Add ability to Whitelist Prefixes

[blocktrron]

This pull request adds the ability to whitelist a certain subnet from which Router Advertisements are accepted from.

Motivation

In networks where local and global unicast prefixes are announced it is not predictable, which one is picked by EtherSia to generate its IPv6 address.
With this PR, it is possible to whitelist the global unicast address space, so EtherSia generates a globally routable IPv6 Address.

[njh]

Hi @blocktrron,

Sorry for taking over a month to respond to this PR.

Can you give a bit more background about this? I don't think I have seen anything like this in other IPv6 stacks. By plugging into a LAN don't you implicitly have to trust the network? If you don't trust the Router Advertisements on the network, is an option to set the address statically instead?

I guess my main concern is that this PR increases the memory usage for everyone, but I am not sure that many people will need/use it.

nick.

[blocktrron]

Hello Nick,

this PR is not primarily about trusting specific RA's. In Networks, where an ULA and a globally Routed Prefix is advertised, EtherSia might pick an undesired prefix.

We have this specific problem in a Mesh network, where the ULA Prefix is advertised by each mesh-node, while the globally routed Prefix is only advertised by nodes bordering to the internet. The globally routed prefix is usually not the first received. We want EtherSia to reliably acquire a globally routed Prefix. 😺

At least here in Germany, many CPEs from ISPs also advertise a globally routed and a ULA prefix.

• blocktrron