lwip - Fixed missing dns servers after bringup with static ipv4 address

Generalized handling of dns servers when brought up with both ipv4 and
ipv6 addresses. Falls back to google dns servers if not dns server is
found through dhcp.

Also added support for the `add_dns_server` method to lwip to support
custom servers.

Author: geky

features/FEATURE_LWIP/lwip-interface/lwip_stack.c

@@ -243,37 +243,43 @@ const ip_addr_t *mbed_lwip_get_ip_addr(bool any_addr, const struct netif *netif)
     return NULL;
 }
 
-#if LWIP_IPV6
 void add_dns_addr(struct netif *lwip_netif)
 {
+    // Do nothing if not brought up
     const ip_addr_t *ip_addr = mbed_lwip_get_ip_addr(true, lwip_netif);
-    if (ip_addr) {
-        if (IP_IS_V6(ip_addr)) {
-            const ip_addr_t *dns_ip_addr;
-            bool dns_addr_exists = false;
-
-            for (char numdns = 0; numdns < DNS_MAX_SERVERS; numdns++) {
-                dns_ip_addr = dns_getserver(numdns);
-                if (!ip_addr_isany(dns_ip_addr)) {
-                   dns_addr_exists = true;
-                   break;
-                }
-            }
+    if (!ip_addr) {
+        return;
+    }
 
-            if (!dns_addr_exists) {
-                /* 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);
-            }
+    // 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)) {
+        /* 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);
+    }
 #endif
 
+#if LWIP_IPV4
+    if (IP_IS_V4(ip_addr)) {
+        /* 8.8.8.8 google */
+        ip_addr_t ipv4_dns_addr = IPADDR4_INIT(0x08080808);
+        dns_setserver(0, &ipv4_dns_addr);
+    }
+#endif
+}
+
 static sys_sem_t lwip_tcpip_inited;
 static void mbed_lwip_tcpip_init_irq(void *eh)
 {
@@ -505,9 +511,7 @@ nsapi_error_t mbed_lwip_bringup(bool dhcp, const char *ip, const char *netmask,
     }
 #endif
 
-#if LWIP_IPV6
     add_dns_addr(&lwip_netif);
-#endif
 
     lwip_connected = true;
     return 0;
@@ -618,6 +622,22 @@ static nsapi_error_t mbed_lwip_gethostbyname(nsapi_stack_t *stack, const char *h
     return 0;
 }
 
+static nsapi_error_t mbed_lwip_add_dns_server(nsapi_stack_t *stack, nsapi_addr_t addr)
+{
+    // Shift all dns servers down to give precedence to new server
+    for (int i = DNS_MAX_SERVERS-1; i > 0; i--) {
+        dns_setserver(i, dns_getserver(i-1));
+    }
+
+    ip_addr_t ip_addr;
+    if (!convert_mbed_addr_to_lwip(&ip_addr, &addr)) {
+        return NSAPI_ERROR_PARAMETER;
+    }
+
+    dns_setserver(0, &ip_addr);
+    return 0;
+}
+
 static nsapi_error_t mbed_lwip_socket_open(nsapi_stack_t *stack, nsapi_socket_t *handle, nsapi_protocol_t proto)
 {
     // check if network is connected
@@ -874,6 +894,7 @@ static void mbed_lwip_socket_attach(nsapi_stack_t *stack, nsapi_socket_t handle,
 /* LWIP network stack */
 const nsapi_stack_api_t lwip_stack_api = {
     .gethostbyname      = mbed_lwip_gethostbyname,
+    .add_dns_server     = mbed_lwip_add_dns_server,
     .socket_open        = mbed_lwip_socket_open,
     .socket_close       = mbed_lwip_socket_close,
     .socket_bind        = mbed_lwip_socket_bind,