Synology documentation sucks big time! It usually only states the blindingly obvious missing off any useful or helpful information.
This post will run through the harder parts of setting up DNS and DHCP on Synology NAS. It is a little tricky the first time you do this especially if you have not come across bind or bind9 before. It seems a little over the top or just plain weird. You will only really need this if you have some local servers, computers or other devices that need to be accessed.
Updated for DSM 6.0, because Synology buggered about with stuff! This may not work on earlier versions.