New method in netutils: get_mac_addr_by_ipv6

This method is practically the reverse of get_ipv6_addr_by_EUI64(),
so we can extract the MAC address from IPv6 addresses that were
generated from interface identifiers.

Change-Id: I48720d38649104f9f2f0a8fd208f2aac7548644e
Related-Change: https://review.opendev.org/718729

oslo_utils/netutils.py

@@ -193,6 +193,35 @@ def get_ipv6_addr_by_EUI64(prefix, mac):
                           'EUI-64: %s') % prefix)
 
 
+def get_mac_addr_by_ipv6(ipv6, dialect=netaddr.mac_unix_expanded):
+    """Extract MAC address from interface identifier based IPv6 address.
+
+    For example from link-local addresses (fe80::/10) generated from MAC.
+
+    :param ipv6: An interface identifier (i.e. mostly MAC) based IPv6
+                 address as a netaddr.IPAddress() object.
+    :param dialect: The netaddr dialect of the the object returned.
+                    Defaults to netaddr.mac_unix_expanded.
+    :returns: A MAC address as a netaddr.EUI() object.
+
+    See also:
+    * https://tools.ietf.org/html/rfc4291#appendix-A
+    * https://tools.ietf.org/html/rfc4291#section-2.5.6
+
+    .. versionadded:: 4.3.0
+    """
+    return netaddr.EUI(int(
+        # out of the lowest 8 bytes (byte positions 8-1)
+        # delete the middle 2 bytes (5-4, 0xff_fe)
+        # by shifting the highest 3 bytes to the right by 2 bytes (8-6 -> 6-4)
+        (((ipv6 & 0xff_ff_ff_00_00_00_00_00) >> 16) +
+         # adding the lowest 3 bytes as they are (3-1)
+         (ipv6 & 0xff_ff_ff)) ^
+        # then invert the universal/local bit
+        0x02_00_00_00_00_00),
+        dialect=dialect)
+
+
 def is_ipv6_enabled():
     """Check if IPv6 support is enabled on the platform.
 

oslo_utils/tests/test_netutils.py

@@ -17,6 +17,7 @@ import contextlib
 import socket
 from unittest import mock
 
+import netaddr
 import netifaces
 from oslotest import base as test_base
 import six
@@ -357,6 +358,38 @@ class IPv6byEUI64TestCase(test_base.BaseTestCase):
                           netutils.get_ipv6_addr_by_EUI64(prefix, mac))
 
 
+class MACbyIPv6TestCase(test_base.BaseTestCase):
+    """Unit tests to extract MAC from IPv6."""
+
+    def test_reverse_generate_IPv6_by_EUI64(self):
+        self.assertEqual(
+            netaddr.EUI('00:16:3e:33:44:55'),
+            netutils.get_mac_addr_by_ipv6(
+                netaddr.IPAddress('2001:db8::216:3eff:fe33:4455')),
+        )
+
+    def test_random_qemu_mac(self):
+        self.assertEqual(
+            netaddr.EUI('52:54:00:42:02:19'),
+            netutils.get_mac_addr_by_ipv6(
+                netaddr.IPAddress('fe80::5054:ff:fe42:219')),
+        )
+
+    def test_local(self):
+        self.assertEqual(
+            netaddr.EUI('02:00:00:00:00:00'),
+            netutils.get_mac_addr_by_ipv6(
+                netaddr.IPAddress('fe80::ff:fe00:0')),
+        )
+
+    def test_universal(self):
+        self.assertEqual(
+            netaddr.EUI('00:00:00:00:00:00'),
+            netutils.get_mac_addr_by_ipv6(
+                netaddr.IPAddress('fe80::200:ff:fe00:0')),
+        )
+
+
 @contextlib.contextmanager
 def mock_file_content(content):
     # Allows StringIO to act like a context manager-enabled file.

releasenotes/notes/netutils-get_mac_addr_by_ipv6-c3ce6a35a69f7c2b.yaml

@@ -0,0 +1,5 @@
+---
+features:
+  - |
+    New method ``netutils.get_mac_addr_by_ipv6(ipv6, dialect)`` extracts
+    the MAC address from IPv6 addresses generated from MACs.