gh-74713: ipaddress: support reverse pointer generation for network objects

[Forst]

Before this change, DNS reverse pointer generation for network objects produced incorrect results, such as:

>>> ipaddress.IPv4Network('192.168.1.0/24').reverse_pointer
'0/24.1.168.192.in-addr.arpa'
>>> ipaddress.IPv6Network('2001:db8:1234::/48').reverse_pointer
'8.4./.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.4.3.2.1.8.b.d.0.1.0.0.2.ip6.arpa'

This change introduces a more generalised reverse pointer generation algorithm, suitable for both address and network objects.

The same code above, after applying this change:

>>> ipaddress.IPv4Network('192.168.1.0/24').reverse_pointer
'1.168.192.in-addr.arpa'
>>> ipaddress.IPv6Network('2001:db8:1234::/48').reverse_pointer
'4.3.2.1.8.b.d.0.1.0.0.2.ip6.arpa'

Getting a reverse pointer for a network, whose prefix size can't be exactly represented in a reverse pointer record, will raise an exception:

>>> ipaddress.IPv4Network('192.168.1.0/28').reverse_pointer
ipaddress.NetmaskValueError: Reverse pointer cannot be generated for given prefix size
>>> ipaddress.IPv6Network('2001:db8:1234::/50').reverse_pointer
ipaddress.NetmaskValueError: Reverse pointer cannot be generated for given prefix size

My approach is a bit different to other PRs proposed to fix bpo-30528:

• Mine is a generic (and perhaps more verbose) implementation, suitable both for address and network objects
• Trying to get a reverse pointer on a prefix size that doesn't align with the reverse pointer granularity will raise an exception

https://bugs.python.org/issue30528

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[MaxwellDupre]

Before

ipaddress.IPv6Network('2001:db8:1234::/48').reverse_pointer
'8.4./.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.4.3.2.1.8.b.d.0.1.0.0.2.ip6.arpa'

After:

ipaddress.IPv6Network('2001:db8:1234::/48').reverse_pointer
'4.3.2.1.8.b.d.0.1.0.0.2.ip6.arpa'

Looks like a few zeros missing!

[Forst]

Before

>>> ipaddress.IPv6Network('2001:db8:1234::/48').reverse_pointer
'8.4./.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.4.3.2.1.8.b.d.0.1.0.0.2.ip6.arpa'

After:

>>> ipaddress.IPv6Network('2001:db8:1234::/48').reverse_pointer
'4.3.2.1.8.b.d.0.1.0.0.2.ip6.arpa'

Looks like a few zeros missing!

@MaxwellDupre, I think you may be confusing IPv6Network with IPv6Address:

>>> ipaddress.IPv6Network('2001:db8:1234::/48').reverse_pointer
'4.3.2.1.8.b.d.0.1.0.0.2.ip6.arpa'
>>> ipaddress.IPv6Address('2001:db8:1234::').reverse_pointer
'0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.4.3.2.1.8.b.d.0.1.0.0.2.ip6.arpa'

For example, if you want to create a DNS zone for a /48 network, you want the zone to be defined as 4.3.2.1.8.b.d.0.1.0.0.2.ip6.arpa. The rest of the "missing" hex digits belong to the host address, not the network prefix, those would be written inside the zone file for each host.

If I misunderstood what you meant, please correct me :)

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