all: leaking queries to OS resolver instead of SRVFAIL

So it would work in more general case than just captive portal network,
which ctrld have supported recently.

Uses who may want no leaking behavior can use a config to turn off this
feature.

cmd/cli/prog.go

@@ -107,9 +107,9 @@ type prog struct {
 	loopMu sync.Mutex
 	loop   map[string]bool
 
-	captivePortalMu          sync.Mutex
-	captivePortalCheckWasRun bool
-	captivePortalDetected    atomic.Bool
+	leakingQueryMu     sync.Mutex
+	leakingQueryWasRun bool
+	leakingQuery       atomic.Bool
 
 	started       chan struct{}
 	onStartedDone chan struct{}
@@ -685,7 +685,7 @@ func (p *prog) dnsWatchdog(iface *net.Interface, nameservers []string, allIfaces
 			mainLog.Load().Debug().Msg("stop dns watchdog")
 			return
 		case <-ticker.C:
-			if p.captivePortalDetected.Load() {
+			if p.leakingQuery.Load() {
 				return
 			}
 			if dnsChanged(iface, ns) {
@@ -742,6 +742,18 @@ func (p *prog) resetDNS() {
 	}
 }
 
+// leakOnUpstreamFailure reports whether ctrld should leak query to OS resolver when failed to connect all upstreams.
+func (p *prog) leakOnUpstreamFailure() bool {
+	if ptr := p.cfg.Service.LeakOnUpstreamFailure; ptr != nil {
+		return *ptr
+	}
+	// Default is false on routers, since this leaking is only useful for devices that move between networks.
+	if router.Name() != "" {
+		return false
+	}
+	return true
+}
+
 func randomLocalIP() string {
 	n := rand.Intn(254-2) + 2
 	return fmt.Sprintf("127.0.0.%d", n)