Re: Experiments with Postgres and SSL

On Wed, Feb 22, 2023 at 4:26 AM Heikki Linnakangas <hlinnaka(at)iki(dot)fi> wrote:
> On 20/01/2023 03:28, Jacob Champion wrote:
> > If you want to multiplex protocols on a port, now is an excellent time
> > to require clients to use ALPN on implicit-TLS connections. (There are
> > no clients that can currently connect via implicit TLS, so you'll
> > never have another chance to force the issue without breaking
> > backwards compatibility.) That should hopefully make it harder to
> > ALPACA yourself or others [2].
>
> Good idea. Do we want to just require the protocol to be "postgres", or
> perhaps "postgres/3.0"? Need to register that with IANA, I guess.

Unless you plan to make the next minor protocol version fundamentally
incompatible, I don't think there's much reason to add '.0'. (And even
if that does happen, 'postgres/3.1' is still distinct from
'postgres/3'. Or 'postgres' for that matter.) The Expert Review
process might provide some additional guidance?

> We implemented a protocol version negotiation mechanism in the libpq
> protocol itself, how would this interact with it? If it's just
> "postgres", then I guess we'd still negotiate the protocol version and
> list of extensions after the TLS handshake.

Yeah. You could choose to replace major version negotiation completely
with ALPN, I suppose, but there might not be any maintenance benefit
if you still have to support plaintext negotiation. Maybe there are
performance implications to handling the negotiation earlier vs.
later?

Note that older versions of TLS will expose the ALPN in plaintext...
but that may not be a factor by the time a postgres/4 shows up, and if
the next protocol is incompatible then it may not be feasible to hide
the differences via transport encryption anyway.

> Regarding Vladimir's comments on how clients can migrate to this, I
> don't have any great suggestions. To summarize, there are several options:
>
> - Add an "fast_tls" option that the user can enable if they know the
> server supports it

I like that such an option could eventually be leveraged for a
postgresqls:// URI scheme (which should not fall back, ever). There
would be other things we'd have to change first to make that a reality
-- postgresqls://example.com?host=evil.local is problematic, for
example -- but it'd be really nice to have an HTTPS equivalent.

--Jacob