Sample mDNS client in Ruby

mdns.rb

# mDNS client
#
# Usage: ruby script.rb "_http._tcp.local."

require "socket"
require "ipaddr"
require "fcntl"
require "resolv"

module DNSSD
  MDNS_PORT = 5353
  MDNS_UNICAST_RESPONSE = 0x8000

  class Query
    attr_reader :name

    TypeValue = 12 # PTR
    ClassValue = 1 | MDNS_UNICAST_RESPONSE

    def initialize name
      @name = name
    end
  end

  def self.browse name, &blk
    send_query [Query.new(name)], &blk
  end

  private_class_method def self.open_client_sockets port
    addrs = Socket.getifaddrs
    addrs.select { |ifa|
      ifa.addr &&
        (ifa.flags & Socket::IFF_UP > 0) &&         # must be up
        (ifa.flags & Socket::IFF_MULTICAST > 0) &&  # must have multicast
        (ifa.flags & Socket::IFF_LOOPBACK == 0) &&  # must not be loopback
        (ifa.flags & Socket::IFF_POINTOPOINT == 0)  # must not be pointopoint
    }.select { |ifa| ifa.addr.ipv4? }.map { |ifa|
      open_ipv4(ifa.addr, port)
    }
  end

  private_class_method def self.send_query queries
    sockets = open_client_sockets 0

    sockets.each { |socket| multiquery_send socket, queries, 0 }

    loop do
      readers, = IO.select(sockets, [], [])
      readers.each do |reader|
        yield query_recv(reader)
      end
    end
  end

  private_class_method def self.open_ipv4 saddr, port
    sock = UDPSocket.new
    sock.setsockopt(Socket::SOL_SOCKET, Socket::SO_REUSEADDR, true)
    sock.setsockopt(Socket::SOL_SOCKET, Socket::SO_REUSEPORT, true)
    sock.setsockopt(Socket::IPPROTO_IP, Socket::IP_MULTICAST_TTL, true)
    sock.setsockopt(Socket::IPPROTO_IP, Socket::IP_MULTICAST_LOOP, true)
    setup_ipv4 sock, saddr, port
    sock
  end

  private_class_method def self.setup_ipv4 sock, saddr, port
    sock.setsockopt(Socket::IPPROTO_IP, Socket::IP_ADD_MEMBERSHIP,
                    IPAddr.new("224.0.0.251").hton + IPAddr.new(saddr.ip_address).hton)
    sock.setsockopt(Socket::IPPROTO_IP, Socket::IP_MULTICAST_IF, IPAddr.new(saddr.ip_address).hton)
    sock.bind saddr.ip_address, port
    flags = sock.fcntl(Fcntl::F_GETFL, 0)
    sock.fcntl(Fcntl::F_SETFL, Fcntl::O_NONBLOCK | flags)
  end

  private_class_method def self.multiquery_send sock, queries, query_id
    query = Resolv::DNS::Message.new query_id

    queries.each { |q| query.add_question q.name, q.class }

    multicast_send sock, query.encode
  end

  private_class_method def self.multicast_send sock, query
    dest = Addrinfo.new(["AF_INET", MDNS_PORT, nil, "224.0.0.251"])
    sock.send(query, 0, dest)
  end

  private_class_method def self.query_recv sock
    buf, = sock.recvfrom 2048
    Resolv::DNS::Message.decode(buf)
  end
end

if __FILE__ == $0
  DNSSD.browse ARGV[0] || "_http._tcp.local." do |recv|
    name = nil
    address = nil
    recv.additional.each do |_, _, data|
      case data
      when Resolv::DNS::Resource::IN::SRV
        name = data.target
      when Resolv::DNS::Resource::IN::A
        address = data.address
      end
    end

    p FOUND: [name, address]
  end
end