windows: Fix NLMTU for ipv6 (#1871) (#1878)

(cherry picked from commit 89178f45ba)

Co-authored-by: Nate Brown <nbrown.us@gmail.com>

overlay/tun_windows.go

@@ -12,6 +12,7 @@ import (
 	"os"
 	"path/filepath"
 	"runtime"
+	"slices"
 	"sync/atomic"
 	"syscall"
 	"unsafe"
@@ -178,6 +179,7 @@ func (t *winTun) addRoutes(logErrors bool) error {
 	luid := winipcfg.LUID(t.tun.LUID())
 	routes := *t.Routes.Load()
 	foundDefault4 := false
+	carriesV6 := slices.ContainsFunc(t.vpnNetworks, func(p netip.Prefix) bool { return p.Addr().Is6() })
 
 	for _, r := range routes {
 		if len(r.Via) == 0 || !r.Install {
@@ -185,6 +187,9 @@ func (t *winTun) addRoutes(logErrors bool) error {
 			continue
 		}
 
+		// A v6 unsafe_route is legal under a v4-only cert; uninstalled ones put nothing on the adapter.
+		carriesV6 = carriesV6 || r.Cidr.Addr().Is6()
+
 		// Add our unsafe route as an on-link route to the nebula tun device.
 		err := luid.AddRoute(r.Cidr, unspecifiedNextHop(r.Cidr), uint32(r.Metric))
 		if err != nil {
@@ -206,6 +211,11 @@ func (t *winTun) addRoutes(logErrors bool) error {
 		}
 	}
 
+	return t.setMTU(luid, foundDefault4, carriesV6)
+}
+
+// setMTU applies tun.mtu per address family. The default route metric rides along on the v4 handle.
+func (t *winTun) setMTU(luid winipcfg.LUID, foundDefault4, carriesV6 bool) error {
 	ipif, err := luid.IPInterface(windows.AF_INET)
 	if err != nil {
 		return fmt.Errorf("failed to get ip interface: %w", err)
@@ -220,6 +230,25 @@ func (t *winTun) addRoutes(logErrors bool) error {
 	if err := ipif.Set(); err != nil {
 		return fmt.Errorf("failed to set ip interface: %w", err)
 	}
+
+	// Windows tracks NLMTU per family and wintun sets neither, so v6 keeps the adapter default of 65535.
+	// Gated so a v4-only overlay under 1280 boots; a v6 one deliberately does not, as linux also refuses.
+	if !carriesV6 {
+		return nil
+	}
+
+	ipif6, err := luid.IPInterface(windows.AF_INET6)
+	if err != nil {
+		// No v6 on the adapter means there is no NLMTU to get wrong. A failed Set below is not the same thing.
+		t.l.Info("Skipping ipv6 MTU, no ipv6 interface on this adapter", "error", err)
+		return nil
+	}
+
+	ipif6.NLMTU = uint32(t.MTU)
+	if err := ipif6.Set(); err != nil {
+		return fmt.Errorf("failed to set ipv6 interface: %w", err)
+	}
+
 	return nil
 }