DNS configuration can be a daunting task. Even seasoned server admins get tripped up over configurations. Yep. Myself included.

Quick Overview of DNS Master and Slave Servers
By default, Virtualmin will set you up with a master server. That works great until the first time your DNS daemon decides not to start or dies. The idea behind a slave DNS server is to provide some redundancy in the event that your primary DNS goes down. Simple concept. This article will assume that you have a working Virtualmin installation running BIND and another server running BIND (to become the slave). You can get Virtualmin GPL for free. If you need a great, reliable linux box I highly recommend Linode. They also offer free DNS service with your account. You can use this as your slave DNS server.

Summary
Create an Access Control List (ACL) to allow transfer to specified servers and configure Virtualmin to notify the slave servers.

Configuration Steps

1. Log into your Virtualmin server. You'll need admin rights since you need to update some Webmin settings.
2. On the upper left side, click on Webmin.
3. Expand Servers from the left side and click on BIND DNS Server.
4. Click on Access Control Lists
5. In the ACL Name box type slaves
6. In the Matching addresses, networks and ACLs box, type in the IP addresses of your slave DNS servers. Type one on each line ending with a semi-colon. (e.g. 69.93.127.10;).
7. Click the Save button to save your changes.
8. Navigate back over to Virtualmin by clicking on the Virtualmin link at the top left of the page.
9. Expand System Settings and click on Server Templates.
10. Click on the Default Settings template (or whichever one you intend to use).
11. From the drop down, choose BIND DNS domain.
12. In the Additional manually configured nameservers box, enter the names of your slave servers (e.g. ns1.example.com). one entry per line.
13. At the bottom you'll see a section called Additional named.conf directives for new domains. This is where you're going to tell BIND to notify your list of slave servers (remember the ACL?) when the zone is updated. Enter the following into the box: notify yes; allow-transfer { slaves; };
14. Click Save

The next part is to configure a slave DNS zone on another DNS server. This will vary depending on the host you choose. For Linode, its simple. Open the DNS manager. Create a new zone choosing slave as the type. Then enter the IP of your DNS master server. Simple.