Защита от DDos

Всем доброго дня.

Имется проблема: есть VPS - на котором находится сайт, подвергается ДДос-атаке, по непонятным причинам (вирусы или конкуренты).

Есть какие-то рекомендации на английсом и предприняты некоторые действия ( проксирование), но хочется понять причину и устранить:

A
public NTP server on your network, running on IP address 92.__.__._
and UDP port 123, participated in a very large-scale attack against a
customer of ours, generating UDP responses to spoofed "monlist"
requests that claimed to be from the attack target.

Please
consider reconfiguring this NTP server in one or more of these ways:

1.
If you run ntpd, upgrading to the latest version, which removes the
"monlist" command that is used for these attacks;
alternately, disabling the monitoring function by adding "disable
monitor" to your /etc/ntp.conf file.

2.
Setting the NTP installation to act as a client only. With ntpd, that
can be done with "restrict default ignore" in
/etc/ntp.conf; other daemons should have a similar configuration
option. More information on configuring different devices can be
found here:
https://www.team-cymru.org/ReadingRoom/Templates/secure-ntp-template.html.

3.
Adjusting your firewall or NTP server configuration so that it only
serves your users and does not respond to outside IP addresses.