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_linux.go

@@ -3,7 +3,6 @@
 package dns
 
 import (
-	"bufio"
 	"bytes"
 	"fmt"
 	"os"
@@ -24,11 +23,15 @@ const (
 )
 
 type fileConfigurator struct {
+	repair *repair
+
 	originalPerms os.FileMode
 }
 
 func newFileConfigurator() (hostManager, error) {
-	return &fileConfigurator{}, nil
+	fc := &fileConfigurator{}
+	fc.repair = newRepair(defaultResolvConfPath, fc.updateConfig)
+	return fc, nil
 }
 
 func (f *fileConfigurator) supportCustomPort() bool {
@@ -59,22 +62,35 @@ func (f *fileConfigurator) applyDNSConfig(config HostDNSConfig) error {
 		}
 	}
 
-	searchDomainList := searchDomains(config)
+	nbSearchDomains := searchDomains(config)
+	nbNameserverIP := config.ServerIP
 
-	originalSearchDomains, nameServers, others, err := originalDNSConfigs(fileDefaultResolvConfBackupLocation)
+	resolvConf, err := parseBackupResolvConf()
 	if err != nil {
 		log.Error(err)
 	}
 
-	searchDomainList = mergeSearchDomains(searchDomainList, originalSearchDomains)
+	f.repair.stopWatchFileChanges()
+
+	err = f.updateConfig(nbSearchDomains, nbNameserverIP, resolvConf)
+	if err != nil {
+		return err
+	}
+	f.repair.watchFileChanges(nbSearchDomains, nbNameserverIP)
+	return nil
+}
+
+func (f *fileConfigurator) updateConfig(nbSearchDomains []string, nbNameserverIP string, cfg *resolvConf) error {
+	searchDomainList := mergeSearchDomains(nbSearchDomains, cfg.searchDomains)
+	nameServers := generateNsList(nbNameserverIP, cfg)
 
 	buf := prepareResolvConfContent(
 		searchDomainList,
-		append([]string{config.ServerIP}, nameServers...),
+		nameServers,
-		others)
+		cfg.others)
 
 	log.Debugf("creating managed file %s", defaultResolvConfPath)
-	err = os.WriteFile(defaultResolvConfPath, buf.Bytes(), f.originalPerms)
+	err := os.WriteFile(defaultResolvConfPath, buf.Bytes(), f.originalPerms)
 	if err != nil {
 		restoreErr := f.restore()
 		if restoreErr != nil {
@@ -88,6 +104,7 @@ func (f *fileConfigurator) applyDNSConfig(config HostDNSConfig) error {
 }
 
 func (f *fileConfigurator) restoreHostDNS() error {
+	f.repair.stopWatchFileChanges()
 	return f.restore()
 }
 
@@ -115,6 +132,18 @@ func (f *fileConfigurator) restore() error {
 	return os.RemoveAll(fileDefaultResolvConfBackupLocation)
 }
 
+// generateNsList generates a list of nameservers from the config and adds the primary nameserver to the beginning of the list
+func generateNsList(nbNameserverIP string, cfg *resolvConf) []string {
+	ns := make([]string, 1, len(cfg.nameServers)+1)
+	ns[0] = nbNameserverIP
+	for _, cfgNs := range cfg.nameServers {
+		if nbNameserverIP != cfgNs {
+			ns = append(ns, cfgNs)
+		}
+	}
+	return ns
+}
+
 func prepareResolvConfContent(searchDomains, nameServers, others []string) bytes.Buffer {
 	var buf bytes.Buffer
 	buf.WriteString(fileGeneratedResolvConfContentHeaderNextLine)
@@ -150,70 +179,6 @@ func searchDomains(config HostDNSConfig) []string {
 	return listOfDomains
 }
 
