[3.10] gh-122792: Make IPv4-mapped IPv6 address properties consistent with IPv4 (GH-122793) (GH-123819)

Make IPv4-mapped IPv6 address properties consistent with IPv4.
(cherry picked from commit 76a1c5d18312712baed4699fe7333abb050ec9b7)

Co-authored-by: Seth Michael Larson <seth@python.org>
Co-authored-by: Łukasz Langa <lukasz@langa.pl>

3 files changed, 49 insertions, 5 deletions

diff --git a/Lib/ipaddress.py b/Lib/ipaddress.py
index 94b1761..ac284ca 100644
--- a/Lib/ipaddress.py
+++ b/Lib/ipaddress.py
@@ -310,7 +310,7 @@ def collapse_addresses(addresses):
                            [IPv4Network('192.0.2.0/24')]
 
     Args:
-        addresses: An iterator of IPv4Network or IPv6Network objects.
+        addresses: An iterable of IPv4Network or IPv6Network objects.
 
     Returns:
         An iterator of the collapsed IPv(4|6)Network objects.
@@ -1840,9 +1840,6 @@ class _BaseV6:
     def _explode_shorthand_ip_string(self):
         """Expand a shortened IPv6 address.
 
-        Args:
-            ip_str: A string, the IPv6 address.
-
         Returns:
             A string, the expanded IPv6 address.
 
@@ -1986,6 +1983,9 @@ class IPv6Address(_BaseV6, _BaseAddress):
             See RFC 2373 2.7 for details.
 
         """
+        ipv4_mapped = self.ipv4_mapped
+        if ipv4_mapped is not None:
+            return ipv4_mapped.is_multicast
         return self in self._constants._multicast_network
 
     @property
@@ -1997,6 +1997,9 @@ class IPv6Address(_BaseV6, _BaseAddress):
             reserved IPv6 Network ranges.
 
         """
+        ipv4_mapped = self.ipv4_mapped
+        if ipv4_mapped is not None:
+            return ipv4_mapped.is_reserved
         return any(self in x for x in self._constants._reserved_networks)
 
     @property
@@ -2007,6 +2010,9 @@ class IPv6Address(_BaseV6, _BaseAddress):
             A boolean, True if the address is reserved per RFC 4291.
 
         """
+        ipv4_mapped = self.ipv4_mapped
+        if ipv4_mapped is not None:
+            return ipv4_mapped.is_link_local
         return self in self._constants._linklocal_network
 
     @property
@@ -2063,6 +2069,9 @@ class IPv6Address(_BaseV6, _BaseAddress):
         ``is_global`` has value opposite to :attr:`is_private`, except for the ``100.64.0.0/10``
         IPv4 range where they are both ``False``.
         """
+        ipv4_mapped = self.ipv4_mapped
+        if ipv4_mapped is not None:
+            return ipv4_mapped.is_global
         return not self.is_private
 
     @property
@@ -2074,6 +2083,9 @@ class IPv6Address(_BaseV6, _BaseAddress):
             RFC 2373 2.5.2.
 
         """
+        ipv4_mapped = self.ipv4_mapped
+        if ipv4_mapped is not None:
+            return ipv4_mapped.is_unspecified
         return self._ip == 0
 
     @property
@@ -2085,6 +2097,9 @@ class IPv6Address(_BaseV6, _BaseAddress):
             RFC 2373 2.5.3.
 
         """
+        ipv4_mapped = self.ipv4_mapped
+        if ipv4_mapped is not None:
+            return ipv4_mapped.is_loopback
         return self._ip == 1
 
     @property
@@ -2201,7 +2216,7 @@ class IPv6Interface(IPv6Address):
 
     @property
     def is_loopback(self):
-        return self._ip == 1 and self.network.is_loopback
+        return super().is_loopback and self.network.is_loopback
 
 
 class IPv6Network(_BaseV6, _BaseNetwork):
@@ -2314,6 +2329,8 @@ class _IPv6Constants:
         IPv6Network('2001:db8::/32'),
         # IANA says N/A, let's consider it not globally reachable to be safe
         IPv6Network('2002::/16'),
+        # RFC 9637: https://www.rfc-editor.org/rfc/rfc9637.html#section-6-2.2
+        IPv6Network('3fff::/20'),
         IPv6Network('fc00::/7'),
         IPv6Network('fe80::/10'),
         ]
diff --git a/Lib/test/test_ipaddress.py b/Lib/test/test_ipaddress.py
index 86f3bb5..47ad8f4 100644
--- a/Lib/test/test_ipaddress.py
+++ b/Lib/test/test_ipaddress.py
@@ -2415,6 +2415,30 @@ class IpaddrUnitTest(unittest.TestCase):
         self.assertEqual(ipaddress.ip_address('::ffff:c0a8:101').ipv4_mapped,
                          ipaddress.ip_address('192.168.1.1'))
 
+    def testIpv4MappedProperties(self):
+        # Test that an IPv4 mapped IPv6 address has
+        # the same properties as an IPv4 address.
+        for addr4 in (
+            "178.62.3.251",     # global
+            "169.254.169.254",  # link local
+            "127.0.0.1",        # loopback
+            "224.0.0.1",        # multicast
+            "192.168.0.1",      # private
+            "0.0.0.0",          # unspecified
+            "100.64.0.1",       # public and not global
+        ):
+            with self.subTest(addr4):
+                ipv4 = ipaddress.IPv4Address(addr4)
+                ipv6 = ipaddress.IPv6Address(f"::ffff:{addr4}")
+
+                self.assertEqual(ipv4.is_global, ipv6.is_global)
+                self.assertEqual(ipv4.is_private, ipv6.is_private)
+                self.assertEqual(ipv4.is_reserved, ipv6.is_reserved)
+                self.assertEqual(ipv4.is_multicast, ipv6.is_multicast)
+                self.assertEqual(ipv4.is_unspecified, ipv6.is_unspecified)
+                self.assertEqual(ipv4.is_link_local, ipv6.is_link_local)
+                self.assertEqual(ipv4.is_loopback, ipv6.is_loopback)
+
     def testIpv4MappedPrivateCheck(self):
         self.assertEqual(
                 True, ipaddress.ip_address('::ffff:192.168.1.1').is_private)
diff --git a/Misc/NEWS.d/next/Security/2024-08-07-10-42-13.gh-issue-122792.oiTMo9.rst b/Misc/NEWS.d/next/Security/2024-08-07-10-42-13.gh-issue-122792.oiTMo9.rst
new file mode 100644
index 0000000..18e293b
--- /dev/null
+++ b/Misc/NEWS.d/next/Security/2024-08-07-10-42-13.gh-issue-122792.oiTMo9.rst
@@ -0,0 +1,3 @@
+Changed IPv4-mapped ``ipaddress.IPv6Address`` to consistently use the mapped IPv4
+address value for deciding properties. Properties which have their behavior fixed
+are ``is_multicast``, ``is_reserved``, ``is_link_local``, ``is_global``, and ``is_unspecified``.