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Async DNS server

Future plans:
    - Better config file format maybe
    - Make sure to differentiate between different classes
    - notice truncation bit

Important: No additional processing is done on some of the record types.
This violates the most basic RFC and is just plain annoying
for resolvers to deal with.  Fix it.

@author: Jp Calderone
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        C{cacheResult} method is called when a response is received from one of
        C{clients}. Defaults to L{None} if no caches are specified. See
        C{caches} of L{__init__} for more details.
    @type cache: L{Cache<twisted.names.cache.CacheResolver>} or L{None}

    @ivar canRecurse: A flag indicating whether this server is capable of
        performing recursive DNS resolution.
    @type canRecurse: L{bool}

    @ivar resolver: A L{resolve.ResolverChain} containing an ordered list of
        C{authorities}, C{caches} and C{clients} to which queries will be
        dispatched.
    @type resolver: L{resolve.ResolverChain}

    @ivar verbose: See L{__init__}

    @ivar connections: A list of all the connected L{DNSProtocol} instances
        using this object as their controller.
    @type connections: C{list} of L{DNSProtocol} instances

    @ivar protocol: A callable used for building a DNS stream protocol. Called
        by L{DNSServerFactory.buildProtocol} and passed the L{DNSServerFactory}
        instance as the one and only positional argument.  Defaults to
        L{dns.DNSProtocol}.
    @type protocol: L{IProtocolFactory} constructor

    @ivar _messageFactory: A response message constructor with an initializer
         signature matching L{dns.Message.__init__}.
    @type _messageFactory: C{callable}
    Nr
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        @param authorities: Resolvers which provide authoritative answers.
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            C{DNSServerFactory.cache} and its C{cacheResult} method will be
            called when a response is received from one of C{clients}.
        @type caches: L{list} of L{Cache<twisted.names.cache.CacheResolver>} instances

        @param clients: Resolvers which are capable of performing recursive DNS
            lookups.
        @type clients: L{list} of L{IResolver} providers

        @param verbose: An integer controlling the verbosity of logging of
            queries and responses. Default is C{0} which means no logging. Set
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        The response will have the same C{maxSize} as the request.

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        @see: L{dns._responseFromMessage}

        @param message: The request message
        @type message: L{Message}

        @param rCode: The response code which will be assigned to the response.
        @type message: L{int}

        @param answers: An optional list of answer records which will be
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        @type answers: L{list} of L{dns.RRHeader}

        @param authority: An optional list of authority records which will be
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        @type authority: L{list} of L{dns.RRHeader}

        @param additional: An optional list of additional records which will be
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        @param failure: The reason for the failed resolution (as reported by
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        @type failure: L{Failure<twisted.python.failure.Failure>}

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        Note: Multiple queries in a single message are not supported because
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        If C{DNSServerFactory.verbose} is C{>0} all received messages will be
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        @param message: The DNS message that was received.
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