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)
 

core/dnsserver/onstartup_test.go

+package dnsserver
+
+import (
+	"testing"
+)
+
+func TestRegex1035PrefSyntax(t *testing.T) {
+
+	testCases := []struct {
+		zone     string
+		expected bool
+	}{
+		{zone: ".", expected: true},
+		{zone: "example.com.", expected: true},
+		{zone: "example.", expected: true},
+		{zone: "example123.", expected: true},
+		{zone: "example123.com.", expected: true},
+		{zone: "abc-123.com.", expected: true},
+		{zone: "an-example.com.", expected: true},
+		{zone: "a.example.com.", expected: true},
+		{zone: "1.0.0.2.ip6.arpa.", expected: true},
+		{zone: "0.10.in-addr.arpa.", expected: true},
+		{zone: "example", expected: false},
+		{zone: "example:.", expected: false},
+		{zone: "-example.com.", expected: false},
+		{zone: ".example.com.", expected: false},
+		{zone: "1.example.com", expected: false},
+		{zone: "abc.123-xyz.", expected: false},
+		{zone: "example-?&^%$.com.", expected: false},
+		{zone: "abc-.example.com.", expected: false},
+		{zone: "abc-%$.example.com.", expected: false},
+		{zone: "123-abc.example.com.", expected: false},
+	}
+
+	for _, testCase := range testCases {
+		if checkZoneSyntax(testCase.zone) != testCase.expected {
+			t.Errorf("Expected %v for %q", testCase.expected, testCase.zone)
+		}
+	}
+}