Incorrect ipaddress.ip_address.reverse_pointer with improved textual representation of IPv4-mapped IPv6 addresses

[kellyjonbrazil]

Bug report

Bug description:

I'm seeing strange results with python 3.13 with the ipaddress.ip_address.reverse_pointer attribute with the new improved textual representation of IPv4-mapped IPv6 addresses:

IP Address: ::FFFF:192.168.1.35

Results in a reverse_pointer of:

5.3...1...8.6.1...2.9.1.f.f.f.f.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.ip6.arpa

instead of the old:

3.2.1.0.8.a.0.c.f.f.f.f.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.ip6.arpa

Python 3.13:

>>> import ipaddress
>>> ip = '::FFFF:192.168.1.35'
>>> ipaddress.ip_address(ip).reverse_pointer
'5.3...1...8.6.1...2.9.1.f.f.f.f.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.ip6.arpa'

Python 3.12:

>>> import ipaddress
>>> ip = '::FFFF:192.168.1.35'
>>> ipaddress.ip_address(ip).reverse_pointer
'3.2.1.0.8.a.0.c.f.f.f.f.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.ip6.arpa'

I'm not sure which one is more valid or if a pointer record is even relevant in the context of mapped addresses. The original way seems more correct to me if nothing for the fact that there are strange extra dots in the new output.

This is related to:

Issue: #87799

PR: #29345

Originally posted by @kellyjonbrazil in #87799 (comment)

CPython versions tested on:

3.13

Operating systems tested on:

Linux

@opavlyuk

Linked PRs

• gh-123409: fix IPv6Address.reverse_pointer output  #123419
• [3.13] gh-123409: fix IPv6Address.reverse_pointer for IPv4-mapped addresses (GH-123419) #123606
• [3.9] gh-123409: fix IPv6Address.reverse_pointer for IPv4-mapped addresses (GH-123419) #135085
• [3.12] gh-123409: fix IPv6Address.reverse_pointer for IPv4-mapped addresses (GH-123419) #135086
• [3.11] gh-123409: fix IPv6Address.reverse_pointer for IPv4-mapped addresses (GH-123419) #135087
• [3.10] gh-123409: fix IPv6Address.reverse_pointer for IPv4-mapped addresses (GH-123419) #135088

[picnixz]

I think there is indeed an issue with the reverse pointer of IPv6. It does not match the RFC. I'll provide a patch.

[opavlyuk]

@kellyjonbrazil Godd catch! Thank you for logging that, and I apologize for missing it during testing.
@picnixz thank you for taking care of this!

[encukou]

Thanks @kellyjonbrazil for the report and @picnixz for the patch!

[picnixz]

You're welcome! (I hope that I didn't misread the RFC because it doesn't tell anything about mapped addresses; however I assumed that we first map IPv4 to IPv6 before taking its reverse pointer, which is what was being done in 3.12.5)