-func originalDNSConfigs(resolvconfFile string) (searchDomains, nameServers, others []string, err error) {
-	file, err := os.Open(resolvconfFile)
-	if err != nil {
-		err = fmt.Errorf(`could not read existing resolv.conf`)
-		return
-	}
-	defer file.Close()
-
-	reader := bufio.NewReader(file)
-
-	for {
-		lineBytes, isPrefix, readErr := reader.ReadLine()
-		if readErr != nil {
-			break
-		}
-
-		if isPrefix {
-			err = fmt.Errorf(`resolv.conf line too long`)
-			return
-		}
-
-		line := strings.TrimSpace(string(lineBytes))
-
-		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
-			}
-
-			searchDomains = splitLines[1:]
-			continue
-		}
-
-		if strings.HasPrefix(line, "nameserver") {
-			splitLines := strings.Fields(line)
-			if len(splitLines) != 2 {
-				continue
-			}
-			nameServers = append(nameServers, splitLines[1])
-			continue
-		}
-
-		others = append(others, line)
-	}
-	return
-}
-
 // merge search Domains lists and cut off the list if it is too long
 func mergeSearchDomains(searchDomains []string, originalSearchDomains []string) []string {
 	lineSize := len("search")
@@ -230,6 +195,19 @@ func mergeSearchDomains(searchDomains []string, originalSearchDomains []string)
 // return with the number of characters in the searchDomains line
 func validateAndFillSearchDomains(initialLineChars int, s *[]string, vs []string) int {
 	for _, sd := range vs {
+		duplicated := false
+		for _, fs := range *s {
+			if fs == sd {
+				duplicated = true
+				break
+			}
+
+		}
+
+		if duplicated {
+			continue
+		}
+
 		tmpCharsNumber := initialLineChars + 1 + len(sd)
 		if tmpCharsNumber > fileMaxLineCharsLimit {
 			// lets log all skipped Domains
@@ -246,6 +224,7 @@ func validateAndFillSearchDomains(initialLineChars int, s *[]string, vs []string
 		}
 		*s = append(*s, sd)
 	}
+
 	return initialLineChars
 }
 
@@ -266,3 +245,18 @@ func copyFile(src, dest string) error {
 	}
 	return nil
 }
+
+func isContains(subList []string, list []string) bool {
+	for _, sl := range subList {
+		var found bool
+		for _, l := range list {
+			if sl == l {
+				found = true
+			}
+		}
+		if !found {
+			return false
+		}
+	}
+	return true
+}

client/internal/dns/file_linux_test.go

@@ -1,3 +1,5 @@
+//go:build !android
+
 package dns
 
 import (
@@ -7,7 +9,7 @@ import (
 
 func Test_mergeSearchDomains(t *testing.T) {
 	searchDomains := []string{"a", "b"}
-	originDomains := []string{"a", "b"}
+	originDomains := []string{"c", "d"}
 	mergedDomains := mergeSearchDomains(searchDomains, originDomains)
 	if len(mergedDomains) != 4 {
 		t.Errorf("invalid len of result domains: %d, want: %d", len(mergedDomains), 4)
@@ -49,6 +51,67 @@ func Test_mergeSearchTooLongDomain(t *testing.T) {
 	}
 }
 
+func Test_isContains(t *testing.T) {
+	type args struct {
+		subList []string
+		list    []string
+	}
+	tests := []struct {
+		args args
+		want bool
+	}{
+		{
+			args: args{
+				subList: []string{"a", "b", "c"},
+				list:    []string{"a", "b", "c"},
+			},
+			want: true,
+		},
+		{
+			args: args{
+				subList: []string{"a"},
+				list:    []string{"a", "b", "c"},
+			},
+			want: true,
+		},
+		{
+			args: args{
+				subList: []string{"d"},
+				list:    []string{"a", "b", "c"},
+			},
+			want: false,
+		},
+		{
+			args: args{
+				subList: []string{"a"},
+				list:    []string{},
+			},
+			want: false,
+		},
+		{
+			args: args{
+				subList: []string{},
+				list:    []string{"b"},
+			},
+			want: true,
+		},
+		{
+			args: args{
+				subList: []string{},
+				list:    []string{},
+			},
+			want: true,
+		},
+	}
+	for _, tt := range tests {
+		t.Run("list check test", func(t *testing.T) {
+			if got := isContains(tt.args.subList, tt.args.list); got != tt.want {
+				t.Errorf("isContains() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}
+
 func getLongLine() string {
 	x := "search "
 	for {

