feat(dns): add validation for cname record-sets (#1019)

relates to #1014 and STACKITTPR-375

stackit/internal/validate/validate.go

@@ -132,6 +132,14 @@ func RecordSet() *Validator {
 					))
 				}
 			case "CNAME":
+				name := req.ConfigValue.ValueString()
+				if name == "" || name[len(name)-1] != '.' {
+					resp.Diagnostics.Append(validatordiag.InvalidAttributeValueDiagnostic(
+						req.Path,
+						"value must be a Fully Qualified Domain Name (FQDN) and end with dot '.'",
+						req.ConfigValue.ValueString(),
+					))
+				}
 			case "NS":
 			case "MX":
 			case "TXT":

stackit/internal/validate/validate_test.go

@@ -264,8 +264,14 @@ func TestRecordSet(t *testing.T) {
 			false,
 		},
 		{
-			"CNAME record",
-			"some-record",
+			"CNAME record Not a Fully Qualified Domain Name",
+			"stackit.de",
+			"CNAME",
+			false,
+		},
+		{
+			"CNAME record ok Fully Qualified Domain Name",
+			"stackit.de.",
 			"CNAME",
 			true,
 		},