Parse original resolv.conf

client/internal/dns/file_linux.go

@@ -3,29 +3,25 @@
 package dns
 
 import (
+	"bufio"
 	"bytes"
 	"fmt"
 	"os"
+	"strings"
 
 	log "github.com/sirupsen/logrus"
 )
 
 const (
-	fileGeneratedResolvConfContentHeader      = "# Generated by NetBird"
-	fileGeneratedResolvConfSearchBeginContent = "search "
-	fileGeneratedResolvConfContentFormat      = fileGeneratedResolvConfContentHeader +
-		"\n# If needed you can restore the original file by copying back %s\n\nnameserver %s\n" +
-		fileGeneratedResolvConfSearchBeginContent + "%s\n\n" +
-		"%s\n"
-)
+	fileGeneratedResolvConfContentHeader         = "# Generated by NetBird"
+	fileGeneratedResolvConfContentHeaderNextLine = fileGeneratedResolvConfContentHeader + `
+# If needed you can restore the original file by copying back ` + fileDefaultResolvConfBackupLocation + "\n\n"
 
-const (
 	fileDefaultResolvConfBackupLocation = defaultResolvConfPath + ".original.netbird"
-	fileMaxLineCharsLimit               = 256
-	fileMaxNumberOfSearchDomains        = 6
-)
 
-var fileSearchLineBeginCharCount = len(fileGeneratedResolvConfSearchBeginContent)
+	fileMaxLineCharsLimit        = 256
+	fileMaxNumberOfSearchDomains = 6
+)
 
 type fileConfigurator struct {
 	originalPerms os.FileMode
@@ -55,58 +51,39 @@ func (f *fileConfigurator) applyDNSConfig(config hostDNSConfig) error {
 		}
 		return fmt.Errorf("unable to configure DNS for this peer using file manager without a nameserver group with all domains configured")
 	}
-	managerType, err := getOSDNSManagerType()
-	if err != nil {
-		return err
-	}
-	switch managerType {
-	case fileManager, netbirdManager:
-		if !backupFileExist {
-			err = f.backup()
-			if err != nil {
-				return fmt.Errorf("unable to backup the resolv.conf file")
-			}
-		}
-	default:
-		// todo improve this and maybe restart DNS manager from scratch
-		return fmt.Errorf("something happened and file manager is not your preferred host dns configurator, restart the agent")
-	}
 
-	var searchDomains string
-	appendedDomains := 0
-	for _, dConf := range config.domains {
-		if dConf.matchOnly || dConf.disabled {
-			continue
-		}
-		if appendedDomains >= fileMaxNumberOfSearchDomains {
-			// lets log all skipped domains
-			log.Infof("already appended %d domains to search list. Skipping append of %s domain", fileMaxNumberOfSearchDomains, dConf.domain)
-			continue
-		}
-		if fileSearchLineBeginCharCount+len(searchDomains) > fileMaxLineCharsLimit {
-			// lets log all skipped domains
-			log.Infof("search list line is larger than %d characters. Skipping append of %s domain", fileMaxLineCharsLimit, dConf.domain)
-			continue
+	if !backupFileExist {
+		err = f.backup()
+		if err != nil {
+			return fmt.Errorf("unable to backup the resolv.conf file")
 		}
-
-		searchDomains += " " + dConf.domain
-		appendedDomains++
 	}
 
-	originalContent, err := os.ReadFile(fileDefaultResolvConfBackupLocation)
+	searchDomainList := searchDomains(config)
+
+	originalSearchDomains, nameServers, others, err := originalDNSConfigs(fileDefaultResolvConfBackupLocation)
 	if err != nil {
-		log.Errorf("Could not read existing resolv.conf")
+		log.Error(err)
 	}
-	content := fmt.Sprintf(fileGeneratedResolvConfContentFormat, fileDefaultResolvConfBackupLocation, config.serverIP, searchDomains, string(originalContent))
-	err = writeDNSConfig(content, defaultResolvConfPath, f.originalPerms)
+
+	searchDomainList = mergeSearchDomains(searchDomainList, originalSearchDomains)
+
+	buf := prepareResolvConfContent(
+		searchDomainList,
+		append([]string{config.serverIP}, nameServers...),
+		others)
+
+	log.Debugf("creating managed file %s", defaultResolvConfPath)
+	err = os.WriteFile(defaultResolvConfPath, buf.Bytes(), f.originalPerms)
 	if err != nil {
-		err = f.restore()
-		if err != nil {
+		restoreErr := f.restore()
+		if restoreErr != nil {
 			log.Errorf("attempt to restore default file failed with error: %s", err)
 		}
-		return err
+		return fmt.Errorf("got an creating resolver file %s. Error: %s", defaultResolvConfPath, err)
 	}
-	log.Infof("created a NetBird managed %s file with your DNS settings. Added %d search domains. Search list: %s", defaultResolvConfPath, appendedDomains, searchDomains)
+
+	log.Infof("created a NetBird managed %s file with your DNS settings. Added %d search domains. Search list: %s", defaultResolvConfPath, len(searchDomainList), searchDomainList)
 	return nil
 }
 
@@ -138,15 +115,138 @@ func (f *fileConfigurator) restore() error {
 	return os.RemoveAll(fileDefaultResolvConfBackupLocation)
 }
 