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
+}

client/internal/dns/file_parser_linux_test.go

@@ -0,0 +1,149 @@
+//go:build !android
+
+package dns
+
+import (
+	"fmt"
+	"os"
+	"testing"
+)
+
+func Test_parseResolvConf(t *testing.T) {
+	testCases := []struct {
+		input          string
+		expectedSearch []string
+		expectedNS     []string
+		expectedOther  []string
+	}{
+		{
+			input: `domain chello.hu
+search chello.hu
+nameserver 192.168.0.1
+`,
+			expectedSearch: []string{"chello.hu"},
+			expectedNS:     []string{"192.168.0.1"},
+			expectedOther:  []string{},
+		},
+		{
+			input: `# This is /run/systemd/resolve/resolv.conf managed by man:systemd-resolved(8).
+# Do not edit.
+#
+# This file might be symlinked as /etc/resolv.conf. If you're looking at
+# /etc/resolv.conf and seeing this text, you have followed the symlink.
+#
+# This is a dynamic resolv.conf file for connecting local clients directly to
+# all known uplink DNS servers. This file lists all configured search domains.
+#
+# Third party programs should typically not access this file directly, but only
+# through the symlink at /etc/resolv.conf. To manage man:resolv.conf(5) in a
+# different way, replace this symlink by a static file or a different symlink.
+#
+# See man:systemd-resolved.service(8) for details about the supported modes of
+# operation for /etc/resolv.conf.
+
+nameserver 192.168.2.1
+nameserver 100.81.99.197
+search netbird.cloud
+`,
+			expectedSearch: []string{"netbird.cloud"},
+			expectedNS:     []string{"192.168.2.1", "100.81.99.197"},
+			expectedOther:  []string{},
+		},
+		{
+			input: `# This is /run/systemd/resolve/resolv.conf managed by man:systemd-resolved(8).
+# Do not edit.
+#
+# This file might be symlinked as /etc/resolv.conf. If you're looking at
+# /etc/resolv.conf and seeing this text, you have followed the symlink.
+#
+# This is a dynamic resolv.conf file for connecting local clients directly to
+# all known uplink DNS servers. This file lists all configured search domains.
+#
+# Third party programs should typically not access this file directly, but only
+# through the symlink at /etc/resolv.conf. To manage man:resolv.conf(5) in a
+# different way, replace this symlink by a static file or a different symlink.
+#
+# See man:systemd-resolved.service(8) for details about the supported modes of
+# operation for /etc/resolv.conf.
+
+nameserver 192.168.2.1
+nameserver 100.81.99.197
+search netbird.cloud
+options debug
+`,
+			expectedSearch: []string{"netbird.cloud"},
+			expectedNS:     []string{"192.168.2.1", "100.81.99.197"},
+			expectedOther:  []string{"options debug"},
+		},
+		{
+			input: `# This is /run/systemd/resolve/resolv.conf managed by man:systemd-resolved(8).
+# Do not edit.
+#
+# This file might be symlinked as /etc/resolv.conf. If you're looking at
+# /etc/resolv.conf and seeing this text, you have followed the symlink.
+#
+# This is a dynamic resolv.conf file for connecting local clients directly to
+# all known uplink DNS servers. This file lists all configured search domains.
+#
+# Third party programs should typically not access this file directly, but only
+# through the symlink at /etc/resolv.conf. To manage man:resolv.conf(5) in a
+# different way, replace this symlink by a static file or a different symlink.
+#
+# See man:systemd-resolved.service(8) for details about the supported modes of
+# operation for /etc/resolv.conf.
+
+nameserver 192.168.2.1
+nameserver 100.81.99.197
+search netbird.cloud
+options debug
+options edns0 trust-ad
+`,
+			expectedSearch: []string{"netbird.cloud"},
+			expectedNS:     []string{"192.168.2.1", "100.81.99.197"},
+			expectedOther:  []string{"options debug", "options edns0 trust-ad"},
+		},
+	}
+
+	for _, testCase := range testCases {
+		testCase := testCase
+		t.Run("test", func(t *testing.T) {
+			t.Parallel()
+			tmpResolvConf := fmt.Sprintf("%s/%s", t.TempDir(), "resolv.conf")
+			err := os.WriteFile(tmpResolvConf, []byte(testCase.input), 0644)
+			if err != nil {
+				t.Fatal(err)
+			}
+			cfg, err := parseResolvConfFile(tmpResolvConf)
+			if err != nil {
+				t.Fatal(err)
+			}
+			ok := compareLists(cfg.searchDomains, testCase.expectedSearch)
+			if !ok {
+				t.Errorf("invalid parse result for search domains, expected: %v, got: %v", testCase.expectedSearch, cfg.searchDomains)
+			}
+
+			ok = compareLists(cfg.nameServers, testCase.expectedNS)
+			if !ok {
+				t.Errorf("invalid parse result for ns domains, expected: %v, got: %v", testCase.expectedNS, cfg.nameServers)
+			}
+
+			ok = compareLists(cfg.others, testCase.expectedOther)
+			if !ok {
+				t.Errorf("invalid parse result for others, expected: %v, got: %v", testCase.expectedOther, cfg.others)
+			}
+		})
+	}
+
+}
+
+func compareLists(search []string, search2 []string) bool {
+	if len(search) != len(search2) {
+		return false
+	}
+	for i, v := range search {
+		if v != search2[i] {
+			return false
+		}
+	}
+	return true
+}

