sock_dns: remove some magic numbers

sys/include/net/dns.h

+/*
+ * Copyright (C) 2019 Freie Universität Berlin
+ *
+ * This file is subject to the terms and conditions of the GNU Lesser
+ * General Public License v2.1. See the file LICENSE in the top level
+ * directory for more details.
+ */
+
+/**
+ * @defgroup    net_dns DNS defines
+ * @ingroup     net
+ * @brief       Generic DNS values
+ * @{
+ *
+ * @file
+ * @brief   Generic DNS values
+ *
+ * @author  Martine Lenders <m.lenders@fu-berlin.de>
+ */
+#ifndef NET_DNS_H
+#define NET_DNS_H
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ * @name    Field lengths
+ * @{
+ */
+#define RR_TYPE_LENGTH      (2U)
+#define RR_CLASS_LENGTH     (2U)
+#define RR_TTL_LENGTH       (4U)
+#define RR_RDLENGTH_LENGTH  (2U)
+/** @} */
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* NET_DNS_H */
+/** @} */

sys/net/application_layer/dns/dns.c

@@ -19,6 +19,7 @@
 #include <string.h>
 #include <stdio.h>
 
+#include "net/dns.h"
 #include "net/sock/udp.h"
 #include "net/sock/dns.h"
 
@@ -114,7 +115,8 @@ static int _parse_dns_reply(uint8_t *buf, size_t len, void* addr_out, int family
             return tmp;
         }
         bufpos += tmp;
-        bufpos += 4;    /* skip type and class of query */
+        /* skip type and class of query */
+        bufpos += (RR_TYPE_LENGTH + RR_CLASS_LENGTH);
     }
 
     for (unsigned n = 0; n < ntohs(hdr->ancount); n++) {
@@ -123,14 +125,14 @@ static int _parse_dns_reply(uint8_t *buf, size_t len, void* addr_out, int family
             return tmp;
         }
         bufpos += tmp;
-        if ((bufpos + 2 + 2 + 4) >= buflim) {
+        if ((bufpos + RR_TYPE_LENGTH + RR_CLASS_LENGTH + RR_TTL_LENGTH) >= buflim) {
             return -EBADMSG;
         }
         uint16_t _type = ntohs(_get_short(bufpos));
-        bufpos += 2;
+        bufpos += RR_TYPE_LENGTH;
         uint16_t class = ntohs(_get_short(bufpos));
-        bufpos += 2;
-        bufpos += 4; /* skip ttl */
+        bufpos += RR_CLASS_LENGTH;
+        bufpos += RR_TTL_LENGTH; /* skip ttl */
 
         unsigned addrlen = ntohs(_get_short(bufpos));
         /* skip unwanted answers */
@@ -154,7 +156,7 @@ static int _parse_dns_reply(uint8_t *buf, size_t len, void* addr_out, int family
              (family == AF_UNSPEC))) {
             return -EBADMSG;
         }
-        bufpos += 2;
+        bufpos += RR_RDLENGTH_LENGTH;
         if ((bufpos + addrlen) >= buflim) {
             return -EBADMSG;
         }