Bug #13769
closed
IPAddr#ipv4_compat incorrect behavior
Description
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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