Develop Reference

Android TRANSPORT_CELLULAR network not available if wifi is connected. How do we make it available?

The moment I get on a wifi connection, the cellular network is completely lost even though the cellular network indicator is definitely on.
This is my network request
val request = NetworkRequest.Builder().run {
addTransportType(NetworkCapabilities.TRANSPORT_CELLULAR)
build()
}
connectivityManager.registerNetworkCallback(request, callback)
I've tried looking in the connectivityManager.allNetworks list and it's no where to be found. Only the wifi network is in there.
What's even weirder is there is one other cellular network that is always there. It does not have the same ID as my cellular network. There's no connection that can be made with it. It never shows up with registerNetworkCallback. The capabilities on it always include "valid" and "internet"
What am I seeing here? Why is my cellular network lost? What is this phantom cellular network?
targetSdkVersion: 29
Device: Galaxy S10 - Android 12

I figured this out.
If you call registerNetworkCallback the above will happen, but if you call requestNetwork with TRANSPORT_CELLULAR,
connectivityManager.requestNetwork(request, callback)
Android will keep the cellular network around. I was so confused because the documentation was so lacking. Once you do that, it will ask you to add the CHANGE_NETWORK_STATE permission.
After this step, the network is available, but you won't be able to make any request with it. You have to call
connectivityManager.bindProcessToNetwork(theCellularNetwork)
to get any connection.
After this is done, the cellular network can be used in tandem with the wifi network. You can even send some traffic to one and some to the other. If you use OkHttp like I do, you just bind the client with the network's socketFactory
val client = OkHttpClient().newBuilder().run {
socketFactory(network.socketFactory)
build()
}
client.newCall(
Request.Builder().url("https://example.com").build()
).execute().let {
Log.i(TAG, "Fetched ${it.body!!.string()}")
}

The cellular network isn't lost, but your app isn't allowed to use it. Once WiFi is connected, everything is forced to use that connection. The only exception to this rule is if your phone has a feature called "Dual Acceleration", which allows the cellular connection to stay active (and obviously, the user would have to enable that feature). Alternatively, you may have a setting in your phone's Developer Options called "Cellular Data Always Active", which will do the same thing.
But needless to say, you can't rely on either of those 2 features being enabled in a production environment. So, just assume that when WiFi is connected, that's the only connection that your app can use

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.

Is the string "captive_portal_detected" from NetworkInfo getExtraInfo() a reliable indication of captive portal?

I have a Samsung 6.0.1 that I am using in my development. I want to know if a network is captive portal. There have been recent changes that make this info known to the developer, which is nice. But there doesn't seem to be a way to just retrieve the data from NetworkInfo.
I have found that if I can get my hands on an instance of Wi-Fi NetworkInfo I can call getExtraInfo and an indication of captive portal will be in there as a string "captive_portal_detected" It's weird this is a string and not a boolean property, but that's not my question.
What version of Android did this string start popping up and is this part of the regular OS or something the Samsung guys popped in for the s7? Can I rely on this string as an indicator that the access point associated with the NetworkInfo is in fact captive portal? There is no documentation on this value that I can find anywhere.

In the documentation it tells that the return of the method getExtraInfo returns the information provided by the lower network layers: http://developer.android.com/intl/pt-br/reference/android/net/NetworkInfo.html#getExtraInfo()
So if getExtraInfo will return if it is in a captive portal its determined by the network hardware, I recommend that you avoid being dependent of detemined hardware.
Here are an example of how to discover if the user are in captive portal: How to check for unrestricted Internet access? (captive portal detection)

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 HttpURLConnection to use mobile network and fallback to WiFi

My application uses HttpURLConnection to connect to my REST services. I log errors and noticed that what happens occasionally is that user get's WiFi connection but it has proxy.
For example, those airport wifi's that redirect you to pay pages and then let you use internet. My code does not follow redirects.
What I really want is to ignore presence of WiFi and force communication over 3G/4G/E whatever. How can I do that on Android?

Switch to mobile network:
As soon as you detect a proxy, pop up a dialog telling the user that your app cannot use that network and hence you are switching to the mobile network. You can switch to a mobile network using ConnectivityManagerclass.
ConnectivityManager cm;
cm = (ConnectivityManager)getSystemService(Context.CONNECTIVITY_SERVICE);
cm.setNetworkPreference(ConnectivityManager.TYPE_MOBILE);
and switch back to the default when you are done:
cm.setNetworkPreference(ConnectivityManager.DEFAULT_NETWORK_PREFERENCE);
Detect a proxy:
Detect proxy using the following snippet
HttpURLConnection conn;
...
if (conn.getResponseCode() == HTTP_PROXY_AUTH){
// You got a '407: Proxy authentication required' response.
// Set the networkPreference() here and retry when
// network connection changes to TYPE_MOBILE.
}
You can check this post to know how to use a HttpURLConnection through a proxy : How do I make HttpURLConnection use a proxy?
Detect a 'network change':
To know how to detect 'network change' see this post :
Android, How to handle change in network (from GPRS to Wi-fi and vice-versa) while polling for data
Update:
If you cannot show a dialog, at least send a status bar Notification so that user knows about the network switch sometime later.

In your activities, Whenever you try to make call to your Web Services
Just Disable the WIFI if it's enabled. There will be many code snippets available on Internet for that like this
Now also Check that if Mobile data network is available or not, and If available make your call, otherwise show user a dialog that this app will require mobile data networks to do the tasks.
and as soon as you complete your HTTP Calls turn the WIFI ON again.