diff --git a/Tethering/Android.bp b/Tethering/Android.bp
index 4efe934..3111ab7 100644
--- a/Tethering/Android.bp
+++ b/Tethering/Android.bp
@@ -16,7 +16,9 @@
 
 java_defaults {
     name: "TetheringAndroidLibraryDefaults",
-    sdk_version: "system_current",
+    // TODO (b/146757305): change to module API once available
+    // TODO (b/148190005): change to module-libs-api-stubs-current once it is ready.
+    sdk_version: "core_platform",
     srcs: [
         "src/**/*.java",
         ":framework-tethering-shared-srcs",
@@ -33,8 +35,15 @@
         "net-utils-framework-common",
     ],
     libs: [
+        // Order matters: framework-tethering needs to be before the system stubs, otherwise
+        // hidden fields in the framework-tethering classes (which are also used to generate stubs)
+        // will not be found.
         "framework-tethering",
+        "android_system_stubs_current",
+        "framework-res",
         "unsupportedappusage",
+        "android_system_stubs_current",
+        "framework-res",
     ],
     plugins: ["java_api_finder"],
     manifest: "AndroidManifestBase.xml",
@@ -82,7 +91,9 @@
 // Common defaults for compiling the actual APK.
 java_defaults {
     name: "TetheringAppDefaults",
-    sdk_version: "system_current",
+    // TODO (b/146757305): change to module API once available
+    // TODO (b/148190005): change to module-libs-api-stubs-current once it is ready.
+    sdk_version: "core_platform",
     privileged: true,
     // Build system doesn't track transitive dependeicies for jni_libs, list all the dependencies
     // explicitly.
@@ -95,7 +106,12 @@
         "res",
     ],
     libs: [
+        // Order matters: framework-tethering needs to be before the system stubs, otherwise
+        // hidden fields in the framework-tethering classes (which are also used to generate stubs)
+        // will not be found.
         "framework-tethering",
+        "android_system_stubs_current",
+        "framework-res",
     ],
     jarjar_rules: "jarjar-rules.txt",
     optimize: {
diff --git a/Tethering/common/TetheringLib/Android.bp b/Tethering/common/TetheringLib/Android.bp
index 8c4f733..cb0de7a 100644
--- a/Tethering/common/TetheringLib/Android.bp
+++ b/Tethering/common/TetheringLib/Android.bp
@@ -41,12 +41,12 @@
 
 java_library {
     name: "framework-tethering",
-    sdk_version: "system_current",
+    // TODO (b/146757305): change to module_app_current once available
+    sdk_version: "core_platform",
     srcs: [
         "src/android/net/TetheredClient.java",
         "src/android/net/TetheringManager.java",
         "src/android/net/TetheringConstants.java",
-        ":framework-tethering-annotations",
     ],
     static_libs: [
         "tethering-aidl-interfaces-java",
@@ -55,6 +55,7 @@
     installable: true,
 
     libs: [
+        "framework-annotations-lib",
         "android_system_stubs_current",
     ],
 
diff --git a/Tethering/common/TetheringLib/jarjar-rules.txt b/Tethering/common/TetheringLib/jarjar-rules.txt
index 1403bba..e459fad 100644
--- a/Tethering/common/TetheringLib/jarjar-rules.txt
+++ b/Tethering/common/TetheringLib/jarjar-rules.txt
@@ -1,2 +1 @@
-rule android.annotation.** com.android.networkstack.tethering.annotation.@1
-rule com.android.internal.annotations.** com.android.networkstack.tethering.annotation.@1
\ No newline at end of file
+# jarjar rules for the bootclasspath tethering framework library here
\ No newline at end of file
diff --git a/Tethering/common/TetheringLib/src/android/net/ITetheringConnector.aidl b/Tethering/common/TetheringLib/src/android/net/ITetheringConnector.aidl
index 5febe73..8be7964 100644
--- a/Tethering/common/TetheringLib/src/android/net/ITetheringConnector.aidl
+++ b/Tethering/common/TetheringLib/src/android/net/ITetheringConnector.aidl
@@ -1,16 +1,16 @@
-/**
- * Copyright (c) 2019, The Android Open Source Project
+/*
+ * Copyright (C) 2020 The Android Open Source Project
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
  * You may obtain a copy of the License at
  *
- *     http://www.apache.org/licenses/LICENSE-2.0
+ *      http://www.apache.org/licenses/LICENSE-2.0
  *
  * Unless required by applicable law or agreed to in writing, software
  * distributed under the License is distributed on an "AS IS" BASIS,
  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing perNmissions and
+ * See the License for the specific language governing permissions and
  * limitations under the License.
  */
 package android.net;
diff --git a/Tethering/common/TetheringLib/src/android/net/ITetheringEventCallback.aidl b/Tethering/common/TetheringLib/src/android/net/ITetheringEventCallback.aidl
index 28a810d..a554193 100644
--- a/Tethering/common/TetheringLib/src/android/net/ITetheringEventCallback.aidl
+++ b/Tethering/common/TetheringLib/src/android/net/ITetheringEventCallback.aidl
@@ -17,6 +17,7 @@
 package android.net;
 
 import android.net.Network;
+import android.net.TetheredClient;
 import android.net.TetheringConfigurationParcel;
 import android.net.TetheringCallbackStartedParcel;
 import android.net.TetherStatesParcel;
@@ -33,4 +34,5 @@
     void onUpstreamChanged(in Network network);
     void onConfigurationChanged(in TetheringConfigurationParcel config);
     void onTetherStatesChanged(in TetherStatesParcel states);
+    void onTetherClientsChanged(in List<TetheredClient> clients);
 }
diff --git a/Tethering/common/TetheringLib/src/android/net/TetheredClient.java b/Tethering/common/TetheringLib/src/android/net/TetheredClient.java
index 779aa3b..8b8b9e5 100644
--- a/Tethering/common/TetheringLib/src/android/net/TetheredClient.java
+++ b/Tethering/common/TetheringLib/src/android/net/TetheredClient.java
@@ -191,6 +191,15 @@
                 return new AddressInfo[size];
             }
         };
+
+        @NonNull
+        @Override
+        public String toString() {
+            return "AddressInfo {"
+                    + mAddress
+                    + (mHostname != null ? ", hostname " + mHostname : "")
+                    + "}";
+        }
     }
 
