Implement RFC4075 Receive SNTP servers via DHCPv6

This adds support for RFC4075 SNTP server configuration via DHCPv6.
The DHCPv6 options transmitted are now conditional on how LwIP is
configured.

A new SNTP application option SNTP_GET_SERVERS_FROM_DHCPV6 is used
to enable. For simplicity this is configured to use the global
LWIP_DHCP6_GET_NTP_SRV configuration setting.

Tests:
 - Check the global options now control the DHCPv6 request sent
   in Wireshark
 - Check against 0, 1 and 3 SNTP servers configured on an odhcpd
   server configured to support RFC 4075 SNTP server lists.
   Verify that the SNTP server list is updated on connection
   establishment on an ESP8266 WeMOS D1.
 - Verify that SNTP packets are sent and recieved from a
   configured server and that system time is updated.

Signed-off-by: David J. Fiddes <D.J@fiddes.net>
(cherry picked from commit 8f2f43f093)

src/apps/sntp/sntp.c

@@ -238,9 +238,9 @@ struct sntp_server {
 };
 static struct sntp_server sntp_servers[SNTP_MAX_SERVERS];
 
-#if SNTP_GET_SERVERS_FROM_DHCP
+#if SNTP_GET_SERVERS_FROM_DHCP || SNTP_GET_SERVERS_FROM_DHCPV6
 static u8_t sntp_set_servers_from_dhcp;
-#endif /* SNTP_GET_SERVERS_FROM_DHCP */
+#endif /* SNTP_GET_SERVERS_FROM_DHCP || SNTP_GET_SERVERS_FROM_DHCPV6 */
 #if SNTP_SUPPORT_MULTIPLE_SERVERS
 /** The currently used server (initialized to 0) */
 static u8_t sntp_current_server;
@@ -648,6 +648,7 @@ void
 sntp_init(void)
 {
   /* LWIP_ASSERT_CORE_LOCKED(); is checked by udp_new() */
+  LWIP_DEBUGF(SNTP_DEBUG_TRACE, ("sntp_init: SNTP initialised\n"));
 
 #ifdef SNTP_SERVER_ADDRESS
 #if SNTP_SERVER_DNS
@@ -750,7 +751,7 @@ sntp_getreachability(u8_t idx)
 }
 #endif /* SNTP_MONITOR_SERVER_REACHABILITY */
 
-#if SNTP_GET_SERVERS_FROM_DHCP
+#if SNTP_GET_SERVERS_FROM_DHCP || SNTP_GET_SERVERS_FROM_DHCPV6
 /**
  * Config SNTP server handling by IP address, name, or DHCP; clear table
  * @param set_servers_from_dhcp enable or disable getting server addresses from dhcp
@@ -764,7 +765,7 @@ sntp_servermode_dhcp(int set_servers_from_dhcp)
     sntp_set_servers_from_dhcp = new_mode;
   }
 }
-#endif /* SNTP_GET_SERVERS_FROM_DHCP */
+#endif /* SNTP_GET_SERVERS_FROM_DHCP || SNTP_GET_SERVERS_FROM_DHCPV6 */
 
 /**
  * @ingroup sntp
@@ -816,6 +817,33 @@ dhcp_set_ntp_servers(u8_t num, const ip4_addr_t *server)
 }
 #endif /* LWIP_DHCP && SNTP_GET_SERVERS_FROM_DHCP */
 
+#if LWIP_IPV6_DHCP6 && SNTP_GET_SERVERS_FROM_DHCPV6
+/**
+ * Initialize one of the NTP servers by IP address, required by DHCPV6
+ *
+ * @param num the number of NTP server addresses to set must be < SNTP_MAX_SERVERS
+ * @param server array of IP address of the NTP servers to set
+ */
+void
+dhcp6_set_ntp_servers(u8_t num_ntp_servers, ip_addr_t* ntp_server_addrs)
+{
+  LWIP_DEBUGF(SNTP_DEBUG_TRACE, ("sntp: %s %u NTP server(s) via DHCPv6\n",
+                                 (sntp_set_servers_from_dhcp ? "Got" : "Rejected"),
+                                 num_ntp_servers));
+  if (sntp_set_servers_from_dhcp && num_ntp_servers) {
+    u8_t i;
+    for (i = 0; (i < num_ntp_servers) && (i < SNTP_MAX_SERVERS); i++) {
+      LWIP_DEBUGF(SNTP_DEBUG_TRACE, ("sntp: NTP server %u: %s\n",
+                                     i, ipaddr_ntoa(&ntp_server_addrs[i])));
+      sntp_setserver(i, &ntp_server_addrs[i]);
+    }
+    for (i = num_ntp_servers; i < SNTP_MAX_SERVERS; i++) {
+      sntp_setserver(i, NULL);
+    }
+  }
+}
+#endif /* LWIP_DHCPv6 && SNTP_GET_SERVERS_FROM_DHCPV6 */
+
 /**
  * @ingroup sntp
  * Obtain one of the currently configured by IP address (or DHCP) NTP servers

src/core/ipv6/dhcp6.c

@@ -451,7 +451,16 @@ dhcp6_msg_finalize(u16_t options_out_len, struct pbuf *p_out)
 static void
 dhcp6_information_request(struct netif *netif, struct dhcp6 *dhcp6)
 {
-  const u16_t requested_options[] = {DHCP6_OPTION_DNS_SERVERS, DHCP6_OPTION_DOMAIN_LIST, DHCP6_OPTION_SNTP_SERVERS};
+  const u16_t requested_options[] = {
+#if LWIP_DHCP6_PROVIDE_DNS_SERVERS
+    DHCP6_OPTION_DNS_SERVERS, 
+    DHCP6_OPTION_DOMAIN_LIST
+#endif
+#if LWIP_DHCP6_GET_NTP_SRV
+    , DHCP6_OPTION_SNTP_SERVERS
+#endif
+  };
+  
   u16_t msecs;
   struct pbuf *p_out;
   u16_t options_out_len;

src/include/lwip/apps/sntp.h

@@ -67,11 +67,11 @@ void sntp_setservername(u8_t idx, const char *server);
 const char *sntp_getservername(u8_t idx);
 #endif /* SNTP_SERVER_DNS */
 
-#if SNTP_GET_SERVERS_FROM_DHCP
+#if SNTP_GET_SERVERS_FROM_DHCP || SNTP_GET_SERVERS_FROM_DHCPV6
 void sntp_servermode_dhcp(int set_servers_from_dhcp);
-#else /* SNTP_GET_SERVERS_FROM_DHCP */
+#else /* SNTP_GET_SERVERS_FROM_DHCP || SNTP_GET_SERVERS_FROM_DHCPV6 */
 #define sntp_servermode_dhcp(x)
-#endif /* SNTP_GET_SERVERS_FROM_DHCP */
+#endif /* SNTP_GET_SERVERS_FROM_DHCP || SNTP_GET_SERVERS_FROM_DHCPV6 */
 
 #ifdef __cplusplus
 }

src/include/lwip/apps/sntp_opts.h

@@ -67,6 +67,12 @@
 #define SNTP_GET_SERVERS_FROM_DHCP LWIP_DHCP_GET_NTP_SRV
 #endif
 
+/** Set this to 1 to implement the callback function called by dhcpv6 when
+ * NTP servers are received. */
+#if !defined SNTP_GET_SERVERS_FROM_DHCPV6 || defined __DOXYGEN__
+#define SNTP_GET_SERVERS_FROM_DHCPV6 LWIP_DHCP6_GET_NTP_SRV
+#endif
+
 /** 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:
  * \#define SNTP_SERVER_ADDRESS "pool.ntp.org"