Exactly, that was my point:

 the 6to4 address 2002:ac8:5ac5:0:b0:46d:fd08:7c9f actually means that the originator is 10.200.90.197 (ac8:5ac5).

Being a private address I don’t think it is valid as a 6to4 address source, because they are meant for public IPv4 addresses:

   The document defines a method for assigning an interim unique IPv6
   address prefix to any site that currently has at least one globally
   unique IPv4 address, and specifies an encapsulation mechanism for
   transmitting IPv6 packets using such a prefix over the global IPv4
   network.

It seems to me is generated by IONOS/Kundenserver. It looks to me like they are using internal email servers for customers and instead of properly configuring IPv6, they allow 6to4 addresses to be generated or may be they do it on purpose. The interesting thing is that afterwards they send the email via IPv4 only!

So either you configure IPv6 properly all the way thru, or not at all, but this way using an invalid 6to4 address is very weird and definitively not a good practice. May be someone has contact with them so they realize this is broken.


Regards,
Jordi

@jordipalet


El 21 ene 2025, a las 19:19, Gert Doering <gert@space.net> escribió:

Hi,

On Tue, Jan 21, 2025 at 04:03:53PM +0100, Marco Moock wrote:
6to4 with private IPv4 address in the inside.

I don't see any problem here.

Why would anyone do this?

If you only have v4, use v4, not pretend to have v6.  If you have real v6,
do not use 2002:

Gert Doering
       -- NetMaster
--
have you enabled IPv6 on something today...?

SpaceNet AG                      Vorstand: Sebastian v. Bomhard, Ingo Lalla,
                                          Karin Schuler, Sebastian Cler
Joseph-Dollinger-Bogen 14        Aufsichtsratsvors.: A. Grundner-Culemann
D-80807 Muenchen                 HRB: 136055 (AG Muenchen)
Tel: +49 (0)89/32356-444         USt-IdNr.: DE813185279
-----
To unsubscribe from this mailing list or change your subscription options, please visit: https://mailman.ripe.net/mailman3/lists/ipv6-wg.ripe.net/
As we have migrated to Mailman 3, you will need to create an account with the email matching your subscription before you can change your settings.
More details at: https://www.ripe.net/membership/mail/mailman-3-migration/


**********************************************
IPv4 is over
Are you ready for the new Internet ?
http://www.theipv6company.com
The IPv6 Company

This electronic message contains information which may be privileged or confidential. The information is intended to be for the exclusive use of the individual(s) named above and further non-explicilty authorized disclosure, copying, distribution or use of the contents of this information, even if partially, including attached files, is strictly prohibited and will be considered a criminal offense. If you are not the intended recipient be aware that any disclosure, copying, distribution or use of the contents of this information, even if partially, including attached files, is strictly prohibited, will be considered a criminal offense, so you must reply to the original sender to inform about this communication and delete it.