T4832: dhcp: Add IPv6-only dhcp option support (RFC 8925)

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

This option is known to work on iOS 15+ and macOS 12.0.1+.

Example command:
```sh
set service dhcp-server shared-network-name LAN6 subnet 192.168.64.0/24 ipv6-only-preferred 0
```

3 files changed, 20 insertions, 0 deletions

diff --git a/data/templates/dhcp-server/dhcpd.conf.j2 b/data/templates/dhcp-server/dhcpd.conf.j2
index 4c2da0aa5..639526532 100644
--- a/data/templates/dhcp-server/dhcpd.conf.j2
+++ b/data/templates/dhcp-server/dhcpd.conf.j2
@@ -22,6 +22,7 @@ ddns-update-style {{ 'interim' if dynamic_dns_update is vyos_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;
 
 # Vendor specific options - Ubiquiti Networks
 option space ubnt;
@@ -127,6 +128,9 @@ shared-network {{ network }} {
 {%                 if subnet_config.wins_server is vyos_defined %}
         option netbios-name-servers {{ subnet_config.wins_server | join(', ') }};
 {%                 endif %}
+{%                 if subnet_config.ipv6_only_preferred is vyos_defined %}
+        option rfc8925-ipv6-only-preferred {{ subnet_config.ipv6_only_preferred }};
+{%                 endif %}
 {%                 if subnet_config.static_route is vyos_defined %}
 {%                     set static_default_route = '' %}
 {%                     if subnet_config.default_router is vyos_defined %}
diff --git a/interface-definitions/dhcp-server.xml.in b/interface-definitions/dhcp-server.xml.in
index 6e1592200..1830cc1ad 100644
--- a/interface-definitions/dhcp-server.xml.in
+++ b/interface-definitions/dhcp-server.xml.in
@@ -373,6 +373,19 @@
                       </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 use the syntax of dhcpd.conf in this text-field. Using this without proper knowledge may result in a crashed DHCP server. Check system log to look for errors.</help>
diff --git a/smoketest/scripts/cli/test_service_dhcp-server.py b/smoketest/scripts/cli/test_service_dhcp-server.py
index 9c9d6d9f1..1f6dc823f 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)