Add DNS servers from cellular PDP to nsapi

UNITTESTS/features/cellular/framework/AT/at_cellularcontext/unittest.cmake

@@ -41,4 +41,5 @@ set(unittest-test-sources
   stubs/SerialBase_stub.cpp
   stubs/CellularContext_stub.cpp
   stubs/CellularUtil_stub.cpp
+  stubs/SocketAddress_stub.cpp
 )

UNITTESTS/stubs/AT_CellularContext_stub.cpp

@@ -287,3 +287,8 @@ void AT_CellularContext::do_connect_with_retry()
 {
 
 }
+
+char *AT_CellularContext::get_interface_name(char *interface_name)
+{
+    return NULL;
+}

features/cellular/framework/AT/AT_CellularContext.cpp

@@ -239,6 +239,16 @@ const char *AT_CellularContext::get_ip_address()
 #endif
 }
 
+char *AT_CellularContext::get_interface_name(char *interface_name)
+{
+    if (_cid < 0) {
+        return NULL;
+    }
+    MBED_ASSERT(interface_name);
+    sprintf(interface_name, "ce%02d", _cid);
+    return interface_name;
+}
+
 void AT_CellularContext::attach(Callback<void(nsapi_event_t, intptr_t)> status_cb)
 {
     _status_cb = status_cb;
@@ -458,7 +468,30 @@ nsapi_error_t AT_CellularContext::do_activate_context()
 
 nsapi_error_t AT_CellularContext::activate_ip_context()
 {
-    return find_and_activate_context();
+    nsapi_error_t ret = find_and_activate_context();
+#if !NSAPI_PPP_AVAILABLE
+    if (ret == NSAPI_ERROR_OK) {
+        pdpContextList_t params_list;
+        if (get_pdpcontext_params(params_list) == NSAPI_ERROR_OK) {
+            pdpcontext_params_t *pdp = params_list.get_head();
+            while (pdp) {
+                SocketAddress addr;
+                if (addr.set_ip_address(pdp->dns_secondary_addr)) {
+                    tr_info("DNS secondary %s", pdp->dns_secondary_addr);
+                    char ifn[5]; // "ce" + two digit _cid + zero
+                    add_dns_server(addr, get_interface_name(ifn));
+                }
+                if (addr.set_ip_address(pdp->dns_primary_addr)) {
+                    tr_info("DNS primary %s", pdp->dns_primary_addr);
+                    char ifn[5]; // "ce" + two digit _cid + zero
+                    add_dns_server(addr, get_interface_name(ifn));
+                }
+                pdp = pdp->next;
+            }
+        }
+    }
+#endif
+    return ret;
 }
 
 nsapi_error_t AT_CellularContext::activate_non_ip_context()

features/cellular/framework/AT/AT_CellularContext.h

@@ -36,6 +36,7 @@ class AT_CellularContext : public CellularContext, public AT_CellularBase {
     virtual nsapi_error_t set_blocking(bool blocking);
     virtual NetworkStack *get_stack();
     virtual const char *get_ip_address();
+    virtual char *get_interface_name(char *interface_name);
     virtual void attach(mbed::Callback<void(nsapi_event_t, intptr_t)> status_cb);
     virtual nsapi_error_t connect();
     virtual nsapi_error_t disconnect();

features/netsocket/nsapi_dns.cpp

@@ -131,6 +131,13 @@ static bool dns_timer_running = false;
 // DNS server configuration
 extern "C" nsapi_error_t nsapi_dns_add_server(nsapi_addr_t addr, const char *interface_name)
 {
+    // check if addr was already added
+    for (int i = 0; i < DNS_SERVERS_SIZE; i++) {
+        if (memcmp(&addr, &dns_servers[i], sizeof(nsapi_addr_t)) == 0) {
+            return NSAPI_ERROR_OK;
+        }
+    }
+
     memmove(&dns_servers[1], &dns_servers[0],
             (DNS_SERVERS_SIZE - 1)*sizeof(nsapi_addr_t));