mdns.rb

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

class MDNS
  IP = "224.0.0.251"
  PORT = 5353
  TTL = 255

  def initialize(hosts=[])
    @host = Socket.gethostname
    @addr = Socket.getaddrinfo(@host, 0, Socket::AF_INET, Socket::SOCK_STREAM)[0][3]

    @hosts = Array(hosts)

    @socket = UDPSocket.new
    @socket.setsockopt(Socket::SOL_SOCKET, Socket::SO_REUSEADDR, true)
    @socket.setsockopt(Socket::SOL_SOCKET, Socket::SO_REUSEPORT, true)
    @socket.setsockopt(Socket::IPPROTO_IP, Socket::IP_ADD_MEMBERSHIP, IPAddr.new(IP).hton + IPAddr.new(addr).hton)
    @socket.setsockopt(Socket::IPPROTO_IP, Socket::IP_TTL, TTL)
    @socket.setsockopt(Socket::IPPROTO_IP, Socket::IP_MULTICAST_TTL, TTL)
    @socket.bind(Socket::INADDR_ANY, PORT)
  end

  attr_reader :host, :addr
  attr_accessor :hosts

  def run
    Socket.udp_server_loop_on([@socket]) do |data, source|
      begin
        request = Resolv::DNS::Message.decode(data)
        p request: request

        if request.qr == 0
          asked_hosts = request.question.map(&:first).map(&:to_s)
          have_hosts = asked_hosts & hosts

          unless have_hosts.empty?
            response = Resolv::DNS::Message.new(request.id)

            response.rd = 0
            response.qr = 1
            response.aa = 1

            have_hosts.each do |host|
              response.add_answer(host, TTL, Resolv::DNS::Resource::IN::A.new(addr))
            end

            p response: response
            source.reply(response.encode)
          end
        end
      rescue
        puts "Error responding to: #{data.inspect} #{source.inspect}:", $!, *$!.backtrace
      end
    end
  end
end

MDNS.new(["github.com", "railscamp.local", "railscamps.local", "rubygems.local", "twetter.local", "twitter.local"]).run