This script uses the Linode API to create a text file backup of your DNS entries. All you need is an API key from Linode.
$ sudo pear install Net_URL2-0.3.1
$ sudo pear install HTTP_Request2-0.5.2
$ sudo pear channel-discover pear.keremdurmus.com
$ sudo pear install krmdrms/Services_Linode
There is a built-in backup script you can call via the command line/cron at backup.php.
Usage:
> php backup.php --api_key=YOUR_API_KEY --outfile=/path/to/output.file