Mark Sanborn . net

Well I just recently hooked up a DSL connection via a DSL modem that was also a router. For regular home users this DSL modem/router is probably good enough but for more complex setups it is useless. In this particular setup we had a custom built firewall. This firewall had a built in dynamic DNS service for ISP’s that rotate IP addresses. The problem however, is that the firewall was behind the DSL modem’s NAT. So it was reporting a private 192.168 number to Dyndns.

We couldn’t turn off NAT on the DSL modem unless we purchased a block of static IPs. The DSL modem, being home orientated, didn’t allow for dynamic DNS.

Fortunately dyndns offers a service to detect your public IP address. They also have an API to change your IP for dynamic DNS. Knowing this I quickly did a search on my favorite perl code repository, CPAN. Sure enough someone made a module that sends updated IP info to dyndns.

Updating DynDNS with Perl

 
The first thing you will want to do is download the module, Net::DNS::DynDNS. On a Linux box it is super easy.

cpan
install Net::DNS::DynDNS

On windows you will need to compile it with dmake and mingw since there are no ppm for Active Perl.

Then create a file called, dyndns.pl, replacing username, password, and yourdomain.selfip with your own values.

#!/usr/bin/perl
use Net::DNS::DynDNS;
print Net::DNS::DynDNS->new('username', 'password')->update('yourdomain.selfip.com');

You can then update your IP by simply doing

perl dyndns.pl

To setup automatic updating we can use our favorite scheduler, cron.

crontab -e

Then add the following line, pointing to where you saved dyndns.pl

1 * * * * /usr/bin/perl /home/youruser/crons/dyndns.pl

This will update every hour which is plenty for my needs. Feel free to change it to fit yours.