Re: ANN: net-mdns, multicast DNS and service discovery (aka "Rendezvous")

Rendezvous is a ( good ) Tibco messaging product, don't get their
lawyers fired up again ;-)



On Thu, 3 Feb 2005 13:00:24 +0900, Sam Roberts <[e-mail]> wrote:
> Hi,
> 
> net-mdns is an extension to the standard 'resolv' resolver library that
> adds support for multicast DNS.  mDNS is an extension of hierarchical,
> unicast DNS to link-local unicast. It is most widely known because it is
> part of Apple's OS X "Rendezvous" system, where it is used to do service
> discovery over local networks.
> 
> I've been planning to do this awhile, but a post by Ben Giddings made me
> think that I should do it sooner rather than later. Thanks!
> 
> It's beta, but it works (for me and my Alpha tester, Ben, anyhow).
> 
> RAA - http://raa.ruby-lang.org/project/net-mdns/
> 
> The rdoc overview is below, to get an idea what its about.
> 
> Cheers,
> Sam
> 
> main:Resolv::MDNS
> title:mDNS - multicast DNS and service discovery (aka "Rendezvous")
> 
> Author::     Sam Roberts <[e-mail]>
> Copyright::  Copyright (C) 2005 Sam Roberts
> License::    May be distributed under the same terms as Ruby
> Version::    0.0
> Homepage::   http://vpim.rubyforge.org/mdns
> Download::   http://vpim.rubyforge.org/mdns/mdns.tgz
> 
> == Summary
> An extension to the standard 'resolv' resolver library that adds support
> for multicast DNS.  mDNS is an extension of hierarchical, unicast DNS to
> link-local multicast. It is most widely known because it is part of Apple's
> OS X "Rendezvous" system, where it is used to do service discovery over
> local networks.
> 
> MDNS can be used for:
> - name to address lookups on local networks
> - address to name lookups on local networks (only for link-local addresses)
> - discovery of services on local networks
> 
> = Example
>   require 'net/http'
>   require 'net/dns/mdns-resolv'
>   require 'resolv-replace'
> 
>   # Address lookup
> 
>   begin
>   puts Resolv.getaddress('example.local')
>   rescue Resolv::ResolvError
>     puts "no such address!"
>   end
> 
>   # Service discovery
> 
>   mdns = Resolv::MDNS.new
> 
>   mdns.each_resource('_http._tcp.local', Resolv::DNS::Resource::IN::PTR) do |rrhttp|
>     service = rrhttp.name
>     host = nil
>     port = nil
>     path = '/'
> 
>     rrsrv = mdns.getresource(rrhttp.name, Resolv::DNS::Resource::IN::SRV)
>     host, port = rrsrv.target.to_s, rrsrv.port
>     rrtxt = mdns.getresource(rrhttp.name, Resolv::DNS::Resource::IN::TXT)
>     if  rrtxt.data =~ /path=(.*)/
>       path = $1
>     end
> 
>     http = Net::HTTP.new(host, port)
> 
>     headers = http.head(path)
> 
>     puts "#{service[0]} on #{host}:#{port}#{path} was last-modified #{headers['last-modified']}"
>   end
> 
> == Address Lookups
> When used for name lookups, it is most useful to add MDNS to the default
> set of resolvers queried when using the 'resolv' module methods. This is
> done by doing:
>   require 'net/dns/mdns-resolv'
>   Resolv.getaddress('localhost') # resolved using Resolv::Hosts("/etc/hosts")
>   Resolv.getaddress('www.example.com') # resolved using Resolv::DNS
>   Resolv.getaddress('example.local') # resolved using Resolv::MDNS
> Using this approach means that both global DNS names and local names can be
> resolved.  When doing this, you may also consider doing:
>   require 'resolv-replace'
> This has the effect of replacing the default ruby implementation of address
> lookup in IPSocket, TCPSocket, UDPSocket, and SOCKSocket with
> Resolv.getaddress, so (if 'net/dns/mdns-resolv' has been required) the
> standard libraries TCP/IP classes will use mDNS for name lookups in the .local domain.
> 
> == Service Discovery
> 
> Service discovery consists of 2 stages:
> - enumerating the names of the instances of the service
> - resolving the instance names
> 
> = Service Enumeration
> 
> To do this query the pointer records (Resolv::DNS::Resource::IN::PTR) for
> names of the form _svc._prot.local. The values of svc and prot for common
> services can be found at http://www.dns-sd.org/ServiceTypes.html.
> The first label of the name returned is suitable for display to peoplem, and
> should be unique in the network.
> 
> = Service Resolution
> 
> In order to resolve a service name query the service record
> (Resolv::DNS::Resource::IN::SRV) for the name. The service record contains
> a host and port to connect to. The host name will have to be resolved to
> an address. This can be done explicitly using mDNS or, if resolv-replace
> and mdns-default have been required, it will be done by the standard library.
> In addition, some services put "extra" information about the service in a
> text (Resolv::DNS::Resource::IN::TXT) record associated with the service name.
> The format of the text record is service-specific.
> 
> == 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
> - http://www.dns-sd.org
> 
> == Comparison to the DNS-SD Extension
> 
> The DNS-SD project at http://dnssd.rubyforge.org/wiki/wiki.pl 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 (they can
> take advantage of the daemon's cache) and likely a better way of doing
> mDNS queries than using pure ruby. Also, the mDNS daemon is capable of
> advertising services over the network, and Resolv::MDNS can't do that,
> though I'm working on it. There's some technical obstacles.
> 
> Currently, the only thing I'm aware of Resolv::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.  There is no
> reason DNS-SD can't do this, and I'll try and add that capability as soon as I
> find a way to install and use DNS-SD, which leads to why you might be
> interested in Resolv::MDNS.
> 
> 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, Resolv::MDNS may be useful to you.
> 
> == Samples
> 
> There are a few samples in the samples/ directory:
> - mdns.rb is useful for finding out as much as possible about services on .local
> - mdns_demo.rb is a sample provided by Ben Giddings, with better docs, showing
>   the call sequence he uses when resolving services. Thanks, Ben!
> 
> == TODO
> 
> - Implement a Resolv:: object that uses the DNS-SD project, so the standard
>   library will use it for .local name lookups.
> - Implement response cacheing and service advertising in Resolv::MDNS.
> - Implement a higher level service discovery API that will work with either
>   DNS-SD or Resolv::MDNS, so either can be used (as available) without code
>   changes.
> - Various API improvements, testing, ...
> 
> == Author
> 
> Any feedback, questions, problems, etc., please contact me, Sam Roberts,
> via [e-mail], or directly.
> 
> 


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