Add a few MAC checks

Notably mac.is_multicast() which is needed for TAPs / LCPs and
interfaces.

Add a test for interface MAC addresses being non-multicast.

config/mac.py

@@ -0,0 +1,48 @@
+#
+# Copyright (c) 2022 Pim van Pelt
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at:
+#     http://www.apache.org/licenses/LICENSE-2.0
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+import logging
+import netaddr
+
+def is_valid(mac):
+    """ Return True if the string given in `mac` is a valid (6-byte) MAC address,
+        as defined by netaddr.EUI """
+    try:
+        addr = netaddr.EUI(mac)
+    except:
+        return False
+    return True
+
+def is_local(mac):
+    """ Return True if a MAC address is a valid locally administered one. """
+    try:
+        addr = netaddr.EUI(mac)
+    except:
+        return False
+    return bool(addr.words[0] & 0b10)
+
+def is_multicast(mac):
+    """ Return True if a MAC address is a valid multicast one. """
+    try:
+        addr = netaddr.EUI(mac)
+    except:
+        return False
+    return bool(addr.words[0] & 0b01)
+
+def is_unicast(mac):
+    """ Return True if a MAC address is a valid unicast one. """
+    try:
+        addr = netaddr.EUI(mac)
+    except:
+        return False
+    return not bool(addr.words[0] & 0b01)