[ruby-core:82500] [Ruby trunk Bug#13769] IPAddr#ipv4_compat incorrect behavior

Issue #13769 has been updated by Glass_saga (Masaki Matsushita).

Target version set to 2.5

RFC4291 says: '"IPv4-Compatible IPv6 address" is now deprecated.'
We can remove #ipv4_compat? or mark it as deprecated.

 Section 2.5.5.1.

 The "IPv4-Compatible IPv6 address" is now deprecated because the
 current IPv6 transition mechanisms no longer use these addresses.
 New or updated implementations are not required to support this
 address type.

----------------------------------------
Bug #13769: IPAddr#ipv4_compat incorrect behavior
https://bugs.ruby-lang.org/issues/13769#change-66309

* Author: arkadiyt (Arkadiy Tetelman)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 2.5
* ruby -v: ruby 2.2.1p85 (2015-02-26 revision 49769) [x86_64-darwin14]
* Backport: 2.2: UNKNOWN, 2.3: UNKNOWN, 2.4: UNKNOWN
----------------------------------------
To ease transition from IPv4 to IPv6, there exist "ipv4-compatible" and "ipv4-mapped" addresses, which are ipv6 addresses that embed an ipv4 address inside them.

Ruby's IPAddr defines several helper functions related to this:
IPAddr#ipv4_mapped? -> return true if the ipaddr is an ipv4-mapped ipv6 address 
IPAddr#ipv4_compat? -> return true if the ipaddr is an ipv4-compatible ipv6 address

These 2 formats are defined in RFC4291 section 2.5.5, here:
https://tools.ietf.org/html/rfc4291#section-2.5.5

Notably for ipv4-compatible addresses, it says the following:

~~~
The "IPv4-Compatible IPv6 address" was defined to assist in the IPv6
   transition.  The format of the "IPv4-Compatible IPv6 address" is as
   follows:

   |                80 bits               | 16 |      32 bits        |
   +--------------------------------------+--------------------------+
   |0000..............................0000|0000|    IPv4 address     |
   +--------------------------------------+----+---------------------+

   Note: The IPv4 address used in the "IPv4-Compatible IPv6 address"
   must be a globally-unique IPv4 unicast address.

~~~

But this is not the behavior of IPAddr#ipv4_compat?, defined here:
https://github.com/ruby/ruby/blob/trunk/lib/ipaddr.rb#L267-L274

Given an ipv6 address it checks that the top 96 bits are 0, but then also that the last 32 bits are not equal to 0 or 1, so we have:

~~~
2.2.1 :002 > IPAddr.new('::0.0.0.0').ipv4_compat?
 => false
2.2.1 :003 > IPAddr.new('::0.0.0.1').ipv4_compat?
 => false
~~~
It seems like those should return true.

Or:

Perhaps this is related to the last sentence of the RFC: "The IPv4 address used in the "IPv4-Compatible IPv6 address must be a globally-unique IPv4 unicast address.". Since 0.0.0.0 and 0.0.0.1 are not globally-unique unicast addresses, that might justify the false return value. Under this reasoning the function is still wrong in that it *only* returns false for those 2 IPv4 addresses, when there are many other non-globally-unique unicast addresses - see here for some:
https://en.wikipedia.org/wiki/IPv4#Special-use_addresses

It's not clear why 0.0.0.0 and 0.0.0.1 are given this special treatment. The commit was made in 2002 and predates Ruby 1.9.3:
https://github.com/ruby/ruby/commit/9ec0a96ad4235f2054976eab6c04efbe62b3c703




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