From 04434b1e82d2aac2fa9d1690dc6d38e3ea6872f6 Mon Sep 17 00:00:00 2001
From: Adam Greene <github@mzpqnxow.com>
Date: Thu, 9 Oct 2025 16:55:08 -0400
Subject: dhcp-server: T7723: add DHCP Option 26 (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 08fbcca4a..0f446c9a9 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>u16:576-9000</format>
+          <description>Client interface MTU</description>
+        </valueHelp>
+        <constraint>
+          <validator name="numeric" argument="--range 576-9000"/>
+        </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 f3334a688..5ce6f47f1 100644
--- a/python/vyos/kea.py
+++ b/python/vyos/kea.py
@@ -45,6 +45,7 @@ kea4_options = {
     '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 94044f8b2..ec8d08388 100755
--- a/smoketest/scripts/cli/test_service_dhcp-server.py
+++ b/smoketest/scripts/cli/test_service_dhcp-server.py
@@ -224,6 +224,7 @@ class TestServiceDHCPServer(VyOSUnitTestSHIM.TestCase):
         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'])
@@ -232,6 +233,7 @@ class TestServiceDHCPServer(VyOSUnitTestSHIM.TestCase):
         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])
@@ -364,6 +366,11 @@ class TestServiceDHCPServer(VyOSUnitTestSHIM.TestCase):
             ['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(
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