post_install() {
  getent group ntp &>/dev/null || groupadd -g 87 ntp >/dev/null
  getent passwd ntp &>/dev/null || useradd -u 87 -g ntp -d /var/lib/ntp \
    -c 'Network Time Protocol' -s /bin/nologin ntp >/dev/null
  ntp_shell=$(getent passwd ntp | cut -d: -f7)
  if [ "$ntp_shell" != '/bin/nologin' ]; then
    chsh -s /bin/nologin ntp &>/dev/null
  fi
  # fix /var/db/ntpd.drift is empty
  echo "0.0" > /var/db/ntpd.drift
}

post_upgrade(){
  post_install
}