net/dns: expose a function for recompiling the DNS configuration (#15346)

updates tailscale/corp#27145

We require a means to trigger a recompilation of the DNS configuration
to pick up new nameservers for platforms where we blend the interface
nameservers from the OS into our DNS config.

Notably, on Darwin, the only API we have at our disposal will, in rare instances,
return a transient error when querying the interface nameservers on a link change if
they have not been set when we get the AF_ROUTE messages for the link
update.

There's a corresponding change in corp for Darwin clients, to track
the interface namservers during NEPathMonitor events, and call this
when the nameservers change.

This will also fix the slightly more obscure bug of changing nameservers
 while tailscaled is running.  That change can now be reflected in
magicDNS without having to stop the client.

Signed-off-by: Jonathan Nobels <jonathan@tailscale.com>

net/dns/manager.go

@@ -35,6 +35,9 @@ import (
 
 var (
 	errFullQueue = errors.New("request queue full")
+	// ErrNoDNSConfig is returned by RecompileDNSConfig when the Manager
+	// has no existing DNS configuration.
+	ErrNoDNSConfig = errors.New("no DNS configuration")
 )
 
 // maxActiveQueries returns the maximal number of DNS requests that can
@@ -91,21 +94,18 @@ func NewManager(logf logger.Logf, oscfg OSConfigurator, health *health.Tracker,
 	}
 
 	// Rate limit our attempts to correct our DNS configuration.
+	// This is done on incoming queries, we don't want to spam it.
 	limiter := rate.NewLimiter(1.0/5.0, 1)
 
 	// This will recompile the DNS config, which in turn will requery the system
 	// DNS settings. The recovery func should triggered only when we are missing
 	// upstream nameservers and require them to forward a query.
 	m.resolver.SetMissingUpstreamRecovery(func() {
-		m.mu.Lock()
-		defer m.mu.Unlock()
-		if m.config == nil {
-			return
-		}
-
 		if limiter.Allow() {
-			m.logf("DNS resolution failed due to missing upstream nameservers.  Recompiling DNS configuration.")
-			m.setLocked(*m.config)
+			m.logf("resolution failed due to missing upstream nameservers.  Recompiling DNS configuration.")
+			if err := m.RecompileDNSConfig(); err != nil {
+				m.logf("config recompilation failed: %v", err)
+			}
 		}
 	})
 
@@ -117,6 +117,26 @@ func NewManager(logf logger.Logf, oscfg OSConfigurator, health *health.Tracker,
 // Resolver returns the Manager's DNS Resolver.
 func (m *Manager) Resolver() *resolver.Resolver { return m.resolver }
 
+// RecompileDNSConfig sets the DNS config to the current value, which has
+// the side effect of re-querying the OS's interface nameservers.  This should be used
+// on platforms where the interface nameservers can change.  Darwin, for example,
+// where the nameservers aren't always available when we process a major interface
+// change event, or platforms where the nameservers may change while tunnel is up.
+//
+// This should be called if it is determined that [OSConfigurator.GetBaseConfig] may
+// give a better or different result than when [Manager.Set] was last called.  The
+// logic for making that determination is up to the caller.
+//
+// It returns [ErrNoDNSConfig] if the [Manager] has no existing DNS configuration.
+func (m *Manager) RecompileDNSConfig() error {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+	if m.config == nil {
+		return ErrNoDNSConfig
+	}
+	return m.setLocked(*m.config)
+}
+
 func (m *Manager) Set(cfg Config) error {
 	m.mu.Lock()
 	defer m.mu.Unlock()