Обсуждение: Frontend/Backend Protocol: SSL / GSS Protocol Negotiation Problem

Hi!

I've received a bug report from a PostgreSQL user that psql 12.1 failed to connect to a PostgreSQL 12.1 server, with
thefollowing error message: 

psql: error: could not connect to server: FATAL: unsupported frontend protocol 1234.5679: server supports 2.0 to 3.0

After inspecting a TCP dump, I realised that libpq apparently sent a GSS startup packet, got 'N' (not supported)
response,then tried a SSL startup packet, at which point the server sent an error. 

The bug report is available at the following URL:
https://github.com/PostgresApp/PostgresApp/issues/537

After inspecting postmaster.c, it seems that postmaster only allows a single negotiation attempt, but libpq doesn't
knowthat. 

I'm not familiar with GSS, but from my naive point of view it would seem that we should fix this issue as follows:

1) On the server side, allow multiple negotiation attempts (eg. allow SSL negotiation after a rejected GSS negotiation
attempt)

2) On the client side, detect an error message after the second negotiation attempt, and reconnect, to ensure
compatibilitywith servers that do not support multiple attempts yet. 

I've attached two proposed patches with these changes.

Best regards,
Jakob

On Fri, Dec 06, 2019 at 02:25:46PM +0100, Jakob Egger wrote:
> I've received a bug report from a PostgreSQL user that psql 12.1
> failed to connect to a PostgreSQL 12.1 server, with the following
> error message:
>
> psql: error: could not connect to server: FATAL: unsupported
> frontend protocol 1234.5679: server supports 2.0 to 3.0

Andrew Gierth has reported this issue, and has provided a patch:
https://www.postgresql.org/message-id/87h82kzwqn.fsf@news-spur.riddles.org.uk
If you could help with it, that would be great.
--
Michael

> On 6. Dec 2019, at 15:08, Michael Paquier <michael@paquier.xyz> wrote:
>
> On Fri, Dec 06, 2019 at 02:25:46PM +0100, Jakob Egger wrote:
>> I've received a bug report from a PostgreSQL user that psql 12.1
>> failed to connect to a PostgreSQL 12.1 server, with the following
>> error message:
>>
>> psql: error: could not connect to server: FATAL: unsupported
>> frontend protocol 1234.5679: server supports 2.0 to 3.0
>
> Andrew Gierth has reported this issue, and has provided a patch:
> https://www.postgresql.org/message-id/87h82kzwqn.fsf@news-spur.riddles.org.uk
> If you could help with it, that would be great.
> --
> Michael


Thanks for pointing me to the right thread! My server side fix is similar to Andrews, but Andrews is maybe a bit more
elegant.

But this also needs to be fixed on the client side as well, otherwise affected clients can't connect to older servers
anymore.

My second patch attempts to fix the issue on the client side.

I'll respond to the other thread as well.

Jakob

>>>>> "Jakob" == Jakob Egger <jakob@eggerapps.at> writes:

 Jakob> But this also needs to be fixed on the client side as well,
 Jakob> otherwise affected clients can't connect to older servers
 Jakob> anymore.

There's a workaround, which is to set PGGSSENCMODE=disable on the
client.

It would be far better to avoid complicating the client side with this
if we can possibly do so.

-- 
Andrew (irc:RhodiumToad)

> On 6. Dec 2019, at 16:45, Andrew Gierth <andrew@tao11.riddles.org.uk> wrote:
>
>>>>>> "Jakob" == Jakob Egger <jakob@eggerapps.at> writes:
>
> Jakob> But this also needs to be fixed on the client side as well,
> Jakob> otherwise affected clients can't connect to older servers
> Jakob> anymore.
>
> There's a workaround, which is to set PGGSSENCMODE=disable on the
> client.
>
> It would be far better to avoid complicating the client side with this
> if we can possibly do so.

As far as I understand, the bug impacts clients version 12.0 or later who have Kerberos when connecting to 12.0 or 12.1
serversthat don't have Kerberos. (Assuming that the bug will be fixed server side in 12.2) 

I don't know how many people use Kerberos, so I can't say if it's worth the additional complexiity to work around the
bug.

In any case, the workaround should probably be documented somewhere:

If you try to connect to a PostgreSQL 12.0 or 12.1 server and you get the following error message:

psql: error: could not connect to server: FATAL: unsupported frontend protocol 1234.5679: server supports 2.0 to 3.0

Then you need to use the connection parameter gssencmode=disable

Is there a place where such workarounds are documented, or do we rely on Google indexing the mailing list archive?

Best regards,

Jakob