diff --git a/Tethering/tests/integration/src/android/net/EthernetTetheringTest.java b/Tethering/tests/integration/src/android/net/EthernetTetheringTest.java
index 971eb10..15f07f2 100644
--- a/Tethering/tests/integration/src/android/net/EthernetTetheringTest.java
+++ b/Tethering/tests/integration/src/android/net/EthernetTetheringTest.java
@@ -47,14 +47,10 @@
 import android.net.TetheringManager.StartTetheringCallback;
 import android.net.TetheringManager.TetheringEventCallback;
 import android.net.TetheringManager.TetheringRequest;
-import android.net.dhcp.DhcpAckPacket;
-import android.net.dhcp.DhcpOfferPacket;
-import android.net.dhcp.DhcpPacket;
 import android.os.Handler;
 import android.os.HandlerThread;
 import android.os.SystemClock;
 import android.os.SystemProperties;
-import android.system.Os;
 import android.util.Log;
 
 import androidx.test.InstrumentationRegistry;
@@ -75,7 +71,6 @@
 import org.junit.runner.RunWith;
 
 import java.io.FileDescriptor;
-import java.net.Inet4Address;
 import java.net.InetAddress;
 import java.net.InterfaceAddress;
 import java.net.NetworkInterface;
@@ -97,15 +92,6 @@
 
     private static final String TAG = EthernetTetheringTest.class.getSimpleName();
     private static final int TIMEOUT_MS = 5000;
-    private static final int PACKET_READ_TIMEOUT_MS = 100;
-    private static final int DHCP_DISCOVER_ATTEMPTS = 10;
-    private static final byte[] DHCP_REQUESTED_PARAMS = new byte[] {
-            DhcpPacket.DHCP_SUBNET_MASK,
-            DhcpPacket.DHCP_ROUTER,
-            DhcpPacket.DHCP_DNS_SERVER,
-            DhcpPacket.DHCP_LEASE_TIME,
-    };
-    private static final String DHCP_HOSTNAME = "testhostname";
     private static final LinkAddress TEST_IP4_ADDR = new LinkAddress("10.0.0.1/8");
     private static final LinkAddress TEST_IP6_ADDR = new LinkAddress("2001:db8:1::101/64");
     private static final InetAddress TEST_IP4_DNS = parseNumericAddress("8.8.8.8");
@@ -254,11 +240,12 @@
 
         FileDescriptor fd = mDownstreamIface.getFileDescriptor().getFileDescriptor();
         mDownstreamReader = makePacketReader(fd, getMTU(mDownstreamIface));
-        DhcpResults dhcpResults = runDhcp(fd, client1);
+        TetheringTester tester = new TetheringTester(mDownstreamReader);
+        DhcpResults dhcpResults = tester.runDhcp(client1);
         assertEquals(new LinkAddress(clientAddr), dhcpResults.ipAddress);
 
         try {
-            runDhcp(fd, client2);
+            tester.runDhcp(client2);
             fail("Only one client should get an IP address");
         } catch (TimeoutException expected) { }
 
@@ -558,38 +545,16 @@
         FileDescriptor fd = iface.getFileDescriptor().getFileDescriptor();
         mDownstreamReader = makePacketReader(fd, mtu);
         mTetheringEventCallback = enableEthernetTethering(iface.getInterfaceName());
-        checkTetheredClientCallbacks(fd);
+        checkTetheredClientCallbacks(mDownstreamReader);
     }
 
-    private DhcpResults runDhcp(FileDescriptor fd, byte[] clientMacAddr) throws Exception {
-        // We have to retransmit DHCP requests because IpServer declares itself to be ready before
-        // its DhcpServer is actually started. TODO: fix this race and remove this loop.
-        DhcpPacket offerPacket = null;
-        for (int i = 0; i < DHCP_DISCOVER_ATTEMPTS; i++) {
-            Log.d(TAG, "Sending DHCP discover");
-            sendDhcpDiscover(fd, clientMacAddr);
-            offerPacket = getNextDhcpPacket();
-            if (offerPacket instanceof DhcpOfferPacket) break;
-        }
-        if (!(offerPacket instanceof DhcpOfferPacket)) {
-            throw new TimeoutException("No DHCPOFFER received on interface within timeout");
-        }
-
-        sendDhcpRequest(fd, offerPacket, clientMacAddr);
-        DhcpPacket ackPacket = getNextDhcpPacket();
-        if (!(ackPacket instanceof DhcpAckPacket)) {
-            throw new TimeoutException("No DHCPACK received on interface within timeout");
-        }
-
-        return ackPacket.toDhcpResults();
-    }
-
-    private void checkTetheredClientCallbacks(FileDescriptor fd) throws Exception {
+    private void checkTetheredClientCallbacks(TapPacketReader packetReader) throws Exception {
         // Create a fake client.
         byte[] clientMacAddr = new byte[6];
         new Random().nextBytes(clientMacAddr);
 
-        DhcpResults dhcpResults = runDhcp(fd, clientMacAddr);
+        TetheringTester tester = new TetheringTester(packetReader);
+        DhcpResults dhcpResults = tester.runDhcp(clientMacAddr);
 
         final Collection<TetheredClient> clients = mTetheringEventCallback.awaitClientConnected();
         assertEquals(1, clients.size());
@@ -602,7 +567,7 @@
         // Check the hostname.
         assertEquals(1, client.getAddresses().size());
         TetheredClient.AddressInfo info = client.getAddresses().get(0);
-        assertEquals(DHCP_HOSTNAME, info.getHostname());
+        assertEquals(TetheringTester.DHCP_HOSTNAME, info.getHostname());
 
