Develop Reference

Android, communicate with mobile data while connected to wifi without internet access

I have an automotive companion app that needs to communicate with both wifi and mobile data networks.
My vehicle control unit provides a wifi network without internet access which exposes an API service that we can call from the app.
In addition to this we need to communicate with another backend reachable from the internet using phone mobile data (3G/4G).
I immediately noticed that some android phones, when connected to a wifi network without internet using android settings menu, show a system dialog informing user that current network has no internet access. Here user have two choises: keep this wifi network or disconnect and switch to another one.
Here some examples:
Samsung J7 - Android 7.0
Motorola moto G7 power - Android 9.0
Xiaomi mi 9T - Android 10
Huawei p9 lite - Android 6.0
After a short analysis I understood that if the user clicks 'NO' option, the system disconnects from the current wifi network and if another network is available connect to this.
If instead user clicks 'YES' option, we can have two differents behavior:
Phone keep connected to wifi network without internet access and all applications cannot communicate anymore with internet, because android try to use the wifi interface.
Phone keep connected to wifi network without internet access but android system rebind all existing sockets and those that will open in the future on moblie data network (if sim is available).
Then i tried same thing but using programmatic connection.
My sample code for differents android versions:
fun connectToWifi(ssid: String, key: String) {
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.Q) {
connectPost10(ssid, key)
} else {
connectPre10(ssid, key)
}
}
#RequiresApi(Build.VERSION_CODES.O)
private fun connectPost10(ssid: String, wpa2Passphrase: String) {
val specifier = WifiNetworkSpecifier.Builder()
.setSsid(ssid)
.setWpa2Passphrase(wpa2Passphrase)
.build()
val request = NetworkRequest.Builder()
.addTransportType(NetworkCapabilities.TRANSPORT_WIFI)
.setNetworkSpecifier(specifier)
.build()
val networkCallback = object: ConnectivityManager.NetworkCallback() {
override fun onAvailable(network: Network) {
val networkSSID = wifiManager.connectionInfo.ssid
.trim()
.removeSurrounding("\"")
if (networkSSID == "MY_NETWORK_WITHOUT_INTERNET_SSID") {
// i'm connected here
}
}
}
connectivityManager.requestNetwork(request, networkCallback)
}
private fun connectPre10(ssid: String, key: String) {
// setup wifi configuration
val config = WifiConfiguration().apply {
SSID = "\"$ssid\""
preSharedKey = "\"$key\""
}
val networkId = wifiManager.addNetwork(config)
wifiManager.disconnect() // disconnect from current (if connected)
wifiManager.enableNetwork(networkId, true) // enable next attempt
wifiManager.reconnect()
}
Please note that in order to read the network ssid android require ACCESS_FINE_LOCATION permission and GPS must be active.
I immediately noticed that using the programmatic connection the native popup didn't show up anymore, but on many devices, after connection, mobile data connectivity was "disabled" by android.
I suppose this behavior is wanted by the system and is determined by the fact that android prefers wifi over metered connections.
I'm ok with that, but what happen in my case where wifi network has no internet access? Other applications that require connectivity stops working because these cannot reach the internet.
I need a solution that allows my application to communicate via both wifi and 4g without preventing other applications from working properly.
My min sdk api level is 23 (Marshmallow), targeting 29 (Android 10).
I managed to solve the problem by saving the networks that come from callbacks registered on the connectivity manager.
val connectivityManager by lazy {
MyApplication.context.getSystemService(Context.CONNECTIVITY_SERVICE) as ConnectivityManager
}
private val wifiNetworkCallback = object : ConnectivityManager.NetworkCallback() {
// Called when the framework connects and has declared a new network ready for use.
override fun onAvailable(network: Network) {
super.onAvailable(network)
listener?.onWifiConnected(network)
}
// Called when a network disconnects or otherwise no longer satisfies this request or callback.
override fun onLost(network: Network) {
super.onLost(network)
