libs/libc/netdb: add dns clear interface

Signed-off-by: chao.an <anchao@xiaomi.com>

libs/libc/netdb/lib_dns.h

@@ -224,6 +224,21 @@ void dns_save_answer(FAR const char *hostname,
                      FAR const union dns_addr_u *addr, int naddr);
 #endif
 
+/****************************************************************************
+ * Name: dns_clear_answer
+ *
+ * Description:
+ *   Clear the resolved hostname in the DNS cache
+ *
+ * Returned Value:
+ *   None
+ *
+ ****************************************************************************/
+
+#if CONFIG_NETDB_DNSCLIENT_ENTRIES > 0
+void dns_clear_answer(void);
+#endif
+
 /****************************************************************************
  * Name: dns_find_answer
  *

libs/libc/netdb/lib_dnsaddserver.c

@@ -270,6 +270,9 @@ int dns_add_nameserver(FAR const struct sockaddr *addr, socklen_t addrlen)
 
   g_dns_nservers = nservers;
   dns_semgive();
+#if CONFIG_NETDB_DNSCLIENT_ENTRIES > 0
+  dns_clear_answer();
+#endif
   dns_notify_nameserver(addr, addrlen);
   return OK;
 }

libs/libc/netdb/lib_dnscache.c

@@ -164,6 +164,31 @@ void dns_save_answer(FAR const char *hostname,
   dns_semgive();
 }
 
+/****************************************************************************
+ * Name: dns_clear_answer
+ *
+ * Description:
+ *   Clear the resolved hostname in the DNS cache
+ *
+ * Returned Value:
+ *   None
+ *
+ ****************************************************************************/
+
+void dns_clear_answer(void)
+{
+  /* Get exclusive access to the DNS cache */
+
+  dns_semtake();
+
+  /* Reset the circular of DNS cache */
+
+  g_dns_head = 0;
+  g_dns_tail = 0;
+
+  dns_semgive();
+}
+
 /****************************************************************************
  * Name: dns_find_answer
  *