPD, try to access [http://update.example.com/update.html] The request should show up in /var/www/logs/dyndns.log

Create a zone file for dyn.example.com

Create a new zone file (e.g. at /var/nsd/zones/dyn.example.com.zone) with the following content

$ORIGIN example.com. 
$TTL 300 
@ IN SOA ns1.example.com. admin.example.com. ( 
300 ; refresh 
900 ; retry 
1209600 ; expire 
1800 ; ttl 

; Name servers 
IN NS ns1.example.com. 
IN NS ns1.example.com. 

; A records 
@ IN A 
update IN A 
dyn IN A 

Don’t forget to set your own domain names, name servers and ip addresses Furthermore, add this zone file to your /var/nsd/etc/nsd.conf

Configure and Install nsd-dyndns

  • Download the scripts from here: https://codeberg.org/mclemens/nsd-dyndns
  • Copy dyndns.conf-dist to /etc/dyndns.conf
    • # cp dyndns.conf-dist /etc/dyndns.conf
  • Edit /etc/dyndns.conf to your needs
  • Copy dyndns.sh to /usr/local/bin/dyndns.sh
    • # cp dyndns.sh /usr/local/bin/dyndns.sh
  • Make the script executable:
    • # chmod u+x /usr/local/bin/dyndns.sh
  • Add /usr/local/bin/dyndns.sh to your crontab

Configure your router

Configure your router to query the following URL:


Don’t forgert to set your own domain name and to replace the string after “?” with the password you configured in the config file.

What it does

When your router gets a new IP and therefore sends an HTTP request to your server, a similar entry should appear in your /var/www/logs/dyndns.log:

update.example.com - - [29/Apr/2018:20:48:19 +0200] "GET /update.html?qwertzuiop1234567890 HTTP/1.1" 200 6 

When the script is executed e.g. via cron, the following happens:

  • It greps the last line of /var/www/logs/dyndns.log where the correct password was found and extracts the requesting IP address
  • It checks if this IP is the same than the last time
  • If it’s a new IP, then it replaces the forth line in your zone file (the line with the version number) with a new version (current unix time stamp)
  • As a second step, it updates the A record of you DynDNS domain (dyn.example.com in our example)
  • It then stores the new IP in the file /tmp/last_dyndns_ip.txt
  • Finally it reloads NSD