listener?.onWifiDisconnected()
}
}
private val mobileNetworkCallback = object : ConnectivityManager.NetworkCallback() {
// Called when the framework connects and has declared a new network ready for use.
override fun onAvailable(network: Network) {
super.onAvailable(network)
connectivityManager.bindProcessToNetwork(network)
listener?.onMobileConnected(network)
}
// Called when a network disconnects or otherwise no longer satisfies this request or callback.
override fun onLost(network: Network) {
super.onLost(network)
connectivityManager.bindProcessToNetwork(null)
listener?.onMobileDisconnected()
}
}
private fun setUpWifiNetworkCallback() {
val request = NetworkRequest.Builder()
.addTransportType(NetworkCapabilities.TRANSPORT_WIFI)
.build()
try {
connectivityManager.unregisterNetworkCallback(wifiNetworkCallback)
} catch (e: Exception) {
Log.d(TAG, "WiFi Network Callback was not registered or already unregistered")
}
connectivityManager.requestNetwork(request, wifiNetworkCallback)
}
private fun setUpMobileNetworkCallback() {
val request = NetworkRequest.Builder()
.addCapability(NetworkCapabilities.NET_CAPABILITY_INTERNET)
.addTransportType(NetworkCapabilities.TRANSPORT_CELLULAR)
.build()
try {
connectivityManager.unregisterNetworkCallback(mobileNetworkCallback)
} catch (e: Exception) {
Log.d(TAG, "Mobile Data Network Callback was not registered or already unregistered")
}
connectivityManager.requestNetwork(request, mobileNetworkCallback)
}
Mark this "connectivityManager.bindProcessToNetwork()" we'll talk about it later.
Subsequently I created two different retrofit services:
Private Retrofit: bound on the network object I receive from the wifi callback, expose api to communicate with my vehicle within the local network.
Public Retrofit: bound on the network object I received from the mobile data callback, expose api to communicate with my backend and everything else that needs the internet.
At this point in my app I am able to redirect the traffic that passes through retrofit,
but how do the libraries that I include in the project understand which network they should use?
Answer: they don't understand it, in fact they try to use the wifi network getting a timeout error.
I noticed this behavior when I added google maps to my application, the canvas showed only an empty grid.
Since it is not possible to redirect google maps traffic through the public retrofit service that I created earlier i had to look for another solution to fix this.
Here it is ConnectivityManager.bindProcessToNetwork() that you have seen before!
https://developer.android.com/reference/android/net/ConnectivityManager#bindProcessToNetwork(android.net.Network).
With this method I am able to tell the process of my application that all the sockets created in the future must use the network that came from the callback of mobile data.
Using this trick, google maps and all the other libraries of which I cannot control connectivity, they will use the data connection to communicate with the internet and therefore they will work properly.
In some phones, especially the older versions of android, there is still the problem of other applications that remain without connectivity because they try to use wifi instead of using mobile data.
As a user I would be quite frustrated if using one app all the others won't work anymore.
So I would like to understand if there is a way to meet all my requirements in such a way that both my app and the others work without problems regardless of the Android version and the phone vendor.
In summary, my questions are:
Why "Network without internet access" popup does not appear if connection is made programmatically?
If Android known that my WiFi has no internet access, why others applications don't use mobile data network as fallback automatically?
Is possible to tell android that other applications must use a certain network to open future sockets?
Each vendor has custom WiFi settings to provide enhanced internet experience (Huawei WiFi+, Samsung Adaptive WiFi, Oppo WiFi Assistant, ...). I have noticed that in some phones activating it solve other applications problem, it seems that these features have permissions to rebind the entire application ecosystem on a specific network interface. How can these features help / hinder me? Is it possible to write some code that does the same thing these features do?

