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Chapter 4: Domain Name System
Host (A) Records
Host records, or A records, simply map a hostname to an IP address. You generally
create host records for each machine in your network.
A sample A record looks like this in a zone file:
colossus A
Using host records, you can implement a load-balancing technique known as round-
robin DNS. Round-robin DNS involves entering multiple A records, all configured
with the same hostname, but with different IP addresses that correspond to different
machines. This way, when computers contact a nameserver for a certain hostname,
they have an equally proportionate chance of receiving any one of the number of
machines with A records. For example, if I have a web site at
and I have three web servers at,, and, I can
configure three A records, all named “www,” but with the three IP addresses men-
tioned earlier. Now, when client computers come to the nameserver and ask for the IP
address of, they have a 33% chance of receiving as
the web server of choice, a 33% chance of receiving, and a 33% chance
of receiving It’s a poor-man’s load-balancing system.
Let’s get a bit more technical: in this scenario, Windows 2000 and
Windows XP clients will continue to attempt a connection to the first
web server that was originally resolved. A DNS cache timeout value on
the client is set to 86,400 seconds (one day) by default. If you change
this value on the client to one second, you have better odds of reach-
ing your server. You can change this value in the registry with the fol-
lowing key:
Change the
MaxCacheEntryTtlLimit to the number of seconds desired.
If the group of machines that serve web sites are on different subnets, the DNS system
can return the “proper” address from a round-robin record set—that is, the one that is
closest to the client requesting it. This functionality is enabled by default. For example, if
you have one A record set up for on IP address, and
another A record set up for the same hostname on IP address, a client com-
puter located on the subnet will receive the A record from his request,
and a client computer located on the subnet will receive the A
record from his request.

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