decouple mtu calculation for the main route

cmd/kindnetd/cni.go

@@ -18,7 +18,6 @@ package main
 
 import (
 	"io"
-	"net"
 	"os"
 	"reflect"
 	"text/template"
@@ -62,21 +61,6 @@ func ComputeCNIConfigInputs(node *corev1.Node) CNIConfigInputs {
 	}
 }
 
-// computeBridgeMTU finds the mtu for the eth0 interface
-// otherwise it defaults to ptp default behavior of being set by kernel
-func computeBridgeMTU() (int, error) {
-	interfaces, err := net.Interfaces()
-	if err != nil {
-		return 0, err
-	}
-	for _, inter := range interfaces {
-		if inter.Name == "eth0" {
-			return inter.MTU, nil
-		}
-	}
-	return 0, errors.New("Found no eth0 device")
-}
-
 // cniConfigPath is where kindnetd will write the computed CNI config
 const cniConfigPath = "/etc/cni/net.d/10-kindnet.conflist"
 

cmd/kindnetd/main.go

@@ -25,6 +25,8 @@ import (
 	"os/signal"
 	"time"
 
+	utilnet "github.com/aojea/kindnet/pkg/net"
+
 	"golang.org/x/sys/unix"
 	"k8s.io/apimachinery/pkg/labels"
 	"k8s.io/apimachinery/pkg/util/sets"
@@ -49,15 +51,14 @@ import (
 // TODO: improve logging & error handling
 
 // IPFamily defines kindnet networking operating model
-type IPFamily string
+type IPFamily int
 
 const (
-	// IPv4Family sets IPFamily to ipv4
-	IPv4Family IPFamily = "ipv4"
-	// IPv6Family sets IPFamily to ipv6
-	IPv6Family IPFamily = "ipv6"
-	// DualStackFamily sets ClusterIPFamily to DualStack
-	DualStackFamily IPFamily = "dualstack"
+	// Family type definitions
+	AllFamily       IPFamily = unix.AF_UNSPEC
+	IPv4Family      IPFamily = unix.AF_INET
+	IPv6Family      IPFamily = unix.AF_INET6
+	DualStackFamily IPFamily = 12 // AF_INET + AF_INET6
 )
 
 func main() {
@@ -111,7 +112,7 @@ func main() {
 		))
 	}
 
-	mtu, err := computeBridgeMTU()
+	mtu, err := utilnet.GetMTU(int(AllFamily))
 	klog.Infof("setting mtu %d for CNI \n", mtu)
 	if err != nil {
 		klog.Infof("Failed to get MTU size from interface eth0, using kernel default MTU size error:%v", err)

pkg/net/host.go

@@ -0,0 +1,73 @@
+package net
+
+import (
+	"fmt"
+	"log"
+	"net"
+
+	"github.com/vishvananda/netlink"
+)
+
+// GetMTU returns the MTU used for the IP family
+func GetMTU(ipFamily int) (int, error) {
+	iface, err := getDefaultGwIf(ipFamily)
+	if err != nil {
+		return 0, err
+	}
+	mtu, err := getInterfaceMTU(iface)
+	if err != nil {
+		return 0, err
+	}
+	return mtu, nil
+}
+
+// getInterfaceMTU finds the mtu for the interface
+func getInterfaceMTU(iface string) (int, error) {
+	interfaces, err := net.Interfaces()
+	if err != nil {
+		return 0, err
+	}
+	for _, inter := range interfaces {
+		if inter.Name == iface {
+			return inter.MTU, nil
+		}
+	}
+	return 0, fmt.Errorf("no %s device found", iface)
+}
+
+func getDefaultGwIf(ipFamily int) (string, error) {
+	routes, err := netlink.RouteList(nil, ipFamily)
+	if err != nil {
+		return "", err
+	}
+
+	for _, r := range routes {
+		// no multipath
+		if len(r.MultiPath) == 0 {
+			if r.Gw == nil {
+				continue
+			}
+			intfLink, err := netlink.LinkByIndex(r.LinkIndex)
+			if err != nil {
+				log.Printf("Failed to get interface link for route %v : %v", r, err)
+				continue
+			}
+			return intfLink.Attrs().Name, nil
+		}
+
+		// multipath, use the first valid entry
+		// xref: https://github.com/vishvananda/netlink/blob/6ffafa9fc19b848776f4fd608c4ad09509aaacb4/route.go#L137-L145
+		for _, nh := range r.MultiPath {
+			if nh.Gw == nil {
+				continue
+			}
+			intfLink, err := netlink.LinkByIndex(r.LinkIndex)
+			if err != nil {
+				log.Printf("Failed to get interface link for route %v : %v", r, err)
+				continue
+			}
+			return intfLink.Attrs().Name, nil
+		}
+	}
+	return "", fmt.Errorf("not routes found")
+}