GuidesLearn AD in 15 Minutes a Week: Microsoft DNS - Part 2...

Learn AD in 15 Minutes a Week: Microsoft DNS – Part 2 Page 2

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DNS Caching

Often, DNS servers will be called on to resolve the same
query multiple times within a short span of time. As an example, if a number of
America Online users, arguably the largest ISP in the world, get an e-mail
that new articles have been posted to
and a number of users begin their day by going to their
browsers to read the new articles, the AOL DNS servers are going to be
continually recalling the resolved address many times within a short time

DNS servers will cache the resolved addresses for a
specific amount of time specified as the Time to Live (TTL) in the returned
data. The DNS server administrator of the zone that contains the data decides on
the TTL for the data. This means that the named administrator of the domain and DNS
servers for sets
the TTL value. This tells the resolving DNS server (in this example, the ones
at AOL) how long to hold that information in its cache. The lower the TTL the
“fresher” the resolution data on the resolving DNS servers.

Once data is cached by a DNS server it will decrease the TTL from its original
value so that it will know when to flush the data from its cache. If another
query for resolution comes in to the DNS server for the URL again, the cached
data will be used and the TTL is reset (in most cases) to the original TTL. (The
only way it wouldn’t be reset to the same TTL value from before would be if the
named administrator of the
domain and DNS server(s) for sets a different TTL.)

DNS Records

The DNS database consists of a number of different resource
records, the most common of which are the address records that hold
computer names and the TCP/IP address of that computer.

Some of the other records held on the DNS server were mentioned briefly in

Microsoft DNS – Part 1
, and we will detail them a little more here.

The Start of Authority Record (SOA)indicates the starting point
of authority for a given DNS zone on a specific DNS server. The SOA resource
record is the first resource record created when you add a new zone. The
following is an example of an SOA record:

@ IN SOA (
                                              1        ; serial number
                                              7200   ; refresh [2h]
                                              900     ; retry [15m]
                                              86400 ; expire [1d]
                                              7200 ) ; min TTL [2h]

The at symbol (@) in a database file indicates “this server.”
IN indicates an Internet record.
Any host name not terminated with a period (.) will be appended with the root
The @ symbol is replaced by a period (.) in the e-mail address of the
Parentheses ( () ) must enclose line breaks that span more than one line.

[NOTES FROM THE FIELD] – The 7200 ; refresh
shows a time period of 2 hours, 900 ; retry [15m] shows a time
period of 15 minutes, 86400 ; expire [1d] shows an expiration time period
of 1 day and 7200 ; min TTL [2h] shows a minimum time to live of 2 hours.

Everything in that record after a ; is a comment,
which is why the line breaks are necessary.

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