Re: [Ntp] Comments on draft-langer-ntp-nts-for-ptp

Miroslav Lichvar <mlichvar@redhat.com> Mon, 08 March 2021 16:00 UTC

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Date: Mon, 8 Mar 2021 17:00:01 +0100
From: Miroslav Lichvar <mlichvar@redhat.com>
To: Heiko Gerstung <heiko.gerstung=40meinberg.de@dmarc.ietf.org>
Cc: Watson Ladd <watsonbladd@gmail.com>, NTP WG <ntp@ietf.org>, "Langer, Martin" <mart.langer@ostfalia.de>
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Subject: Re: [Ntp] Comments on draft-langer-ntp-nts-for-ptp
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On Mon, Mar 08, 2021 at 02:31:04PM +0100, Heiko Gerstung wrote:
> >    If you don't have any client-specific state on the server (and
> >    hardcode the address in the cookie), how do you prevent replay
> >    attacks, e.g. canceling a previous request, or changing the message
> >    rate to a previous value, or requesting unicast transmissions for
> >    clients that no longer exist to cause a DoS attack on the server?
> 
> Not sure what you mean with "hardcode the address in the cookie", 

That the client's address is included in the cookie to prevent
replaying from other addresses.

> but in order to change the message rate or cancel a unicast transmission, an NTS4PTP client would have to present a valid cookie. Replay attacks could be prevented by adding an expiration field into the cookie after which a GM will not accept it anymore. 

I don't see how could that work.

How would the server know when will be the cookie used? Even if it had
only one cookie, it could be used at any time, e.g. to cancel the
grant when it is shutting down. If it was started soon again, that
cookie which authenticated the cancelation request would still be
valid.

-- 
Miroslav Lichvar