Enhance DNS failover reliability (#1637)

* Fix using wrong array index in log to avoid potential panic

* Increase gRPC connection timeout and add the timeout resolv.conf option

This makes sure the dns client is able to failover to a second
configured nameserver, if present. That is the case then when using the
dns `file` manager and a resolv.conf file generated for netbird.

* On file backup restore, remove the first NS if it's the netbird NS

* Bump dns mangager discovery message from debug to info to ease debugging

client/internal/dns/file_parser_linux.go

@@ -5,6 +5,7 @@ package dns
 import (
 	"fmt"
 	"os"
+	"regexp"
 	"strings"
 
 	log "github.com/sirupsen/logrus"
@@ -14,6 +15,9 @@ const (
 	defaultResolvConfPath = "/etc/resolv.conf"
 )
 
+var timeoutRegex = regexp.MustCompile(`timeout:\d+`)
+var attemptsRegex = regexp.MustCompile(`attempts:\d+`)
+
 type resolvConf struct {
 	nameServers   []string
 	searchDomains []string
@@ -103,3 +107,62 @@ func parseResolvConfFile(resolvConfFile string) (*resolvConf, error) {
 	}
 	return rconf, nil
 }
+
+// prepareOptionsWithTimeout appends timeout to existing options if it doesn't exist,
+// otherwise it adds a new option with timeout and attempts.
+func prepareOptionsWithTimeout(input []string, timeout int, attempts int) []string {
+	configs := make([]string, len(input))
+	copy(configs, input)
+
+	for i, config := range configs {
+		if strings.HasPrefix(config, "options") {
+			config = strings.ReplaceAll(config, "rotate", "")
+			config = strings.Join(strings.Fields(config), " ")
+
+			if strings.Contains(config, "timeout:") {
+				config = timeoutRegex.ReplaceAllString(config, fmt.Sprintf("timeout:%d", timeout))
+			} else {
+				config = strings.Replace(config, "options ", fmt.Sprintf("options timeout:%d ", timeout), 1)
+			}
+
+			if strings.Contains(config, "attempts:") {
+				config = attemptsRegex.ReplaceAllString(config, fmt.Sprintf("attempts:%d", attempts))
+			} else {
+				config = strings.Replace(config, "options ", fmt.Sprintf("options attempts:%d ", attempts), 1)
+			}
+
+			configs[i] = config
+			return configs
+		}
+	}
+
+	return append(configs, fmt.Sprintf("options timeout:%d attempts:%d", timeout, attempts))
+}
+
+// removeFirstNbNameserver removes the given nameserver from the given file if it is in the first position
+// and writes the file back to the original location
+func removeFirstNbNameserver(filename, nameserverIP string) error {
+	resolvConf, err := parseResolvConfFile(filename)
+	if err != nil {
+		return fmt.Errorf("parse backup resolv.conf: %w", err)
+	}
+	content, err := os.ReadFile(filename)
+	if err != nil {
+		return fmt.Errorf("read %s: %w", filename, err)
+	}
+
+	if len(resolvConf.nameServers) > 1 && resolvConf.nameServers[0] == nameserverIP {
+		newContent := strings.Replace(string(content), fmt.Sprintf("nameserver %s\n", nameserverIP), "", 1)
+
+		stat, err := os.Stat(filename)
+		if err != nil {
+			return fmt.Errorf("stat %s: %w", filename, err)
+		}
+		if err := os.WriteFile(filename, []byte(newContent), stat.Mode()); err != nil {
+			return fmt.Errorf("write %s: %w", filename, err)
+		}
+
+	}
+
+	return nil
+}