ifconfig: T2912: add helper to verify interface min/max supported MTU

Currently the MTU size of an interface is only checked when entered via CLI but
if the interface supportes the configured MTU at all is not verified at all.

New helper functions get_min_mtu(), get_max_mtu() and verify_mtu() have been
added to provide a central API for validation.

2 files changed, 23 insertions, 2 deletions

diff --git a/python/vyos/configverify.py b/python/vyos/configverify.py
index afa6c7f06..6e5ba1df0 100644
--- a/python/vyos/configverify.py
+++ b/python/vyos/configverify.py
@@ -23,6 +23,27 @@
 
 from vyos import ConfigError
 
+def verify_mtu(config):
+    """
+    Common helper function used by interface implementations to perform
+    recurring validation if the specified MTU can be used by the underlaying
+    hardware.
+    """
+    from vyos.ifconfig import Interface
+    if 'mtu' in config:
+        mtu = int(config['mtu'])
+
+        tmp = Interface(config['ifname'])
+        min_mtu = tmp.get_min_mtu()
+        max_mtu = tmp.get_max_mtu()
+
+        if mtu < min_mtu:
+            raise ConfigError(f'Interface MTU too low, ' \
+                              f'minimum supported MTU is {min_mtu}!')
+        if mtu > max_mtu:
+            raise ConfigError(f'Interface MTU too high, ' \
+                              f'maximum supported MTU is {max_mtu}!')
+
 def verify_vrf(config):
     """
     Common helper function used by interface implementations to perform
diff --git a/python/vyos/ifconfig/interface.py b/python/vyos/ifconfig/interface.py
index a0f0ffe04..d200fc7a8 100644
--- a/python/vyos/ifconfig/interface.py
+++ b/python/vyos/ifconfig/interface.py
@@ -299,7 +299,7 @@ class Interface(Control):
         >>> Interface('eth0').get_min_mtu()
         '60'
         """
-        return self.get_interface('min_mtu')
+        return int(self.get_interface('min_mtu'))
 
     def get_max_mtu(self):
         """
@@ -310,7 +310,7 @@ class Interface(Control):
         >>> Interface('eth0').get_max_mtu()
         '9000'
         """
-        return self.get_interface('max_mtu')
+        return int(self.get_interface('max_mtu'))
 
     def get_mtu(self):
         """