client/internal/dns/file_repair_linux.go

@@ -0,0 +1,151 @@
+//go:build !android
+
+package dns
+
+import (
+	"fmt"
+	"path"
+	"sync"
+
+	"github.com/fsnotify/fsnotify"
+	log "github.com/sirupsen/logrus"
+)
+
+var (
+	eventTypes = []fsnotify.Op{
+		fsnotify.Create,
+		fsnotify.Write,
+		fsnotify.Remove,
+		fsnotify.Rename,
+	}
+)
+
+type repairConfFn func([]string, string, *resolvConf) error
+
+type repair struct {
+	operationFile string
+	updateFn      repairConfFn
+	watchDir      string
+
+	inotify   *fsnotify.Watcher
+	inotifyWg sync.WaitGroup
+}
+
+func newRepair(operationFile string, updateFn repairConfFn) *repair {
+	return &repair{
+		operationFile: operationFile,
+		watchDir:      path.Dir(operationFile),
+		updateFn:      updateFn,
+	}
+}
+
+func (f *repair) watchFileChanges(nbSearchDomains []string, nbNameserverIP string) {
+	if f.inotify != nil {
+		return
+	}
+
+	log.Infof("start to watch resolv.conf")
+	inotify, err := fsnotify.NewWatcher()
+	if err != nil {
+		log.Errorf("failed to start inotify watcher for resolv.conf: %s", err)
+		return
+	}
+	f.inotify = inotify
+
+	f.inotifyWg.Add(1)
+	go func() {
+		defer f.inotifyWg.Done()
+		for event := range f.inotify.Events {
+			if !f.isEventRelevant(event) {
+				continue
+			}
+
+			log.Tracef("resolv.conf changed, check if it is broken")
+
+			rConf, err := parseResolvConfFile(f.operationFile)
+			if err != nil {
+				log.Warnf("failed to parse resolv conf: %s", err)
+				continue
+			}
+
+			log.Debugf("check resolv.conf parameters: %s", rConf)
+			if !isNbParamsMissing(nbSearchDomains, nbNameserverIP, rConf) {
+				log.Tracef("resolv.conf still correct, skip the update")
+				continue
+			}
+			log.Info("broken params in resolv.conf, repairing it...")
+
+			err = f.inotify.Remove(f.watchDir)
+			if err != nil {
+				log.Errorf("failed to rm inotify watch for resolv.conf: %s", err)
+			}
+
+			err = f.updateFn(nbSearchDomains, nbNameserverIP, rConf)
+			if err != nil {
+				log.Errorf("failed to repair resolv.conf: %v", err)
+			}
+
+			err = f.inotify.Add(f.watchDir)
+			if err != nil {
+				log.Errorf("failed to readd inotify watch for resolv.conf: %s", err)
+				return
+			}
+		}
+	}()
+
+	err = f.inotify.Add(f.watchDir)
+	if err != nil {
+		log.Errorf("failed to add inotify watch for resolv.conf: %s", err)
+		return
+	}
+}
+
+func (f *repair) stopWatchFileChanges() {
+	if f.inotify == nil {
+		return
+	}
+	err := f.inotify.Close()
+	if err != nil {
+		log.Warnf("failed to close resolv.conf inotify: %v", err)
+	}
+	f.inotifyWg.Wait()
+	f.inotify = nil
+}
+
+func (f *repair) isEventRelevant(event fsnotify.Event) bool {
+	var ok bool
+	for _, et := range eventTypes {
+		if event.Has(et) {
+			ok = true
+			break
+		}
+	}
+	if !ok {
+		return false
+	}
+
+	operationFileSymlink := fmt.Sprintf("%s~", f.operationFile)
+	if event.Name == f.operationFile || event.Name == operationFileSymlink {
+		return true
+	}
+	return false
+}
+
+// nbParamsAreMissing checks if the resolv.conf file contains all the parameters that NetBird needs
+// check the NetBird related nameserver IP at the first place
+// check the NetBird related search domains in the search domains list
+func isNbParamsMissing(nbSearchDomains []string, nbNameserverIP string, rConf *resolvConf) bool {
+	if !isContains(nbSearchDomains, rConf.searchDomains) {
+		return true
+	}
+
+	if len(rConf.nameServers) == 0 {
+		return true
+	}
+
+	if rConf.nameServers[0] != nbNameserverIP {
+		return true
+	}
+
+	return false
+}

