Merge branch 'feature/add_reserved_dns_server' into 'master'

ladd reserved DNS server to avoid DHCP changing all DNS server address

See merge request sdk/ESP8266_RTOS_SDK!958

components/lwip/Kconfig

@@ -275,10 +275,18 @@ config LWIP_IGMP
     bool "Enable IGMP module"
     default y
 
+config ESP_DNS
+    bool "Enable espressif advansed DNS"
+    default y
+    help
+        Enable this option, espressif advansed DNS functions will be enable. User can set the
+        reserved DNS server(its index is DNS_FALLBACK_SERVER_INDEX) which will not be changed by DHCP.
+
 config DNS_MAX_SERVERS
     int "The maximum of DNS servers"
-    range 1 5
-    default 2
+    range 2 5
+    default 3 if ESP_DNS
+    default 2 if !ESP_DNS
 
 menuconfig LWIP_NETIF_LOOPBACK
     bool "Enable per-interface loopback"

components/lwip/lwip/src/core/ipv4/dhcp.c

@@ -657,6 +657,12 @@ dhcp_handle_ack(struct netif *netif)
 #if LWIP_DHCP_PROVIDE_DNS_SERVERS
   /* DNS servers */
   for (n = 0; (n < LWIP_DHCP_PROVIDE_DNS_SERVERS) && dhcp_option_given(dhcp, DHCP_OPTION_IDX_DNS_SERVER + n); n++) {
+#if ESP_DNS
+    if (n == DNS_FALLBACK_SERVER_INDEX) {
+        continue;
+    }
+#endif
+
     ip_addr_t dns_addr;
     ip_addr_set_ip4_u32(&dns_addr, lwip_htonl(dhcp_get_option_value(dhcp, DHCP_OPTION_IDX_DNS_SERVER + n)));
     dns_setserver(n, &dns_addr);

components/lwip/port/esp8266/include/lwipopts.h

@@ -54,6 +54,12 @@
 
 #define ESP_LWIP 1
 
+#ifdef CONFIG_ESP_DNS
+#define ESP_DNS  1
+#else
+#define ESP_DNS  0
+#endif
+
 #ifdef CONFIG_LWIP_IPV6_MLD_SOCK
 #define ESP_LWIP_IPV6_MLD 1
 #else
@@ -831,6 +837,7 @@ size_t memp_malloc_get_size(size_t type);
  * DNS_SERVER_ADDRESS(ipaddr), where 'ipaddr' is an 'ip_addr_t*'
  */
 #define DNS_MAX_SERVERS                 CONFIG_DNS_MAX_SERVERS
+#define DNS_FALLBACK_SERVER_INDEX       (DNS_MAX_SERVERS - 1)
 
 /** DNS do a name checking between the query and the response. */
 #define DNS_DOES_NAME_CHECK             1