[client] Always register NetBird with plain Linux DNS and use original servers as upstream (#3967)

client/internal/dns/file_unix.go

@@ -8,7 +8,6 @@ import (
 	"net/netip"
 	"os"
 	"strings"
-	"time"
 
 	log "github.com/sirupsen/logrus"
 
@@ -18,7 +17,7 @@ import (
 const (
 	fileGeneratedResolvConfContentHeader         = "# Generated by NetBird"
 	fileGeneratedResolvConfContentHeaderNextLine = fileGeneratedResolvConfContentHeader + `
-# If needed you can restore the original file by copying back ` + fileDefaultResolvConfBackupLocation + "\n\n"
+# The original file can be restored from ` + fileDefaultResolvConfBackupLocation + "\n\n"
 
 	fileDefaultResolvConfBackupLocation = defaultResolvConfPath + ".original.netbird"
 
@@ -26,16 +25,11 @@ const (
 	fileMaxNumberOfSearchDomains = 6
 )
 
-const (
-	dnsFailoverTimeout  = 4 * time.Second
-	dnsFailoverAttempts = 1
-)
-
 type fileConfigurator struct {
-	repair *repair
-
-	originalPerms  os.FileMode
-	nbNameserverIP string
+	repair              *repair
+	originalPerms       os.FileMode
+	nbNameserverIP      netip.Addr
+	originalNameservers []string
 }
 
 func newFileConfigurator() (*fileConfigurator, error) {
@@ -49,22 +43,9 @@ func (f *fileConfigurator) supportCustomPort() bool {
 }
 
 func (f *fileConfigurator) applyDNSConfig(config HostDNSConfig, stateManager *statemanager.Manager) error {
-	backupFileExist := f.isBackupFileExist()
-	if !config.RouteAll {
-		if backupFileExist {
-			f.repair.stopWatchFileChanges()
-			err := f.restore()
-			if err != nil {
-				return fmt.Errorf("restoring the original resolv.conf file return err: %w", err)
-			}
-		}
-		return ErrRouteAllWithoutNameserverGroup
-	}
-
-	if !backupFileExist {
-		err := f.backup()
-		if err != nil {
-			return fmt.Errorf("unable to backup the resolv.conf file: %w", err)
+	if !f.isBackupFileExist() {
+		if err := f.backup(); err != nil {
+			return fmt.Errorf("backup resolv.conf: %w", err)
 		}
 	}
 
@@ -76,6 +57,8 @@ func (f *fileConfigurator) applyDNSConfig(config HostDNSConfig, stateManager *st
 		log.Errorf("could not read original search domains from %s: %s", fileDefaultResolvConfBackupLocation, err)
 	}
 
+	f.originalNameservers = resolvConf.nameServers
+
 	f.repair.stopWatchFileChanges()
 
 	err = f.updateConfig(nbSearchDomains, f.nbNameserverIP, resolvConf, stateManager)
@@ -86,15 +69,19 @@ func (f *fileConfigurator) applyDNSConfig(config HostDNSConfig, stateManager *st
 	return nil
 }
 
-func (f *fileConfigurator) updateConfig(nbSearchDomains []string, nbNameserverIP string, cfg *resolvConf, stateManager *statemanager.Manager) error {
-	searchDomainList := mergeSearchDomains(nbSearchDomains, cfg.searchDomains)
-	nameServers := generateNsList(nbNameserverIP, cfg)
+// getOriginalNameservers returns the nameservers that were found in the original resolv.conf
+func (f *fileConfigurator) getOriginalNameservers() []string {
+	return f.originalNameservers
+}
+
+func (f *fileConfigurator) updateConfig(nbSearchDomains []string, nbNameserverIP netip.Addr, cfg *resolvConf, stateManager *statemanager.Manager) error {
+	searchDomainList := mergeSearchDomains(nbSearchDomains, cfg.searchDomains)
 
-	options := prepareOptionsWithTimeout(cfg.others, int(dnsFailoverTimeout.Seconds()), dnsFailoverAttempts)
 	buf := prepareResolvConfContent(
 		searchDomainList,
-		nameServers,
-		options)
+		[]string{nbNameserverIP.String()},
+		cfg.others,
+	)
 
 	log.Debugf("creating managed file %s", defaultResolvConfPath)
 	err := os.WriteFile(defaultResolvConfPath, buf.Bytes(), f.originalPerms)
@@ -197,38 +184,28 @@ func restoreResolvConfFile() error {
 	return nil
 }
 
-// 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)
 
 	for _, cfgLine := range others {
 		buf.WriteString(cfgLine)
-		buf.WriteString("\n")
+		buf.WriteByte('\n')
 	}
 
 	if len(searchDomains) > 0 {
 		buf.WriteString("search ")
 		buf.WriteString(strings.Join(searchDomains, " "))
-		buf.WriteString("\n")
+		buf.WriteByte('\n')
 	}
 
 	for _, ns := range nameServers {
 		buf.WriteString("nameserver ")
 		buf.WriteString(ns)
-		buf.WriteString("\n")
+		buf.WriteByte('\n')
 	}
+
 	return buf
 }