     @Override
@@ -212,4 +221,13 @@
             return new TetheredClient[size];
         }
     };
+
+    @NonNull
+    @Override
+    public String toString() {
+        return "TetheredClient {hwAddr " + mMacAddress
+                + ", addresses " + mAddresses
+                + ", tetheringType " + mTetheringType
+                + "}";
+    }
 }
diff --git a/Tethering/common/TetheringLib/src/android/net/TetheringCallbackStartedParcel.aidl b/Tethering/common/TetheringLib/src/android/net/TetheringCallbackStartedParcel.aidl
index 14ee2d3..c064aa4 100644
--- a/Tethering/common/TetheringLib/src/android/net/TetheringCallbackStartedParcel.aidl
+++ b/Tethering/common/TetheringLib/src/android/net/TetheringCallbackStartedParcel.aidl
@@ -17,6 +17,7 @@
 package android.net;
 
 import android.net.Network;
+import android.net.TetheredClient;
 import android.net.TetheringConfigurationParcel;
 import android.net.TetherStatesParcel;
 
@@ -29,4 +30,5 @@
     Network upstreamNetwork;
     TetheringConfigurationParcel config;
     TetherStatesParcel states;
+    List<TetheredClient> tetheredClients;
 }
\ No newline at end of file
diff --git a/Tethering/common/TetheringLib/src/android/net/TetheringManager.java b/Tethering/common/TetheringLib/src/android/net/TetheringManager.java
index cbe492d..fd9f713 100644
--- a/Tethering/common/TetheringLib/src/android/net/TetheringManager.java
+++ b/Tethering/common/TetheringLib/src/android/net/TetheringManager.java
@@ -375,6 +375,9 @@
             mTetherStatesParcel = states;
         }
 
+        @Override
+        public void onTetherClientsChanged(List<TetheredClient> clients) { }
+
         public void waitForStarted() {
             mWaitForCallback.block(DEFAULT_TIMEOUT_MS);
             throwIfPermissionFailure(mError);
@@ -921,6 +924,7 @@
                         sendRegexpsChanged(parcel.config);
                         maybeSendTetherableIfacesChangedCallback(parcel.states);
                         maybeSendTetheredIfacesChangedCallback(parcel.states);
+                        callback.onClientsChanged(parcel.tetheredClients);
                     });
                 }
 
@@ -951,6 +955,11 @@
                         maybeSendTetheredIfacesChangedCallback(states);
                     });
                 }
+
+                @Override
+                public void onTetherClientsChanged(final List<TetheredClient> clients) {
+                    executor.execute(() -> callback.onClientsChanged(clients));
+                }
             };
             getConnector(c -> c.registerTetheringEventCallback(remoteCallback, callerPkg));
             mTetheringEventCallbacks.put(callback, remoteCallback);
diff --git a/Tethering/src/android/net/ip/IpServer.java b/Tethering/src/android/net/ip/IpServer.java
index f39e7af..b4d49c0 100644
--- a/Tethering/src/android/net/ip/IpServer.java
+++ b/Tethering/src/android/net/ip/IpServer.java
@@ -19,6 +19,7 @@
 import static android.net.InetAddresses.parseNumericAddress;
 import static android.net.RouteInfo.RTN_UNICAST;
 import static android.net.dhcp.IDhcpServer.STATUS_SUCCESS;
+import static android.net.shared.Inet4AddressUtils.intToInet4AddressHTH;
 import static android.net.util.NetworkConstants.FF;
 import static android.net.util.NetworkConstants.RFC7421_PREFIX_LENGTH;
 import static android.net.util.NetworkConstants.asByte;
@@ -29,11 +30,15 @@
 import android.net.IpPrefix;
 import android.net.LinkAddress;
 import android.net.LinkProperties;
+import android.net.MacAddress;
 import android.net.RouteInfo;
+import android.net.TetheredClient;
 import android.net.TetheringManager;
+import android.net.dhcp.DhcpLeaseParcelable;
 import android.net.dhcp.DhcpServerCallbacks;
 import android.net.dhcp.DhcpServingParamsParcel;
 import android.net.dhcp.DhcpServingParamsParcelExt;
+import android.net.dhcp.IDhcpLeaseCallbacks;
 import android.net.dhcp.IDhcpServer;
 import android.net.ip.RouterAdvertisementDaemon.RaParams;
 import android.net.shared.NetdUtils;
@@ -48,6 +53,8 @@
 import android.util.Log;
 import android.util.SparseArray;
 
+import androidx.annotation.NonNull;
+
 import com.android.internal.util.MessageUtils;
 import com.android.internal.util.State;
 import com.android.internal.util.StateMachine;
@@ -57,7 +64,10 @@
 import java.net.InetAddress;
 import java.net.UnknownHostException;
 import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.Collections;
 import java.util.HashSet;
+import java.util.List;
 import java.util.Objects;
 import java.util.Random;
 import java.util.Set;
@@ -130,6 +140,11 @@
          * @param newLp the new LinkProperties to report
          */
         public void updateLinkProperties(IpServer who, LinkProperties newLp) { }
+
+        /**
+         * Notify that the DHCP leases changed in one of the IpServers.
+         */
+        public void dhcpLeasesChanged() { }
     }
 
