vyos.ifconfig: get_mac_synthetic() must generate a stable "MAC"

Commit b7d30137b1 ("vyos.ifconfig: provide generic get_mac_synthetic() method")
provided a common helper to generate MAC addresses used by EUI64 addresses for
interfaces not having a layer2 interface (WireGuard or ip tunnel).

The problem is that every call to the helper always yielded a new MAC address.
This becomes problematic when IPv6 link-local addresses are generated and
modified on the interface as multiple link-local (fe80::/64) addresses can
easily be added to the interface leaving ... a mess.

This commit changes the way how the "synthetic" MAC is generated, we generate a
UUID which is stable as it is based on the interface name. We take out the last
48 bits of the UUID and form the "MAC" address.

(cherry picked from commit 081e23996feb60ad903caf8b0a4587f5dacc69bf)

1 files changed, 10 insertions, 4 deletions

diff --git a/python/vyos/ifconfig/interface.py b/python/vyos/ifconfig/interface.py
index 2629729f8..de46d3d66 100644
--- a/python/vyos/ifconfig/interface.py
+++ b/python/vyos/ifconfig/interface.py
@@ -27,6 +27,8 @@ from netifaces import ifaddresses
 # this is not the same as socket.AF_INET/INET6
 from netifaces import AF_INET
 from netifaces import AF_INET6
+from uuid import uuid3
+from uuid import NAMESPACE_DNS
 
 from vyos import ConfigError
 from vyos.configdict import list_diff
@@ -56,7 +58,6 @@ from vyos.ifconfig import Section
 
 from netaddr import EUI
 from netaddr import mac_unix_expanded
-from random import getrandbits
 
 class Interface(Control):
     # This is the class which will be used to create
@@ -438,9 +439,14 @@ class Interface(Control):
         >>> Interface('eth0').get_mac()
         '00:50:ab:cd:ef:00'
         """
-        # we choose 40 random bytes for the MAC address, this gives
-        # us e.g. EUI('00-EA-EE-D6-A3-C8') or EUI('00-41-B9-0D-F2-2A')
-        tmp = EUI(getrandbits(48)).value
+        # calculate a UUID based on the interface name - this is as predictable
+        # as an interface MAC address and thus can be used in the same way
+        tmp = uuid3(NAMESPACE_DNS, self.ifname)
+        # take the last 48 bits from the UUID string
+        tmp = str(tmp).split('-')[-1]
+        # Convert pseudo random string into EUI format which now represents a
+        # MAC address
+        tmp = EUI(tmp).value
         # set locally administered bit in MAC address
         tmp |= 0xf20000000000
         # convert integer to "real" MAC address representation