Let netdb use a memp pool for allocating memory when getaddrinfo() is called.

src/include/lwip/dns.h

@@ -66,6 +66,13 @@
 #define DNS_RRCLASS_HS            4     /* Hesiod [Dyer 87] */
 #define DNS_RRCLASS_FLUSH         0x800 /* Flush bit */
 
+/* The size used for the next line is rather a hack, but it prevents including socket.h in all files
+   that include memp.h, and that would possibly break portability (since socket.h defines some types
+   and constants possibly already define by the OS).
+   Calculation rule:
+   sizeof(struct addrinfo) + sizeof(struct sockaddr_in) + DNS_MAX_NAME_LENGTH + 1 byte zero-termination */
+#define NETDB_ELEM_SIZE           (32 + 16 + DNS_MAX_NAME_LENGTH + 1)
+
 /** Callback which is invoked when a hostname is found.
  * A function of this type must be implemented by the application using the DNS resolver.
  * @param name pointer to the name that was looked up.
@@ -75,15 +82,10 @@
 */
 typedef void (*dns_found_callback)(const char *name, struct ip_addr *ipaddr, void *callback_arg);
 
-
 void           dns_init(void);
-
 void           dns_tmr(void);
-
 void           dns_setserver(u8_t numdns, struct ip_addr *dnsserver);
-
 struct ip_addr dns_getserver(u8_t numdns);
-
 err_t          dns_gethostbyname(const char *hostname, struct ip_addr *addr,
                                  dns_found_callback found, void *callback_arg);
 

src/include/lwip/memp_std.h

@@ -74,6 +74,9 @@ LWIP_MEMPOOL(SNMP_NODE,      MEMP_NUM_SNMP_NODE,       sizeof(struct mib_list_no
 LWIP_MEMPOOL(SNMP_VARBIND,   MEMP_NUM_SNMP_VARBIND,    sizeof(struct snmp_varbind),      "SNMP_VARBIND")
 LWIP_MEMPOOL(SNMP_VALUE,     MEMP_NUM_SNMP_VALUE,      SNMP_MAX_VALUE_SIZE,              "SNMP_VALUE")
 #endif /* LWIP_SNMP */
+#if LWIP_DNS && LWIP_SOCKET
+LWIP_MEMPOOL(NETDB,          MEMP_NUM_NETDB,           NETDB_ELEM_SIZE,               "NETDB")
+#endif /* LWIP_DNS && LWIP_SOCKET */
 
 /*
  * A list of pools of pbuf's used by LWIP.

src/include/lwip/opt.h

@@ -362,6 +362,14 @@
 #define MEMP_NUM_SNMP_VALUE             3
 #endif
 
+/**
+ * MEMP_NUM_NETDB: the number of concurrently running lwip_addrinfo() calls
+ * (before freeing the corresponding memory using lwip_freeaddrinfo()).
+ */
+#ifndef MEMP_NUM_NETDB
+#define MEMP_NUM_NETDB                  1
+#endif
+
 /**
  * PBUF_POOL_SIZE: the number of buffers in the pbuf pool. 
  */