dns: Add API to clear dns cache

src/core/dns.c

@@ -396,6 +396,12 @@ dns_clear_servers(bool keep_fallback)
     dns_setserver(numdns, NULL);
   }
 }
+
+void
+dns_clear_cache(void)
+{
+  memset(dns_table, 0, sizeof(struct dns_table_entry) * DNS_TABLE_SIZE);
+}
 #endif
 
 /**

src/include/lwip/dns.h

@@ -113,6 +113,7 @@ err_t            dns_gethostbyname_addrtype(const char *hostname, ip_addr_t *add
                                    u8_t dns_addrtype);
 #if ESP_DNS
 void             dns_clear_servers(bool keep_fallback);
+void             dns_clear_cache(void);
 #endif
 
 #if DNS_LOCAL_HOSTLIST