client/internal/dns/file_repair_linux_test.go

@@ -0,0 +1,130 @@
+//go:build !android
+
+package dns
+
+import (
+	"context"
+	"os"
+	"testing"
+	"time"
+
+	"github.com/netbirdio/netbird/util"
+)
+
+func TestMain(m *testing.M) {
+	_ = util.InitLog("debug", "console")
+	code := m.Run()
+	os.Exit(code)
+}
+
+func Test_newRepairtmp(t *testing.T) {
+	type args struct {
+		resolvConfContent  string
+		touchedConfContent string
+		wantChange         bool
+	}
+	tests := []args{
+		{
+			resolvConfContent: `
+nameserver 10.0.0.1
+nameserver 8.8.8.8
+searchdomain netbird.cloud something`,
+
+			touchedConfContent: `
+nameserver 8.8.8.8
+searchdomain netbird.cloud something`,
+			wantChange: true,
+		},
+		{
+			resolvConfContent: `
+nameserver 10.0.0.1
+nameserver 8.8.8.8
+searchdomain netbird.cloud something`,
+
+			touchedConfContent: `
+nameserver 10.0.0.1
+nameserver 8.8.8.8
+searchdomain netbird.cloud something somethingelse`,
+			wantChange: false,
+		},
+		{
+			resolvConfContent: `
+nameserver 10.0.0.1
+nameserver 8.8.8.8
+searchdomain netbird.cloud something`,
+
+			touchedConfContent: `
+nameserver 10.0.0.1
+searchdomain netbird.cloud something`,
+			wantChange: false,
+		},
+		{
+			resolvConfContent: `
+nameserver 10.0.0.1
+nameserver 8.8.8.8
+searchdomain netbird.cloud something`,
+
+			touchedConfContent: `
+searchdomain something`,
+			wantChange: true,
+		},
+		{
+			resolvConfContent: `
+nameserver 10.0.0.1
+nameserver 8.8.8.8
+searchdomain netbird.cloud something`,
+
+			touchedConfContent: `
+nameserver 10.0.0.1`,
+			wantChange: true,
+		},
+		{
+			resolvConfContent: `
+nameserver 10.0.0.1
+nameserver 8.8.8.8
+searchdomain netbird.cloud something`,
+
+			touchedConfContent: `
+nameserver 8.8.8.8`,
+			wantChange: true,
+		},
+	}
+
+	for _, tt := range tests {
+		tt := tt
+		t.Run("test", func(t *testing.T) {
+			t.Parallel()
+			workDir := t.TempDir()
+			operationFile := workDir + "/resolv.conf"
+			err := os.WriteFile(operationFile, []byte(tt.resolvConfContent), 0755)
+			if err != nil {
+				t.Fatalf("failed to write out resolv.conf: %s", err)
+			}
+
+			var changed bool
+			ctx, cancel := context.WithTimeout(context.Background(), time.Second)
