wgengine/router: add a setting to disable SNAT for subnet routes.

Part of #320.

Signed-off-by: David Anderson <danderson@tailscale.com>

cmd/tailscale/tailscale.go

@@ -56,6 +56,7 @@ func main() {
 	upf.BoolVar(&upArgs.shieldsUp, "shields-up", false, "don't allow incoming connections")
 	upf.StringVar(&upArgs.advertiseRoutes, "advertise-routes", "", "routes to advertise to other nodes (comma-separated, e.g. 10.0.0.0/8,192.168.0.0/24)")
 	upf.StringVar(&upArgs.advertiseTags, "advertise-tags", "", "ACL tags to request (comma-separated, e.g. eng,montreal,ssh)")
+	upf.BoolVar(&upArgs.noSNAT, "no-snat", false, "disable SNAT of traffic to local routes advertised with -advertise-routes")
 	upf.StringVar(&upArgs.authKey, "authkey", "", "node authorization key")
 	upCmd := &ffcli.Command{
 		Name:       "up",
@@ -105,6 +106,7 @@ var upArgs struct {
 	shieldsUp       bool
 	advertiseRoutes string
 	advertiseTags   string
+	noSNAT          bool
 	authKey         string
 }
 
@@ -191,6 +193,7 @@ func runUp(ctx context.Context, args []string) error {
 	prefs.ShieldsUp = upArgs.shieldsUp
 	prefs.AdvertiseRoutes = routes
 	prefs.AdvertiseTags = tags
+	prefs.NoSNAT = upArgs.noSNAT
 
 	c, bc, ctx, cancel := connect(ctx)
 	defer cancel()

ipn/local.go

@@ -707,7 +707,7 @@ func (b *LocalBackend) authReconfig() {
 		log.Fatalf("WGCfg: %v", err)
 	}
 
-	err = b.e.Reconfig(cfg, dom, uc.AdvertiseRoutes)
+	err = b.e.Reconfig(cfg, dom, uc.AdvertiseRoutes, uc.NoSNAT)
 	if err == wgengine.ErrNoChanges {
 		return
 	}
@@ -736,7 +736,7 @@ func (b *LocalBackend) enterState(newState State) {
 		b.blockEngineUpdates(true)
 		fallthrough
 	case Stopped:
-		err := b.e.Reconfig(&wgcfg.Config{}, nil, nil)
+		err := b.e.Reconfig(&wgcfg.Config{}, nil, nil, false)
 		if err != nil {
 			b.logf("Reconfig(down): %v", err)
 		}
@@ -812,7 +812,7 @@ func (b *LocalBackend) stateMachine() {
 
 func (b *LocalBackend) stopEngineAndWait() {
 	b.logf("stopEngineAndWait...")
-	b.e.Reconfig(&wgcfg.Config{}, nil, nil)
+	b.e.Reconfig(&wgcfg.Config{}, nil, nil, false)
 	b.requestEngineStatusAndWait()
 	b.logf("stopEngineAndWait: done.")
 }

ipn/prefs.go

@@ -53,6 +53,15 @@ type Prefs struct {
 	// the control server will allow you to take on the rights for that
 	// tag.
 	AdvertiseTags []string
+	// NoSNAT specifies whether to source NAT traffic going to
+	// destinations in AdvertiseRoutes. The default is to apply source
+	// NAT, which makes the traffic appear to come from the router
+	// machine rather than the peer's Tailscale IP.
+	//
+	// Disabling SNAT requires additional manual configuration in your
+	// network to route Tailscale traffic back to the subnet relay
+	// machine.
+	NoSNAT bool
 
