tunnel: check for WeakHostSend/Forwarding in pitfalls

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

manager/service.go

@@ -55,7 +55,6 @@ func (service *managerService) Execute(args []string, r <-chan svc.ChangeRequest
 	}
 
 	services.PrintStarting()
-	checkForPitfalls()
 
 	path, err := os.Executable()
 	if err != nil {

tunnel/interfacewatcher.go

@@ -80,6 +80,7 @@ func (iw *interfaceWatcher) setup(family winipcfg.AddressFamily) {
 		iw.errors <- interfaceWatcherError{services.ErrorSetNetConfig, err}
 		return
 	}
+	evaluateDynamicPitfalls(family, iw.conf, iw.luid)
 
 	iw.started <- family
 }

manager/pitfalls.go → tunnel/pitfalls.go

@@ -3,7 +3,7 @@
  * Copyright (C) 2019-2021 WireGuard LLC. All Rights Reserved.
  */
 
-package manager
+package tunnel
 
 import (
 	"log"
@@ -12,15 +12,24 @@ import (
 
 	"golang.org/x/sys/windows"
 	"golang.org/x/sys/windows/svc/mgr"
+	"golang.zx2c4.com/go118/netip"
+	"golang.zx2c4.com/wireguard/windows/conf"
+	"golang.zx2c4.com/wireguard/windows/tunnel/winipcfg"
 )
 
-func checkForPitfalls() {
+func evaluateStaticPitfalls() {
 	go func() {
 		pitfallDnsCacheDisabled()
 		pitfallVirtioNetworkDriver()
 	}()
 }
 
+func evaluateDynamicPitfalls(family winipcfg.AddressFamily, conf *conf.Config, luid winipcfg.LUID) {
+	go func() {
+		pitfallWeakHostSend(family, conf, luid)
+	}()
+}
+
 func pitfallDnsCacheDisabled() {
 	scm, err := mgr.Connect()
 	if err != nil {
@@ -92,3 +101,80 @@ func pitfallVirtioNetworkDriver() {
 		return
 	}
 }
+
+func pitfallWeakHostSend(family winipcfg.AddressFamily, conf *conf.Config, ourLUID winipcfg.LUID) {
+	routingTable, err := winipcfg.GetIPForwardTable2(family)
+	if err != nil {
+		return
+	}
+	type endpointRoute struct {
+		addr         netip.Addr
+		name         string
+		lowestMetric uint32
+		highestCIDR  uint8
+		weakHostSend bool
+		finalIsOurs  bool
+	}
+	endpoints := make([]endpointRoute, 0, len(conf.Peers))
+	for _, peer := range conf.Peers {
+		addr, err := netip.ParseAddr(peer.Endpoint.Host)
+		if err != nil {
+			continue
+		}
+		endpoints = append(endpoints, endpointRoute{addr: addr, lowestMetric: ^uint32(0)})
+	}
+	for i := range routingTable {
+		var (
+			ifrow    *winipcfg.MibIfRow2
+			ifacerow *winipcfg.MibIPInterfaceRow
+			metric   uint32
+		)
+		for j := range endpoints {
+			r, e := &routingTable[i], &endpoints[j]
+			if (e.addr.Is4() && family != windows.AF_INET) || (e.addr.Is6() && family != windows.AF_INET6) {
+				continue
+			}
+			if r.DestinationPrefix.PrefixLength < e.highestCIDR {
+				continue
+			}
+			if !r.DestinationPrefix.Prefix().Contains(e.addr) {
+				continue
+			}
+			if ifrow == nil {
+				ifrow, err = r.InterfaceLUID.Interface()
+				if err != nil {
+					continue
+				}
+			}
+			if ifrow.OperStatus != winipcfg.IfOperStatusUp {
+				continue
+			}
+			if ifacerow == nil {
+				ifacerow, err = r.InterfaceLUID.IPInterface(family)
+				if err != nil {
+					continue
+				}
+				metric = r.Metric + ifacerow.Metric
+			}
+			if r.DestinationPrefix.PrefixLength == e.highestCIDR && metric > e.lowestMetric {
+				continue
+			}
+			e.lowestMetric = metric
+			e.highestCIDR = r.DestinationPrefix.PrefixLength
+			e.finalIsOurs = r.InterfaceLUID == ourLUID
+			if !e.finalIsOurs {
+				e.name = ifrow.Alias()
+				e.weakHostSend = ifacerow.ForwardingEnabled || ifacerow.WeakHostSend
+			}
+		}
+	}
+	problematicInterfaces := make(map[string]bool, len(endpoints))
+	for _, e := range endpoints {
+		if e.weakHostSend && e.finalIsOurs {
+			problematicInterfaces[e.name] = true
+		}
+	}
+	for iface := range problematicInterfaces {
+		log.Printf("Warning: the %q interface has Forwarding/WeakHostSend enabled, which will cause routing loops", iface)
+	}
+}

tunnel/service.go

@@ -135,6 +135,8 @@ func (service *tunnelService) Execute(args []string, r <-chan svc.ChangeRequest,
 		}
 	}
 
+	evaluateStaticPitfalls()
+
 	log.Println("Watching network interfaces")
 	watcher, err = watchInterface()
 	if err != nil {