This is a complete rewrite of net-mdns-0.0. The major additions are a
mDNS responder (thus ability to advertise services) and a
DNSSD-compatible high-level API.

Only tested on Mac OS X 10.3. Multicast socket options and behaviours
vary across different operating systems. It will take some tweaking, I'm
sure, so I would appreciate feedback from people with other kinds of
systems.

Cheers,
Sam

-- stuff ripped from the on-line documentation --

Where?

  Homepage::   http://dnssd.rubyforge.org/net-mdns
  Download::   http://rubyforge.org/frs/?group_id=316

What?

  An implementation of a multicast DNS (mDNS) responder.  mDNS is an
  extension of hierarchical, unicast DNS to link-local multicast, used to
  do service discovery and address lookups over local networks. It is
  most widely known because it is part of Apple's OS X.

  net-mdns consists of:
  - Net::DNS::MDNSSD: a high-level API for browsing, resolving, and advertising
    services using DNS-SD over mDNS that aims to be compatible with DNSSD, see
    below for more information.
  - Resolv::MDNS: an extension to the 'resolv' resolver library that adds
    support for multicast DNS.
  - Net::DNS::MDNS: the low-level APIs and mDNS responder at the core of
    Resolv::MDNS and Net::DNS::MDNSSD.

  net-mdns can be used for:
  - name to address lookups on local networks
  - address to name lookups on local networks
  - discovery of services on local networks
  - advertisement of services on local networks


Example?

  # Advertise a webrick server over mDNS.

  require 'webrick'
  require 'net/dns/mdns-sd'

  DNSSD = Net::DNS::MDNSSD

  class HelloServlet < WEBrick::HTTPServlet::AbstractServlet
    def do_GET(req, resp)   
      resp.body = "hello, world\n"
      resp['content-type'] = 'text/plain'
      raise WEBrick::HTTPStatus::OK
    end
  end

  server = WEBrick::HTTPServer.new( :Port => 8080 )

  server.mount( '/hello/', HelloServlet )

  handle = DNSSD.register("hello", '_http._tcp', 'local', 8080, 'path' => '/hello/')

  ['INT', 'TERM'].each { |signal| 
    trap(signal) { server.shutdown; handle.stop; }
  }

  server.start


How does it compare to the DNSSD project?

  The DNS-SD project at http://dnssd.rubyforge.org is another
  approach to mDNS and service discovery.

  DNS-SD is a compiled ruby extension implemented on top of the dns_sd.h APIs
  published by Apple. These APIs work by contacting a local mDNS daemon
  (through unix domain sockets) and should be more efficient since they
  use a daemon written in C by a dedicated team at Apple.

  Currently, the only thing I'm aware of net-mdns doing that DNS-SD
  doesn't is integrate into the standard library so that link-local domain
  names can be used throughout the standard networking classes, and allow
  querying of arbitrary DNS record types. There is no reason DNS-SD can't
  do this, it just needs to wrap DNSServiceQueryRecord() and expose it, and
  that will happen sometime soon.

  Since net-mdns doesn't do significantly more than DNSSD, why would you be
  interested in it?

  The DNS-SD extension requires the dns_sd.h C language APIs for the Apple
  mDNS daemon. Installing the Apple responder can be quite difficult, and
  requires a running daemon.  It also requires compiling the extension. If
  you need a pure ruby implementation, or if building DNS-SD turns out to be
  difficult for you, net-mdns may be useful to you.