Support disabled resolved stub server mode (#1493)

In the case of disabled stub listeren the list of name servers is unordered. The solution is to configure the resolv.conf file directly instead of dbus API.
Because third-party services also can manipulate the DNS settings the agent watch the resolv.conf file and keep it up to date.

- apply file type DNS manager if in the name server list does not exist the 127.0.0.53 address
- watching the resolv.conf file with inotify service and overwrite all the time if the configuration has changed and it invalid
- fix resolv.conf generation algorithm

client/internal/dns/host_linux.go

@@ -11,10 +11,6 @@ import (
 	log "github.com/sirupsen/logrus"
 )
 
-const (
-	defaultResolvConfPath = "/etc/resolv.conf"
-)
-
 const (
 	netbirdManager osManagerType = iota
 	fileManager
@@ -85,7 +81,11 @@ func getOSDNSManagerType() (osManagerType, error) {
 			return networkManager, nil
 		}
 		if strings.Contains(text, "systemd-resolved") && isDbusListenerRunning(systemdResolvedDest, systemdDbusObjectNode) {
-			return systemdManager, nil
+			if checkStub() {
+				return systemdManager, nil
+			} else {
+				return fileManager, nil
+			}
 		}
 		if strings.Contains(text, "resolvconf") {
 			if isDbusListenerRunning(systemdResolvedDest, systemdDbusObjectNode) {
@@ -103,3 +103,20 @@ func getOSDNSManagerType() (osManagerType, error) {
 	}
 	return fileManager, nil
 }
+
+// checkStub checks if the stub resolver is disabled in systemd-resolved. If it is disabled, we fall back to file manager.
+func checkStub() bool {
+	rConf, err := parseDefaultResolvConf()
+	if err != nil {
+		log.Warnf("failed to parse resolv conf: %s", err)
+		return true
+	}
+
+	for _, ns := range rConf.nameServers {
+		if ns == "127.0.0.53" {
+			return true
+		}
+	}
+
+	return false
+}