Add modules for RFC8925 and RFC2563

[candlerb]

These modules support DHCPv4 responses for IPv6-only or IPv6-mostly networks.

In particular: macOS / iOS / Android will configure their built-in CLAT if you provide the RFC8925 DHCPv4 response and advertise a RFC8781 NAT64 prefix in your Router Advertisements.

[Natolumin]

Thanks! This is very nice; sorry for missing it the first time you posted it

There are a few questions and follow-ups inline and a couple changes I'd like to see, but otherwise this is a nice approach

[candlerb]

Thank you for your thorough code review!

This is excellent timing - right now I'm at APRICOT 2024 in Bangkok and I'm using this code for the IPv6-only SSID in the conference network. I'll go through all your notes tomorrow, and should be able to deploy the updated code in advance of the conference starting on Tuesday.

I added the MAC address logging to be able to report on how many clients support these DHCP options.

[candlerb]

I have pushed the code which would be used if the latest version of insomniacslk/dhcp#524 is accepted (and tested it in a go workspace)

I have left AutoConfigure as it is, pending feedback on insomniacslk/dhcp#525

[Natolumin]

This is good to merge as far as I'm concerned, as soon as the upstream discussions are resolved

[candlerb]

The changes to insomniacslk/dhcp have been accepted, and I've done go get ...@latest to update go.mod. Build and test succeeds locally. Please kick off CI.

Note: I did a go mod tidy and it has removed a whole load of stuff from go.sum, including old versions of libraries. If you want to keep them, let me know and I'll try some other way of doing this.

[codecov]

Codecov Report

Attention: Patch coverage is 58.18182% with 23 lines in your changes are missing coverage. Please review.

Project coverage is 47.11%. Comparing base (321f8a6) to head (eb3cff2).

Files	Patch %	Lines
plugins/ipv6only/plugin.go	47.82%	12 Missing ⚠️
plugins/autoconfigure/plugin.go	65.62%	11 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #170      +/-   ##
==========================================
+ Coverage   46.58%   47.11%   +0.52%     
==========================================
  Files          16       18       +2     
  Lines        1157     1212      +55     
==========================================
+ Hits          539      571      +32     
- Misses        568      591      +23     
  Partials       50       50              

Flag	Coverage Δ
integtests	∅ <ø> (?)
unittests	47.11% <58.18%> (+0.52%)	⬆️

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[Natolumin]

Thank you for your contribution!

[candlerb]

FYI, a few stats from the IPv6-only SSID at APRICOT 2024, using coredhcp + radvd + jool:

• 142 unique MAC addresses seen
• Of those, 115 (81%) requested option 108 (IPv6-only Preferred)
• Of the remaining 27, none sent option 116 (AutoConfigure).

It appears that RFC8925 support is widespread, but RFC2563 is non-existent.