Dynamic DNS for NSD

Introduction

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

Requirements

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

Installation

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. ( 
1524952218 
300 ; refresh 
900 ; retry 
1209600 ; expire 
1800 ; ttl 
) 

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

; A records 
@ IN A 123.123.123.123 
update IN A 123.123.123.123 
dyn IN A 123.123.123.123 

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

  • 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:

http://update.example.com/update.html?qwertzuiop1234567890 

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 123.123.123.123 - - [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