dname: add maximal label length constant

src/libknot/consts.h

@@ -34,8 +34,9 @@
 /*!
  * \brief Basic limits for domain names (RFC 1035).
  */
-#define KNOT_DNAME_MAXLEN 255     /*!< 1-byte maximum. */
-#define KNOT_DNAME_MAXLABELS 127  /*!< 1-char labels. */
+#define KNOT_DNAME_MAXLEN      255 /*!< 1-byte maximum. */
+#define KNOT_DNAME_MAXLABELS   127 /*!< 1-char labels. */
+#define KNOT_DNAME_MAXLABELLEN  63 /*!< 2^6 - 1 */
 
 /*!
  * \brief Address family numbers.

src/libknot/dname.c

@@ -73,8 +73,8 @@ int knot_dname_wire_check(const uint8_t *name, const uint8_t *endp,
 				is_compressed = true;
 			}
 		} else {
-			/* Check label length (maximum 63 bytes allowed). */
-			if (*name > 63)
+			/* Check label length. */
+			if (*name > KNOT_DNAME_MAXLABELLEN)
 				return KNOT_EMALF;
 			/* Check if there's enough space. */
 			int lblen = *name + 1;