wgengine/router: print Docker warning when stateful filtering is enabled

When Docker is detected on the host and stateful filtering is enabled,
Docker containers may be unable to reach Tailscale nodes (depending on
the network settings of a container). Detect Docker when stateful
filtering is enabled and print a health warning to aid users in noticing
this issue.

We avoid printing the warning if the current node isn't advertising any
subnet routes and isn't an exit node, since without one of those being
true, the node wouldn't have the correct AllowedIPs in WireGuard to
allow a Docker container to connect to another Tailscale node anyway.

Updates #12070

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: Idef538695f4d101b0ef6f3fb398c0eaafc3ae281

wgengine/router/router_linux.go

@@ -42,6 +42,7 @@ type linuxRouter struct {
 	logf              func(fmt string, args ...any)
 	tunname           string
 	netMon            *netmon.Monitor
+	health            *health.Tracker
 	unregNetMon       func()
 	addrs             map[netip.Prefix]bool
 	routes            map[netip.Prefix]bool
@@ -81,15 +82,16 @@ func newUserspaceRouter(logf logger.Logf, tunDev tun.Device, netMon *netmon.Moni
 		ambientCapNetAdmin: useAmbientCaps(),
 	}
 
-	return newUserspaceRouterAdvanced(logf, tunname, netMon, cmd)
+	return newUserspaceRouterAdvanced(logf, tunname, netMon, cmd, health)
 }
 
-func newUserspaceRouterAdvanced(logf logger.Logf, tunname string, netMon *netmon.Monitor, cmd commandRunner) (Router, error) {
+func newUserspaceRouterAdvanced(logf logger.Logf, tunname string, netMon *netmon.Monitor, cmd commandRunner, health *health.Tracker) (Router, error) {
 	r := &linuxRouter{
 		logf:          logf,
 		tunname:       tunname,
 		netfilterMode: netfilterOff,
 		netMon:        netMon,
+		health:        health,
 
 		cmd: cmd,
 
@@ -420,6 +422,7 @@ func (r *linuxRouter) Set(cfg *Config) error {
 		}
 	}
 	r.statefulFiltering = cfg.StatefulFiltering
+	r.updateStatefulFilteringWithDockerWarning(cfg)
 
 	// Issue 11405: enable IP forwarding on gokrazy.
 	advertisingRoutes := len(cfg.SubnetRoutes) > 0
@@ -430,6 +433,53 @@ func (r *linuxRouter) Set(cfg *Config) error {
 	return multierr.New(errs...)
 }
 
+var warnStatefulFilteringWithDocker = health.NewWarnable()
+
+func (r *linuxRouter) updateStatefulFilteringWithDockerWarning(cfg *Config) {
+	// If stateful filtering is disabled, clear the warning.
+	if !r.statefulFiltering {
+		r.health.SetWarnable(warnStatefulFilteringWithDocker, nil)
+		return
+	}
+
+	advertisingRoutes := len(cfg.SubnetRoutes) > 0
+
+	// TODO(andrew-d,maisem): we might want to check if we're running in a
+	// container, since, if so, stateful filtering might prevent other
+	// containers from connecting through the Tailscale in this container.
+	//
+	// For now, just check for the case where we're running Tailscale on
+	// the host and Docker is also running.
+
+	// If this node isn't a subnet router or exit node, then we would never
+	// have allowed traffic from a Docker container in to Tailscale, since
+	// there wouldn't be an AllowedIP for the container's source IP. So we
+	// don't need to warn in this case.
+	//
+	// cfg.SubnetRoutes contains all subnet routes for the node, including
+	// the default route (0.0.0.0/0 or ::/0) if this node is an exit node.
+	if advertisingRoutes {
+		// Check for the presence of a Docker interface and warn if it's found
+		// on the system.
+		//
+		// TODO(andrew-d): do a better job at detecting Docker, e.g. by looking
+		// for it in the $PATH or by checking for the presence of the Docker
+		// socket/daemon/etc.
+		ifstate := r.netMon.InterfaceState()
+		if _, found := ifstate.Interface["docker0"]; found {
+			r.health.SetWarnable(warnStatefulFilteringWithDocker, fmt.Errorf(""+
+				"Stateful filtering is enabled and Docker was detected; this may prevent Docker containers "+
+				"on this host from connecting to Tailscale nodes. "+
+				"See https://tailscale.com/s/stateful-docker",
+			))
+			return
+		}
+	}
+
+	// If we get here, then we have no warnings; clear anything existing.
+	r.health.SetWarnable(warnStatefulFilteringWithDocker, nil)
+}
+
 // UpdateMagicsockPort implements the Router interface.
 func (r *linuxRouter) UpdateMagicsockPort(port uint16, network string) error {
 	if r.nfr == nil {

wgengine/router/router_linux_test.go

@@ -22,6 +22,7 @@ import (
 	"github.com/tailscale/wireguard-go/tun"
 	"github.com/vishvananda/netlink"
 	"go4.org/netipx"
+	"tailscale.com/health"
 	"tailscale.com/net/netmon"
 	"tailscale.com/net/tsaddr"
 	"tailscale.com/tstest"
@@ -369,7 +370,8 @@ ip route add throw 192.168.0.0/24 table 52` + basic,
 	defer mon.Close()
 
 	fake := NewFakeOS(t)
-	router, err := newUserspaceRouterAdvanced(t.Logf, "tailscale0", mon, fake)
+	ht := new(health.Tracker)
+	router, err := newUserspaceRouterAdvanced(t.Logf, "tailscale0", mon, fake, ht)
 	router.(*linuxRouter).nfr = fake.nfr
 	if err != nil {
 		t.Fatalf("failed to create router: %v", err)