     /** Capture IpServer dependencies, for injection. */
@@ -205,6 +220,8 @@
     private IDhcpServer mDhcpServer;
     private RaParams mLastRaParams;
     private LinkAddress mIpv4Address;
+    @NonNull
+    private List<TetheredClient> mDhcpLeases = Collections.emptyList();
 
     public IpServer(
             String ifaceName, Looper looper, int interfaceType, SharedLog log,
@@ -262,6 +279,14 @@
         return new LinkProperties(mLinkProperties);
     }
 
+    /**
+     * Get the latest list of DHCP leases that was reported. Must be called on the IpServer looper
+     * thread.
+     */
+    public List<TetheredClient> getAllLeases() {
+        return Collections.unmodifiableList(mDhcpLeases);
+    }
+
     /** Stop this IpServer. After this is called this IpServer should not be used any more. */
     public void stop() {
         sendMessage(CMD_INTERFACE_DOWN);
@@ -334,7 +359,7 @@
 
                 mDhcpServer = server;
                 try {
-                    mDhcpServer.start(new OnHandlerStatusCallback() {
+                    mDhcpServer.startWithCallbacks(new OnHandlerStatusCallback() {
                         @Override
                         public void callback(int startStatusCode) {
                             if (startStatusCode != STATUS_SUCCESS) {
@@ -342,7 +367,7 @@
                                 handleError();
                             }
                         }
-                    });
+                    }, new DhcpLeaseCallback());
                 } catch (RemoteException e) {
                     throw new IllegalStateException(e);
                 }
@@ -355,6 +380,48 @@
         }
     }
 
