interfaces: restructure PPP to allow complex IPv6-only deployments and remove "Use IPv4 connectivity" by making it the default

[fichtner]

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TODO:

• [x] The ironic truth about the option is that "Use IPv4 connectivity" opt-in is that the opt-in is actually the default so the default gets inverted, but only when PPPoE is involved in the first place. It would be nice to restrict this to PPPoE and invert or migrate the current default in order to make the most sense and avoid a bit of code (and feature taint having the option always available causing undefined behaviour on non-PPPoE connections).
• [x] While here the side quest is to restructure PPP to be address family agnostic and allow for things like IPv6-only DHCPv6 over PPPoE which isn't possible at the moment.
• [x] Restructure the PPP code to be IP family agnostic (will backport to 24.7.x)
• [ ] Prevent selection of wrong IPv4 modes for PPP devices in interface settings (will backport to 24.7.x)
• [ ] Remove edit of PPP devices in interface settings.

[doktornotor]

Ok, appears there was a user using 6rd over PPPoE... This https://github.com/opnsense/core/commit/b2d9372b0f8b2583da90e1e294d88996539e1983 did not make him happy...

https://forum.opnsense.org/index.php?topic=42081.msg209929#msg209929

[doktornotor]

On another note - while doing all these PPP cleanups, just found this in the backlog. Those impossible to work options are still there.

https://github.com/opnsense/core/issues/7005

[jfieber]

Ok, appears there was a user using 6rd over PPPoE... This b2d9372 did not make him happy...

To be completely clear, I’m far more unhappy with the ISP (CenturyLink) on this issue than with opnsense. It would be helpful if the removal was called out in the 24.7.2 release notes, and a statement of if there is a plan to restore it in a future release.

The other facet is that the previous stf device configuration under the hood mostly survives the upgrade process, but doesn’t work quite right (I didn’t spend much time digging into why before reverting to 24.1) and has no UI I could find to fix it. That is a poor experience.

[doktornotor]

This 6rd-over-PPPoE implementation would be my favorite if there is a "worst ways to deploy IPv6" competition among ISPs.

The option is trivial to restore, I'd suppose no call was made because no one expected such a thing to be in use.

[fichtner]

@jfieber hello, thanks for reaching out. I wrote the bulk in the forum, see https://forum.opnsense.org/index.php?topic=42081.msg209986#msg209986

For anyone else with that issue at the moment use 947e61b

# opnsense-patch 947e61b

It would be best to bring this into 24.7.3 as well.

The implications for the setup going forward are in the forum, but since is mostly a documentation effort done before releasing 25.1 I'll refrain from adding that to the current discussion here.