code: public byte[] getHardwareAddress()
    {
      if (CurrentTime.isTest() || System.getProperty("test.mac") != null) {
        return new byte[] { 10, 0, 0, 0, 0, 10 };
      }
      
      for (NetworkInterfaceBase nic : getNetworkInterfaces()) {
        if (! nic.isLoopback()) {
          return nic.getHardwareAddress();
        }
      }
      
      try {
        InetAddress localHost = InetAddress.getLocalHost();
        
        return localHost.getAddress();
      } catch (Exception e) {
        log.log(Level.FINER, e.toString(), e);
      }
      
      return new byte[0];
    }
docs: Returns a unique identifying byte array for the server, generally
  the mac address.