+    private class DhcpLeaseCallback extends IDhcpLeaseCallbacks.Stub {
+        @Override
+        public void onLeasesChanged(List<DhcpLeaseParcelable> leaseParcelables) {
+            final ArrayList<TetheredClient> leases = new ArrayList<>();
+            for (DhcpLeaseParcelable lease : leaseParcelables) {
+                final LinkAddress address = new LinkAddress(
+                        intToInet4AddressHTH(lease.netAddr), lease.prefixLength);
+
+                final MacAddress macAddress;
+                try {
+                    macAddress = MacAddress.fromBytes(lease.hwAddr);
+                } catch (IllegalArgumentException e) {
+                    Log.wtf(TAG, "Invalid address received from DhcpServer: "
+                            + Arrays.toString(lease.hwAddr));
+                    return;
+                }
+
+                final TetheredClient.AddressInfo addressInfo = new TetheredClient.AddressInfo(
+                        address, lease.hostname, lease.expTime);
+                leases.add(new TetheredClient(
+                        macAddress,
+                        Collections.singletonList(addressInfo),
+                        mInterfaceType));
+            }
+
+            getHandler().post(() -> {
+                mDhcpLeases = leases;
+                mCallback.dhcpLeasesChanged();
+            });
+        }
+
+        @Override
+        public int getInterfaceVersion() {
+            return this.VERSION;
+        }
+
+        @Override
+        public String getInterfaceHash() throws RemoteException {
+            return this.HASH;
+        }
+    }
+
     private boolean startDhcp(Inet4Address addr, int prefixLen) {
         if (mUsingLegacyDhcp) {
             return true;
@@ -388,6 +455,8 @@
                             mLastError = TetheringManager.TETHER_ERROR_DHCPSERVER_ERROR;
                             // Not much more we can do here
                         }
+                        mDhcpLeases.clear();
+                        getHandler().post(mCallback::dhcpLeasesChanged);
                     }
                 });
                 mDhcpServer = null;
@@ -756,7 +825,7 @@
                 final IpPrefix ipv4Prefix = new IpPrefix(mIpv4Address.getAddress(),
                         mIpv4Address.getPrefixLength());
                 NetdUtils.tetherInterface(mNetd, mIfaceName, ipv4Prefix);
-            } catch (RemoteException | ServiceSpecificException e) {
+            } catch (RemoteException | ServiceSpecificException | IllegalStateException e) {
                 mLog.e("Error Tethering: " + e);
                 mLastError = TetheringManager.TETHER_ERROR_TETHER_IFACE_ERROR;
                 return;
diff --git a/Tethering/src/com/android/server/connectivity/tethering/ConnectedClientsTracker.java b/Tethering/src/com/android/server/connectivity/tethering/ConnectedClientsTracker.java
new file mode 100644
index 0000000..cdd1a5d
--- /dev/null
+++ b/Tethering/src/com/android/server/connectivity/tethering/ConnectedClientsTracker.java
@@ -0,0 +1,183 @@
+/*
+ * Copyright (C) 2020 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.android.server.connectivity.tethering;
+
+import static android.net.TetheringManager.TETHERING_WIFI;
+
+import android.net.MacAddress;
+import android.net.TetheredClient;
+import android.net.TetheredClient.AddressInfo;
+import android.net.ip.IpServer;
+import android.net.wifi.WifiClient;
+import android.os.SystemClock;
+
+import androidx.annotation.NonNull;
+import androidx.annotation.Nullable;
+import androidx.annotation.VisibleForTesting;
+
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Map;
+import java.util.Set;
+
+/**
+ * Tracker for clients connected to downstreams.
+ *
+ * <p>This class is not thread safe, it is intended to be used only from the tethering handler
+ * thread.
+ */
+public class ConnectedClientsTracker {
+    private final Clock mClock;
+
+    @NonNull
+    private List<WifiClient> mLastWifiClients = Collections.emptyList();
+    @NonNull
+    private List<TetheredClient> mLastTetheredClients = Collections.emptyList();
+
+    @VisibleForTesting
+    static class Clock {
+        public long elapsedRealtime() {
+            return SystemClock.elapsedRealtime();
+        }
+    }
+
+    public ConnectedClientsTracker() {
+        this(new Clock());
+    }
+
+    @VisibleForTesting
+    ConnectedClientsTracker(Clock clock) {
+        mClock = clock;
+    }
+
+    /**
+     * Update the tracker with new connected clients.
+     *
+     * <p>The new list can be obtained through {@link #getLastTetheredClients()}.
+     * @param ipServers The IpServers used to assign addresses to clients.
+     * @param wifiClients The list of L2-connected WiFi clients. Null for no change since last
+     *                    update.
+     * @return True if the list of clients changed since the last calculation.
+     */
+    public boolean updateConnectedClients(
+            Iterable<IpServer> ipServers, @Nullable List<WifiClient> wifiClients) {
+        final long now = mClock.elapsedRealtime();
+
+        if (wifiClients != null) {
+            mLastWifiClients = wifiClients;
+        }
+        final Set<MacAddress> wifiClientMacs = getClientMacs(mLastWifiClients);
+
+        // Build the list of non-expired leases from all IpServers, grouped by mac address
+        final Map<MacAddress, TetheredClient> clientsMap = new HashMap<>();
+        for (IpServer server : ipServers) {
+            for (TetheredClient client : server.getAllLeases()) {
+                if (client.getTetheringType() == TETHERING_WIFI
+                        && !wifiClientMacs.contains(client.getMacAddress())) {
+                    // Skip leases of WiFi clients that are not (or no longer) L2-connected
+                    continue;
+                }
+                final TetheredClient prunedClient = pruneExpired(client, now);
+                if (prunedClient == null) continue; // All addresses expired
+
+                addLease(clientsMap, prunedClient);
+            }
+        }
+
+        // TODO: add IPv6 addresses from netlink
+
+        // Add connected WiFi clients that do not have any known address
+        for (MacAddress client : wifiClientMacs) {
+            if (clientsMap.containsKey(client)) continue;
+            clientsMap.put(client, new TetheredClient(
+                    client, Collections.emptyList() /* addresses */, TETHERING_WIFI));
+        }
+
+        final HashSet<TetheredClient> clients = new HashSet<>(clientsMap.values());
+        final boolean clientsChanged = clients.size() != mLastTetheredClients.size()
+                || !clients.containsAll(mLastTetheredClients);
+        mLastTetheredClients = Collections.unmodifiableList(new ArrayList<>(clients));
+        return clientsChanged;
+    }
+
+    private static void addLease(Map<MacAddress, TetheredClient> clientsMap, TetheredClient lease) {
+        final TetheredClient aggregateClient = clientsMap.getOrDefault(
+                lease.getMacAddress(), lease);
+        if (aggregateClient == lease) {
+            // This is the first lease with this mac address
+            clientsMap.put(lease.getMacAddress(), lease);
+            return;
+        }
+
+        // Only add the address info; this assumes that the tethering type is the same when the mac
+        // address is the same. If a client is connected through different tethering types with the
+        // same mac address, connected clients callbacks will report all of its addresses under only
+        // one of these tethering types. This keeps the API simple considering that such a scenario
+        // would really be a rare edge case.
+        clientsMap.put(lease.getMacAddress(), aggregateClient.addAddresses(lease));
+    }
+
+    /**
+     * Get the last list of tethered clients, as calculated in {@link #updateConnectedClients}.
+     *
+     * <p>The returned list is immutable.
+     */
+    @NonNull
+    public List<TetheredClient> getLastTetheredClients() {
+        return mLastTetheredClients;
+    }
+
+    private static boolean hasExpiredAddress(List<AddressInfo> addresses, long now) {
+        for (AddressInfo info : addresses) {
+            if (info.getExpirationTime() <= now) {
+                return true;
+            }
+        }
+        return false;
+    }
+
+    @Nullable
+    private static TetheredClient pruneExpired(TetheredClient client, long now) {
+        final List<AddressInfo> addresses = client.getAddresses();
+        if (addresses.size() == 0) return null;
+        if (!hasExpiredAddress(addresses, now)) return client;
+
+        final ArrayList<AddressInfo> newAddrs = new ArrayList<>(addresses.size() - 1);
+        for (AddressInfo info : addresses) {
+            if (info.getExpirationTime() > now) {
+                newAddrs.add(info);
+            }
+        }
+
+        if (newAddrs.size() == 0) {
+            return null;
+        }
+        return new TetheredClient(client.getMacAddress(), newAddrs, client.getTetheringType());
+    }
+
+    @NonNull
+    private static Set<MacAddress> getClientMacs(@NonNull List<WifiClient> clients) {
+        final Set<MacAddress> macs = new HashSet<>(clients.size());
+        for (WifiClient c : clients) {
+            macs.add(c.getMacAddress());
+        }
+        return macs;
+    }
+}
diff --git a/Tethering/src/com/android/server/connectivity/tethering/Tethering.java b/Tethering/src/com/android/server/connectivity/tethering/Tethering.java
index 64c16e4..6261def 100644
--- a/Tethering/src/com/android/server/connectivity/tethering/Tethering.java
+++ b/Tethering/src/com/android/server/connectivity/tethering/Tethering.java
@@ -24,6 +24,7 @@
 import static android.net.ConnectivityManager.ACTION_RESTRICT_BACKGROUND_CHANGED;
 import static android.net.ConnectivityManager.CONNECTIVITY_ACTION;
 import static android.net.ConnectivityManager.EXTRA_NETWORK_INFO;
+import static android.net.NetworkStack.PERMISSION_MAINLINE_NETWORK_STACK;
 import static android.net.TetheringManager.ACTION_TETHER_STATE_CHANGED;
 import static android.net.TetheringManager.EXTRA_ACTIVE_LOCAL_ONLY;
 import static android.net.TetheringManager.EXTRA_ACTIVE_TETHER;
@@ -79,6 +80,7 @@
 import android.net.Network;
 import android.net.NetworkInfo;
 import android.net.TetherStatesParcel;
+import android.net.TetheredClient;
 import android.net.TetheringCallbackStartedParcel;
 import android.net.TetheringConfigurationParcel;
 import android.net.TetheringRequestParcel;
@@ -89,6 +91,7 @@
 import android.net.util.PrefixUtils;
 import android.net.util.SharedLog;
 import android.net.util.VersionedBroadcastListener;
+import android.net.wifi.WifiClient;
 import android.net.wifi.WifiManager;
 import android.net.wifi.p2p.WifiP2pGroup;
 import android.net.wifi.p2p.WifiP2pInfo;
@@ -128,8 +131,10 @@
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collection;
-import java.util.HashSet;
+import java.util.Collections;
 import java.util.Iterator;
+import java.util.LinkedHashSet;
+import java.util.List;
 import java.util.Set;
 import java.util.concurrent.Executor;
 import java.util.concurrent.RejectedExecutionException;
@@ -145,6 +150,10 @@
     private static final boolean DBG = false;
     private static final boolean VDBG = false;
 
