ipn/ipnlocal,net/dns/resolver: use the user dialer and routes for DNS forwarding by default, except on iOS and Android

In this PR, we make the "user-dial-routes" behavior default on all platforms except for iOS and Android.
It can be disabled by setting the TS_DNS_FORWARD_USE_ROUTES envknob to 0 or false.

Updates #12027
Updates #13837

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

cmd/k8s-operator/depaware.txt

@@ -840,7 +840,7 @@ tailscale.com/cmd/k8s-operator dependencies: (generated by github.com/tailscale/
         tailscale.com/net/dns/publicdns                              from tailscale.com/net/dns+
         tailscale.com/net/dns/recursive                              from tailscale.com/net/dnsfallback
         tailscale.com/net/dns/resolvconffile                         from tailscale.com/cmd/k8s-operator+
-        tailscale.com/net/dns/resolver                               from tailscale.com/net/dns
+        tailscale.com/net/dns/resolver                               from tailscale.com/net/dns+
         tailscale.com/net/dnscache                                   from tailscale.com/control/controlclient+
         tailscale.com/net/dnsfallback                                from tailscale.com/control/controlclient+
         tailscale.com/net/flowtrack                                  from tailscale.com/net/packet+

cmd/tailscaled/depaware.txt

@@ -318,7 +318,7 @@ tailscale.com/cmd/tailscaled dependencies: (generated by github.com/tailscale/de
         tailscale.com/net/dns/publicdns                              from tailscale.com/net/dns+
         tailscale.com/net/dns/recursive                              from tailscale.com/net/dnsfallback
         tailscale.com/net/dns/resolvconffile                         from tailscale.com/net/dns+
-        tailscale.com/net/dns/resolver                               from tailscale.com/net/dns
+        tailscale.com/net/dns/resolver                               from tailscale.com/net/dns+
         tailscale.com/net/dnscache                                   from tailscale.com/control/controlclient+
         tailscale.com/net/dnsfallback                                from tailscale.com/cmd/tailscaled+
         tailscale.com/net/flowtrack                                  from tailscale.com/net/packet+

ipn/ipnlocal/local.go

@@ -64,6 +64,7 @@ import (
 	"tailscale.com/logpolicy"
 	"tailscale.com/net/captivedetection"
 	"tailscale.com/net/dns"
+	"tailscale.com/net/dns/resolver"
 	"tailscale.com/net/dnscache"
 	"tailscale.com/net/dnsfallback"
 	"tailscale.com/net/ipset"
@@ -4908,7 +4909,6 @@ func (b *LocalBackend) authReconfig() {
 	nm := cn.NetMap()
 	hasPAC := b.prevIfState.HasPAC()
 	disableSubnetsIfPAC := cn.SelfHasCap(tailcfg.NodeAttrDisableSubnetsIfPAC)
-	userDialUseRoutes := cn.SelfHasCap(tailcfg.NodeAttrUserDialUseRoutes)
 	dohURL, dohURLOK := cn.exitNodeCanProxyDNS(prefs.ExitNodeID())
 	dcfg := cn.dnsConfigForNetmap(prefs, b.keyExpired, b.logf, version.OS())
 	// If the current node is an app connector, ensure the app connector machine is started
@@ -4969,7 +4969,7 @@ func (b *LocalBackend) authReconfig() {
 	}
 	b.logf("[v1] authReconfig: ra=%v dns=%v 0x%02x: %v", prefs.RouteAll(), prefs.CorpDNS(), flags, err)
 
-	if userDialUseRoutes {
+	if resolver.ShouldUseRoutes(b.ControlKnobs()) {
 		b.dialer.SetRoutes(rcfg.Routes, rcfg.LocalRoutes)
 	} else {
 		b.dialer.SetRoutes(nil, nil)

net/dns/resolver/forwarder.go

@@ -17,6 +17,7 @@ import (
 	"net/http"
 	"net/netip"
 	"net/url"
+	"runtime"
 	"sort"
 	"strings"
 	"sync"
@@ -740,18 +741,37 @@ func (f *forwarder) sendUDP(ctx context.Context, fq *forwardQuery, rr resolverAn
 	return out, nil
 }
 
+var optDNSForwardUseRoutes = envknob.RegisterOptBool("TS_DNS_FORWARD_USE_ROUTES")
+
+// ShouldUseRoutes reports true if the DNS resolver should use the peer or system dialer
+// for forwarding DNS queries to upstream nameservers via TCP, based on the destination
+// address and configured routes. Currently, this requires maintaining a [bart.Table],
+// resulting in a slightly higher memory usage.
+//
+// It reports false if the system dialer should always be used, regardless of the
+// destination address.
+//
+// TODO(nickkhyl): Update [tsdial.Dialer] to reuse the bart.Table we create in net/tstun.Wrapper
+// to avoid having two bart tables in memory, especially on iOS. Once that's done,
+// we can get rid of the nodeAttr/control knob and always use UserDial for DNS.
+//
+// See https://github.com/tailscale/tailscale/issues/12027.
+func ShouldUseRoutes(knobs *controlknobs.Knobs) bool {
+	switch runtime.GOOS {
+	case "android", "ios":
+		// On mobile platforms with lower memory limits (e.g., 50MB on iOS),
+		// this behavior is still gated by the "user-dial-routes" nodeAttr.
+		return knobs != nil && knobs.UserDialUseRoutes.Load()
+	default:
+		// On all other platforms, it is the default behavior,
+		// but it can be overridden with the "TS_DNS_FORWARD_USE_ROUTES" env var.
+		doNotUseRoutes := optDNSForwardUseRoutes().EqualBool(false)
+		return !doNotUseRoutes
+	}
+}
+
 func (f *forwarder) getDialerType() netx.DialFunc {
-	if f.controlKnobs != nil && f.controlKnobs.UserDialUseRoutes.Load() {
+	if ShouldUseRoutes(f.controlKnobs) {
-		// It is safe to use UserDial as it dials external servers without going through Tailscale
-		// and closes connections on interface change in the same way as SystemDial does,
-		// thus preventing DNS resolution issues when switching between WiFi and cellular,
-		// but can also dial an internal DNS server on the Tailnet or via a subnet router.
-		//
-		// TODO(nickkhyl): Update tsdial.Dialer to reuse the bart.Table we create in net/tstun.Wrapper
-		// to avoid having two bart tables in memory, especially on iOS. Once that's done,
-		// we can get rid of the nodeAttr/control knob and always use UserDial for DNS.
-		//
-		// See https://github.com/tailscale/tailscale/issues/12027.
 		return f.dialer.UserDial
 	}
 	return f.dialer.SystemDial

tsnet/depaware.txt

@@ -271,7 +271,7 @@ tailscale.com/tsnet dependencies: (generated by github.com/tailscale/depaware)
         tailscale.com/net/dns/publicdns                              from tailscale.com/net/dns+
         tailscale.com/net/dns/recursive                              from tailscale.com/net/dnsfallback
         tailscale.com/net/dns/resolvconffile                         from tailscale.com/net/dns+
-        tailscale.com/net/dns/resolver                               from tailscale.com/net/dns
+        tailscale.com/net/dns/resolver                               from tailscale.com/net/dns+
         tailscale.com/net/dnscache                                   from tailscale.com/control/controlclient+
         tailscale.com/net/dnsfallback                                from tailscale.com/control/controlclient+
         tailscale.com/net/flowtrack                                  from tailscale.com/net/packet+