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Amount of change you anticipate for the records within the zone
Amount of time you can withstand an outage that might require changing an IP
address
Amount of traffic you believe the DNS server can handle
Resolvers query the name server every time the TTL expires for a given record. A low TTL,
say 60 seconds, can burden the name server, especially for popular DNS records. (DNS
queries aren't particularly intensive for a server to handle, but they can add up quickly if
you mistakenly use 60 seconds instead of 600 seconds for the TTL on a popular record.) Set
a low TTL only when you need to respond quickly to a changing environment.
A high TTL, say 604,800 seconds (that's one week), means that if you need to make a
change to the DNS record, clients might not see the change for up to a week. This consid-
eration is especially important when making changes to the network, and it's one that's
all too frequently overlooked. I can't count the number of times I've worked with clients
who had recently made a DNS change to a new IP for their email or website only to ask
why it's not working for some clients. The answer can be found in the TTL value. If the
record is being cached, then the only thing that can solve their problem is time.
You should choose a TTL that's appropriate for your environment. Take the following
factors into account:
The amount of time that you can afford to be offline if you need to make a change to
a DNS record that's being cached
The amount of load that a low TTL will cause on the DNS server
In addition, you should plan well ahead of any major infrastructure changes and change
the TTL to a lower value to lessen the effect of the downtime by reducing the amount of
time that the record(s) can be cached.
Introducing DNS Database Zones
As mentioned earlier in this chapter, a DNS zone is a portion of the DNS namespace over
which a specific DNS server has authority. Within a given DNS zone, there are resource
records (RRs) that define the hosts and other types of information that make up the
database for the zone. You can choose from several different zone types. Understanding the
characteristics of each will help you choose which is right for your organization.
The DNS zones discussed in this topic are all Microsoft Windows Server
2012 / 2012 R2 zones. Non-Windows (for example, Unix) systems set up
their DNS zones differently.
 
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