First, regarding your questions:
This behavior is reserved to the system app.
Android knows there is a healthy connection to the WiFi network. It does not check further to verify that there is no connection to the outside world. It is actually not always the desired behavior btw.
Yes, see below
In some aspects yes, see below
It seems to me that what you're looking for is to alter the default routing mechanism of Android.
That is, you would like all the traffic to the server(s) inside the WiFi network be routed to the WiFi network, while all other traffic be routed via the mobile data interface. There are a couple of ways to achieve this:
If your app is part of the infotainment system of the vehicle, and can possess system privileges, or alternatively, on a rooted Android phones, you can directly alter the routing table, using ip route commands.
What you described is actually part of the functionality of a Virtual Private Network (VPN). You can implement a VPN service yourself server side and client side, based on open source solutions such as OpenVPN, in which the VPN server would be inside the wifi network. Android has prebuilt infrastructure for implementing the VPN client: https://developer.android.com/guide/topics/connectivity/vpn
You can use commercial VPN solutions. Some of them allow the configuration you're looking for, and I believe will meet the needs you described.

Use multiple network interfaces in an app

I wrote an app that is triggering a Sony qx smartphone attachable camera over wifi. However I need to transfer the images off the phone over another local network in real time. Since the wifi card is being used for qx connection I need to be able to use ethernet over usb for transferring images off the phone. Http requests will be used to trigger the camera and send the images off the phone.
Is it possible in one android app on a phone with two network interfaces setup to specify for certain http requests to use one network interface and for others to use another network interface ? Does this need to be done through routing tables, not java?
The phone I'm using is a rooted nexus 6p.
Update:
Currently, I was able to get an Ethernet adapter working with the device (Nexus 6P). The device is connected to a local network over Ethernet. When the Wi-Fi interface is off, I can ping all devices on the local network the device is connected to over Ethernet. However, I am unable to access the web servers (Not using DNS) of any of the devices on that network (which I know they are running), i.e. Http via a browser app. The nexus 6p is connected to the network over Ethernet via a Ubiquiti Station. This seems to be a routing issue.
I can tether(usb interface) and use Wi-Fi in one app, so that leads me to believe it is possible to use Ethernet and Wi-Fi.
Update2:
After more testing, it seems to be that it is a permissions issue. Since when I ping the network the device is connected to over Ethernet without first running su in the terminal the network doesn't exist. However, when I run su then ping, I can ping the network. Thus it seems my app needs to get superuser permission before accessing Ethernet. I've granted it superuser access, but nothing has changed. I read that simply running su isn't enough from one of the comments in this post. This is because su just spawns a root shell that dies. This also explains why I couldn't access any of the web servers on this network via a browser app. Is it possible to grant my app access to the Ethernet interface when making HTTP calls like give HttpURLConnection root access, if that makes any sense (running su doesn't work)? There seems to definitely be a solution since HttpURLConnection can make calls over the USB tethering interface (Nexus 6P calls it rndis0) fine.
Update 3:
I found online here , that I can make my app a System app (thought this might grant the app eth0 access). I just moved my app to /system/app and then rebooted. However, this didn't seem to give the app anymore privileges (thus not solving the problem) , or there is something else required to make the app system than just copying it to /system/app.
Update 4:
So I was able to get Ethernet working on every app without root permissions! It seemed to be that it only works over DHCP and does not like static connections, which I was using. It works with Wi-Fi enabled, however, I cannot contact any of the devices on the Wi-Fi network when Ethernet is enabled. Is there a way around this? Does it have to do with setting two default gateways?

Since you were programming in Nexus 6P, you can try to use the new API added in ConnectivityManager to select the ethernet as your preferred network connection for your process.
Since I can't build the similar environment like yours, I am not sure if it works. It's just a suggested solution, totally not tested and verified.
ConnectivityManager connectivityManager = (ConnectivityManager) getSystemService(CONNECTIVITY_SERVICE);
Network etherNetwork = null;
for (Network network : connectivityManager.getAllNetworks()) {
NetworkInfo networkInfo = connectivityManager.getNetworkInfo(network);
if (networkInfo.getType() == ConnectivityManager.TYPE_ETHERNET) {
etherNetwork = network;
}
}
Network boundNetwork = connectivityManager.getBoundNetworkForProcess();
if (boundNetwork != null) {
NetworkInfo boundNetworkInfo = connectivityManager.getNetworkInfo(boundNetwork);
if (boundNetworkInfo.getType() != ConnectivityManager.TYPE_ETHERNET) {
if (etherNetwork != null) {
connectivityManager.bindProcessToNetwork(etherNetwork);
}
}
}

