Validate upstream reachability on first DNS configuration (#1489)

* Test upstream DNS for availability as soon as they are received from management

* Use root zone for DNS tests

client/internal/dns/server.go

@@ -32,6 +32,7 @@ type Server interface {
 	UpdateDNSServer(serial uint64, update nbdns.Config) error
 	OnUpdatedHostDNSServer(strings []string)
 	SearchDomains() []string
+	ProbeAvailability()
 }
 
 type registeredHandlerMap map[string]handlerWithStop
@@ -63,6 +64,7 @@ type DefaultServer struct {
 type handlerWithStop interface {
 	dns.Handler
 	stop()
+	probeAvailability()
 }
 
 type muxUpdate struct {
@@ -248,6 +250,14 @@ func (s *DefaultServer) SearchDomains() []string {
 	return searchDomains
 }
 
+// ProbeAvailability tests each upstream group's servers for availability
+// and deactivates the group if no server responds
+func (s *DefaultServer) ProbeAvailability() {
+	for _, mux := range s.dnsMuxMap {
+		mux.probeAvailability()
+	}
+}
+
 func (s *DefaultServer) applyConfiguration(update nbdns.Config) error {
 	// is the service should be Disabled, we stop the listener or fake resolver
 	// and proceed with a regular update to clean up the handlers and records
@@ -378,6 +388,7 @@ func (s *DefaultServer) buildUpstreamHandlerUpdate(nameServerGroups []*nbdns.Nam
 			})
 		}
 	}
+
 	return muxUpdates, nil
 }
 
@@ -488,13 +499,13 @@ func (s *DefaultServer) upstreamCallbacks(
 		}
 
 		l := log.WithField("nameservers", nsGroup.NameServers)
-		l.Debug("reactivate temporary Disabled nameserver group")
+		l.Debug("reactivate temporary disabled nameserver group")
 
 		if nsGroup.Primary {
 			s.currentConfig.RouteAll = true
 		}
 		if err := s.hostManager.applyDNSConfig(s.currentConfig); err != nil {
-			l.WithError(err).Error("reactivate temporary Disabled nameserver group, DNS update apply")
+			l.WithError(err).Error("reactivate temporary disabled nameserver group, DNS update apply")
 		}
 	}
 	return