Re: [Ntp] Antwort: Re: Symmetric mode

[Danny Mayer <[e-mail]>]

On 9/22/22 3:40 AM, Miroslav Lichvar wrote:
> 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.
You forget that a symmetric peer knows about the other peer and it keeps 
that information. It's not really ephemeral since as a client of that 
other node it keeps the information of that peer.
>> 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.
When a peer is acting as the client it knows if it already has received 
the message. If the peer is acting as the server then it acts as if 
that's a new request from a client and responds appropriately.
>
> 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).
No, as explained above.
>
>> 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.
First of all you know what IP addresses you are peering with, so that 
wouldn't work anyway. I assume that you are talking about the peer 
acting as a client so if it's a replay attack it wouldn't work as the 
peer acting as a client has already received the packet so it would 
discard the replay packet.
> If you restrict a key to an address and port, the advantage of the
> ephemeral association over persistent association is lost.
I'm not sure what you mean.
>   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.

No, that's not true. Symmetric peer mode uses the same number of packets 
as client/server and as I said above it's no more vulnerable than 
client/server.

Danny