Just to give a little more explanation on how this finally got solved.
Utilizing #alijandro's answer I was able to switch back and forth between Ethernet and Wi-Fi in one app. For some reason for the Ethernet to work it required the network gateway to supply DHCP address, not static. Then since the bindProcessToNetwork, used in #alijandro's answer is per-process, I decided to split communications with the QX camera into a Service that runs in a separate Process. The main Application (another process) would post images over Ethernet to a local network. I was successfully able to contact the devices on the local network via HTTP over Ethernet while simultaneously triggering the QX over Wi-Fi. Currently, I used Messenger to communicate using IPC to tell the QX triggering Service what methods to call.

Most of android tv boxes can use wifi and ethernet together. In my device, i can enable ethernet from this path ---
Settings -> More ... > Ethernet ---
But your device wont have a menu like that as i understand. So you should make an app to do that. This application needs to access some system specific resources so your device needs to be rooted or application needs to signed with system signature.
Also this topic can help you link

There is an easy way to do this that will answer the OP's original question about how to do this with a single application (not two separate app processes) using ConnectivityManager.requestNetwork().
The docs for ConnectivityManager.requestNetwork() allude to this:
... For example, an application could use this method to obtain a
connected cellular network even if the device currently has a data
connection over Ethernet. This may cause the cellular radio to consume
additional power. Or, an application could inform the system that it
wants a network supporting sending MMSes and have the system let it
know about the currently best MMS-supporting network through the
provided NetworkCallback. ...
For OP's scenario of using Wi-Fi for some traffic and ethernet for other traffic one only needs to call ConnectivityManager.requestNetwork() twice with two separate requests. One for TRANSPORT_WIFI and one for TRANSPORT_ETHERNET. The operative item here is we need a way to uniquely identify these networks. For OP's scenario, we can use transport type.
final NetworkRequest requestForWifi =
new NetworkRequest.Builder()
.addTransportType(NetworkCapabilities.TRANSPORT_WIFI)
.build();
final NetworkRequest requestForEthernet =
new NetworkRequest.Builder()
.addTransportType(NetworkCapabilities.TRANSPORT_ETHERNET)
.build();
final ConnectivityManager connectivityManager = (ConnectivityManager)
context.getSystemService(Context.CONNECTIVITY_SERVICE);
final NetworkCallback networkCallbackWifi = new NetworkCallback() {
#Override
void onAvailable(Network network) {
// Triggers when this network is available so you can bind to it.
}
#Override
void onLost(Network network) {
// Triggers when this network is lost.
}
};
final NetworkCallback networkCallbackEthernet = new NetworkCallback() {
#Override
void onAvailable(Network network) {
// Triggers when this network is available so you can bind to it.
}
#Override
void onLost(Network network) {
// Triggers when this network is lost.
}
};
connectivityManager.requestNetwork(requestForWifi, networkCallbackWifi);
connectivityManager.requestNetwork(requestForEthernet, networkCallbackEthernet);
Then, once the callbacks trigger, you can then in the pertinent code (e.g. OP's code for transferring images), listen for onAvailable(Network network) and use the provided Network with Network.OpenConnection() to connect to an HTTP server using that network.
This would allow you to connect to two separate Networks from the same application.

How to stay connected through mobile network after WIFI is connected on Android?

I noticed that while streaming audio from a remote server through 3G (mobile) connection and while the WIFI is disconnected or OFF, as soon as WIFI is activated and connected, connection through 3G is dropped.
I want the app keep using 3G even if WIFI is connected too now. I want to do this to keep continuity. (User may opt-in/out to/from this behaviour).
Is there a special flag, lock, etc.. For this purpose?

