WiFi Presence (source: https://github.com/holman/feedback/issues/59)

wifi_presence

require 'net/http'
out = `arp-scan -lq`

macs =
out.split("\n").map do |line|
  if line =~ /([0-9A-F][0-9A-F :]{16})/i
    puts $1
    $1.split(' ').join(':').upcase
  end
end.compact.join(',')