Something I wrote a lifetime ago that'll send a Wake On LAN magic packet to a specified computer when a specified device joins the WiFi zone.

wakeonphone.rb

require 'rubygems'
require 'snmp'
require 'socket'

iPhones = [ "00:23:32:f4:c4:a1" ]
pcs = [ "00:1B:21:29:70:1D" ]

wakeUp = Hash::new
woken = Hash::new
seen = Hash::new
iPhones.each do |s|
   wakeUp[s] = pcs[iPhones.index(s)]
   woken[s] = false;
   seen[s] = false;
end

wakeUp.each do |k,v|
   puts "When I see \"#{k}\" I'll wake up \"#{v}\"."
end

def wake(mac)
   wol_magic=(0xff.chr)*6+(mac.split(/:/).pack("H*H*H*H*H*H*"))*16
   socket = UDPSocket.open()
   socket.setsockopt(Socket::SOL_SOCKET,Socket::SO_BROADCAST,1)
   10.times { socket.send(wol_magic, 0, "192.168.1.255", "discard") }
   socket.close()
end


ipTable_columns = ["ipNetToMediaPhysAddress"]
while true do
   SNMP::Manager.open(:Host => '192.168.1.254', :Community => "secret") do |manager|
      manager.walk(ipTable_columns) do |row|
         row.each do |vb|
            mac = vb.value.unpack("H2H2H2H2H2H2").join(":")
            #puts mac
            iPhones.each do |s|  
               if (s == mac)
                  if woken[s]
                     seen[s] = true
                     next
                  end
                  puts "#{mac} found!  Waking #{wakeUp[mac]}"
                  wake(pcs[iPhones.index(s)])
                  woken[s] = true
                  seen[s] = true
                  break
               end
            end
         end
      end
   end
   #puts "---"
   iPhones.each do |s|
      if seen[s] != true
         woken[s] = false
      end
      seen[s] = false
   end
   sleep(5)
end