From c934c8fb1192b13496f817f908cfef9a88a46b84 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 |  7 +++---
 pkg/util/net.go       | 54 +++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 58 insertions(+), 3 deletions(-)

diff --git a/pkg/config/ovn/ovn.go b/pkg/config/ovn/ovn.go
index 8adf9f38afb..1373d89782e 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"
 )
@@ -70,10 +71,10 @@ func (o *OVNKubernetesConfig) validateConfig() error {
 	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)
+	link, err := util.FindDefaultRouteInterface()
 	if err == nil && link.MTU > 0 {
 		o.MTU = link.MTU
 	} else {
diff --git a/pkg/util/net.go b/pkg/util/net.go
index fcdc4918955..c759bf86bca 100644
--- a/pkg/util/net.go
+++ b/pkg/util/net.go
@@ -266,3 +266,57 @@ func GetHostIPv6(ipHint string) (string, error) {
 
 	return "", fmt.Errorf("unable to find host IPv6 address")
 }
+func FindDefaultRouteInterface() (*tcpnet.Interface, error) {
+	nodeIP, err := GetHostIP("")
+	if err != nil {
+		return nil, fmt.Errorf("failed to get host IP: %v", err)
+	}
+
+	ip := tcpnet.ParseIP(nodeIP)
+	if ip == nil {
+		return nil, fmt.Errorf("invalid IP address: %s", nodeIP)
+	}
+
+	ifaces, err := tcpnet.Interfaces()
+
+	if err != nil {
+		return nil, err
+	}
+
+	isDown := func(iface tcpnet.Interface) bool {
+		return iface.Flags&1 == 0
+	}
+
+	for _, iface := range ifaces {
+		if isDown(iface) {
+			continue
+		}
+		found, err := ipAddrExistsAtInterface(ip, iface)
+		if err != nil {
+			return nil, err
+		}
+		if !found {
+			continue
+		}
+
+		return &iface, nil
+	}
+	return nil, fmt.Errorf("no usable interface found")
+}
+
+func ipAddrExistsAtInterface(ipAddr tcpnet.IP, iface tcpnet.Interface) (bool, error) {
+	addrs, err := iface.Addrs()
+
+	if err != nil {
+		return false, err
+	}
+
+	for _, a := range addrs {
+		if ipnet, ok := a.(*tcpnet.IPNet); ok {
+			if ipnet.IP.Equal(ipAddr) {
+				return true, nil
+			}
+		}
+	}
+	return false, nil
+}