T8302: This PR adds Router Advertisement (RA) base-interface support for IPv6 wildcard prefix derivation and includes validation and smoketests.

Included changes:
- Add `base-interface` under:
  - `service router-advert interface <lan-if> prefix ::/64 base-interface <wan-if>`
- Render `Base6Interface <wan-if>;` in `radvd.conf` when configured.
- Add validation guard:
  - `base-interface` is only allowed with wildcard prefix `::/64`.
- Extend smoketests:
  - Positive case: `::/64` + `base-interface` commits and renders correctly.
  - Negative case: non-`::/64` prefix with `base-interface` fails commit.

4 files changed, 35 insertions, 0 deletions

diff --git a/data/templates/router-advert/radvd.conf.j2 b/data/templates/router-advert/radvd.conf.j2
index 15febd0dd..3da405688 100644
--- a/data/templates/router-advert/radvd.conf.j2
+++ b/data/templates/router-advert/radvd.conf.j2
@@ -84,6 +84,9 @@ interface {{ iface }} {
         AdvPreferredLifetime {{ prefix_options.preferred_lifetime }};
         DeprecatePrefix {{ 'on' if prefix_options.deprecate_prefix is vyos_defined else 'off' }};
         DecrementLifetimes {{ 'on' if prefix_options.decrement_lifetime is vyos_defined else 'off' }};
+{%                 if prefix_options.base_interface is vyos_defined %}
+        Base6Interface {{ prefix_options.base_interface }};
+{%                 endif %}
     };
 {%             endfor %}
 {%         endif %}
diff --git a/interface-definitions/service_router-advert.xml.in b/interface-definitions/service_router-advert.xml.in
index dc7164abc..878b53e78 100644
--- a/interface-definitions/service_router-advert.xml.in
+++ b/interface-definitions/service_router-advert.xml.in
@@ -305,6 +305,21 @@
                       <valueless/>
                     </properties>
                   </leafNode>
+                  <leafNode name="base-interface">
+                    <properties>
+                      <help>Prefix will be combined with IPv6 address of specified interface</help>
+                      <completionHelp>
+                        <script>${vyos_completion_dir}/list_interfaces</script>
+                      </completionHelp>
+                      <valueHelp>
+                        <format>txt</format>
+                        <description>Interface name</description>
+                      </valueHelp>
+                      <constraint>
+                        #include <include/constraint/interface-name.xml.i>
+                      </constraint>
+                    </properties>
+                  </leafNode>
                   <leafNode name="preferred-lifetime">
                     <properties>
                       <help>Time in seconds that the prefix will remain preferred</help>
diff --git a/smoketest/scripts/cli/test_service_router-advert.py b/smoketest/scripts/cli/test_service_router-advert.py
index 54133b5c7..52f0db4ab 100755
--- a/smoketest/scripts/cli/test_service_router-advert.py
+++ b/smoketest/scripts/cli/test_service_router-advert.py
@@ -376,5 +376,19 @@ class TestServiceRADVD(VyOSUnitTestSHIM.TestCase):
         tmp = f'autoignoreprefixes' + ' {'
         self.assertNotIn(tmp, config)
 
+    def test_base_interface(self):
+        self.cli_set(base_path + ['prefix', '::/64'])
+        self.cli_set(base_path + ['prefix', '::/64', 'base-interface', 'eth0'])
+        self.cli_commit()
+
+        config = read_file(RADVD_CONF)
+        self.assertIn('Base6Interface eth0;', config)
+
+        self.cli_set(base_path + ['prefix', '2001:db8:1234::/64'])
+        self.cli_set(
+            base_path + ['prefix', '2001:db8:1234::/64', 'base-interface', 'eth0']
+        )
+        with self.assertRaises(ConfigSessionError):
+            self.cli_commit()
 if __name__ == '__main__':
     unittest.main(verbosity=2, failfast=VyOSUnitTestSHIM.TestCase.debug_on())
diff --git a/src/conf_mode/service_router-advert.py b/src/conf_mode/service_router-advert.py
index 4345bea1d..86b0f8dd5 100755
--- a/src/conf_mode/service_router-advert.py
+++ b/src/conf_mode/service_router-advert.py
@@ -64,6 +64,9 @@ def verify(rtradv):
                 if not (int(valid_lifetime) >= int(preferred_lifetime)):
                     raise ConfigError('Prefix valid-lifetime must be greater then or equal to preferred-lifetime')
 
+                if 'base_interface' in prefix_config and prefix != '::/64':
+                    raise ConfigError('Prefix base-interface can only be used together with the wildcard prefix "::/64"')
+
         if 'nat64prefix' in interface_config:
             nat64_supported_lengths = [32, 40, 48, 56, 64, 96]
             for prefix, prefix_config in interface_config['nat64prefix'].items():