Re: [Ntp] Symmetric mode

[Miroslav Lichvar <[e-mail]>]

On Sun, Sep 18, 2022 at 07:46:14PM -0700, Hal Murray wrote:
> Is symmetric mode interesting enough that we should try to fix that?  If so, 
> would you please say a few words about why it is interesting?

It's not very useful. I think the most interesting thing about it is
that sources using the symmetric mode are marked differently in tools
like ntpq, so it's more obvious to the admin that synchronization can
work in both directions.

At the protocol level it's just trouble. It's difficult to implement
if you want to handle all the corner cases and difficult to secure.
The main problem is that there is no response for each request, so the
peers have to select to which request they respond, i.e. guess which
one was genuine. Authentication with a symmetric key doesn't prevent
a replay attack.

There are ephemeral associations, where only one peer is configured
with the address of the other peer. This enables attackers to replay an
authenticated message to create an unlimited number of associations on
the peer. In the ntp.org implementation there is a possibility to
limit keys to IP addresses to prevent that, but in that case it's
easier to just specify the address directly as a peer in the
configuration file and you don't have to worry about associations
created on different ports of the address.

-- 
Miroslav Lichvar