feat: add support for custom DNS server

resolves zmap#419

Author: developStorm

config.go

@@ -24,6 +24,7 @@ type Config struct {
 	ConnectionsPerHost int             `long:"connections-per-host" default:"1" description:"Number of times to connect to each host (results in more output)"`
 	ReadLimitPerHost   int             `long:"read-limit-per-host" default:"96" description:"Maximum total kilobytes to read for a single host (default 96kb)"`
 	Prometheus         string          `long:"prometheus" description:"Address to use for Prometheus server (e.g. localhost:8080). If empty, Prometheus is disabled."`
+	CustomDNS          string          `long:"dns" description:"Address of a custom DNS server for lookups. Default port is 53."`
 	Multiple           MultipleCommand `command:"multiple" description:"Multiple module actions"`
 	inputFile          *os.File
 	outputFile         *os.File
@@ -128,6 +129,14 @@ func validateFrameworkConfiguration() {
 	if config.ReadLimitPerHost > 0 {
 		DefaultBytesReadLimit = config.ReadLimitPerHost * 1024
 	}
+
+	// Validate custom DNS
+	if config.CustomDNS != "" {
+		var err error
+		if config.CustomDNS, err = AddDefaultPortToDNSServerName(config.CustomDNS); err != nil {
+			log.Fatalf("invalid DNS server address: %s", err)
+		}
+	}
 }
 
 // GetMetaFile returns the file to which metadata should be output

conn.go

@@ -341,6 +341,16 @@ func (d *Dialer) SetDefaults() *Dialer {
 			KeepAlive: d.Timeout,
 			DualStack: true,
 		}
+
+		// Use custom DNS as default if set
+		if config.CustomDNS != "" {
+			d.Dialer.Resolver = &net.Resolver{
+				PreferGo: true,
+				Dial: func(ctx context.Context, network, address string) (net.Conn, error) {
+					return net.Dial(network, config.CustomDNS)
+				},
+			}
+		}
 	}
 	return d
 }

utility.go

@@ -2,16 +2,18 @@ package zgrab2
 
 import (
 	"errors"
+	"fmt"
 	"net"
 	"regexp"
 	"strconv"
 	"strings"
 
 	"time"
 
-	"github.com/zmap/zflags"
-	"github.com/sirupsen/logrus"
 	"runtime/debug"
+
+	"github.com/sirupsen/logrus"
+	flags "github.com/zmap/zflags"
 )
 
 var parser *flags.Parser
@@ -214,8 +216,9 @@ func IsTimeoutError(err error) bool {
 // doing anything. Otherwise, it logs the stacktrace, the panic error, and the provided message
 // before re-raising the original panic.
 // Example:
-//     defer zgrab2.LogPanic("Error decoding body '%x'", body)
-func LogPanic(format string, args...interface{}) {
+//
+//	defer zgrab2.LogPanic("Error decoding body '%x'", body)
+func LogPanic(format string, args ...interface{}) {
 	err := recover()
 	if err == nil {
 		return
@@ -224,3 +227,25 @@ func LogPanic(format string, args...interface{}) {
 	logrus.Errorf(format, args...)
 	panic(err)
 }
+
+func AddDefaultPortToDNSServerName(inAddr string) (string, error) {
+	// Try to split host and port to see if the port is already specified.
+	host, port, err := net.SplitHostPort(inAddr)
+	if err != nil {
+		// might mean there's no port specified
+		host = inAddr
+	}
+
+	// Validate the host part as an IP address.
+	ip := net.ParseIP(host)
+	if ip == nil {
+		return "", fmt.Errorf("invalid IP address")
+	}
+
+	// If the original input does not have a port, specify port 53
+	if port == "" {
+		port = "53"
+	}
+
+	return net.JoinHostPort(ip.String(), port), nil
+}