+    // TODO: add the below permissions to @SystemApi
+    private static final String PERMISSION_NETWORK_SETTINGS = "android.permission.NETWORK_SETTINGS";
+    private static final String PERMISSION_NETWORK_STACK = "android.permission.NETWORK_STACK";
+
     private static final Class[] sMessageClasses = {
             Tethering.class, TetherMasterSM.class, IpServer.class
     };
@@ -176,6 +185,17 @@
         }
     }
 
+    /**
+     * Cookie added when registering {@link android.net.TetheringManager.TetheringEventCallback}.
+     */
+    private static class CallbackCookie {
+        public final boolean hasListClientsPermission;
+
+        private CallbackCookie(boolean hasListClientsPermission) {
+            this.hasListClientsPermission = hasListClientsPermission;
+        }
+    }
+
     private final SharedLog mLog = new SharedLog(TAG);
     private final RemoteCallbackList<ITetheringEventCallback> mTetheringEventCallbacks =
             new RemoteCallbackList<>();
@@ -191,7 +211,8 @@
     private final UpstreamNetworkMonitor mUpstreamNetworkMonitor;
     // TODO: Figure out how to merge this and other downstream-tracking objects
     // into a single coherent structure.
-    private final HashSet<IpServer> mForwardedDownstreams;
+    // Use LinkedHashSet for predictable ordering order for ConnectedClientsTracker.
+    private final LinkedHashSet<IpServer> mForwardedDownstreams;
     private final VersionedBroadcastListener mCarrierConfigChange;
     private final TetheringDependencies mDeps;
     private final EntitlementManager mEntitlementMgr;
@@ -200,6 +221,7 @@
     private final NetdCallback mNetdCallback;
     private final UserRestrictionActionListener mTetheringRestriction;
     private final ActiveDataSubIdListener mActiveDataSubIdListener;
+    private final ConnectedClientsTracker mConnectedClientsTracker;
     private int mActiveDataSubId = INVALID_SUBSCRIPTION_ID;
     // All the usage of mTetheringEventCallback should run in the same thread.
     private ITetheringEventCallback mTetheringEventCallback = null;
@@ -234,6 +256,7 @@
         mPublicSync = new Object();
 
         mTetherStates = new ArrayMap<>();
+        mConnectedClientsTracker = new ConnectedClientsTracker();
 
         mTetherMasterSM = new TetherMasterSM("TetherMaster", mLooper, deps);
         mTetherMasterSM.start();
@@ -246,7 +269,7 @@
                 statsManager, mLog);
         mUpstreamNetworkMonitor = mDeps.getUpstreamNetworkMonitor(mContext, mTetherMasterSM, mLog,
                 TetherMasterSM.EVENT_UPSTREAM_CALLBACK);
-        mForwardedDownstreams = new HashSet<>();
+        mForwardedDownstreams = new LinkedHashSet<>();
 
         IntentFilter filter = new IntentFilter();
         filter.addAction(ACTION_CARRIER_CONFIG_CHANGED);
@@ -291,6 +314,9 @@
 
         startStateMachineUpdaters(mHandler);
         startTrackDefaultNetwork();
+        getWifiManager().registerSoftApCallback(
+                mHandler::post /* executor */,
+                new TetheringSoftApCallback());
     }
 
     private void startStateMachineUpdaters(Handler handler) {
@@ -385,6 +411,24 @@
         }
     }
 
+    private class TetheringSoftApCallback implements WifiManager.SoftApCallback {
+        // TODO: Remove onStateChanged override when this method has default on
+        // WifiManager#SoftApCallback interface.
+        // Wifi listener for state change of the soft AP
+        @Override
+        public void onStateChanged(final int state, final int failureReason) {
+            // Nothing
+        }
+
+        // Called by wifi when the number of soft AP clients changed.
+        @Override
+        public void onConnectedClientsChanged(final List<WifiClient> clients) {
+            if (mConnectedClientsTracker.updateConnectedClients(mForwardedDownstreams, clients)) {
+                reportTetherClientsChanged(mConnectedClientsTracker.getLastTetheredClients());
+            }
+        }
+    }
+
     void interfaceStatusChanged(String iface, boolean up) {
         // Never called directly: only called from interfaceLinkStateChanged.
         // See NetlinkHandler.cpp: notifyInterfaceChanged.
@@ -1938,14 +1982,21 @@
 
