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/file_parser_linux.go

@@ -0,0 +1,105 @@
+//go:build !android
+
+package dns
+
+import (
+	"fmt"
+	"os"
+	"strings"
+
+	log "github.com/sirupsen/logrus"
+)
+
+const (
+	defaultResolvConfPath = "/etc/resolv.conf"
+)
+
+type resolvConf struct {
+	nameServers   []string
+	searchDomains []string
+	others        []string
+}
+
+func (r *resolvConf) String() string {
+	return fmt.Sprintf("search domains: %v, name servers: %v, others: %s", r.searchDomains, r.nameServers, r.others)
+}
+
+func parseDefaultResolvConf() (*resolvConf, error) {
+	return parseResolvConfFile(defaultResolvConfPath)
+}
+
+func parseBackupResolvConf() (*resolvConf, error) {
+	return parseResolvConfFile(fileDefaultResolvConfBackupLocation)
+}
+
+func parseResolvConfFile(resolvConfFile string) (*resolvConf, error) {
+	file, err := os.Open(resolvConfFile)
+	if err != nil {
+		return nil, fmt.Errorf("failed to open %s file: %w", resolvConfFile, err)
+	}
+	defer func() {
+		if err := file.Close(); err != nil {
+			log.Errorf("failed closing %s: %s", resolvConfFile, err)
+		}
+	}()
+
+	cur, err := os.ReadFile(resolvConfFile)
+	if err != nil {
+		return nil, fmt.Errorf("failed to read %s file: %w", resolvConfFile, err)
+	}
+
+	if len(cur) == 0 {
+		return nil, fmt.Errorf("file is empty")
+	}
+
+	rconf := &resolvConf{
+		searchDomains: make([]string, 0),
+		nameServers:   make([]string, 0),
+		others:        make([]string, 0),
+	}
+
+	for _, line := range strings.Split(string(cur), "\n") {
+		line = strings.TrimSpace(line)
+
+		if strings.HasPrefix(line, "#") {
+			continue
+		}
+
+		if strings.HasPrefix(line, "domain") {
+			continue
+		}
+
+		if strings.HasPrefix(line, "options") && strings.Contains(line, "rotate") {
+			line = strings.ReplaceAll(line, "rotate", "")
+			splitLines := strings.Fields(line)
+			if len(splitLines) == 1 {
+				continue
+			}
+			line = strings.Join(splitLines, " ")
+		}
+
+		if strings.HasPrefix(line, "search") {
+			splitLines := strings.Fields(line)
+			if len(splitLines) < 2 {
+				continue
+			}
+
+			rconf.searchDomains = splitLines[1:]
+			continue
+		}
+
+		if strings.HasPrefix(line, "nameserver") {
+			splitLines := strings.Fields(line)
+			if len(splitLines) != 2 {
+				continue
+			}
+			rconf.nameServers = append(rconf.nameServers, splitLines[1])
+			continue
+		}
+
+		if line != "" {
+			rconf.others = append(rconf.others, line)
+		}
+	}
+	return rconf, nil
+}