 	// NotepadURLs is a debugging setting that opens OAuth URLs in
 	// notepad.exe on Windows, rather than loading them in a browser.
@@ -83,9 +92,9 @@ func (p *Prefs) Pretty() string {
 	} else {
 		pp = "Persist=nil"
 	}
-	return fmt.Sprintf("Prefs{ra=%v mesh=%v dns=%v want=%v notepad=%v derp=%v shields=%v routes=%v %v}",
+	return fmt.Sprintf("Prefs{ra=%v mesh=%v dns=%v want=%v notepad=%v derp=%v shields=%v routes=%v snat=%v %v}",
 		p.RouteAll, p.AllowSingleHosts, p.CorpDNS, p.WantRunning,
-		p.NotepadURLs, !p.DisableDERP, p.ShieldsUp, p.AdvertiseRoutes, pp)
+		p.NotepadURLs, !p.DisableDERP, p.ShieldsUp, p.AdvertiseRoutes, !p.NoSNAT, pp)
 }
 
 func (p *Prefs) ToBytes() []byte {
@@ -113,6 +122,7 @@ func (p *Prefs) Equals(p2 *Prefs) bool {
 		p.NotepadURLs == p2.NotepadURLs &&
 		p.DisableDERP == p2.DisableDERP &&
 		p.ShieldsUp == p2.ShieldsUp &&
+		p.NoSNAT == p2.NoSNAT &&
 		compareIPNets(p.AdvertiseRoutes, p2.AdvertiseRoutes) &&
 		compareStrings(p.AdvertiseTags, p2.AdvertiseTags) &&
 		p.Persist.Equals(p2.Persist)

ipn/prefs_test.go

@@ -20,7 +20,7 @@ func fieldsOf(t reflect.Type) (fields []string) {
 }
 
 func TestPrefsEqual(t *testing.T) {
-	prefsHandles := []string{"ControlURL", "RouteAll", "AllowSingleHosts", "CorpDNS", "WantRunning", "ShieldsUp", "AdvertiseRoutes", "AdvertiseTags", "NotepadURLs", "DisableDERP", "Persist"}
+	prefsHandles := []string{"ControlURL", "RouteAll", "AllowSingleHosts", "CorpDNS", "WantRunning", "ShieldsUp", "AdvertiseRoutes", "AdvertiseTags", "NoSNAT", "NotepadURLs", "DisableDERP", "Persist"}
 	if have := fieldsOf(reflect.TypeOf(Prefs{})); !reflect.DeepEqual(have, prefsHandles) {
 		t.Errorf("Prefs.Equal check might be out of sync\nfields: %q\nhandled: %q\n",
 			have, prefsHandles)
@@ -111,6 +111,17 @@ func TestPrefsEqual(t *testing.T) {
 			true,
 		},
 
+		{
+			&Prefs{NoSNAT: true},
+			&Prefs{NoSNAT: false},
+			false,
+		},
+		{
+			&Prefs{NoSNAT: true},
+			&Prefs{NoSNAT: true},
+			true,
+		},
+
 		{
 			&Prefs{NotepadURLs: true},
 			&Prefs{NotepadURLs: false},

wgengine/router/router.go

@@ -43,4 +43,5 @@ type Settings struct {
 	DNSDomains   []string
 	Routes       []netaddr.IPPrefix // routes to point into the Tailscale interface
 	SubnetRoutes []netaddr.IPPrefix // subnets being advertised to other Tailscale nodes
+	NoSNAT       bool               // don't SNAT traffic to local subnets
 }

wgengine/router/router_linux.go

@@ -54,6 +54,7 @@ type linuxRouter struct {
 	addrs        map[netaddr.IPPrefix]bool
 	routes       map[netaddr.IPPrefix]bool
 	subnetRoutes map[netaddr.IPPrefix]bool
+	noSNAT       bool
 
 	ipt4 *iptables.IPTables
 }
@@ -72,6 +73,7 @@ func newUserspaceRouter(logf logger.Logf, _ *device.Device, tunDev tun.Device) (
 	return &linuxRouter{
 		logf:    logf,
 		tunname: tunname,
+		noSNAT:  true,
 		ipt4:    ipt4,
 	}, nil
 }
@@ -193,9 +195,23 @@ func (r *linuxRouter) Set(rs Settings) error {
 		errq = err
 	}
 