     /** Register tethering event callback */
     void registerTetheringEventCallback(ITetheringEventCallback callback) {
+        final boolean hasListPermission =
+                hasCallingPermission(PERMISSION_NETWORK_SETTINGS)
+                        || hasCallingPermission(PERMISSION_MAINLINE_NETWORK_STACK)
+                        || hasCallingPermission(PERMISSION_NETWORK_STACK);
         mHandler.post(() -> {
-            mTetheringEventCallbacks.register(callback);
+            mTetheringEventCallbacks.register(callback, new CallbackCookie(hasListPermission));
             final TetheringCallbackStartedParcel parcel = new TetheringCallbackStartedParcel();
             parcel.tetheringSupported = mDeps.isTetheringSupported();
             parcel.upstreamNetwork = mTetherUpstream;
             parcel.config = mConfig.toStableParcelable();
             parcel.states =
                     mTetherStatesParcel != null ? mTetherStatesParcel : emptyTetherStatesParcel();
+            parcel.tetheredClients = hasListPermission
+                    ? mConnectedClientsTracker.getLastTetheredClients()
+                    : Collections.emptyList();
             try {
                 callback.onCallbackStarted(parcel);
             } catch (RemoteException e) {
@@ -1965,6 +2016,10 @@
         return parcel;
     }
 
+    private boolean hasCallingPermission(@NonNull String permission) {
+        return mContext.checkCallingPermission(permission) == PERMISSION_GRANTED;
+    }
+
     /** Unregister tethering event callback */
     void unregisterTetheringEventCallback(ITetheringEventCallback callback) {
         mHandler.post(() -> {
@@ -2018,6 +2073,24 @@
         }
     }
 
+    private void reportTetherClientsChanged(List<TetheredClient> clients) {
+        final int length = mTetheringEventCallbacks.beginBroadcast();
+        try {
+            for (int i = 0; i < length; i++) {
+                try {
+                    final CallbackCookie cookie =
+                            (CallbackCookie) mTetheringEventCallbacks.getBroadcastCookie(i);
+                    if (!cookie.hasListClientsPermission) continue;
+                    mTetheringEventCallbacks.getBroadcastItem(i).onTetherClientsChanged(clients);
+                } catch (RemoteException e) {
+                    // Not really very much to do here.
+                }
+            }
+        } finally {
+            mTetheringEventCallbacks.finishBroadcast();
+        }
+    }
+
     void dump(@NonNull FileDescriptor fd, @NonNull PrintWriter writer, @Nullable String[] args) {
         // Binder.java closes the resource for us.
         @SuppressWarnings("resource")
@@ -2109,6 +2182,14 @@
             public void updateLinkProperties(IpServer who, LinkProperties newLp) {
                 notifyLinkPropertiesChanged(who, newLp);
             }
+
+            @Override
+            public void dhcpLeasesChanged() {
+                if (mConnectedClientsTracker.updateConnectedClients(
+                        mForwardedDownstreams, null /* wifiClients */)) {
+                    reportTetherClientsChanged(mConnectedClientsTracker.getLastTetheredClients());
+                }
+            }
         };
     }
 
diff --git a/Tethering/tests/unit/src/android/net/ip/IpServerTest.java b/Tethering/tests/unit/src/android/net/ip/IpServerTest.java
index f29ad78..acedfab 100644
--- a/Tethering/tests/unit/src/android/net/ip/IpServerTest.java
+++ b/Tethering/tests/unit/src/android/net/ip/IpServerTest.java
@@ -469,7 +469,8 @@
 
