From eef83e2aa78b8cd1c1b281911b51edae7ab49f19 Mon Sep 17 00:00:00 2001
From: Evgeny Slutsky <eslutsky@redhat.com>
Date: Wed, 4 Dec 2024 16:04:36 +0100
Subject: [PATCH] OCPBUGS-38468: use default route mtu

Signed-off-by: Evgeny Slutsky <eslutsky@redhat.com>
---
 pkg/config/ovn/ovn.go | 29 ++++++---------------
 pkg/util/net.go       | 60 +++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 68 insertions(+), 21 deletions(-)

diff --git a/pkg/config/ovn/ovn.go b/pkg/config/ovn/ovn.go
index 7616a7b34cf..bb8d8fe29ac 100644
--- a/pkg/config/ovn/ovn.go
+++ b/pkg/config/ovn/ovn.go
@@ -8,6 +8,7 @@ import (
 	"path/filepath"
 	"regexp"
 
+	"github.com/openshift/microshift/pkg/util"
 	"k8s.io/klog/v2"
 	"sigs.k8s.io/yaml"
 )
@@ -40,10 +41,6 @@ func (o *OVNKubernetesConfig) Validate() error {
 	if err != nil {
 		return fmt.Errorf("failed to validate OVS bridge: %w", err)
 	}
-	err = o.validateConfig()
-	if err != nil {
-		return fmt.Errorf("failed to validate OVN-K configuration: %w", err)
-	}
 	return nil
 }
 
@@ -56,27 +53,17 @@ func (o *OVNKubernetesConfig) validateOVSBridge() error {
 	return nil
 }
 
-// validateConfig validates the user defined configuration in /etc/microshift/ovn.yaml
-func (o *OVNKubernetesConfig) validateConfig() error {
-	// validate MTU conf
-	iface, err := net.InterfaceByName(OVNGatewayInterface)
-	if err != nil {
-		return fmt.Errorf("failed to find OVN gateway interface %q: %w", OVNGatewayInterface, err)
-	}
-
-	if iface.MTU < o.MTU {
-		return fmt.Errorf("interface MTU (%d) is too small for specified overlay (%d)", iface.MTU, o.MTU)
-	}
-	return nil
-}
-
-// getClusterMTU retrieves MTU from ovn-kubernetes gateway interface "br-ex",
-// and falls back to use 1500 when "br-ex" mtu is unable to get or less than 0.
+// getClusterMTU retrieves MTU from the default route network interface,
+// and falls back to use 1500 when unable to get the mtu or less than 0.
 func (o *OVNKubernetesConfig) getClusterMTU(multinode bool) {
-	link, err := net.InterfaceByName(OVNGatewayInterface)
+	klog.Infof("getClusterMTU: finding default route interface")
+	link, err := util.FindDefaultRouteIface()
+
 	if err == nil && link.MTU > 0 {
+		klog.Infof("getClusterMTU: Using Interface %s with MTU %d as source default route.", link.Name, link.MTU)
 		o.MTU = link.MTU
 	} else {
+		klog.Infof("getClusterMTU: Couldnt extract MTU from the default route interface , Using Default MTU.")
 		o.MTU = defaultMTU
 	}
 
diff --git a/pkg/util/net.go b/pkg/util/net.go
index fcdc4918955..8ab61e71250 100644
--- a/pkg/util/net.go
+++ b/pkg/util/net.go
@@ -34,6 +34,14 @@ import (
 
 var previousGatewayIP string = ""
 
+type routeStruct struct {
+	// Name of interface
+	Iface string
+
+	// big-endian hex string
+	Gateway string
+}
+
 // Remember whether we have successfully found the hard-coded nodeIP
 // on this host.
 var foundHardCodedNodeIP bool
@@ -266,3 +274,55 @@ func GetHostIPv6(ipHint string) (string, error) {
 
 	return "", fmt.Errorf("unable to find host IPv6 address")
 }
+
+// Find the Default route Interface based on /proc/net/route or /proc/net/ipv6_route
+func FindDefaultRouteIface() (iface *tcpnet.Interface, err error) {
+
+	// first try IPv4
+	parsedStruct, err := findDefaultRouteForFamily(netlink.FAMILY_V4)
+	if err != nil {
+
+		// try IPv6
+		parsedStruct, err = findDefaultRouteForFamily(netlink.FAMILY_V6)
+		if err != nil {
+			return nil, err
+		}
+	}
+
+	iface, err = tcpnet.InterfaceByName(parsedStruct.Iface)
+	if err != nil {
+		return nil, err
+	}
+
+	return iface, nil
+}
+
+func findDefaultRouteForFamily(family int) (routeStruct, error) {
+	handle, err := netlink.NewHandle()
+	if err != nil {
+		fmt.Printf("error opening New Handle %w ", err)
+	}
+
+	routeList, err := handle.RouteList(nil, family)
+	if err != nil {
+		fmt.Printf("error listing routes %w \n ", err)
+	}
+
+	for _, route := range routeList {
+
+		//  for Default route the Destination should be nil (0)
+		if route.Dst == nil {
+			link, err := handle.LinkByIndex(route.LinkIndex)
+			if err != nil {
+				fmt.Printf("error listing Link by Index %w \n ", err)
+			}
+			return routeStruct{
+				Iface:   link.Attrs().Name,
+				Gateway: route.Gw.String(),
+			}, nil
+
+		}
+
+	}
+	return routeStruct{}, fmt.Errorf("no default gateway found")
+}