[Ntp] Antw: [EXT] Re: Antwort: Re: Symmetric mode

[Ulrich Windl <[e-mail]>]

>>> Miroslav Lichvar <[e-mail]> schrieb am 22.09.2022 um 09:40 in
Nachricht <YywRhCmTD4mt8KTy@localhost>:
> On Wed, Sep 21, 2022 at 07:30:06PM ‑0400, Danny Mayer wrote:
>> > Again, could you qualify which of these statements you disagree with
>> > and/or how so?
>> 
>> Symmetric peer associations are not ephemeral since both sides keep
>> information of each other the same way that any client does.
> 
> Quoting from RFC 5905:
>   Ephemeral associations are mobilized upon the arrival of a
>   packet and are demobilized upon error or timeout.
> 
>> An attacker
>> cannot really replay an authenticated message as it would be rejected as
>> already received.
> 
> The peer doesn't know if a received message is a new one or it was
> already sent before by one of the peers. It doesn't have an infinite
> storage and there can be messages lost in the network.
> 
> ntpd remembers only the last received packet. You can replay any
> previous message from the other peer, or the peer's own messages (last
> or previous).
> 
>> I'm not sure what is meant by limiting IP addresses to
>> prevent that since a replay attack would have to use the same IP address
to
>> send the packet.
> 
> The IP addresses and ports are not included in the data authenticated
> by the NTP MAC. You can replay an authenticated NTP messages from any
> address and any port.

But this problem isn't specific to peer messages, right? So what is the
specific peer issue?

> 
> If you restrict a key to an address and port, the advantage of the
> ephemeral association over persistent association is lost. And if you
> specify the other peer on both ends, you can just as well use the
> client/server mode. It will double the NTP traffic, but it will be
> more secure.
> 
> ‑‑ 
> Miroslav Lichvar
> 
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