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

[Danny Mayer <[e-mail]>]

On 9/22/22 6:06 AM, Ulrich Windl wrote:
>>>> Hal Murray <[e-mail]> schrieb am 22.09.2022 um 11:43 in
> Nachricht
> <[e-mail]>:
>
>> [e-mail]‑regensburg.de said:
>>>> 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?
>> With client/server, the client can put a nonce in the packet.  That prevents
>> replay attacks.
>>
>> With NTP v3+v4, there isn't an official slot for a nonce in the packet
>> format,
>> but as long as the client checks that the transmit time stamp (xmt) is
>> returned in the originate time stamp (org) slot that will catch replay
>> attacks.
>>
>> That assumes the client put a time stamp in the xmt slot.  It can also be
>> used
>> as a nonce since the server doesn't use it after copying it over to the org
>> slot of the reply packet.
> I guess someone in the past didn't think that it would be easy to replay
> authenticated packets from a different address.
> Wouldn't it be easier to include the server's IP (and port, maybe version,
> too) as extension field, so that the address would be considered when checking
> the MAC? Then a client or server would have to (or at least: can) check whether
> the IP packet's sending address matches the authenticated address. But still
> that (IP source address) may be fake, too...
It would be a nice idea if it wasn't for the fact that some packet may 
be going through a NAT and the IP address that the server sees may be 
different from the one that the client uses to send the packet.

Danny