DNS server support

Support for writing DNS servers, and various API changes to accomplish that, specifically:

 - `DnsType` -> `DnsType<RecordType>` where RecordType is a subclass of `DnsEntry`, and has payload decoders for specific types
 - Introduced record-specific subclasses of `DnsEntry` for common types, with fallback to the ByteBuf-based `DnsResource`
 - `DnsEntry` gains public `writeTo()` and `writePayload()` to write itself correctly into a ByteBuf
 - Added `DnsQueryDecoder` and `DnsResponseEncoder`
 - Introduced `NameWriter` for writing names, so that it's possible to implement a stateful `NameWriter` to do DNS name compression
 - `DnsMessage` is threadsafe (needed for proxy DNS servers where multiple calls to remote DNS servers may contribute partial responses)
    - If someone really feels strongly that creating the `List` instances it uses internally should be lazy, that could be done, but it would be good to have some evidence that really accomplishes something - feels like premature optimization
 - `DnsQuestion` is no longer a subclass of `DnsEntry` (which now has a time-to-live, since that is common to all other record types).
 - Added the package io.netty.handler.codec.dns.server with `DnsServerHandler`, a `SimpleChannelInboundHandler<DatagramPacket>` that handles the common plumbing of writing a DNS server that is unlikely to differ between implementations
 - Corrected error codes in `DnsResponseCode` to match https://tools.ietf.org/html/rfc2929 (they were sequential, but in fact 11-15 are unused and numbering restarts at 16)
 - Added error codes that exist but were not included in `DnsResponseCode` but are part of the spec
 - Added `DnsDecoderException` which can specify a `DnsResponseCode`
 - `DnsQuery` implements `Iterable<DnsQuestion>`, simply because it's intuitive
 - Reasonable `toString()` implementations on entity classes for logging purposes
 - Added to netty-examples:  DNS server, DNS client, proxy DNS server

In a line-comment, really? :-)

On Mon, Dec 1, 2014 at 3:57 AM, Norman Maurer <notifications@github.com>
wrote:

> In example/src/main/java/io/netty/example/dns/client/DnsClient.java:
>
> > +import java.util.concurrent.atomic.AtomicInteger;
> > +
> > +/**
> > + * A simple DNS client. Note that this is for <i>demonstration</i> purposes - in
> > + * particular in a production implementation you would want to deal with
> > + * failures where the remote server never responds at all, fail over across
> > + * multiple servers; and sequentially incrementing IDs are a bad idea.
> > + * <p>
> > + * Caching is left as an exercise for the reader.
> > + */
> > +public class DnsClient {
> > +
> > +    // Map of Callbacks for IDs.  Note that in a production server you would
> > +    // want to track when a callback was submitted, and discard callbacks
> > +    // that have been waiting for an answer for too long - this will leak
> > +    // if the remote server does not respond at all
>
> Missing '.' at end of line
>
> —
> Reply to this email directly or view it on GitHub
> <https://github.com/netty/netty/pull/3146/files#r21076703>.
>



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