Warn when domain names are not in RFC1035 preferred syntax (#5414)

Warn when domain names are not in RFC1035 preferred syntax 

Signed-off-by: Md Sahil <Mohdssahil1@gmail.com>

core/dnsserver/onstartup.go

@@ -2,9 +2,22 @@ package dnsserver
 
 import (
 	"fmt"
+	"regexp"
 	"sort"
+
+	"github.com/coredns/coredns/plugin/pkg/dnsutil"
 )
 
+// checkZoneSyntax() checks whether the given string match 1035 Preferred Syntax or not.
+// The root zone, and all reverse zones always return true even though they technically don't meet 1035 Preferred Syntax
+func checkZoneSyntax(zone string) bool {
+	if zone == "." || dnsutil.IsReverse(zone) != 0 {
+		return true
+	}
+	regex1035PreferredSyntax, _ := regexp.MatchString(`^(([A-Za-z]([A-Za-z0-9-]*[A-Za-z0-9])?)\.)+$`, zone)
+	return regex1035PreferredSyntax
+}
+
 // startUpZones creates the text that we show when starting up:
 // grpc://example.com.:1055
 // example.com.:1053 on 127.0.0.1
@@ -13,6 +26,7 @@ func startUpZones(protocol, addr string, zones map[string]*Config) string {
 
 	keys := make([]string, len(zones))
 	i := 0
+
 	for k := range zones {
 		keys[i] = k
 		i++
@@ -20,6 +34,9 @@ func startUpZones(protocol, addr string, zones map[string]*Config) string {
 	sort.Strings(keys)
 
 	for _, zone := range keys {
+		if !checkZoneSyntax(zone) {
+			s += fmt.Sprintf("Warning: Domain %q does not follow RFC1035 preferred syntax\n", zone)
+		}
 		// split addr into protocol, IP and Port
 		_, ip, port, err := SplitProtocolHostPort(addr)