Add MTU to CNI result

This commit allow CNIs to expose the MTU of interface

Signed-off-by: Sebastian Sch <sebassch@gmail.com>

SPEC.md

@@ -567,6 +567,7 @@ Plugins must output a JSON object with the following keys upon a successful `ADD
 - `interfaces`: An array of all interfaces created by the attachment, including any host-level interfaces:
     - `name`: The name of the interface.
     - `mac`: The hardware address of the interface (if applicable).
+    - `mtu`: The MTU of the interface (if applicable).
     - `sandbox`: The isolation domain reference (e.g. path to network namespace) for the interface, or empty if on the host. For interfaces created inside the container, this should be the value passed via `CNI_NETNS`.
 - `ips`: IPs assigned by this attachment. Plugins may include IPs assigned external to the container.
     - `address` (string): an IP address in CIDR notation (eg "192.168.1.3/24").

pkg/types/100/types.go

@@ -270,6 +270,7 @@ func (r *Result) PrintTo(writer io.Writer) error {
 type Interface struct {
 	Name    string `json:"name"`
 	Mac     string `json:"mac,omitempty"`
+	Mtu     int    `json:"mtu,omitempty"`
 	Sandbox string `json:"sandbox,omitempty"`
 }
 

pkg/types/100/types_test.go

@@ -53,6 +53,7 @@ func testResult() *current.Result {
 			{
 				Name:    "eth0",
 				Mac:     "00:11:22:33:44:55",
+				Mtu:     1500,
 				Sandbox: "/proc/3553/ns/net",
 			},
 		},
@@ -106,6 +107,7 @@ var _ = Describe("Current types operations", func() {
         {
             "name": "eth0",
             "mac": "00:11:22:33:44:55",
+			"mtu": 1500,
             "sandbox": "/proc/3553/ns/net"
         }
     ],