Add Socket::IPAddress#link_local?

[GeopJr]

This PR adds Socket::IPAddress#link_local?like Ruby's

(the spec examples are also from Ruby)

fix partially: #13198

[Fryguy]

Does private? have to be changed to use the new link_local? method? Or is that for another PR? (asking because you said this fixes #13198)

EDIT: Actually it's not that way on the Ruby side, either, so I don't understand where/how this method gets used.

[GeopJr]

The thread that the issue author links (who is also the author of) is about detecting non-public ip addresses, mainly 169.254.0.0/16. #link_local? should allow them to detect it as non-public.

I don't think it should be under #private? as rfc 6890 mentions 169.254.0.0/16 as Link Local

[Blacksmoke16]

I don't think it should be under #private? as rfc 6890 mentions 169.254.0.0/16 as Link Local

I think it makes sense to have a dedicated method for it, but my understanding is while 169.254.0.0/16 is Link Local, link local is also a private IP, so it would make sense to do something like return false if self.link_local? within #private such that you do not have to || a bunch of separate calls together. E.g. from the OP #private is missing a few others in additional to this link local range.

[GeopJr]

I'd personally vote for #public? being something like !link_local? && !private? && !loopback? && !unspecified? instead of breaking compatibility with ruby:

irb(main):002:0> IPAddr.new('169.254.169.254').link_local?
=> true
irb(main):003:0> IPAddr.new('169.254.169.254').private?
=> false

edit:
I re-read the comment, I'll unlink the issue for now since this requires more thought

[straight-shoota]

I commented some clarification in #13198 (comment)
Please open a new issue for a #public? method (although I'm pretty convinced we need a better name because public? not being the inverse of #private? would be confusing).

[GeopJr]

I don't really have any strong arguments in favor of a #public?-like method instead of !private? && ... so I'll pass on creating an issue