net: dns: Allow user to specify max answer length via Kconfig

If the default max. answer length of 512 bytes is not enough,
user can specify it by setting CONFIG_DNS_RESOLVER_MAX_ANSWER_SIZE
to suitable value.

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>

include/zephyr/net/dns_resolve.h

@@ -56,7 +56,11 @@ enum dns_query_type {
 #define DNS_BUF_TIMEOUT K_MSEC(500) /* ms */
 
 /* This value is recommended by RFC 1035 */
+#if defined(CONFIG_DNS_RESOLVER_MAX_ANSWER_SIZE)
+#define DNS_RESOLVER_MAX_BUF_SIZE CONFIG_DNS_RESOLVER_MAX_ANSWER_SIZE
+#else
 #define DNS_RESOLVER_MAX_BUF_SIZE 512
+#endif /* CONFIG_DNS_RESOLVER_MAX_ANSWER_SIZE */
 
 /* Make sure that we can compile things even if CONFIG_DNS_RESOLVER
  * is not enabled.

subsys/net/lib/dns/Kconfig

@@ -61,6 +61,14 @@ config DNS_RESOLVER_MAX_SERVERS
 	  DNS server is enough. Each connection to DNS server will use one
 	  network context.
 
+config DNS_RESOLVER_MAX_ANSWER_SIZE
+	int "Max length of a DNS answer"
+	range 1 $(UINT16_MAX)
+	default 512
+	help
+	  Max length of the returned DNS answer we can process.
+	  Recommended value by RFC 1035 is 512 bytes.
+
 config DNS_RESOLVER_MAX_QUERY_LEN
 	int "Max length of a DNS query"
 	range 1 $(UINT8_MAX)

subsys/net/lib/dns/llmnr_responder.c

@@ -53,7 +53,6 @@ static struct net_mgmt_event_callback mgmt_cb;
 #define BUF_ALLOC_TIMEOUT K_MSEC(100)
 
 /* This value is recommended by RFC 1035 */
-#define DNS_RESOLVER_MAX_BUF_SIZE	512
 #define DNS_RESOLVER_MIN_BUF		2
 #define DNS_RESOLVER_BUF_CTR	(DNS_RESOLVER_MIN_BUF + \
 				 CONFIG_LLMNR_RESOLVER_ADDITIONAL_BUF_CTR)

tests/net/lib/dns_packet/src/main.c

@@ -1144,7 +1144,6 @@ static void run_dns_valid_response(const char *test_case,
 		      test_case, ret);
 }
 
-#define DNS_RESOLVER_MAX_BUF_SIZE	512
 #define DNS_RESOLVER_MIN_BUF	1
 #define DNS_RESOLVER_BUF_CTR	(DNS_RESOLVER_MIN_BUF + 1)
 #define DNS_MAX_NAME_LEN	255