ipn/ipnlocal, net/tsdial: plumb routes into tsdial and use them in UserDial

We'd like to use tsdial.Dialer.UserDial instead of SystemDial for DNS over TCP.
This is primarily necessary to properly dial internal DNS servers accessible
over Tailscale and subnet routes. However, to avoid issues when switching
between Wi-Fi and cellular, we need to ensure that we don't retain connections
to any external addresses on the old interface. Therefore, we need to determine
which dialer to use internally based on the configured routes.

This plumbs routes and localRoutes from router.Config to tsdial.Dialer,
and updates UserDial to use either the peer dialer or the system dialer,
depending on the network address and the configured routes.

Updates tailscale/corp#18725
Fixes #4529

Signed-off-by: Nick Khyl <nickk@tailscale.com>

cmd/tailscaled/depaware.txt

@@ -89,7 +89,7 @@ tailscale.com/cmd/tailscaled dependencies: (generated by github.com/tailscale/de
   LW 💣 github.com/digitalocean/go-smbios/smbios                     from tailscale.com/posture
      💣 github.com/djherbis/times                                    from tailscale.com/drive/driveimpl
         github.com/fxamacker/cbor/v2                                 from tailscale.com/tka
-        github.com/gaissmai/bart                                     from tailscale.com/net/tstun
+        github.com/gaissmai/bart                                     from tailscale.com/net/tstun+
         github.com/go-json-experiment/json/internal                  from github.com/go-json-experiment/json/internal/jsonflags+
         github.com/go-json-experiment/json/internal/jsonflags        from github.com/go-json-experiment/json/internal/jsonopts+
         github.com/go-json-experiment/json/internal/jsonopts         from github.com/go-json-experiment/json/jsontext

ipn/ipnlocal/local.go

@@ -3590,6 +3590,7 @@ func (b *LocalBackend) authReconfig() {
 	nm := b.netMap
 	hasPAC := b.prevIfState.HasPAC()
 	disableSubnetsIfPAC := nm.HasCap(tailcfg.NodeAttrDisableSubnetsIfPAC)
+	userDialUseRoutes := nm.HasCap(tailcfg.NodeAttrUserDialUseRoutes)
 	dohURL, dohURLOK := exitNodeCanProxyDNS(nm, b.peers, prefs.ExitNodeID())
 	dcfg := dnsConfigForNetmap(nm, b.peers, prefs, b.logf, version.OS())
 	// If the current node is an app connector, ensure the app connector machine is started
@@ -3647,6 +3648,12 @@ func (b *LocalBackend) authReconfig() {
 	}
 	b.logf("[v1] authReconfig: ra=%v dns=%v 0x%02x: %v", prefs.RouteAll(), prefs.CorpDNS(), flags, err)
 
+	if userDialUseRoutes {
+		b.dialer.SetRoutes(rcfg.Routes, rcfg.LocalRoutes)
+	} else {
+		b.dialer.SetRoutes(nil, nil)
+	}
+
 	b.initPeerAPIListener()
 }
 

net/tsdial/tsdial.go

@@ -14,9 +14,11 @@
 	"runtime"
 	"strings"
 	"sync"
+	"sync/atomic"
 	"syscall"
 	"time"
 
+	"github.com/gaissmai/bart"
 	"tailscale.com/net/dnscache"
 	"tailscale.com/net/netknob"
 	"tailscale.com/net/netmon"
@@ -66,6 +68,8 @@ type Dialer struct {
 	netnsDialerOnce sync.Once
 	netnsDialer     netns.Dialer
 
+	routes atomic.Pointer[bart.Table[bool]] // or nil if UserDial should not use routes. `true` indicates routes that point into the Tailscale interface
+
 	mu               sync.Mutex
 	closed           bool
 	dns              dnsMap
@@ -129,6 +133,23 @@ func (d *Dialer) SetExitDNSDoH(doh string) {
 	}
 }
 
+// SetRoutes configures the dialer to dial the specified routes via Tailscale,
+// and the specified localRoutes using the default interface.
+func (d *Dialer) SetRoutes(routes, localRoutes []netip.Prefix) {
+	var rt *bart.Table[bool]
+	if len(routes) > 0 || len(localRoutes) > 0 {
+		rt = &bart.Table[bool]{}
+		for _, r := range routes {
+			rt.Insert(r, true)
+		}
+		for _, r := range localRoutes {
+			rt.Insert(r, false)
+		}
+	}
+
+	d.routes.Store(rt)
+}
+
 func (d *Dialer) Close() error {
 	d.mu.Lock()
 	defer d.mu.Unlock()
@@ -387,6 +408,15 @@ func (d *Dialer) UserDial(ctx context.Context, network, addr string) (net.Conn,
 		}
 		return d.NetstackDialTCP(ctx, ipp)
 	}
+
+	if routes := d.routes.Load(); routes != nil {
+		if isTailscaleRoute, _ := routes.Get(ipp.Addr()); isTailscaleRoute {
+			return d.getPeerDialer().DialContext(ctx, network, ipp.String())
+		}
+
+		return d.SystemDial(ctx, network, ipp.String())
+	}
+
 	// Workaround for macOS for now: dial Tailscale IPs with peer dialer.
 	// TODO(bradfitz): fix dialing subnet routers, public IPs via exit nodes,
 	// etc. This is a temporary partial for macOS. We need to plumb ART tables &
@@ -424,7 +454,7 @@ func (d *Dialer) dialPeerAPI(ctx context.Context, network, addr string) (net.Con
 }
 
 // getPeerDialer returns the *net.Dialer to use to dial peers (e.g. for peerapi,
-// or "tailscale nc")
+// "tailscale nc", or querying internal DNS servers over Tailscale)
 //
 // This is not used in netstack mode.
 //

tailcfg/tailcfg.go

@@ -132,7 +132,8 @@
 //   - 89: 2024-03-23: Client no longer respects deleted PeerChange.Capabilities (use CapMap)
 //   - 90: 2024-04-03: Client understands PeerCapabilityTaildrive.
 //   - 91: 2024-04-24: Client understands PeerCapabilityTaildriveSharer.
-const CurrentCapabilityVersion CapabilityVersion = 91
+//   - 92: 2024-05-06: Client understands NodeAttrUserDialUseRoutes.
+const CurrentCapabilityVersion CapabilityVersion = 92
 
 type StableID string
 
@@ -2259,6 +2260,10 @@ type Oauth2Token struct {
 
 	// NodeAttrSuggestExitNodeUI allows the currently suggested exit node to appear in the client GUI.
 	NodeAttrSuggestExitNodeUI NodeCapability = "suggest-exit-node-ui"
+
+	// NodeAttrUserDialUseRoutes makes UserDial use either the peer dialer or the system dialer,
+	// depending on the destination address and the configured routes.
+	NodeAttrUserDialUseRoutes NodeCapability = "user-dial-routes"
 )
 
 // SetDNSRequest is a request to add a DNS record.