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_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)
+			}
+		})
+	}
+
+}