-func writeDNSConfig(content, fileName string, permissions os.FileMode) error {
-	log.Debugf("creating managed file %s", fileName)
+func prepareResolvConfContent(searchDomains, nameServers, others []string) bytes.Buffer {
 	var buf bytes.Buffer
-	buf.WriteString(content)
-	err := os.WriteFile(fileName, buf.Bytes(), permissions)
+	buf.WriteString(fileGeneratedResolvConfContentHeaderNextLine)
+
+	for _, cfgLine := range others {
+		buf.WriteString(cfgLine)
+		buf.WriteString("\n")
+	}
+
+	if len(searchDomains) > 0 {
+		buf.WriteString("search ")
+		buf.WriteString(strings.Join(searchDomains, " "))
+		buf.WriteString("\n")
+	}
+
+	for _, ns := range nameServers {
+		buf.WriteString("nameserver ")
+		buf.WriteString(ns)
+		buf.WriteString("\n")
+	}
+	return buf
+}
+
+func searchDomains(config hostDNSConfig) []string {
+	listOfDomains := make([]string, 0)
+	for _, dConf := range config.domains {
+		if dConf.matchOnly || dConf.disabled {
+			continue
+		}
+
+		listOfDomains = append(listOfDomains, dConf.domain)
+	}
+	return listOfDomains
+}
+
+func originalDNSConfigs(resolvconfFile string) (searchDomains, nameServers, others []string, err error) {
+	file, err := os.Open(resolvconfFile)
 	if err != nil {
-		return fmt.Errorf("got an creating resolver file %s. Error: %s", fileName, err)
+		err = fmt.Errorf(`could not read existing resolv.conf`)
+		return
 	}
-	return nil
+	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")
+	searchDomainsList := make([]string, 0, len(searchDomains)+len(originalSearchDomains))
+
+	lineSize = validateAndFillSearchDomains(lineSize, &searchDomainsList, searchDomains)
+	_ = validateAndFillSearchDomains(lineSize, &searchDomainsList, originalSearchDomains)
+
+	return searchDomainsList
+}
+
+// validateAndFillSearchDomains checks if the search domains list is not too long and if the line is not too long
+// extend s slice with vs elements
+// return with the number of characters in the searchDomains line
+func validateAndFillSearchDomains(initialLineChars int, s *[]string, vs []string) int {
+	for _, sd := range vs {
+		tmpCharsNumber := initialLineChars + 1 + len(sd)
+		if tmpCharsNumber > fileMaxLineCharsLimit {
+			// lets log all skipped domains
+			log.Infof("search list line is larger than %d characters. Skipping append of %s domain", fileMaxLineCharsLimit, sd)
+			continue
+		}
+
+		initialLineChars = tmpCharsNumber
+
+		if len(*s) >= fileMaxNumberOfSearchDomains {
+			// lets log all skipped domains
+			log.Infof("already appended %d domains to search list. Skipping append of %s domain", fileMaxNumberOfSearchDomains, sd)
+			continue
+		}
+		*s = append(*s, sd)
+	}
+	return initialLineChars
 }
 
 func copyFile(src, dest string) error {

client/internal/dns/file_linux_test.go

@@ -0,0 +1,62 @@
+package dns
+
+import (
+	"fmt"
+	"testing"
+)
+
+func Test_mergeSearchDomains(t *testing.T) {
+	searchDomains := []string{"a", "b"}
+	originDomains := []string{"a", "b"}
+	mergedDomains := mergeSearchDomains(searchDomains, originDomains)
+	if len(mergedDomains) != 4 {
+		t.Errorf("invalid len of result domains: %d, want: %d", len(mergedDomains), 4)
+	}
+}
+
+func Test_mergeSearchTooMuchDomains(t *testing.T) {
+	searchDomains := []string{"a", "b", "c", "d", "e", "f", "g"}
+	originDomains := []string{"h", "i"}
+	mergedDomains := mergeSearchDomains(searchDomains, originDomains)
+	if len(mergedDomains) != 6 {
+		t.Errorf("invalid len of result domains: %d, want: %d", len(mergedDomains), 6)
+	}
+}
+
+func Test_mergeSearchTooMuchDomainsInOrigin(t *testing.T) {
+	searchDomains := []string{"a", "b"}
+	originDomains := []string{"c", "d", "e", "f", "g"}
+	mergedDomains := mergeSearchDomains(searchDomains, originDomains)
+	if len(mergedDomains) != 6 {
+		t.Errorf("invalid len of result domains: %d, want: %d", len(mergedDomains), 6)
+	}
+}
+
+func Test_mergeSearchTooLongDomain(t *testing.T) {
+	searchDomains := []string{getLongLine()}
+	originDomains := []string{"b"}
+	mergedDomains := mergeSearchDomains(searchDomains, originDomains)
+	if len(mergedDomains) != 1 {
+		t.Errorf("invalid len of result domains: %d, want: %d", len(mergedDomains), 1)
+	}
+
+	searchDomains = []string{"b"}
+	originDomains = []string{getLongLine()}
+
+	mergedDomains = mergeSearchDomains(searchDomains, originDomains)
+	if len(mergedDomains) != 1 {
+		t.Errorf("invalid len of result domains: %d, want: %d", len(mergedDomains), 1)
+	}
+}
+
+func getLongLine() string {
+	x := "search "
+	for {
+		for i := 0; i <= 9; i++ {
+			if len(x) > fileMaxLineCharsLimit {
+				return x
+			}
+			x = fmt.Sprintf("%s%d", x, i)
+		}
+	}
+}