osx_dns.rb

require 'plist'

name_servers = ['172.17.42.1', '8.8.4.4', '8.8.8.8']
plist_file = '/Library/Preferences/SystemConfiguration/preferences.plist'
plist = Plist::parse_xml(plist_file)
current_set_key = plist['CurrentSet'].split('/').at(-1)
service_key = nil
plist['NetworkServices'].each do |key, dict|
  if (addresses = dict['DNS']['ServerAddresses'])
    addresses = addresses + (name_servers - addresses)
    dict['DNS']['ServerAddresses'] = addresses
    plist['NetworkServices'][key] = dict
    service_key = key
    break
  end
end
plist['Sets'][current_set_key]['Network']['Global']['IPv4']['ServiceOrder'].delete_if do |s|
  s == service_key
end
plist['Sets'][current_set_key]['Network']['Global']['IPv4']['ServiceOrder'].unshift(service_key)
File.open(plist_file, 'w') do |file|
  file.write(plist.to_plist)
end
system("defaults read #{plist_file} > /dev/null")