Created attachment 180933 [details]
Screenshot of the Wifi and Networking configuration window

SUMMARY
The Wifi and Internet section of the System Settings app currently does not have a field to set the NetworkManager connection option `ipv6.addr-gen-mode`.

From networkmanager.conf(5):

> CONNECTION SECTION
>        ipv6.addr-gen-mode
>           If the per-profile setting is either "default" or "default-or-eui64", the global default is used.
>           If the default is unspecified, the fallback value is either "stable-privacy" or "eui64", depending
>           on whether the per-profile setting is "default" or "default-or-eui64, respectively.

I cannot use the default, `stable-privacy`, because I have a dynamic GUA address — which means that the interface identifiers are constantly changing, and I need a "stable" address for the router firewall rules to work.

So I edit my connection file at `/etc/NetworkManager/system-connections/`:

```diff
[connection]
id=my_connection
uuid=my_uuid
type=ethernet
autoconnect-priority=-100

[ethernet]

[ipv4]
method=auto

[ipv6]
-addr-gen-mode=stable-privacy
+addr-gen-mode=eui64
method=auto

[proxy]
```

After restarting the NetworkManager service, I can see the new IPv6 IID based on the MAC address — it works. But as soon as you decide to edit the connection through the System Settings app, it will reset `addr-gen-mode` to `stable-privacy` again. The question is: WHY?

While we're at it, I'd like to request the addition of this field to the GUI app. For instance, the GTK-based "nm-connection-editor" has it (will attach a screenshot of it).

STEPS TO REPRODUCE
1. In the System Settings app, go to Wi-Fi & Internet, then Wi-Fi & Networking.
2. Pick your connection from the list and rename it — to make sure it is created at `/etc/NetworkManager/system-connections/` (e.g. "my_connection"). Remember to "Apply" the changes.
3. Edit `/etc/NetworkManager/system-connections/my_connection.nmconnection` and set `addr-gen-mode` in the `ipv6` section to `eui64`.
4. Restart NetworkManager: `sudo systemctl restart NetworkManager`
5. Repeat step 1 and 2, then inspect the connection file: `cat /etc/NetworkManager/system-connections/my_connection.nmconnection`

OBSERVED RESULT
```
[ipv6]
addr-gen-mode=stable-privacy
```

EXPECTED RESULT
```
[ipv6]
addr-gen-mode=eui64
```

SOFTWARE/OS VERSIONS
Windows: n/a
macOS: n/a
(available in the Info Center app, or by running `kinfo` in a terminal window)
Linux/KDE Plasma: Fedora 42 (KDE Plasma Edition) (amd64)
KDE Plasma Version: 6.3.4
KDE Frameworks Version: 6.13.0
Qt Version: 6.9.0

ADDITIONAL INFORMATION