gh-74713: Lib/ipaddress: support reverse pointer generation for networks

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

Author: Forst

Lib/ipaddress.py

@@ -1252,13 +1252,20 @@ def _string_from_ip_int(cls, ip_int):
         return '.'.join(map(str, ip_int.to_bytes(4, 'big')))
 
     def _reverse_pointer(self):
-        """Return the reverse DNS pointer name for the IPv4 address.
+        """Return the reverse DNS pointer name for the IPv4 address or network.
 
         This implements the method described in RFC1035 3.5.
 
         """
-        reverse_octets = str(self).split('.')[::-1]
-        return '.'.join(reverse_octets) + '.in-addr.arpa'
+        prefix = getattr(self, 'prefixlen', IPV4LENGTH)
+        octet_count, remainder = divmod(prefix, 8)  # each octet is 8 bits long
+        if remainder:
+            raise NetmaskValueError(
+                'Reverse pointer cannot be generated for given prefix size')
+        address_exploded = getattr(self, 'network_address', self).exploded
+        reverse_octets = address_exploded.split('.')[:octet_count][::-1]
+        reverse_octets.append('in-addr.arpa')
+        return '.'.join(reverse_octets)
 
 class IPv4Address(_BaseV4, _BaseAddress):
 
@@ -1880,13 +1887,20 @@ def _explode_shorthand_ip_string(self):
         return ':'.join(parts)
 
     def _reverse_pointer(self):
-        """Return the reverse DNS pointer name for the IPv6 address.
+        """Return the reverse DNS pointer name for the IPv6 address or network.
 
         This implements the method described in RFC3596 2.5.
 
         """
-        reverse_chars = self.exploded[::-1].replace(':', '')
-        return '.'.join(reverse_chars) + '.ip6.arpa'
+        prefix = getattr(self, 'prefixlen', IPV6LENGTH)
+        char_count, remainder = divmod(prefix, 4)  # each char is 4 bits long
+        if remainder:
+            raise NetmaskValueError(
+                'Reverse pointer cannot be generated for given prefix size')
+        address_exploded = getattr(self, 'network_address', self).exploded
+        reverse_chars = list(address_exploded.replace(':', '')[:char_count][::-1])
+        reverse_chars.append('ip6.arpa')
+        return '.'.join(reverse_chars)
 
     @staticmethod
     def _split_scope_id(ip_str):

Lib/test/test_ipaddress.py

@@ -723,6 +723,24 @@ def test_subnet_of_mixed_types(self):
             ipaddress.IPv6Network('::1/128').subnet_of(
                 ipaddress.IPv4Network('10.0.0.0/30'))
 
+    def test_reverse_pointer(self):
+        self.assertEqual(self.factory('0.0.0.0/0').reverse_pointer,
+                         'in-addr.arpa')
+        self.assertEqual(self.factory('127.0.0.0/8').reverse_pointer,
+                         '127.in-addr.arpa')
+        self.assertEqual(self.factory('127.12.0.0/16').reverse_pointer,
+                         '12.127.in-addr.arpa')
+        self.assertEqual(self.factory('127.12.34.0/24').reverse_pointer,
+                         '34.12.127.in-addr.arpa')
+        self.assertEqual(self.factory('127.12.34.56/32').reverse_pointer,
+                         '56.34.12.127.in-addr.arpa')
+
+        with self.assertRaises(ipaddress.NetmaskValueError):
+            self.factory('127.0.0.0/12').reverse_pointer
+
+        with self.assertRaises(ipaddress.NetmaskValueError):
+            self.factory('127.23.0.0/17').reverse_pointer
+
 
 class NetmaskTestMixin_v6(CommonTestMixin_v6):
     """Input validation on interfaces and networks is very similar"""
@@ -879,6 +897,24 @@ def test_supernet_of(self):
             self.factory('2000:aaa::/48').supernet_of(
                 self.factory('2000:aaa::/56')))
 
+    def test_reverse_pointer(self):
+        self.assertEqual(self.factory('::/0').reverse_pointer, 'ip6.arpa')
+        self.assertEqual(self.factory('2000::/4').reverse_pointer, '2.ip6.arpa')
+        self.assertEqual(self.factory('2000::/8').reverse_pointer,
+                         '0.2.ip6.arpa')
+        self.assertEqual(self.factory('2001:db8::/32').reverse_pointer,
+                         '8.b.d.0.1.0.0.2.ip6.arpa')
+        self.assertEqual(self.factory('2001:db8:1234:5678::/64').reverse_pointer,
+                         '8.7.6.5.4.3.2.1.8.b.d.0.1.0.0.2.ip6.arpa')
+        self.assertEqual(self.factory('2001:db8::1/128').reverse_pointer,
+                         '1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0' +
+                         '.0.0.0.0.0.0.0.0.8.b.d.0.1.0.0.2.ip6.arpa')
+
+        with self.assertRaises(ipaddress.NetmaskValueError):
+            self.factory('2000::/7').reverse_pointer
+
+        with self.assertRaises(ipaddress.NetmaskValueError):
+            self.factory('2001:db8::10/127').reverse_pointer
 
 class FactoryFunctionErrors(BaseTestCase):
 

Misc/NEWS.d/next/Library/2021-10-17-18-47-46.gh-issue-74713.-BnGug.rst

@@ -0,0 +1,2 @@
+Fix reverse pointer generation for :class:`ipaddress.IPv4Network` and
+:class:`ipaddress.IPv6Network` objects.