1 files changed, 113 insertions, 0 deletions
diff --git a/tunnel/mtumonitor.go b/tunnel/mtumonitor.go
new file mode 100644
index 00000000..c07823a2
--- /dev/null
+++ b/tunnel/mtumonitor.go
@@ -0,0 +1,113 @@
+/* SPDX-License-Identifier: MIT
+ *
+ * Copyright (C) 2019-2022 WireGuard LLC. All Rights Reserved.
+ */
+
+package tunnel
+
+import (
+	"golang.org/x/sys/windows"
+	"golang.zx2c4.com/wireguard/windows/tunnel/winipcfg"
+)
+
+func findDefaultLUID(family winipcfg.AddressFamily, ourLUID winipcfg.LUID, lastLUID *winipcfg.LUID, lastIndex *uint32) error {
+	r, err := winipcfg.GetIPForwardTable2(family)
+	if err != nil {
+		return err
+	}
+	lowestMetric := ^uint32(0)
+	index := uint32(0)
+	luid := winipcfg.LUID(0)
+	for i := range r {
+		if r[i].DestinationPrefix.PrefixLength != 0 || r[i].InterfaceLUID == ourLUID {
+			continue
+		}
+		ifrow, err := r[i].InterfaceLUID.Interface()
+		if err != nil || ifrow.OperStatus != winipcfg.IfOperStatusUp {
+			continue
+		}
+
+		iface, err := r[i].InterfaceLUID.IPInterface(family)
+		if err != nil {
+			continue
+		}
+
+		if r[i].Metric+iface.Metric < lowestMetric {
+			lowestMetric = r[i].Metric + iface.Metric
+			index = r[i].InterfaceIndex
+			luid = r[i].InterfaceLUID
+		}
+	}
+	if luid == *lastLUID && index == *lastIndex {
+		return nil
+	}
+	*lastLUID = luid
+	*lastIndex = index
+	return nil
+}
+
+func monitorMTU(family winipcfg.AddressFamily, ourLUID winipcfg.LUID) ([]winipcfg.ChangeCallback, error) {
+	var minMTU uint32
+	if family == windows.AF_INET {
+		minMTU = 576
+	} else if family == windows.AF_INET6 {
+		minMTU = 1280
+	}
+	lastLUID := winipcfg.LUID(0)
+	lastIndex := ^uint32(0)
+	lastMTU := uint32(0)
+	doIt := func() error {
+		err := findDefaultLUID(family, ourLUID, &lastLUID, &lastIndex)
+		if err != nil {
+			return err
+		}
+		mtu := uint32(0)
+		if lastLUID != 0 {
+			iface, err := lastLUID.Interface()
+			if err != nil {
+				return err
+			}
+			if iface.MTU > 0 {
+				mtu = iface.MTU
+			}
+		}
+		if mtu > 0 && lastMTU != mtu {
+			iface, err := ourLUID.IPInterface(family)
+			if err != nil {
+				return err
+			}
+			iface.NLMTU = mtu - 80
+			if iface.NLMTU < minMTU {
+				iface.NLMTU = minMTU
+			}
+			err = iface.Set()
+			if err != nil {
+				return err
+			}
+			lastMTU = mtu
+		}
+		return nil
+	}
+	err := doIt()
+	if err != nil {
+		return nil, err
+	}
+	cbr, err := winipcfg.RegisterRouteChangeCallback(func(notificationType winipcfg.MibNotificationType, route *winipcfg.MibIPforwardRow2) {
+		if route != nil && route.DestinationPrefix.PrefixLength == 0 {
+			doIt()
+		}
+	})
+	if err != nil {
+		return nil, err
+	}
+	cbi, err := winipcfg.RegisterInterfaceChangeCallback(func(notificationType winipcfg.MibNotificationType, iface *winipcfg.MibIPInterfaceRow) {
+		if notificationType == winipcfg.MibParameterNotification {
+			doIt()
+		}
+	})
+	if err != nil {
+		cbr.Unregister()
+		return nil, err
+	}
+	return []winipcfg.ChangeCallback{cbr, cbi}, nil
+}