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Chapter 4: Domain Name System
Forwarding, in the simplest terms, is the process by which a nameserver passes on
requests it cannot answer locally to another server. You can make forwarding work
to your advantage so that you effectively combine the resolver caches for many
nameservers into one. By doing this, you allow clients to resolve previously retrieved
sites from that “mega-cache” before requiring a true refresh lookup of the informa-
tion from authoritative nameservers on the public Internet.
Here’s how it works. DNS behavior by default is to consult the preferred nameserver
first to see if it has the necessary zone information for which the client is searching. It
doesn’t matter to the client if the preferred nameserver has the zone information but
isn’t authoritative; having the information is enough for the client, and it takes the
returned results and makes the connection. But if the server doesn’t have the zone
recorded in its files, it must go upstream, to the public Internet, to ask other
nameservers for the zone information that’s needed. This takes time because it adds
a delay to the initial resolution while the preferred nameserver is searching the Inter-
net for the answer. However, after the nameserver looks up the information once, it
stores it in its cache of resolved names so that the next user looking for the same
resolver information doesn’t incur that delay: the preferred nameserver can simply
answer out of its cache and return the data nearly instantaneously.
Forwarding takes this cache and expands it to multiple nameservers. Consider an
organization with four or five nameservers. Clients likely will have different pre-
ferred nameservers, set to one of each of those four or five. So, when one client wants
information that’s not in his nameserver’s cache, his preferred nameserver will search
it out and return it, and all future users of that particular preferred nameserver will
Figure 4-22. Controlling DNS replication in Active Directory

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