Re: Recursion
- From: "Herb Martin" <news@xxxxxxxxxxxxxx>
- Date: Wed, 11 May 2005 08:28:29 -0500
"Ricardo" <Ricardo@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote in message
news:987DE20E-E7F8-4E69-B346-E66DCD15572C@xxxxxxxxxxxxxxxx
> What does recursion mean in a DNS server ?
Recursion is simply when a DNS server works it's
way down the DNS hierarchy, starting from a root
DNS server, where it finds the top level DNS server
(e.g., .com, .edu, country codes), then continues down
the hierarchy until it locates an authoritative server
for the request -- or proves that the requested
record does not exist.
[The remainder is perhaps more than you wish to know
about recursion....]
'Recursion' is a programming (or mathematical) term for
repeating the same process using new context establish
by the previous execution of that same process.
Since the process can be performed at any level of the
DNS hierarchy (once primed by root hints, a cache file,
or by a previously cached result such as the address of
the .Com DNS servers), the term recursion is appropriate.
In conjunction with this, there is another term which is
sometimes confused: a "recursive query" (or request.)
A particular DNS query is marked by the requested as
either recursive or 'iterative' -- in the former case is
asking the receiving DNS server to find the answer if
it doesn't know that answer. In the latter (iterative) the
requester is asking ONLY for what the receiving server
already knows.
Some confuse the two terms, assuming (incorrectly) that
a "recursive query" will cause recursion when it may not:
A recursive query may be resolved by the receiving server
forwarding to another server -- and either of them may
actually have the answers in cache or even be authoritive
for the record.
So a recursive query is a type of query create by a DNS client
(even another DNS server can be a "client"), and the actual
or physical recursion is a way of handling or carrying out that
resolution.
Most client request from ordinary stations are going to be marked
as recursive queries, while it is common for the clients DNS
server to actually perform the resolution by doing recursion.
When one server is performing recursion, it usually makes only
iterative requests to each DNS server in the hierarchy until it
finds the authoritative server for that record.
Alternatively, a DNS server may forward the recursive request
to a "forwarder". In some sense, the forwarding DNS server
becomes a DNS 'client' of the Forwarder DNS server which
not performs the same type of resolution choices: authoritive,
cached, recursion, or forwarding to ANOTHER forwarder...
--
Herb Martin, MCSE, MVP
Accelerated MCSE
http://www.LearnQuick.Com
[phone number on web site]
.
- References:
- Recursion
- From: Ricardo
- Recursion
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