+	switch {
+	case rs.NoSNAT == r.noSNAT:
+		// state already correct, nothing to do.
+	case rs.NoSNAT:
+		if err := r.delSNATRule(); err != nil && errq == nil {
+			errq = err
+		}
+	default:
+		if err := r.addSNATRule(); err != nil && errq == nil {
+			errq = err
+		}
+	}
+
 	r.addrs = newAddrs
 	r.routes = newRoutes
 	r.subnetRoutes = newSubnetRoutes
+	r.noSNAT = rs.NoSNAT
 
 	// TODO: this:
 	if false {
@@ -558,13 +574,12 @@ func (r *linuxRouter) addBaseNetfilter4() error {
 		return fmt.Errorf("adding %v in filter/ts-input: %v", args, err)
 	}
 
-	// Forward and masquerade packets that have the Tailscale subnet
+	// Forward and mark packets that have the Tailscale subnet route
-	// route bit set. The bit gets set by rules inserted into
+	// bit set. The bit gets set by rules inserted into filter/FORWARD
-	// filter/FORWARD later on. We use packet marks here so both
+	// later on. We use packet marks here so both filter/FORWARD and
-	// filter/FORWARD and nat/POSTROUTING can match on these packets
+	// nat/POSTROUTING can match on these packets of interest.
-	// of interest.
 	//
-	// In particular, we only want to apply masquerading in
+	// In particular, we only want to apply SNAT rules in
 	// nat/POSTROUTING to packets that originated from the Tailscale
 	// interface, but we can't match on the inbound interface in
 	// POSTROUTING. So instead, we match on the inbound interface and
@@ -579,11 +594,26 @@ func (r *linuxRouter) addBaseNetfilter4() error {
 	if err := r.ipt4.Append("filter", "ts-forward", args...); err != nil {
 		return fmt.Errorf("adding %v in filter/ts-forward: %v", args, err)
 	}
-	// TODO(danderson): this should be optional.
+
-	args = []string{"-m", "mark", "--mark", tailscaleSubnetRouteMark, "-j", "MASQUERADE"}
+	return nil
+}
+
+// addSNATRule adds a netfilter rule to SNAT traffic destined for
+// local subnets.
+func (r *linuxRouter) addSNATRule() error {
+	args := []string{"-m", "mark", "--mark", tailscaleSubnetRouteMark, "-j", "MASQUERADE"}
 	if err := r.ipt4.Append("nat", "ts-postrouting", args...); err != nil {
 		return fmt.Errorf("adding %v in nat/ts-postrouting: %v", args, err)
 	}
+	return nil
+}
 
+// delSNATRule removes the netfilter rule to SNAT traffic destined for
+// local subnets. Fails if the rule does not exist.
+func (r *linuxRouter) delSNATRule() error {
+	args := []string{"-m", "mark", "--mark", tailscaleSubnetRouteMark, "-j", "MASQUERADE"}
+	if err := r.ipt4.Delete("nat", "ts-postrouting", args...); err != nil {
+		return fmt.Errorf("deleting %v in nat/ts-postrouting: %v", args, err)
+	}
 	return nil
 }

wgengine/userspace.go

@@ -242,6 +242,8 @@ func newUserspaceEngineAdvanced(logf logger.Logf, tundev tun.Device, routerGen R
 		e.wgdev.Close()
 		return nil, err
 	}
+	// TODO(danderson): we should delete this. It's pointless to apply
+	// a no-op settings here.
 	if err := e.router.Set(router.Settings{}); err != nil {
 		e.wgdev.Close()
 		return nil, err
@@ -325,14 +327,14 @@ func (e *userspaceEngine) pinger(peerKey wgcfg.Key, ips []wgcfg.IP) {
 	}
 }
 
