T4832: dhcp: Add dhcp option to signal IPv6-only support

Clients supporting this DHCP option (DHCP option 108, per RFC 8925) will
disable its IPv4 network stack for configured number of seconds
and operate in IPv6-only mode.

Example clients supporting this option including iOS 15+ and macOS 12.0.1+.

3 files changed, 23 insertions, 0 deletions

diff --git a/data/templates/dhcp-server/dhcpd.conf.tmpl b/data/templates/dhcp-server/dhcpd.conf.tmpl
index 0f0c622d4..13272ade3 100644
--- a/data/templates/dhcp-server/dhcpd.conf.tmpl
+++ b/data/templates/dhcp-server/dhcpd.conf.tmpl
@@ -22,6 +22,7 @@ ddns-update-style {{ 'interim' if dynamic_dns_update is defined else 'none' }};
 option rfc3442-static-route code 121 = array of integer 8;
 option windows-static-route code 249 = array of integer 8;
 option wpad-url code 252 = text;
+option rfc8925-ipv6-only-preferred code 108 = unsigned integer 32;
 
 {% if global_parameters is defined and global_parameters is not none %}
 # The following {{ global_parameters | length }} line(s) have been added as
@@ -78,6 +79,9 @@ shared-network {{ network }} {
 {%     if network_config.ntp_server is defined and network_config.ntp_server is not none %}
     option ntp-servers {{ network_config.ntp_server | join(', ') }};
 {%     endif %}
+{%     if network_config.ntp_server is defined and network_config.ntp_server is not none %}
+    option ntp-servers {{ network_config.ntp_server | join(', ') }};
+{%     endif %}
 {%     if network_config.ping_check is defined %}
     ping-check true;
 {%     endif %}
@@ -118,6 +122,9 @@ shared-network {{ network }} {
 {%         if subnet_config.wins_server is defined and subnet_config.wins_server is not none %}
         option netbios-name-servers {{ subnet_config.wins_server | join(', ') }};
 {%         endif %}
+{%         if subnet_config.ipv6_only_preferred is defined and subnet_config.ipv6_only_preferred is not none %}
+        option rfc8925-ipv6-only-preferred {{ subnet_config.ipv6_only_preferred }};
+{%         endif %}
 {%         if subnet_config.static_route is defined and subnet_config.static_route is not none %}
 {%           set static_default_route = '' %}
 {%           if subnet_config.default_router and subnet_config.default_router is not none %}
diff --git a/interface-definitions/dhcp-server.xml.in b/interface-definitions/dhcp-server.xml.in
index 8c10ccf99..c1f2fe2fd 100644
--- a/interface-definitions/dhcp-server.xml.in
+++ b/interface-definitions/dhcp-server.xml.in
@@ -357,6 +357,19 @@
                       </leafNode>
                     </children>
                   </tagNode >
+                  <leafNode name="ipv6-only-preferred">
+                    <properties>
+                      <help>Disable IPv4 on IPv6 only hosts (RFC 8925)</help>
+                      <valueHelp>
+                        <format>u32</format>
+                        <description>Seconds</description>
+                      </valueHelp>
+                      <constraint>
+                        <validator name="numeric" argument="--range 0-4294967295"/>
+                      </constraint>
+                      <constraintErrorMessage>Seconds must be between 0 and 4294967295 (49 days)</constraintErrorMessage>
+                    </properties>
+                  </leafNode>
                   <leafNode name="subnet-parameters">
                     <properties>
                       <help>Additional subnet parameters for DHCP server. You must
diff --git a/smoketest/scripts/cli/test_service_dhcp-server.py b/smoketest/scripts/cli/test_service_dhcp-server.py
index 9adb9c042..8568d96eb 100755
--- a/smoketest/scripts/cli/test_service_dhcp-server.py
+++ b/smoketest/scripts/cli/test_service_dhcp-server.py
@@ -112,6 +112,7 @@ class TestServiceDHCPServer(VyOSUnitTestSHIM.TestCase):
         bootfile_server = '192.0.2.1'
         wpad            = 'http://wpad.vyos.io/foo/bar'
         server_identifier = bootfile_server
+        ipv6_only_preferred = 300
 
         pool = base_path + ['shared-network-name', shared_net_name, 'subnet', subnet]
         # we use the first subnet IP address as default gateway
@@ -132,6 +133,7 @@ class TestServiceDHCPServer(VyOSUnitTestSHIM.TestCase):
         self.cli_set(pool + ['server-identifier', server_identifier])
 
         self.cli_set(pool + ['static-route', '10.0.0.0/24', 'next-hop', '192.0.2.1'])
+        self.cli_set(pool + ['ipv6-only-preferred', ipv6_only_preferred])
 
         # check validate() - No DHCP address range or active static-mapping set
         with self.assertRaises(ConfigSessionError):
@@ -169,6 +171,7 @@ class TestServiceDHCPServer(VyOSUnitTestSHIM.TestCase):
         self.assertIn(f'max-lease-time 86400;', config)
         self.assertIn(f'range {range_0_start} {range_0_stop};', config)
         self.assertIn(f'set shared-networkname = "{shared_net_name}";', config)
+        self.assertIn(f'option rfc8925-ipv6-only-preferred {ipv6_only_preferred};', config)
 
         # weird syntax for those static routes
         self.assertIn(f'option rfc3442-static-route 24,10,0,0,192,0,2,1, 0,192,0,2,1;', config)