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

By Jason Zandri (Send Email)
Posted Jan 6, 2003


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 2000trainers.com 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 period.

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 2000trainers.com domain and DNS servers for 2000trainers.com 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 2000trainers.com domain and DNS server(s) for 2000trainers.com 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 server1.zandri.net. (
                                              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 domain.
The @ symbol is replaced by a period (.) in the e-mail address of the administrator.
Parentheses ( () ) must enclose line breaks that span more than one line.

[NOTES FROM THE FIELD] - The 7200 ; refresh [2h] 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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