net/dns: always attempt to read the OS config on macOS/iOS

Also reconfigure DNS on iOS/macOS on link changes.

Signed-off-by: Maisem Ali <maisem@tailscale.com>

net/dns/manager.go

@@ -265,22 +265,40 @@ func (m *Manager) compileConfig(cfg Config) (rcfg resolver.Config, ocfg OSConfig
 	rcfg.Routes = routes
 	ocfg.Nameservers = []netip.Addr{cfg.serviceIP()}
 
-	if m.os.SupportsSplitDNS() {
-		ocfg.MatchDomains = cfg.matchDomains()
-	} else {
+	var baseCfg *OSConfig // base config; non-nil if/when known
+
+	// Even though Apple devices can do split DNS, they don't provide a way to
+	// selectively answer ExtraRecords, and ignore other DNS traffic. As a
+	// workaround, we read the existing default resolver configuration and use
+	// that as the forwarder for all DNS traffic that quad-100 doesn't handle.
+	const isApple = runtime.GOOS == "darwin" || runtime.GOOS == "ios"
+
+	if isApple || !m.os.SupportsSplitDNS() {
 		// If the OS can't do native split-dns, read out the underlying
 		// resolver config and blend it into our config.
-		bcfg, err := m.os.GetBaseConfig()
-		if err != nil {
+		cfg, err := m.os.GetBaseConfig()
+		if err == nil {
+			baseCfg = &cfg
+		} else if isApple && err == ErrGetBaseConfigNotSupported {
+			// This is currently (2022-10-13) expected on certain iOS and macOS
+			// builds.
+		} else {
 			health.SetDNSOSHealth(err)
 			return resolver.Config{}, OSConfig{}, err
 		}
+	}
+
+	if baseCfg == nil || isApple && len(baseCfg.Nameservers) == 0 {
+		// If there was no base config, or if we're on Apple and the base
+		// config is empty, then we need to fallback to SplitDNS mode.
+		ocfg.MatchDomains = cfg.matchDomains()
+	} else {
 		var defaultRoutes []*dnstype.Resolver
-		for _, ip := range bcfg.Nameservers {
+		for _, ip := range baseCfg.Nameservers {
 			defaultRoutes = append(defaultRoutes, &dnstype.Resolver{Addr: ip.String()})
 		}
 		rcfg.Routes["."] = defaultRoutes
-		ocfg.SearchDomains = append(ocfg.SearchDomains, bcfg.SearchDomains...)
+		ocfg.SearchDomains = append(ocfg.SearchDomains, baseCfg.SearchDomains...)
 	}
 
 	return rcfg, ocfg, nil

wgengine/userspace.go

@@ -1187,15 +1187,18 @@ func (e *userspaceEngine) linkChange(changed bool, cur *interfaces.State) {
 	// suspend/resume or whenever NetworkManager is started, it
 	// nukes all systemd-resolved configs. So reapply our DNS
 	// config on major link change.
-	if (runtime.GOOS == "linux" || runtime.GOOS == "android") && changed {
-		e.wgLock.Lock()
-		dnsCfg := e.lastDNSConfig
-		e.wgLock.Unlock()
-		if dnsCfg != nil {
-			if err := e.dns.Set(*dnsCfg); err != nil {
-				e.logf("wgengine: error setting DNS config after major link change: %v", err)
-			} else {
-				e.logf("wgengine: set DNS config again after major link change")
+	if changed {
+		switch runtime.GOOS {
+		case "linux", "android", "ios", "darwin":
+			e.wgLock.Lock()
+			dnsCfg := e.lastDNSConfig
+			e.wgLock.Unlock()
+			if dnsCfg != nil {
+				if err := e.dns.Set(*dnsCfg); err != nil {
+					e.logf("wgengine: error setting DNS config after major link change: %v", err)
+				} else {
+					e.logf("wgengine: set DNS config again after major link change")
+				}
 			}
 		}
 	}