     private void assertDhcpStarted(IpPrefix expectedPrefix) throws Exception {
         verify(mDependencies, times(1)).makeDhcpServer(eq(IFACE_NAME), any(), any());
-        verify(mDhcpServer, timeout(MAKE_DHCPSERVER_TIMEOUT_MS).times(1)).start(any());
+        verify(mDhcpServer, timeout(MAKE_DHCPSERVER_TIMEOUT_MS).times(1)).startWithCallbacks(
+                any(), any());
         final DhcpServingParamsParcel params = mDhcpParamsCaptor.getValue();
         // Last address byte is random
         assertTrue(expectedPrefix.contains(intToInet4AddressHTH(params.serverAddr)));
diff --git a/Tethering/tests/unit/src/com/android/server/connectivity/tethering/ConnectedClientsTrackerTest.kt b/Tethering/tests/unit/src/com/android/server/connectivity/tethering/ConnectedClientsTrackerTest.kt
new file mode 100644
index 0000000..56f3e21
--- /dev/null
+++ b/Tethering/tests/unit/src/com/android/server/connectivity/tethering/ConnectedClientsTrackerTest.kt
@@ -0,0 +1,157 @@
+/*
+ * Copyright (C) 2020 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.android.server.connectivity.tethering
+
+import android.net.LinkAddress
+import android.net.MacAddress
+import android.net.TetheredClient
+import android.net.TetheredClient.AddressInfo
+import android.net.TetheringManager.TETHERING_USB
+import android.net.TetheringManager.TETHERING_WIFI
+import android.net.ip.IpServer
+import android.net.wifi.WifiClient
+import androidx.test.filters.SmallTest
+import androidx.test.runner.AndroidJUnit4
+import org.junit.Test
+import org.junit.runner.RunWith
+import org.mockito.Mockito.doReturn
+import org.mockito.Mockito.mock
+import kotlin.test.assertEquals
+import kotlin.test.assertFalse
+import kotlin.test.assertTrue
+
+@RunWith(AndroidJUnit4::class)
+@SmallTest
+class ConnectedClientsTrackerTest {
+
+    private val server1 = mock(IpServer::class.java)
+    private val server2 = mock(IpServer::class.java)
+    private val servers = listOf(server1, server2)
+
+    private val clock = TestClock(1324L)
+
+    private val client1Addr = MacAddress.fromString("01:23:45:67:89:0A")
+    private val client1 = TetheredClient(client1Addr, listOf(
+            AddressInfo(LinkAddress("192.168.43.44/32"), null /* hostname */, clock.time + 20)),
+            TETHERING_WIFI)
+    private val wifiClient1 = makeWifiClient(client1Addr)
+    private val client2Addr = MacAddress.fromString("02:34:56:78:90:AB")
+    private val client2Exp30AddrInfo = AddressInfo(
+            LinkAddress("192.168.43.45/32"), "my_hostname", clock.time + 30)
+    private val client2 = TetheredClient(client2Addr, listOf(
+            client2Exp30AddrInfo,
+            AddressInfo(LinkAddress("2001:db8:12::34/72"), "other_hostname", clock.time + 10)),
+            TETHERING_WIFI)
+    private val wifiClient2 = makeWifiClient(client2Addr)
+    private val client3Addr = MacAddress.fromString("03:45:67:89:0A:BC")
+    private val client3 = TetheredClient(client3Addr,
+            listOf(AddressInfo(LinkAddress("2001:db8:34::34/72"), "other_other_hostname",
+                    clock.time + 10)),
+            TETHERING_USB)
+
+    @Test
+    fun testUpdateConnectedClients() {
+        doReturn(emptyList<TetheredClient>()).`when`(server1).allLeases
+        doReturn(emptyList<TetheredClient>()).`when`(server2).allLeases
+
+        val tracker = ConnectedClientsTracker(clock)
+        assertFalse(tracker.updateConnectedClients(servers, null))
+
+        // Obtain a lease for client 1
+        doReturn(listOf(client1)).`when`(server1).allLeases
+        assertSameClients(listOf(client1), assertNewClients(tracker, servers, listOf(wifiClient1)))
+
+        // Client 2 L2-connected, no lease yet
+        val client2WithoutAddr = TetheredClient(client2Addr, emptyList(), TETHERING_WIFI)
+        assertSameClients(listOf(client1, client2WithoutAddr),
+                assertNewClients(tracker, servers, listOf(wifiClient1, wifiClient2)))
+
+        // Client 2 lease obtained
+        doReturn(listOf(client1, client2)).`when`(server1).allLeases
+        assertSameClients(listOf(client1, client2), assertNewClients(tracker, servers, null))
+
+        // Client 3 lease obtained
+        doReturn(listOf(client3)).`when`(server2).allLeases
+        assertSameClients(listOf(client1, client2, client3),
+                assertNewClients(tracker, servers, null))
+
+        // Client 2 L2-disconnected
+        assertSameClients(listOf(client1, client3),
+                assertNewClients(tracker, servers, listOf(wifiClient1)))
+
+        // Client 1 L2-disconnected
+        assertSameClients(listOf(client3), assertNewClients(tracker, servers, emptyList()))
+
+        // Client 1 comes back
+        assertSameClients(listOf(client1, client3),
+                assertNewClients(tracker, servers, listOf(wifiClient1)))
+
+        // Leases lost, client 1 still L2-connected
+        doReturn(emptyList<TetheredClient>()).`when`(server1).allLeases
+        doReturn(emptyList<TetheredClient>()).`when`(server2).allLeases
+        assertSameClients(listOf(TetheredClient(client1Addr, emptyList(), TETHERING_WIFI)),
+                assertNewClients(tracker, servers, null))
+    }
+
+    @Test
+    fun testUpdateConnectedClients_LeaseExpiration() {
+        val tracker = ConnectedClientsTracker(clock)
+        doReturn(listOf(client1, client2)).`when`(server1).allLeases
+        doReturn(listOf(client3)).`when`(server2).allLeases
+        assertSameClients(listOf(client1, client2, client3), assertNewClients(
+                tracker, servers, listOf(wifiClient1, wifiClient2)))
+
+        clock.time += 20
+        // Client 3 has no remaining lease: removed
+        val expectedClients = listOf(
+                // Client 1 has no remaining lease but is L2-connected
+                TetheredClient(client1Addr, emptyList(), TETHERING_WIFI),
+                // Client 2 has some expired leases
+                TetheredClient(
+                        client2Addr,
+                        // Only the "t + 30" address is left, the "t + 10" address expired
+                        listOf(client2Exp30AddrInfo),
+                        TETHERING_WIFI))
+        assertSameClients(expectedClients, assertNewClients(tracker, servers, null))
+    }
+
+    private fun assertNewClients(
+        tracker: ConnectedClientsTracker,
+        ipServers: Iterable<IpServer>,
+        wifiClients: List<WifiClient>?
+    ): List<TetheredClient> {
+        assertTrue(tracker.updateConnectedClients(ipServers, wifiClients))
+        return tracker.lastTetheredClients
+    }
+
+    private fun assertSameClients(expected: List<TetheredClient>, actual: List<TetheredClient>) {
+        val expectedSet = HashSet(expected)
+        assertEquals(expected.size, expectedSet.size)
+        assertEquals(expectedSet, HashSet(actual))
+    }
+
+    private fun makeWifiClient(macAddr: MacAddress): WifiClient {
+        // Use a mock WifiClient as the constructor is not part of the WiFi module exported API.
+        return mock(WifiClient::class.java).apply { doReturn(macAddr).`when`(this).macAddress }
+    }
+
+    private class TestClock(var time: Long) : ConnectedClientsTracker.Clock() {
+        override fun elapsedRealtime(): Long {
+            return time
+        }
+    }
+}
\ No newline at end of file
diff --git a/Tethering/tests/unit/src/com/android/server/connectivity/tethering/TetheringTest.java b/Tethering/tests/unit/src/com/android/server/connectivity/tethering/TetheringTest.java
index 6d49e20..8e5aaf2 100644
--- a/Tethering/tests/unit/src/com/android/server/connectivity/tethering/TetheringTest.java
+++ b/Tethering/tests/unit/src/com/android/server/connectivity/tethering/TetheringTest.java
@@ -88,6 +88,7 @@
 import android.net.NetworkRequest;
 import android.net.RouteInfo;
 import android.net.TetherStatesParcel;
+import android.net.TetheredClient;
 import android.net.TetheringCallbackStartedParcel;
 import android.net.TetheringConfigurationParcel;
 import android.net.TetheringRequestParcel;
@@ -142,6 +143,7 @@
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collection;
+import java.util.List;
 import java.util.Vector;
 
