lwip: menuconfig option to enable ntp servers option request via DHCP

This could be toggled on/off, off is the default.
SNTP debug option. Example update for ntp via DHCP

Signed-off-by: Emil Muratov <gpm@hotplug.ru>

components/lwip/Kconfig

@@ -846,13 +846,22 @@ menu "LWIP"
                 First argument of sntp_setserver/sntp_setservername functions
                 is limited to this value.
 
+        config LWIP_DHCP_GET_NTP_SRV
+            bool "Request NTP servers from DHCP"
+            default n
+            help
+                If enabled, LWIP will add 'NTP' to Parameter-Request Option sent via DHCP-request.
+                DHCP server might reply with an NTP server address in option 42.
+                SNTP callback for such replies should be set accordingly (see sntp_servermode_dhcp() func.)
+
         config LWIP_DHCP_MAX_NTP_SERVERS
             int "Maximum number of NTP servers aquired via DHCP"
             default 1
             range 1 16
+            depends on LWIP_DHCP_GET_NTP_SRV
             help
                 Set maximum number of NTP servers aquired via DHCP-offer.
-                Must be less or equal to total "Maximum number of NTP servers".
+                Should be less or equal to "Maximum number of NTP servers", any extra servers would be just ignored.
 
         config LWIP_SNTP_UPDATE_DELAY
             int "Request interval to update time (ms)"
@@ -1033,4 +1042,9 @@ menu "LWIP"
         depends on LWIP_DEBUG
         default n
 
+    config LWIP_SNTP_DEBUG
+        bool "Enable SNTP debug messages"
+        depends on LWIP_DEBUG
+        default n
+
 endmenu

components/lwip/port/esp32/include/lwipopts.h

@@ -910,6 +910,15 @@
 #define TCP_DEBUG                        LWIP_DBG_OFF
 #endif
 
+/**
+ * SNTP_DEBUG: Enable debugging for SNTP.
+ */
+#ifdef CONFIG_LWIP_SNTP_DEBUG
+#define SNTP_DEBUG                       LWIP_DBG_ON
+#else
+#define SNTP_DEBUG                       LWIP_DBG_OFF
+#endif
+
 /**
  * MEMP_DEBUG: Enable debugging in memp.c.
  */
@@ -1075,9 +1084,13 @@ LWIP_FORWARD_DECLARE_C_CXX void sntp_sync_time(struct timeval *tv);
 LWIP_FORWARD_DECLARE_C_CXX uint32_t sntp_get_sync_interval(void);
 
 // Max number of SNTP servers handled (default equal to LWIP_DHCP_MAX_NTP_SERVERS)
-#if CONFIG_LWIP_SNTP_MAX_SERVERS_DEFINED && !defined SNTP_MAX_SERVERS
+#if defined CONFIG_LWIP_SNTP_MAX_SERVERS
 #define SNTP_MAX_SERVERS                CONFIG_LWIP_SNTP_MAX_SERVERS
-#endif // CONFIG_LWIP_SNTP_MAX_SERVERS_DEFINED
+#endif // CONFIG_LWIP_SNTP_MAX_SERVERS
+
+#ifdef CONFIG_LWIP_DHCP_GET_NTP_SRV
+#define LWIP_DHCP_GET_NTP_SRV           CONFIG_LWIP_DHCP_GET_NTP_SRV
+#endif // CONFIG_LWIP_DHCP_GET_NTP_SRV
 
 /** Set this to 1 to support DNS names (or IP address strings) to set sntp servers
  * One server address/name can be defined as default if SNTP_SERVER_DNS == 1:

examples/protocols/sntp/main/sntp_example_main.c

@@ -122,6 +122,14 @@ static void obtain_time(void)
     ESP_ERROR_CHECK(esp_netif_init());
     ESP_ERROR_CHECK( esp_event_loop_create_default() );
 
+    /**
+     * NTP server address could be aquired via DHCP,
+     * see LWIP_DHCP_GET_NTP_SRV menuconfig option
+     */
+#ifdef LWIP_DHCP_GET_NTP_SRV
+    sntp_servermode_dhcp(1);
+#endif
+
     /* This helper function configures Wi-Fi or Ethernet, as selected in menuconfig.
      * Read "Establishing Wi-Fi or Ethernet Connection" section in
      * examples/protocols/README.md for more information about this function.