This isn't possible on devices before Android 5.0 (Lollipop). The OS only keeps one interface up at a time, and applications don't have any control over this choice.
On devices running Android 5.0 or newer, you can use the new multi-networking APIs to pick which interface you want to use for network traffic.
Here's the steps to do this, from the Android 5.0 changelog:
To select and connect to a network dynamically from your app, follow these steps:
Create a ConnectivityManager.
Use the NetworkRequest.Builder class to create an NetworkRequest object and specify the network features and transport type your app is interested in.
To scan for suitable networks, call requestNetwork() or registerNetworkCallback(), and pass in the NetworkRequest object and an implementation of ConnectivityManager.NetworkCallback. Use the requestNetwork() method if you want to actively switch to a suitable network once it’s detected; to receive only notifications for scanned networks without actively switching, use the registerNetworkCallback() method instead.
When the system detects a suitable network, it connects to the network and invokes the onAvailable() callback. You can use the Network object from the callback to get additional information about the network, or to direct traffic to use the selected network.
Specifically, if you want to force your traffic over 3G/LTE, even if there's a WiFi signal present, you'd use something like this:
ConnectivityManager cm = (ConnectivityManager) context.getSystemService(
Context.CONNECTIVITY_SERVICE);
NetworkRequest.Builder req = new NetworkRequest.Builder();
req.addTransportType(NetworkCapabilities.TRANSPORT_CELLULAR);
cm.requestNetwork(req.build(), new ConnectivityManager.NetworkCallback() {
#Override
public void onAvailable(Network network) {
// If you want to use a raw socket...
network.bindSocket(...);
// Or if you want a managed URL connection...
URLConnection conn = network.openConnection(...);
}
// Be sure to override other options in NetworkCallback() too...
}

Force the use of the cellular connection on android while connected to WiFi

I own a diagnostic device that also acts as a Wifi access point. Any device can connect to that access point and request information from the device. The device obviously does not provide internet access over that network.
I tried to create an android app that requests information from the device, using the provided Wifi network, and then uploads the data to some server in the internet using the cellular connection.
Android, however, seems to deactivate the cellular connection whenever a Wifi network is connected. Since the Wifi network is not connected to the internet, the data cannot be uploaded, and the app is useless.
So far I tried to use the ConnectivityManager to request the mobile network and bind it to my communication socket. Also, I tried to iterate all network interfaces and bind the communication socket to the IP address of the cellular interface. However, both requests failed since I was not able to query the cellular network interface from the app.
How could I manage to forward the data from the device in the Wifi network to some server over the cellular internet connection?

I filed a bug report for this issue.
The issue should be fixed in Android version 5.1.0

Here is the right sample code.
NetworkRequest cellularRequest = new NetworkRequest.Builder()
.addTransportType(NetworkCapabilities.TRANSPORT_CELLULAR)
.addCapability(NetworkCapabilities.NET_CAPABILITY_INTERNET).build();
if (connectivityManager != null) {
connectivityManager.requestNetwork(cellularRequest, new ConnectivityManager.NetworkCallback() {
#Override
public void onAvailable(#NonNull Network network) {
super.onAvailable(network);
// do request with the network
OkHttpClient.Builder builder = new OkHttpClient.Builder();
builder.socketFactory(network.getSocketFactory());
OkHttpClient client = builder.build();
Call call = client.newCall(request);
Response response = call.execute();
// do remove callback. if you forget to remove it, you will received callback when cellular connect again.
connectivityManager.unregisterNetworkCallback(this);
}
#Override
public void onUnavailable() {
super.onUnavailable();
// do remove callback
connectivityManager.unregisterNetworkCallback(this);
}
});
}

How to detect if a network is (configured as) a mobile hotspot on Android?

