Subject: Re: How to configure client-side TLS ciphers for streaming replication?

From: Laurenz Albe <[email-masked]>
Date: Tue, 26 Aug 2025 22:16:39 +0200

On Tue, 2025-08-26 at 20:34 +0800, xx Z wrote:
> Thanks for your suggestion.
> But I still want to know why we can't set "ssl_ciphers" on the client side.

I'd say because nobody implemented it, perhaps because nobody felt the need.

> This is still considered a security issue in some cases, and PostgreSQL has
> mature capabilities on the master side to implement this functionality.

That sounds to me like some moderately clueful security auditor is looking
for a nit to pick.  If you do streaming replication, and you control the
ciphers on the primary server, what added security benefit do you get by
controlling the ciphers on the standby server (the client) as well?

Yours,
Laurenz Albe

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From: Daniel Gustafsson <[email-masked]>
Date: Wed, 27 Aug 2025 21:59:18 +0200

> On 26 Aug 2025, at 22:16, Laurenz Albe <[email-masked]> wrote:
> 

I think the former is a highly likely suspect here.

> controlling the ciphers on the standby server (the client) as well?

I would place this above nitpicking, but I also don't have a clear idea of an
attack (if I did I'd fix it..).  TLS is riddled with weird cases involving
network middleboxes (usually very enterprisy) so insisting on control isn't
necessarily a bad thing.

--
Daniel Gustafsson