Improved manual DNS address addition for dual stack case

If DHCP or PPP does not provide DNS addresses, for dual stack, adds both ipv4
and ipv6 DNS addresses to DNS list.

Author: Mika Leppänen

features/FEATURE_LWIP/lwip-interface/lwip_stack.c

@@ -259,39 +259,88 @@ const ip_addr_t *mbed_lwip_get_ip_addr(bool any_addr, const struct netif *netif)
     return NULL;
 }
 
-void add_dns_addr(struct netif *lwip_netif)
+static void add_dns_addr_to_dns_list_index(const u8_t addr_type, const u8_t index)
 {
-    // Do nothing if not brought up
-    const ip_addr_t *ip_addr = mbed_lwip_get_ip_addr(true, lwip_netif);
-    if (!ip_addr) {
-        return;
-    }
-
-    // Check for existing dns server
-    for (char numdns = 0; numdns < DNS_MAX_SERVERS; numdns++) {
-        const ip_addr_t *dns_ip_addr = dns_getserver(numdns);
-        if (!ip_addr_isany(dns_ip_addr)) {
-            return;
-        }
-    }
-
 #if LWIP_IPV6
-    if (IP_IS_V6(ip_addr)) {
+    if (addr_type == IPADDR_TYPE_V6) {
         /* 2001:4860:4860::8888 google */
         ip_addr_t ipv6_dns_addr = IPADDR6_INIT(
                 PP_HTONL(0x20014860UL),
                 PP_HTONL(0x48600000UL),
                 PP_HTONL(0x00000000UL),
                 PP_HTONL(0x00008888UL));
-        dns_setserver(0, &ipv6_dns_addr);
+        dns_setserver(index, &ipv6_dns_addr);
     }
 #endif
-
 #if LWIP_IPV4
-    if (IP_IS_V4(ip_addr)) {
+    if (addr_type == IPADDR_TYPE_V4) {
         /* 8.8.8.8 google */
         ip_addr_t ipv4_dns_addr = IPADDR4_INIT(0x08080808);
-        dns_setserver(0, &ipv4_dns_addr);
+        dns_setserver(index, &ipv4_dns_addr);
+    }
+#endif
+}
+
+static int get_ip_addr_type(const ip_addr_t *ip_addr)
+{
+#if LWIP_IPV6
+    if (IP_IS_V6(ip_addr)) {
+        return IPADDR_TYPE_V6;
+    }
+#endif
+#if LWIP_IPV4
+    if (IP_IS_V4(ip_addr)) {
+        return IPADDR_TYPE_V4;
+    }
+#endif
+    return IPADDR_TYPE_ANY;
+}
+
+void add_dns_addr(struct netif *lwip_netif)
+{
+    // Check for existing dns address
+    for (char numdns = 0; numdns < DNS_MAX_SERVERS; numdns++) {
+        const ip_addr_t *dns_ip_addr = dns_getserver(numdns);
+        if (!ip_addr_isany(dns_ip_addr)) {
+            return;
+        }
+    }
+
+    // Get preferred ip version
+    const ip_addr_t *ip_addr = mbed_lwip_get_ip_addr(false, lwip_netif);
+    u8_t addr_type = IPADDR_TYPE_ANY;
+
+    // Add preferred ip version dns address to index 0
+    if (ip_addr) {
+        addr_type = get_ip_addr_type(ip_addr);
+        add_dns_addr_to_dns_list_index(addr_type, 0);
+    }
+
+#if LWIP_IPV4 && LWIP_IPV6
+    if (!ip_addr) {
+        // Get address for any ip version
+        ip_addr = mbed_lwip_get_ip_addr(true, lwip_netif);
+        if (!ip_addr) {
+            return;
+        }
+        addr_type = get_ip_addr_type(ip_addr);
+        // Add the dns address to index 0
+        add_dns_addr_to_dns_list_index(addr_type, 0);
+    }
+
+    if (addr_type == IPADDR_TYPE_V4) {
+        // If ipv4 is preferred and ipv6 is available add ipv6 dns address to index 1
+        ip_addr = mbed_lwip_get_ipv6_addr(lwip_netif);
+    } else if (addr_type == IPADDR_TYPE_V6) {
+        // If ipv6 is preferred and ipv4 is available add ipv4 dns address to index 1
+        ip_addr = mbed_lwip_get_ipv4_addr(lwip_netif);
+    } else {
+        ip_addr = NULL;
+    }
+
+    if (ip_addr) {
+        addr_type = get_ip_addr_type(ip_addr);
+        add_dns_addr_to_dns_list_index(addr_type, 1);
     }
 #endif
 }