Module: Net::DNS::MDNS

Defined in:: lib/net/dns/mdns.rb

Overview

:main:Net::DNS::MDNS :title:net-mdns - multicast DNS and DNS service discovery

Author: Sam Roberts <[email protected]>
Copyright: Copyright © 2005 Sam Roberts
License: May be distributed under the same terms as Ruby
Version: 0.4
Homepage: dnssd.rubyforge.org/net-mdns
Download: rubyforge.org/frs/?group_id=316

Summary

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

Client Example

This is an example of finding all _http._tcp services, connecting to them, and printing the ‘Server’ field of the HTTP headers using Net::HTTP (from exhttp.txt):

require 'net/http'
require 'thread'
require 'pp'

# For MDNSSD
require 'net/dns/mdns-sd'

# To make Resolv aware of mDNS
require 'net/dns/resolv-mdns'

# To make TCPSocket use Resolv, not the C library resolver.
require 'net/dns/resolv-replace'

# Use a short name.
DNSSD = Net::DNS::MDNSSD

# Sync stdout, and don't write to console from multiple threads.
$stdout.sync
$lock = Mutex.new

# Be quiet.
debug = false

DNSSD.browse('_http._tcp') do |b|
  $lock.synchronize { pp b } if debug
  DNSSD.resolve(b.name, b.type) do |r|
    $lock.synchronize { pp r } if debug
    begin
      http = Net::HTTP.new(r.target, r.port)

      path = r.text_record['path'] || '/'

      headers = http.head(path)

      $lock.synchronize do
        puts "#{r.name.inspect} on #{r.target}:#{r.port}#{path} was last-modified #{headers['server']}"
      end
    rescue
      $lock.synchronize { puts $!; puts $!.backtrace }
    end
  end
end

# Hit enter when you think that's all.
STDIN.gets

Server Example

This is an example of advertising a webrick server using DNS-SD (from exwebrick.txt).

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

Samples

There are a few command line utilities in the samples/ directory:

mdns.txt, mdns.rb is a command line interface for to Net::DNS::MDNSSD (or to DNSSD)
v1demo.txt, v1demo.rb is a sample provided by Ben Giddings showing the call sequences to use with Resolv::MDNS for service resolution. This predates Net::DNS::MDNSSD, so while its a great sample, you might want to look at mdns.rb instead.
v1mdns.txt, v1mdns.rb is a low-level utility for exercising Resolv::MDNS.
mdns-watch.txt, mdns-watch.rb is a utility that dumps all mDNS traffic, useful for debugging.

Comparison to the DNS-SD Extension

The DNS-SD project at 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.

For More Information

See the following:

draft-cheshire-dnsext-multicastdns-04.txt for a description of mDNS
RFC 2782 for a description of DNS SRV records
draft-cheshire-dnsext-dns-sd-02.txt for a description of how to use SRV, PTR, and TXT records for service discovery
www.dns-sd.org (a list of services is at www.dns-sd.org/ServiceTypes.html).
dnssd.rubyforge.org - for DNSSD, a C extension for communicating with Apple’s mDNSResponder daemon.

TODO

See TODO.

Thanks

to Tanaka Akira for resolv.rb, I learned a lot about meta-programming and ruby idioms from it, as well as getting an almost-complete implementation of the DNS message format and a resolver framework I could plug mDNS support into.
to Charles Mills for letting me add net-mdns to DNS-SD’s Rubyforge project when he hardly knew me, and hadn’t even seen any code yet.
to Ben Giddings for promising to use this if I wrote it, which was the catalyst for resuming a year-old prototype.

Author

Any feedback, questions, problems, etc., please contact me, Sam Roberts, via [email protected], or directly.

Defined Under Namespace

Modules: QueryImp Classes: Answer, BackgroundQuery, Cache, Query, Question, Responder, Service