fix(mapper): prevent index out of bounds in ToEndpointName with multi-dot suffix (#6433)

When a multi-dot txt-suffix (e.g. ".foo.bar") is configured,
ToEndpointName panics on DNS names with fewer labels than the
suffix expects:

  1. parts[:1+dc] panics when len(parts) <= dc
  2. parts[1+dc] panics when len(parts) == 1+dc

Add bounds checks before both slice accesses and return ("", "")
directly for the too-few-labels case, with a debug log for
operator visibility. Return the correct result when the name has
exactly enough labels to match the suffix but no trailing labels.

This generalizes the single-label fix from #4885, which only
guarded the zero-dot case via strings.Contains.

Signed-off-by: Jathavedhan M <jathavedhan.m@ibm.com>

Author: carterpewpew

registry/mapper/mapper.go

@@ -19,6 +19,8 @@ package mapper
 import (
 	"strings"
 
+	log "github.com/sirupsen/logrus"
+
 	"sigs.k8s.io/external-dns/endpoint"
 )
 
@@ -74,40 +76,42 @@ func (a AffixNameMapper) ToEndpointName(dns string) (string, string) {
 	// drop suffix
 	if a.isSuffix() {
 		dc := strings.Count(a.suffix, ".")
-		DNSName := strings.SplitN(lowerDNSName, ".", 2+dc)
-		domainWithSuffix := strings.Join(DNSName[:1+dc], ".")
-
-		r, rType := a.dropAffixExtractType(domainWithSuffix)
-		if !strings.Contains(lowerDNSName, ".") {
+		parts := strings.SplitN(lowerDNSName, ".", 2+dc)
+		if len(parts) <= dc {
+			log.Debugf("skipping TXT record %q: too few labels for suffix %q", dns, a.suffix)
+			return "", ""
+		}
+		r, rType := a.dropAffixExtractType(strings.Join(parts[:1+dc], "."))
+		if len(parts) <= 1+dc {
 			return r, rType
 		}
-		return r + "." + DNSName[1+dc], rType
+		return r + "." + parts[1+dc], rType
 	}
 	return "", ""
 }
 
 func (a AffixNameMapper) ToTXTName(dns, recordType string) string {
-	DNSName := strings.SplitN(dns, ".", 2)
+	parts := strings.SplitN(dns, ".", 2)
 	recordType = strings.ToLower(recordType)
 	recordT := recordType + "-"
 
 	prefix := a.normalizeAffixTemplate(a.prefix, recordType)
 	suffix := a.normalizeAffixTemplate(a.suffix, recordType)
 
 	// If specified, replace a leading asterisk in the generated txt record name with some other string
-	if a.wildcardReplacement != "" && DNSName[0] == "*" {
-		DNSName[0] = a.wildcardReplacement
+	if a.wildcardReplacement != "" && parts[0] == "*" {
+		parts[0] = a.wildcardReplacement
 	}
 
 	if !a.recordTypeInAffix() {
-		DNSName[0] = recordT + DNSName[0]
+		parts[0] = recordT + parts[0]
 	}
 
-	if len(DNSName) < 2 {
-		return prefix + DNSName[0] + suffix
+	if len(parts) < 2 {
+		return prefix + parts[0] + suffix
 	}
 
-	return prefix + DNSName[0] + suffix + "." + DNSName[1]
+	return prefix + parts[0] + suffix + "." + parts[1]
 }
 
 func (a AffixNameMapper) recordTypeInAffix() bool {

registry/mapper/mapper_test.go

@@ -107,6 +107,27 @@ func TestAffixNameMapper_ToEndpointName(t *testing.T) {
 			wantEndpointName: "foo.example.com",
 			wantRecordType:   endpoint.RecordTypeCNAME,
 		},
+		{
+			name:             "suffix with multiple dots and trailing labels",
+			mapper:           NewAffixNameMapper("", ".foo.bar", ""),
+			input:            "a-example.foo.bar.com",
+			wantEndpointName: "example.com",
+			wantRecordType:   endpoint.RecordTypeA,
+		},
+		{
+			name:             "suffix with multiple dots and no trailing labels",
+			mapper:           NewAffixNameMapper("", ".foo.bar", ""),
+			input:            "a-example.foo.bar",
+			wantEndpointName: "example",
+			wantRecordType:   endpoint.RecordTypeA,
+		},
+		{
+			name:             "suffix with multiple dots and too few labels does not panic",
+			mapper:           NewAffixNameMapper("", ".foo.bar", ""),
+			input:            "a-example.foo",
+			wantEndpointName: "",
+			wantRecordType:   "",
+		},
 		{
 			name:             "no affix with A record",
 			mapper:           NewAffixNameMapper("", "", ""),