Despite popular belief, there is no dynamic update of host records in the way you are describing. Dynamic update refers only to how DNS primaries and secondaries keep in sync.
During normal BIND (name resolution) operations, DNS records are provided by primary and secondary DNS servers when asked for and are cached for the duration of their Time-To-Live (TTL) value.
Their a two points of view you must look at when configuring DNS. First, for your situation, external resolution of your domain namespace (don't get confused here with the word "domain" -- we aren't talking at all about Windows NT Domain today) is the ability of your DNS server to respond to requests from Internet users for the IP addresses of your hosts (e.g., www.yourco.com, ftp.yourco.com and mail.yourco.com). The other point of view is from that of your in-house users for which your DNS server will provide name resolution.
Without discussing the miriad ways these two tasks can be accomplished, lets examine a normal name lookup in a most standard sense. We'll take the point of view of your in-house user making a request for a resource on the Internet (we'll see the first point of view from the perspective of the DNS server of the domain from which our user is making the request). Here goes...
Sam opens his web browser of choice (IE 5.5 sp1, of course) and types http://www.yahoo.com into the address bar and hits enter.
Via configuration of IP on Sam's local machine, the DNS server (your server providing viewpoint #2) is queried for the IP address of www.yahoo.com.
If a request (from one of Sam's co-workers) for this address has been received from Yahoo's DNS server since the TTL value in the response, that response is used to fulfillSam's request; Otherwise, DNS begins to break down the address left-to-right, looking for known DNS (learned from cached responses and retained for the duration of each response's
