The nameserver infrastructure that we've configured so far is sufficient to get the domain to function, but it is currently a single point of failure. In order to deal with your existing nameserver being unreachable for some reason, we're going to want to add at least one additional nameserver for your network.
Now, maybe your initial thought would be to configure the nameservers identically and create some method to synchronize the zone files across the systems. Luckily, this isn't needed. Rather, bind/named can handle the synchronization internally, through the use of zone transfer (AXFR) requests or incremental zone transfer (IXFR) requests secured with the same type of HMAC keys utilized by the DHCP server to send updates to the DNS server. Rather than making changes to a single record though, zone transfers send the entire zone file, including all records.
