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Update description of forwarding behavior in ARM

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Suzanne Goldlust 2020-07-23 13:05:43 +00:00 committed by Michał Kępień
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commit 30e126ad02
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doc/arm/introduction.rst
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@ -269,14 +269,26 @@ recursive lookup itself. Instead, it can *forward* some or all of the
queries that it cannot satisfy from its cache to another caching name
server, commonly referred to as a *forwarder*.
There may be one or more forwarders, and they are queried in turn until
the list is exhausted or an answer is found. Forwarders are typically
used when it is undesirable for all the servers at a given site to interact
directly with the rest of the Internet's servers. A typical scenario
involves internal DNS servers and an Internet firewall.
Servers unable to pass packets through the firewall forward their requests to the
server that can, and that server queries the Internet DNS
servers on the internal servers' behalf.
Forwarders are typically used when an administrator does not wish for
all the servers at a given site to interact directly with the rest of
the Internet. For example, a common scenario is when multiple internal
DNS servers are behind an Internet firewall. Servers behind the firewall
forward their requests to the server with external access, which queries
Internet DNS servers on the internal servers' behalf.
Another scenario (largely now superseded by Response Policy Zones) is to
send queries first to a custom server for RBL processing before
forwarding them to the wider Internet.
There may be one or more forwarders in a given setup. The order in which
the forwarders are listed in ``named.conf`` does not determine the
sequence in which they are queried; rather, ``named`` uses the response
times from previous queries to select the server that is likely to
respond the most quickly. A server that has not yet been queried is
given an initial small random response time to ensure that it is tried
at least once. Dynamic adjustment of the recorded response times ensures
that all forwarders are queried, even those with slower response times.
This permits changes in behavior based on server responsiveness.
.. _multi_role: