Re: [Ntp] Antwort: Re: Symmetric mode

[Miroslav Lichvar <[e-mail]>]

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.

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