Re: [Ntp] DDoS meets NTP

Miroslav Lichvar <mlichvar@redhat.com> Tue, 20 April 2021 07:23 UTC

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Date: Tue, 20 Apr 2021 09:23:18 +0200
From: Miroslav Lichvar <mlichvar@redhat.com>
To: Hal Murray <hmurray@megapathdsl.net>
Cc: NTP WG <ntp@ietf.org>
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Subject: Re: [Ntp] DDoS meets NTP
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On Mon, Apr 19, 2021 at 10:38:23AM -0700, Hal Murray wrote:
> 
> mlichvar@redhat.com said:
> > What is worse, it can be exploited for a DoS attack on real NTP clients if it
> > doesn't randomly let some packets through. 
> 
> How many is "some"?  What fraction of the responses does a client need?
> 
> Is this a solvable problem?  If I let 1/N through, is there a value of N that 
> lets through enough real replies without also letting through enough bogus 
> traffic to make traditional DDoS practical?

Address-specific rate limiting in NTP can be circumvented. I think its
only purpose can be saving network traffic on broken and misconfigured
clients. That's all.

As for the highest usable N, it depends on the client implementation.
For a public server, N=4 (a leak rate of 25%) seems to be about the
highest value where most clients still appear to do something useful.
This can save up to 37.5% of network traffic (TX+RX).

-- 
Miroslav Lichvar