-func configSignature(cfg *wgcfg.Config, dnsDomains []string, localRoutes []wgcfg.CIDR) (string, error) {
+func configSignature(cfg *wgcfg.Config, dnsDomains []string, localRoutes []wgcfg.CIDR, noSNAT bool) (string, error) {
 	// TODO(apenwarr): get rid of uapi stuff for in-process comms
 	uapi, err := cfg.ToUAPI()
 	if err != nil {
 		return "", err
 	}
 
-	return fmt.Sprintf("%s %v %v", uapi, dnsDomains, localRoutes), nil
+	return fmt.Sprintf("%s %v %v %v", uapi, dnsDomains, localRoutes, noSNAT), nil
 }
 
 // TODO(apenwarr): dnsDomains really ought to be in wgcfg.Config.
@@ -344,7 +346,7 @@ func configSignature(cfg *wgcfg.Config, dnsDomains []string, localRoutes []wgcfg
 // hand. Feels like we either need a wgengine.Config type, or make
 // router and wgengine siblings of each other that interact via glue
 // in ipn.
-func (e *userspaceEngine) Reconfig(cfg *wgcfg.Config, dnsDomains []string, localRoutes []wgcfg.CIDR) error {
+func (e *userspaceEngine) Reconfig(cfg *wgcfg.Config, dnsDomains []string, localRoutes []wgcfg.CIDR, noSNAT bool) error {
 	e.wgLock.Lock()
 	defer e.wgLock.Unlock()
 
@@ -357,7 +359,7 @@ func (e *userspaceEngine) Reconfig(cfg *wgcfg.Config, dnsDomains []string, local
 	}
 	e.mu.Unlock()
 
-	rc, err := configSignature(cfg, dnsDomains, localRoutes)
+	rc, err := configSignature(cfg, dnsDomains, localRoutes, noSNAT)
 	if err != nil {
 		return err
 	}
@@ -403,6 +405,7 @@ func (e *userspaceEngine) Reconfig(cfg *wgcfg.Config, dnsDomains []string, local
 		DNS:          wgIPToNetaddr(cfg.DNS),
 		DNSDomains:   dnsDomains,
 		SubnetRoutes: wgCIDRToNetaddr(localRoutes),
+		NoSNAT:       noSNAT,
 	}
 	for _, peer := range cfg.Peers {
 		rs.Routes = append(rs.Routes, wgCIDRToNetaddr(peer.AllowedIPs)...)

wgengine/watchdog.go

@@ -61,8 +61,8 @@ func (e *watchdogEngine) watchdog(name string, fn func()) {
 	})
 }
 
-func (e *watchdogEngine) Reconfig(cfg *wgcfg.Config, dnsDomains []string, localRoutes []wgcfg.CIDR) error {
+func (e *watchdogEngine) Reconfig(cfg *wgcfg.Config, dnsDomains []string, localRoutes []wgcfg.CIDR, noSNAT bool) error {
-	return e.watchdogErr("Reconfig", func() error { return e.wrap.Reconfig(cfg, dnsDomains, localRoutes) })
+	return e.watchdogErr("Reconfig", func() error { return e.wrap.Reconfig(cfg, dnsDomains, localRoutes, noSNAT) })
 }
 func (e *watchdogEngine) GetFilter() *filter.Filter {
 	var x *filter.Filter

wgengine/wgengine.go

@@ -59,7 +59,7 @@ type Engine interface {
 	// sends an updated network map.
 	//
 	// The returned error is ErrNoChanges if no changes were made.
-	Reconfig(cfg *wgcfg.Config, dnsDomains []string, localSubnets []wgcfg.CIDR) error
+	Reconfig(cfg *wgcfg.Config, dnsDomains []string, localSubnets []wgcfg.CIDR, noSNAT bool) error
 
 	// GetFilter returns the current packet filter, if any.
 	GetFilter() *filter.Filter