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

On Tue, 2025-08-26 at 19:48 +0800, xx Z wrote:
> Is there a way for a streaming replication standby (client) to restrict its list
> of supported TLS ciphers, similar to how the ssl_ciphers parameter works on the
> primary server?
> We need this for security compliance but can't find an equivalent setting for
> the client-side connection in primary_conninfo.

I don't think that there is a way to do that on the client side.
But the streaming replication primary is surely under your control, so it should
be sufficient to set "ssl_siphers" there.

Yours,
Laurenz Albe

> On Aug 26, 2025, at 5:35 AM, xx Z <xxz030811@gmail.com> wrote:
>
> 
> Thanks for your suggestion.
> But I still want to know why we can't set "ssl_ciphers" on the client side.
> This is still considered a security issue in some cases, and PostgreSQL has mature capabilities on the master side to
implementthis functionality. 
>
> Greetings,
> Yunfei Zhou
>

What is your attack/exposure scenario?

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

> On 26 Aug 2025, at 22:16, Laurenz Albe <laurenz.albe@cybertec.at> wrote:
>
> 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.

I think the former is a highly likely suspect here.

>> 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?

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