Cluster slave server

A quick first answer, maybe it helps you already, otherwise there will certainly be a more extensive reply from the developers. :)

To add a Webmin server as DNS Cluster Slave, you first need to add the server as a "Webmin Remote Server". You do that under "Webmin -> Webmin -> Webmin Servers Index". There you click "Register a new server" and enter its data. Make sure to set "Link type" to "Login via Webmin with username" and "Make fast RPC calls" to "Yes".

Once the server connection is okay, you can add that server as a DNS Cluster Slave in the Webmin BIND Module, icon "Cluster Slave Servers". Choose the server to add from the dropdown box, and set the "Create" options to your desire (or ask here if you're unsure what they do).

In your case, since you wish to use hostnames like "ns1/2.abc.com" for you nameservers, make sure to enter the proper name in the "Name for NS record" field. This will cause Virtualmin to use that name for the slave server in zones it creates.

Finally, in the Virtualmin Server Template, section "BIND DNS Domain", enter the desired hostname of the master server in the "Master DNS server hostname" area (set the radio button from "Automatic" to "Hostname"). This will cause Virtualmin to enter that hostname in the zone file instead of the server's system hostname.

The cluster slave servers do not need to be entered here. They will be added automatically as nameservers into the zone, with the hostname you previously entered in the BIND module.

Locutus's explanation of how to setup cluster slaves is correct - also, we have docs on this at http://www.virtualmin.com/documentation/dns/slave-configuration

It is quite possible for 3 virtual systems to use each other as slaves, so that each system is the primary for its own domains and the secondary for domains created on the other two.