android: correctly grab DNS settings

Closes #ENG-3005

Signed-off-by: Percy Wegmann <percy@tailscale.com>

android/src/main/java/com/tailscale/ipn/App.kt

@@ -23,12 +23,12 @@ import android.content.res.Configuration
 import android.net.ConnectivityManager
 import android.net.LinkProperties
 import android.net.Network
+import android.net.NetworkCapabilities
+import android.net.NetworkRequest
 import android.net.Uri
 import android.net.VpnService
 import android.os.Build
 import android.os.Environment
-import android.os.Handler
-import android.os.Looper
 import android.provider.MediaStore
 import android.provider.Settings
 import android.util.Log
@@ -64,7 +64,6 @@ import java.util.Objects
 
 class App : Application(), libtailscale.AppContext {
   private val applicationScope = CoroutineScope(SupervisorJob() + Dispatchers.Default)
-  var dnsConfigObj = DnsConfig()
 
   companion object {
     const val STATUS_CHANNEL_ID = "tailscale-status"
@@ -75,11 +74,13 @@ class App : Application(), libtailscale.AppContext {
     private const val FILE_CHANNEL_ID = "tailscale-files"
     private const val FILE_NOTIFICATION_ID = 3
     private const val TAG = "App"
-    private val mainHandler = Handler(Looper.getMainLooper())
+    private val networkConnectivityRequest =
+        NetworkRequest.Builder()
+            .addCapability(NetworkCapabilities.NET_CAPABILITY_INTERNET)
+            .addCapability(NetworkCapabilities.NET_CAPABILITY_NOT_VPN)
+            .build()
     lateinit var appInstance: App
 
-    private fun isEmpty(str: String?) = str.isNullOrEmpty()
-
     @JvmStatic
     fun startActivityForResult(act: Activity, intent: Intent?, request: Int) {
       val f: Fragment = act.getFragmentManager().findFragmentByTag(PEER_TAG)
@@ -143,33 +144,36 @@ class App : Application(), libtailscale.AppContext {
   }
 
   // requestNetwork attempts to find the best network that matches the passed NetworkRequest. It is
-  // possible that
-  // this might return an unusuable network, eg a captive portal.
+  // possible that this might return an unusuable network, eg a captive portal.
   private fun setAndRegisterNetworkCallbacks() {
     connectivityManager.requestNetwork(
-        dnsConfigObj.dnsConfigNetworkRequest,
+        networkConnectivityRequest,
         object : ConnectivityManager.NetworkCallback() {
           override fun onAvailable(network: Network) {
             super.onAvailable(network)
+
             val sb = StringBuilder()
             val linkProperties: LinkProperties? = connectivityManager.getLinkProperties(network)
-            val dnsList: MutableList<InetAddress> =
-                linkProperties?.getDnsServers() ?: mutableListOf()
+            val dnsList: MutableList<InetAddress> = linkProperties?.dnsServers ?: mutableListOf()
             for (ip in dnsList) {
-              sb.append(ip.getHostAddress()).append(" ")
+              sb.append(ip.hostAddress).append(" ")
             }
-            val searchDomains: String? = linkProperties?.getDomains()
+            val searchDomains: String? = linkProperties?.domains
             if (searchDomains != null) {
               sb.append("\n")
               sb.append(searchDomains)
             }
-            dnsConfigObj.updateDNSFromNetwork(sb.toString())
-            Libtailscale.onDnsConfigChanged()
+
+            if (dns.updateDNSFromNetwork(sb.toString())) {
+              Libtailscale.onDnsConfigChanged()
+            }
           }
 
           override fun onLost(network: Network) {
             super.onLost(network)
-            Libtailscale.onDnsConfigChanged()
+            if (dns.updateDNSFromNetwork("")) {
+              Libtailscale.onDnsConfigChanged()
+            }
           }
         })
   }

android/src/main/java/com/tailscale/ipn/DnsConfig.java

@@ -3,9 +3,6 @@
 
 package com.tailscale.ipn;
 
-import android.net.NetworkCapabilities;
-import android.net.NetworkRequest;
-
 // Tailscale DNS Config retrieval
 //
 // Tailscale's DNS support can either override the local DNS servers with a set of servers
@@ -44,14 +41,14 @@ public class DnsConfig {
         }
     }
 
-    void updateDNSFromNetwork(String dnsConfigs) {
+    boolean updateDNSFromNetwork(String dnsConfigs) {
         synchronized (this) {
-            this.dnsConfigs = dnsConfigs;
+            if (!dnsConfigs.equals(this.dnsConfigs)) {
+                this.dnsConfigs = dnsConfigs;
+                return true;
+            } else {
+                return false;
+            }
         }
     }
-
-    NetworkRequest getDNSConfigNetworkRequest() {
-        // Request networks that are able to reach the Internet.
-        return new NetworkRequest.Builder().addCapability(NetworkCapabilities.NET_CAPABILITY_INTERNET).build();
-    }
 }