From 46b71412433a50e534cb0d03147f77ec46f30a9c Mon Sep 17 00:00:00 2001
From: Adam Greene <github@mzpqnxow.com>
Date: Mon, 18 Aug 2025 09:05:03 -0400
Subject: [PATCH 1/3] Changes to support DHCP option 26 (client interface mtu)

---
 interface-definitions/include/dhcp/option-v4.xml.i | 12 ++++++++++++
 python/vyos/kea.py                                 |  1 +
 smoketest/scripts/cli/test_service_dhcp-server.py  |  7 +++++++
 3 files changed, 20 insertions(+)

diff --git a/interface-definitions/include/dhcp/option-v4.xml.i b/interface-definitions/include/dhcp/option-v4.xml.i
index 08fbcca4a0..4a9548a043 100644
--- a/interface-definitions/include/dhcp/option-v4.xml.i
+++ b/interface-definitions/include/dhcp/option-v4.xml.i
@@ -83,6 +83,18 @@
         </constraint>
       </properties>
     </leafNode>
+    <leafNode name="interface-mtu">
+      <properties>
+        <help>Interface MTU</help>
+        <valueHelp>
+          <format>u32:1-16</format>
+          <description>Client interface MTU</description>
+        </valueHelp>
+        <constraint>
+          <validator name="numeric" argument="--range 1200-9000"/>
+        </constraint>
+      </properties>
+    </leafNode>
     <leafNode name="ip-forwarding">
       <properties>
         <help>Enable IP forwarding on client</help>
diff --git a/python/vyos/kea.py b/python/vyos/kea.py
index 15c8564b04..a24fc2748e 100644
--- a/python/vyos/kea.py
+++ b/python/vyos/kea.py
@@ -45,6 +45,7 @@
     'ipv6_only_preferred': 'v6-only-preferred',
     'captive_portal': 'v4-captive-portal',
     'capwap_controller': 'capwap-ac-v4',
+    'interface_mtu': 'interface-mtu',
 }
 
 kea6_options = {
diff --git a/smoketest/scripts/cli/test_service_dhcp-server.py b/smoketest/scripts/cli/test_service_dhcp-server.py
index c59fc4efd8..add2408b16 100755
--- a/smoketest/scripts/cli/test_service_dhcp-server.py
+++ b/smoketest/scripts/cli/test_service_dhcp-server.py
@@ -225,6 +225,7 @@ def test_dhcp_single_pool_options(self):
         server_identifier = bootfile_server
         ipv6_only_preferred = '300'
         capwap_access_controller = '192.168.2.125'
+        interface_mtu = '1420'
 
         pool = base_path + ['shared-network-name', shared_net_name, 'subnet', subnet]
         self.cli_set(pool + ['subnet-id', '1'])
@@ -233,6 +234,7 @@ def test_dhcp_single_pool_options(self):
         self.cli_set(pool + ['option', 'name-server', dns_1])
         self.cli_set(pool + ['option', 'name-server', dns_2])
         self.cli_set(pool + ['option', 'domain-name', domain_name])
+        self.cli_set(pool + ['option', 'interface-mtu', interface_mtu])
         self.cli_set(pool + ['option', 'ip-forwarding'])
         self.cli_set(pool + ['option', 'smtp-server', smtp_server])
         self.cli_set(pool + ['option', 'pop-server', smtp_server])
@@ -365,6 +367,11 @@ def test_dhcp_single_pool_options(self):
             ['Dhcp4', 'shared-networks', 0, 'subnet4', 0, 'option-data'],
             {'name': 'ip-forwarding', 'data': 'true'},
         )
+        self.verify_config_object(
+            obj,
+            ['Dhcp4', 'shared-networks', 0, 'subnet4', 0, 'option-data'],
+            {'name': 'interface-mtu', 'data': interface_mtu},
+        )
 
         # Time zone
         self.verify_config_object(

From 5fc7ec82dd429413eb5037525a89a9a66ee64308 Mon Sep 17 00:00:00 2001
From: Adam Greene <github@mzpqnxow.com>
Date: Fri, 29 Aug 2025 18:02:16 -0400
Subject: [PATCH 2/3] fix type for interface-mtu

---
 interface-definitions/include/dhcp/option-v4.xml.i | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/interface-definitions/include/dhcp/option-v4.xml.i b/interface-definitions/include/dhcp/option-v4.xml.i
index 4a9548a043..fb8667c175 100644
--- a/interface-definitions/include/dhcp/option-v4.xml.i
+++ b/interface-definitions/include/dhcp/option-v4.xml.i
@@ -87,7 +87,7 @@
       <properties>
         <help>Interface MTU</help>
         <valueHelp>
-          <format>u32:1-16</format>
+          <format>u16:1200-9000</format>
           <description>Client interface MTU</description>
         </valueHelp>
         <constraint>

From 1bb88d808481e1fa7ed8a0d53a343673753aeafc Mon Sep 17 00:00:00 2001
From: Adam Greene <github@mzpqnxow.com>
Date: Fri, 29 Aug 2025 18:23:14 -0400
Subject: [PATCH 3/3] use minimum MTU specified in RFC 790 as the lower bound

---
 interface-definitions/include/dhcp/option-v4.xml.i | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/interface-definitions/include/dhcp/option-v4.xml.i b/interface-definitions/include/dhcp/option-v4.xml.i
index fb8667c175..0f446c9a95 100644
--- a/interface-definitions/include/dhcp/option-v4.xml.i
+++ b/interface-definitions/include/dhcp/option-v4.xml.i
@@ -87,11 +87,11 @@
       <properties>
         <help>Interface MTU</help>
         <valueHelp>
-          <format>u16:1200-9000</format>
+          <format>u16:576-9000</format>
           <description>Client interface MTU</description>
         </valueHelp>
         <constraint>
-          <validator name="numeric" argument="--range 1200-9000"/>
+          <validator name="numeric" argument="--range 576-9000"/>
         </constraint>
       </properties>
     </leafNode>