Fix DNS_KEEP_NAMESERVER behavior

internal/dns/nameserver.go

@@ -35,23 +35,20 @@ func (c *configurator) UseDNSSystemWide(ip net.IP, keepNameserver bool) error {
 		_ = file.Close()
 		return err
 	}
+
 	s := strings.TrimSuffix(string(data), "\n")
-	lines := strings.Split(s, "\n")
-	if len(lines) == 1 && lines[0] == "" {
-		lines = nil
+
+	lines := []string{
+		"nameserver " + ip.String(),
 	}
-	found := false
-	if !keepNameserver { // default
-		for i := range lines {
-			if strings.HasPrefix(lines[i], "nameserver ") {
-				lines[i] = "nameserver " + ip.String()
-				found = true
-			}
+	for _, line := range strings.Split(s, "\n") {
+		if line == "" ||
+			(!keepNameserver && strings.HasPrefix(line, "nameserver ")) {
+			continue
 		}
+		lines = append(lines, line)
 	}
-	if !found {
-		lines = append(lines, "nameserver "+ip.String())
-	}
+
 	s = strings.Join(lines, "\n") + "\n"
 	_, err = file.WriteString(s)
 	if err != nil {

internal/dns/nameserver_test.go

@@ -18,18 +18,22 @@ import (
 func Test_UseDNSSystemWide(t *testing.T) {
 	t.Parallel()
 	tests := map[string]struct {
-		data        []byte
-		writtenData string
-		openErr     error
-		readErr     error
-		writeErr    error
-		closeErr    error
-		err         error
+		ip             net.IP
+		keepNameserver bool
+		data           []byte
+		writtenData    string
+		openErr        error
+		readErr        error
+		writeErr       error
+		closeErr       error
+		err            error
 	}{
 		"no data": {
+			ip:          net.IP{127, 0, 0, 1},
 			writtenData: "nameserver 127.0.0.1\n",
 		},
 		"open error": {
+			ip:      net.IP{127, 0, 0, 1},
 			openErr: fmt.Errorf("error"),
 			err:     fmt.Errorf("error"),
 		},
@@ -38,17 +42,26 @@ func Test_UseDNSSystemWide(t *testing.T) {
 			err:     fmt.Errorf("error"),
 		},
 		"write error": {
+			ip:          net.IP{127, 0, 0, 1},
 			writtenData: "nameserver 127.0.0.1\n",
 			writeErr:    fmt.Errorf("error"),
 			err:         fmt.Errorf("error"),
 		},
 		"lines without nameserver": {
+			ip:          net.IP{127, 0, 0, 1},
 			data:        []byte("abc\ndef\n"),
-			writtenData: "abc\ndef\nnameserver 127.0.0.1\n",
+			writtenData: "nameserver 127.0.0.1\nabc\ndef\n",
 		},
 		"lines with nameserver": {
+			ip:          net.IP{127, 0, 0, 1},
 			data:        []byte("abc\nnameserver abc def\ndef\n"),
-			writtenData: "abc\nnameserver 127.0.0.1\ndef\n",
+			writtenData: "nameserver 127.0.0.1\nabc\ndef\n",
+		},
+		"keep nameserver": {
+			ip:             net.IP{127, 0, 0, 1},
+			keepNameserver: true,
+			data:           []byte("abc\nnameserver abc def\ndef\n"),
+			writtenData:    "nameserver 127.0.0.1\nabc\nnameserver abc def\ndef\n",
 		},
 	}
 	for name, tc := range tests {
@@ -89,12 +102,12 @@ func Test_UseDNSSystemWide(t *testing.T) {
 			}
 
 			logger := mock_logging.NewMockLogger(mockCtrl)
-			logger.EXPECT().Info("using DNS address %s system wide", "127.0.0.1")
+			logger.EXPECT().Info("using DNS address %s system wide", tc.ip.String())
 			c := &configurator{
 				openFile: openFile,
 				logger:   logger,
 			}
-			err := c.UseDNSSystemWide(net.IP{127, 0, 0, 1}, false)
+			err := c.UseDNSSystemWide(tc.ip, tc.keepNameserver)
 			if tc.err != nil {
 				require.Error(t, err)
 				assert.Equal(t, tc.err.Error(), err.Error())