As of Android 4.1, your device can detect if it's connected to a mobile hotspot (given that the mobile hotspot is also running Android 4.1 or higher). Also, you have the option to flag networks as mobile hotspots (under Settings / Data Usage / Overflow menu / Mobile Hotspots).
But how do I detect this as a -user- I meant developer? It's not stored in the WifiConfiguration, so where is it?
Some context: I want to build a simple tool for Android that checks if you are connected to a network that you or Android has flagged as a mobile hotspot. If so, it will check if no other (non-hotspot) networks are available. If so, it should connect to these other networks since those should be much faster and have no data cap. Why? Because my phones and tablets connect to (mobile) hotspots quite often, even when a better network is available.
Here is some pseudo code of what I'm looking for:
// Check if android has detected mobile hotspot
WifiManager wifiMgr = getSystemService(Context.WIFI_SERVICE);
WifiInfo wifiInfo = wifiMgr .getConnectionInfo();
boolean isMobileHotspot = wifiInfo.isMobileHotspot;
UPDATE Jul 3rd 2014
Okay so Matiash' answer is good but ConnectivityManager.isActiveNetworkMetered() will only return the value for the current network. I do need that, so it helped me along, but it bring me to the next part in my tool/app:
IF the device is connected to a mobile hotspot (or a 'metered network' as Android calls it) I want to check if any of the nearby access points is a better option. So I need to know whether any of the known AP's (WifiManager.getConfiguredNetworks()) is also flagged as such before I connect to it...
I have a List<ScanResult> and a List<WifiConfiguration>, looks like neither of them has this information.
Which bring me back to my initial question: Is there a way to retrieve the Mobile Hotspots (as configured by Android and/or user) under Data Usage? And this time I mean ALL of them.
UPDATE Jul 7th 2014
I've posted a feature request in the AOSP Issue Tracker for access (readonly) to the NetworkPolicyManager. Plz vote on it here: https://code.google.com/p/android/issues/detail?id=73206&thanks=73206&ts=1404719243

You can access this information by calling ConnectivityManager.isActiveNetworkMetered().
This will return whether the active connection is a hotspot (as defined in Data Usage -> Mobile Hotspots).
About the second part, I'm sorry but I don't think that's possible. The flag is not public, and even if you get the object that could be used to retrieve it (android.net.NetworkPolicyManager) by reflection:
Object npm = Class.forName("android.net.NetworkPolicyManager").getDeclaredMethod("from", Context.class).invoke(null, this);
Object policies = npm.getClass().getDeclaredMethod("getNetworkPolicies").invoke(npm);
calling getNetworkPolicies() requires the MANAGE_NETWORK_POLICY permission, which cannot be obtained by non-system apps, because it has a "signature" protection level.
I hope to be proved incorrect though. :) Maybe looking at the source code of the Android activity that manages this information (https://github.com/android/platform_packages_apps_settings/blob/master/src/com/android/settings/net/DataUsageMeteredSettings.java), in particular the buildWifiPref() method, will provide some clue.

I do not know if what you want is possible but you can check whether your device is connected to a network by checking the ip.
You can use the tool below to see if you has ip, and shalt know if he is connected to a network or not.
public static Boolean check_connection(final Context _context)
{
boolean connected;
ConnectivityManager conectivtyManager = (ConnectivityManager) _context
.getSystemService(Context.CONNECTIVITY_SERVICE);
if (conectivtyManager.getActiveNetworkInfo() != null
&& conectivtyManager.getActiveNetworkInfo().isAvailable()
&& conectivtyManager.getActiveNetworkInfo().isConnected())
{
connected = true;
} else
{
connected = false;
}
return connected;
}

//Check if hotspot tethering is enabled
try {
ConnectivityManager connectivityManager = (ConnectivityManager)getSystemService(Context.CONNECTIVITY_SERVICE);
boolean isMobileData = connectivityManager.isActiveNetworkMetered();
if(isMobileData) {
List<NetworkInterface> interfaces = Collections.list(NetworkInterface.getNetworkInterfaces());
for (NetworkInterface networkInterface : interfaces) {
if (networkInterface.getName().equals("ap0")) {
//Tethering is enabled
SendHotspotEnabledHandler sendHotspotEnabledHandler = new SendHotspotEnabledHandler(new WeakReference<Context>(SendInstalledAppsService.this));
sendHotspotEnabledHandler.execute();
break;
}
}
}
} catch (SocketException e) {
}