How do I perform maintenance on a master DNS/DHCP (BIND9/ISC-DHCP) Ubuntu server?

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http://www.linuxquestions.org – Hi everyone, I'm running non-GUI Ubuntu Server 12.04.2 LTS (64-bit) on my 2 DNS/DHCP servers. Both servers are running BIND9 9.8.1-P1 and ISC-DHCP-SERVER 4.1.ESV-R4-0ubuntu5.6. Please note that for DNS I'm running them in a master/slave configuration, and for DHCP they are running a failover configuration. Ideally, I'd like to perform maintenance on the master/primary first, and once it's back up and running, I'd like to then take care of the secondary/slave. Maintenance will consist of installing various updates and performing a restart. I'm uncertain how to gracefully bring each machine dow (HowTos)