SERVERFILE=/var/lib/dhcp/chrony.servers.$interface

chrony_config() {
    rm -f $SERVERFILE
    for server in $new_ntp_servers; do
        echo "$server iburst" >> $SERVERFILE
    done
    /usr/lib/chrony/chrony-helper update-daemon || :
}

chrony_restore() {
    if [ -f $SERVERFILE ]; then
        rm -f $SERVERFILE
        /usr/lib/chrony/chrony-helper update-daemon || :
    fi
}

case $reason in
    BOUND|RENEW|REBIND|REBOOT)
        chrony_config
        ;;
    EXPIRE|FAIL|RELEASE|STOP)
        chrony_restore
        ;;
esac