 @RunWith(AndroidJUnit4.class)
@@ -470,6 +472,7 @@
                 ArgumentCaptor.forClass(PhoneStateListener.class);
         verify(mTelephonyManager).listen(phoneListenerCaptor.capture(),
                 eq(PhoneStateListener.LISTEN_ACTIVE_DATA_SUBSCRIPTION_ID_CHANGE));
+        verify(mWifiManager).registerSoftApCallback(any(), any());
         mPhoneStateListener = phoneListenerCaptor.getValue();
     }
 
@@ -728,7 +731,8 @@
 
         sendIPv6TetherUpdates(upstreamState);
         verify(mRouterAdvertisementDaemon, never()).buildNewRa(any(), notNull());
-        verify(mDhcpServer, timeout(DHCPSERVER_START_TIMEOUT_MS).times(1)).start(any());
+        verify(mDhcpServer, timeout(DHCPSERVER_START_TIMEOUT_MS).times(1)).startWithCallbacks(
+                any(), any());
     }
 
     @Test
@@ -764,7 +768,8 @@
         verify(mNetd, times(1)).tetherAddForward(TEST_USB_IFNAME, TEST_MOBILE_IFNAME);
         verify(mNetd, times(1)).ipfwdAddInterfaceForward(TEST_USB_IFNAME, TEST_MOBILE_IFNAME);
         verify(mRouterAdvertisementDaemon, times(1)).start();
-        verify(mDhcpServer, timeout(DHCPSERVER_START_TIMEOUT_MS).times(1)).start(any());
+        verify(mDhcpServer, timeout(DHCPSERVER_START_TIMEOUT_MS).times(1)).startWithCallbacks(
+                any(), any());
 
         sendIPv6TetherUpdates(upstreamState);
         verify(mRouterAdvertisementDaemon, times(1)).buildNewRa(any(), notNull());
@@ -778,7 +783,8 @@
 
         verify(mNetd, times(1)).tetherAddForward(TEST_USB_IFNAME, TEST_XLAT_MOBILE_IFNAME);
         verify(mNetd, times(1)).tetherAddForward(TEST_USB_IFNAME, TEST_MOBILE_IFNAME);
-        verify(mDhcpServer, timeout(DHCPSERVER_START_TIMEOUT_MS).times(1)).start(any());
+        verify(mDhcpServer, timeout(DHCPSERVER_START_TIMEOUT_MS).times(1)).startWithCallbacks(
+                any(), any());
         verify(mNetd, times(1)).ipfwdAddInterfaceForward(TEST_USB_IFNAME, TEST_XLAT_MOBILE_IFNAME);
         verify(mNetd, times(1)).ipfwdAddInterfaceForward(TEST_USB_IFNAME, TEST_MOBILE_IFNAME);
 
@@ -794,7 +800,8 @@
         runUsbTethering(upstreamState);
 
         verify(mNetd, times(1)).tetherAddForward(TEST_USB_IFNAME, TEST_MOBILE_IFNAME);
-        verify(mDhcpServer, timeout(DHCPSERVER_START_TIMEOUT_MS).times(1)).start(any());
+        verify(mDhcpServer, timeout(DHCPSERVER_START_TIMEOUT_MS).times(1)).startWithCallbacks(
+                any(), any());
         verify(mNetd, times(1)).ipfwdAddInterfaceForward(TEST_USB_IFNAME, TEST_MOBILE_IFNAME);
 
         // Then 464xlat comes up
@@ -817,7 +824,8 @@
         verify(mNetd, times(1)).tetherAddForward(TEST_USB_IFNAME, TEST_MOBILE_IFNAME);
         verify(mNetd, times(1)).ipfwdAddInterfaceForward(TEST_USB_IFNAME, TEST_MOBILE_IFNAME);
         // DHCP not restarted on downstream (still times(1))
-        verify(mDhcpServer, timeout(DHCPSERVER_START_TIMEOUT_MS).times(1)).start(any());
+        verify(mDhcpServer, timeout(DHCPSERVER_START_TIMEOUT_MS).times(1)).startWithCallbacks(
+                any(), any());
     }
 
     @Test
@@ -847,7 +855,8 @@
     public void workingNcmTethering() throws Exception {
         runNcmTethering();
 
-        verify(mDhcpServer, timeout(DHCPSERVER_START_TIMEOUT_MS).times(1)).start(any());
+        verify(mDhcpServer, timeout(DHCPSERVER_START_TIMEOUT_MS).times(1)).startWithCallbacks(
+                any(), any());
     }
 
     @Test
@@ -1171,6 +1180,11 @@
         }
 
         @Override
+        public void onTetherClientsChanged(List<TetheredClient> clients) {
+            // TODO: check this
+        }
+
+        @Override
         public void onCallbackStarted(TetheringCallbackStartedParcel parcel) {
             mActualUpstreams.add(parcel.upstreamNetwork);
             mTetheringConfigs.add(parcel.config);