+			updateFn := func([]string, string, *resolvConf) error {
+				changed = true
+				cancel()
+				return nil
+			}
+
+			r := newRepair(operationFile, updateFn)
+			r.watchFileChanges([]string{"netbird.cloud"}, "10.0.0.1")
+
+			err = os.WriteFile(operationFile, []byte(tt.touchedConfContent), 0755)
+			if err != nil {
+				t.Fatalf("failed to write out resolv.conf: %s", err)
+			}
+
+			<-ctx.Done()
+
+			r.stopWatchFileChanges()
+
+			if changed != tt.wantChange {
+				t.Errorf("unexpected result: want: %v, got: %v", tt.wantChange, changed)
+			}
+		})
+	}
+
+}

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) {
+			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
+}

client/internal/dns/resolvconf_linux.go

@@ -22,16 +22,16 @@ type resolvconf struct {
 
 // supported "openresolv" only
 func newResolvConfConfigurator(wgInterface WGIface) (hostManager, error) {
-	originalSearchDomains, nameServers, others, err := originalDNSConfigs("/etc/resolv.conf")
+	resolvConfEntries, err := parseDefaultResolvConf()
 	if err != nil {
 		log.Error(err)
 	}
 
 	return &resolvconf{
 		ifaceName:             wgInterface.Name(),
-		originalSearchDomains: originalSearchDomains,
+		originalSearchDomains: resolvConfEntries.searchDomains,
-		originalNameServers:   nameServers,
+		originalNameServers:   resolvConfEntries.nameServers,
-		othersConfigs:         others,
+		othersConfigs:         resolvConfEntries.others,
 	}, nil
 }
 

go.mod

@@ -39,6 +39,7 @@ require (
 	github.com/coreos/go-iptables v0.7.0
 	github.com/creack/pty v1.1.18
 	github.com/eko/gocache/v3 v3.1.1
+	github.com/fsnotify/fsnotify v1.6.0
 	github.com/getlantern/systray v1.2.1
 	github.com/gliderlabs/ssh v0.3.4
 	github.com/godbus/dbus/v5 v5.1.0
@@ -99,7 +100,6 @@ require (
 	github.com/dgraph-io/ristretto v0.1.1 // indirect
 	github.com/dgryski/go-rendezvous v0.0.0-20200823014737-9f7001d12a5f // indirect
 	github.com/fredbi/uri v0.0.0-20181227131451-3dcfdacbaaf3 // indirect
-	github.com/fsnotify/fsnotify v1.6.0 // indirect
 	github.com/getlantern/context v0.0.0-20190109183933-c447772a6520 // indirect
 	github.com/getlantern/errors v0.0.0-20190325191628-abdb3e3e36f7 // indirect
 	github.com/getlantern/golog v0.0.0-20190830074920-4ef2e798c2d7 // indirect