vm/mix/net: make vm.gateway6 consistent with vm.gateway

Use VM's actual IP address as a gateway for other VMs, instead of
hardcoded link-local address. This is important for sys-net generated
ICMP diagnostics packets - those must _not_ have link-local source
address, otherwise wouldn't be properly forwarded back to the right VM.

qubes/tests/vm/qubesvm.py

@@ -993,7 +993,7 @@ class TC_90_QubesVM(QubesVMTestsMixin, qubes.tests.QubesTestCase):
             expected['/qubes-ip6'] = \
                 qubes.config.qubes_ipv6_prefix.replace(':0000', '') + \
                 '::a89:3'
-            expected['/qubes-gateway6'] = 'fe80::fcff:ffff:feff:ffff'
+            expected['/qubes-gateway6'] = expected['/qubes-ip6'][:-1] + '2'
             vm.create_qdb_entries()
             self.assertEqual(test_qubesdb.data, expected)
 
@@ -1043,9 +1043,9 @@ class TC_90_QubesVM(QubesVMTestsMixin, qubes.tests.QubesTestCase):
         test_qubesdb.data.clear()
         with self.subTest('proxy_ipv6'):
             netvm.features['ipv6'] = True
-            expected['/qubes-netvm-gateway6'] = 'fe80::fcff:ffff:feff:ffff'
             ip6 = qubes.config.qubes_ipv6_prefix.replace(
                 ':0000', '') + '::a89:3'
+            expected['/qubes-netvm-gateway6'] = ip6[:-1] + '2'
             expected['/qubes-firewall/' + ip6] = ''
             expected['/qubes-firewall/' + ip6 + '/0000'] = 'action=accept'
             expected['/qubes-firewall/' + ip6 + '/policy'] = 'drop'

qubes/vm/mix/net.py

@@ -192,7 +192,7 @@ class NetVMMixin(qubes.events.Emitter):
     def gateway6(self):
         '''Gateway (IPv6) for other domains that use this domain as netvm.'''
         if self.features.check_with_netvm('ipv6', False):
-            return 'fe80::fcff:ffff:feff:ffff' if self.provides_network else \
+            return self.visible_ip6 if self.provides_network else \
                 None
         return None