Fix parsing AMTRELAY no gateway relay

CHANGELOG.md

@@ -13,6 +13,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 - Correct lengths for GOST R 34.10-2012 and SM3 delegation signer (DS) digest
   algorithms
+- Require the AMTRELAY relay field to be `.` for the no gateway relay type as
+  specified by RFC 8777 (#257)
 
 ## [0.2.3] - 2025-09-03
 

src/generic/types.h

@@ -2752,29 +2752,33 @@ static int32_t parse_amtrelay_rdata(
   if ((code = parse_int8(parser, type, &fields[2], rdata, token)) < 0)
     return code;
 
-  if (octets[1]) {
-    if ((code = take_contiguous(parser, type, &fields[3], token)) < 0)
-      return code;
-    switch (octets[1]) {
-      case 1: /* IPv4 address */
-        type = (const type_info_t *)amtrelay_ipv4;
-        fields = type->rdata.fields;
-        if ((code = parse_ip4(parser, type, &fields[3], rdata, token)) < 0)
-          return code;
-        break;
-      case 2: /* IPv6 address */
-        type = (const type_info_t *)amtrelay_ipv6;
-        fields = type->rdata.fields;
-        if ((code = parse_ip6(parser, type, &fields[3], rdata, token)) < 0)
-          return code;
-        break;
-      case 3: /* domain name */
-        if ((code = parse_name(parser, type, &fields[3], rdata, token)) < 0)
-          return code;
-        break;
-      default:
+  if ((code = take_contiguous(parser, type, &fields[3], token)) < 0)
+    return code;
+  switch (octets[1]) {
+    case 0:
+      /* no gateway requires a '.' as the relay in presentation format
+       * without parsing it into wireformat rdata */
+      if (!(token->length == 1 && *token->data == '.'))
         SYNTAX_ERROR(parser, "Invalid %s in %s", NAME(&fields[3]), NAME(type));
-    }
+      break;
+    case 1: /* IPv4 address */
+      type = (const type_info_t *)amtrelay_ipv4;
+      fields = type->rdata.fields;
+      if ((code = parse_ip4(parser, type, &fields[3], rdata, token)) < 0)
+        return code;
+      break;
+    case 2: /* IPv6 address */
+      type = (const type_info_t *)amtrelay_ipv6;
+      fields = type->rdata.fields;
+      if ((code = parse_ip6(parser, type, &fields[3], rdata, token)) < 0)
+        return code;
+      break;
+    case 3: /* domain name */
+      if ((code = parse_name(parser, type, &fields[3], rdata, token)) < 0)
+        return code;
+      break;
+    default:
+      SYNTAX_ERROR(parser, "Invalid %s in %s", NAME(&fields[3]), NAME(type));
   }
   octets[1] |= D;
   if ((code = take_delimiter(parser, type, token)) < 0)

tests/types.c

@@ -995,7 +995,7 @@ static const rdata_t doa2_rdata =
         );
 
 static const char amtrelay_text[] =
-  PAD("foo. AMTRELAY 200 0 0");
+  PAD("foo. AMTRELAY 200 0 0 .");
 static const char amtrelay_generic_text[] =
   PAD("foo. AMTRELAY \\# 2 c8 00");
 static const rdata_t amtrelay_rdata =