Add AdvSNACRouterFlag option to set the SNAC router bit

[arkku]

Adds a new option: AdvSNACRouterFlag. When enabled, this sets RA option bit 6, a formerly reserved bit, that is seems to be now allocated to "SNAC router".

The reason for this addition is that I noticed Apple's HomeKit devices setting this flag to 1 when advertising themselves as a Thread border router. I have multiple VLANs, and I wanted to use radvd to re-advertise the route with identical options (but different router address).

In 2019 this same bit has been proposed to stand for IPv6 only, but that does not seem to have been adopted as standard. However, tcpdump still decodes the bit as "ipv6 only". These two uses aren't necessarily contradictory, since the Stub Network advertised as such is indeed IPv6 only.

[stappersg]

On Mon, May 19, 2025 at 08:53:47PM -0700, Robin H. Johnson wrote:

arkku:

+When set, indicates the router is a SNAC router (stub network auto +configuration, e.g., for IPv6-only stub networks).

Please link to the RFC here

I like would to see a real commit message, not just the commit summary.

Plus "Sign off"

Groeten Geert Stappers

Silence is hard to parse

[Neustradamus]

@arkku: Good job but have you seen latest comments?

Can you update your PR?

Thanks in advance.

[Neustradamus]

@arkku: Good job but have you seen latest comments?

Can you update your PR?

Thanks in advance.

[Neustradamus]

@arkku: Good job but have you seen latest @stappersg comments?

Can you update your PR?

Thanks in advance.

[Neustradamus]

@systemcrash, @Noltari. @Alphix: What do you think about this radvd PR?