it will only appear prior to the routers option if it is present on the
  Parameter-Request-List.  The subnet-mask option will also only be
  included by default (if it is not on the PRL) in response to DISCOVER
  or REQUEST messages.
  [ISC-Bugs #17117]
- The FQDN option is only supplied if the client supplied an FQDN option or
  if the FQDN option was explicitly requested on the PRL.
  [ISC-Bugs #17117]