manager: make multiple tunnels mode automatic

Rather than having to set a registry knob to enable multiple tunnels, it
is now automatic. If an additional activated tunnel has the same route
subnets or interface IP addresses as a previous tunnel, that previous
one is stopped. But if there's no overlap, then they coexist.

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

3 files changed, 74 insertions, 46 deletions

diff --git a/conf/config.go b/conf/config.go
index a6266851..c35e6664 100644
--- a/conf/config.go
+++ b/conf/config.go
@@ -94,6 +94,43 @@ func (r *IPCidr) MaskSelf() {
 	}
 }
 
+func (conf *Config) IntersectsWith(other *Config) bool {
+	type hashableIPCidr struct {
+		ip   string
+		cidr byte
+	}
+	allRoutes := make(map[hashableIPCidr]bool, len(conf.Interface.Addresses)*2+len(conf.Peers)*3)
+	for _, a := range conf.Interface.Addresses {
+		allRoutes[hashableIPCidr{string(a.IP), byte(len(a.IP) * 8)}] = true
+		a.MaskSelf()
+		allRoutes[hashableIPCidr{string(a.IP), a.Cidr}] = true
+	}
+	for i := range conf.Peers {
+		for _, a := range conf.Peers[i].AllowedIPs {
+			a.MaskSelf()
+			allRoutes[hashableIPCidr{string(a.IP), a.Cidr}] = true
+		}
+	}
+	for _, a := range other.Interface.Addresses {
+		if allRoutes[hashableIPCidr{string(a.IP), byte(len(a.IP) * 8)}] {
+			return true
+		}
+		a.MaskSelf()
+		if allRoutes[hashableIPCidr{string(a.IP), a.Cidr}] {
+			return true
+		}
+	}
+	for i := range other.Peers {
+		for _, a := range other.Peers[i].AllowedIPs {
+			a.MaskSelf()
+			if allRoutes[hashableIPCidr{string(a.IP), a.Cidr}] {
+				return true
+			}
+		}
+	}
+	return false
+}
+
 func (e *Endpoint) String() string {
 	if strings.IndexByte(e.Host, ':') > 0 {
 		return fmt.Sprintf("[%s]:%d", e.Host, e.Port)
diff --git a/docs/adminregistry.md b/docs/adminregistry.md
index 34033446..6d8838f6 100644
--- a/docs/adminregistry.md
+++ b/docs/adminregistry.md
@@ -38,20 +38,6 @@ executing these scripts.
 > reg add HKLM\Software\WireGuard /v DangerousScriptExecution /t REG_DWORD /d 1 /f
 ```
 
-#### `HKLM\Software\WireGuard\MultipleSimultaneousTunnels`
-
-When this key is set to `DWORD(1)`, the UI may start multiple tunnels at the
-same time; otherwise, an existing tunnel is stopped when a new one is started.
-Note that it is always possible, regardless of this key, to start multiple
-tunnels using `wireguard /installtunnelservice`; this controls only the semantics
-of tunnel start requests coming from the UI. If all goes well, this key will be
-removed and the logic of whether to stop existing tunnels will be based on
-overlapping routes, but for now, this key provides a manual override.
-
-```
-> reg add HKLM\Software\WireGuard /v MultipleSimultaneousTunnels /t REG_DWORD /d 1 /f
-```
-
 #### `HKLM\Software\WireGuard\ExperimentalKernelDriver`
 
 When this key is set to `DWORD(1)`, an experimental kernel driver from the
diff --git a/manager/ipc_server.go b/manager/ipc_server.go
index fe195094..ba785ced 100644
--- a/manager/ipc_server.go
+++ b/manager/ipc_server.go
@@ -108,44 +108,49 @@ func (s *ManagerService) RuntimeConfig(tunnelName string) (*conf.Config, error)
 }
 
 func (s *ManagerService) Start(tunnelName string) error {
-	// TODO: Rather than being lazy and gating this behind a knob (yuck!), we should instead keep track of the routes
-	// of each tunnel, and only deactivate in the case of a tunnel with identical routes being added.
-	if !conf.AdminBool("MultipleSimultaneousTunnels") {
-		trackedTunnelsLock.Lock()
-		tt := make([]string, 0, len(trackedTunnels))
-		var inTransition string
-		for t, state := range trackedTunnels {
-			tt = append(tt, t)
-			if len(t) > 0 && (state == TunnelStarting || state == TunnelUnknown) {
-				inTransition = t
-				break
-			}
+	c, err := conf.LoadFromName(tunnelName)
+	if err != nil {
+		return err
+	}
+
+	// Figure out which tunnels have intersecting addresses/routes and stop those.
+	trackedTunnelsLock.Lock()
+	tt := make([]string, 0, len(trackedTunnels))
+	var inTransition string
+	for t, state := range trackedTunnels {
+		c2, err := conf.LoadFromName(t)
+		if err != nil || !c.IntersectsWith(c2) {
+			// If we can't get the config, assume it doesn't intersect.
+			continue
+		}
+		tt = append(tt, t)
+		if len(t) > 0 && (state == TunnelStarting || state == TunnelUnknown) {
+			inTransition = t
+			break
 		}
-		trackedTunnelsLock.Unlock()
-		if len(inTransition) != 0 {
-			return fmt.Errorf("Please allow the tunnel ‘%s’ to finish activating", inTransition)
+	}
+	trackedTunnelsLock.Unlock()
+	if len(inTransition) != 0 {
+		return fmt.Errorf("Please allow the tunnel ‘%s’ to finish activating", inTransition)
+	}
+
+	// Stop those intersecting tunnels asynchronously.
+	go func() {
+		for _, t := range tt {
+			s.Stop(t)
 		}
-		go func() {
-			for _, t := range tt {
+		for _, t := range tt {
+			state, err := s.State(t)
+			if err == nil && (state == TunnelStarted || state == TunnelStarting) {
+				log.Printf("[%s] Trying again to stop zombie tunnel", t)
 				s.Stop(t)
+				time.Sleep(time.Millisecond * 100)
 			}
-			for _, t := range tt {
-				state, err := s.State(t)
-				if err == nil && (state == TunnelStarted || state == TunnelStarting) {
-					log.Printf("[%s] Trying again to stop zombie tunnel", t)
-					s.Stop(t)
-					time.Sleep(time.Millisecond * 100)
-				}
-			}
-		}()
-	}
+		}
+	}()
 	time.AfterFunc(time.Second*10, cleanupStaleNetworkInterfaces)
 
-	// After that process is started -- it's somewhat asynchronous -- we install the new one.
-	c, err := conf.LoadFromName(tunnelName)
-	if err != nil {
-		return err
-	}
+	// After the stop process has begun, but before it's finished, we install the new one.
 	path, err := c.Path()
 	if err != nil {
 		return err