         // Check the address is the one that was handed out in the DHCP ACK.
         assertLinkAddressMatches(dhcpResults.ipAddress, info.getAddress());
@@ -615,18 +580,6 @@
         assertTrue(msg, Math.abs(dhcpResults.leaseDuration - actualLeaseDuration) < 10);
     }
 
-    private DhcpPacket getNextDhcpPacket() throws ParseException {
-        byte[] packet;
-        while ((packet = mDownstreamReader.popPacket(PACKET_READ_TIMEOUT_MS)) != null) {
-            try {
-                return DhcpPacket.decodeFullPacket(packet, packet.length, DhcpPacket.ENCAP_L2);
-            } catch (DhcpPacket.ParseException e) {
-                // Not a DHCP packet. Continue.
-            }
-        }
-        return null;
-    }
-
     private static final class TetheredInterfaceRequester implements TetheredInterfaceCallback {
         private final Handler mHandler;
         private final EthernetManager mEm;
@@ -670,31 +623,6 @@
         }
     }
 
-    private void sendDhcpDiscover(FileDescriptor fd, byte[] macAddress) throws Exception {
-        ByteBuffer packet = DhcpPacket.buildDiscoverPacket(DhcpPacket.ENCAP_L2,
-                new Random().nextInt() /* transactionId */, (short) 0 /* secs */,
-                macAddress,  false /* unicast */, DHCP_REQUESTED_PARAMS,
-                false /* rapid commit */,  DHCP_HOSTNAME);
-        sendPacket(fd, packet);
-    }
-
-    private void sendDhcpRequest(FileDescriptor fd, DhcpPacket offerPacket, byte[] macAddress)
-            throws Exception {
-        DhcpResults results = offerPacket.toDhcpResults();
-        Inet4Address clientIp = (Inet4Address) results.ipAddress.getAddress();
-        Inet4Address serverIdentifier = results.serverAddress;
-        ByteBuffer packet = DhcpPacket.buildRequestPacket(DhcpPacket.ENCAP_L2,
-                0 /* transactionId */, (short) 0 /* secs */, DhcpPacket.INADDR_ANY /* clientIp */,
-                false /* broadcast */, macAddress, clientIp /* requestedIpAddress */,
-                serverIdentifier, DHCP_REQUESTED_PARAMS, DHCP_HOSTNAME);
-        sendPacket(fd, packet);
-    }
-
-    private void sendPacket(FileDescriptor fd, ByteBuffer packet) throws Exception {
-        assertNotNull("Only tests on virtual interfaces can send packets", fd);
-        Os.write(fd, packet);
-    }
-
     public void assertLinkAddressMatches(LinkAddress l1, LinkAddress l2) {
         // Check all fields except the deprecation and expiry times.
         String msg = String.format("LinkAddresses do not match. expected: %s actual: %s", l1, l2);
diff --git a/Tethering/tests/integration/src/android/net/TetheringTester.java b/Tethering/tests/integration/src/android/net/TetheringTester.java
new file mode 100644
index 0000000..38d74ad
--- /dev/null
+++ b/Tethering/tests/integration/src/android/net/TetheringTester.java
@@ -0,0 +1,155 @@
+/*
+ * Copyright (C) 2021 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 android.net;
+
+import static org.junit.Assert.fail;
+
+import android.net.dhcp.DhcpAckPacket;
+import android.net.dhcp.DhcpOfferPacket;
+import android.net.dhcp.DhcpPacket;
+import android.util.ArrayMap;
+import android.util.Log;
+
+import com.android.testutils.TapPacketReader;
+
+import java.net.Inet4Address;
+import java.nio.ByteBuffer;
+import java.util.Random;
+import java.util.concurrent.TimeoutException;
+import java.util.function.Function;
+
+/**
+ * A class simulate tethered client. When caller create TetheringTester, it would connect to
+ * tethering module that do the dhcp and slaac to obtain ipv4 and ipv6 address. Then caller can
+ * send/receive packets by this class.
+ */
+public final class TetheringTester {
+    private static final String TAG = TetheringTester.class.getSimpleName();
+    private static final int PACKET_READ_TIMEOUT_MS = 100;
+    private static final int DHCP_DISCOVER_ATTEMPTS = 10;
+    private static final byte[] DHCP_REQUESTED_PARAMS = new byte[] {
+            DhcpPacket.DHCP_SUBNET_MASK,
+            DhcpPacket.DHCP_ROUTER,
+            DhcpPacket.DHCP_DNS_SERVER,
+            DhcpPacket.DHCP_LEASE_TIME,
+    };
+
+    public static final String DHCP_HOSTNAME = "testhostname";
+
+    private final ArrayMap<MacAddress, TetheredDevice> mTetheredDevices;
+    private final TapPacketReader mDownstreamReader;
+
+    public TetheringTester(TapPacketReader downstream) {
+        if (downstream == null) fail("Downstream reader could not be NULL");
+
+        mDownstreamReader = downstream;
+        mTetheredDevices = new ArrayMap<>();
+    }
+
+    public TetheredDevice createTetheredDevice(MacAddress macAddr) throws Exception {
+        if (mTetheredDevices.get(macAddr) != null) {
+            fail("Tethered device already created");
+        }
+
+        TetheredDevice tethered = new TetheredDevice(macAddr);
+        mTetheredDevices.put(macAddr, tethered);
+
+        return tethered;
+    }
+
+    public class TetheredDevice {
+        private final MacAddress mMacAddr;
+
+        public final Inet4Address mIpv4Addr;
+
+        private TetheredDevice(MacAddress mac) throws Exception {
+            mMacAddr = mac;
+
+            DhcpResults dhcpResults = runDhcp(mMacAddr.toByteArray());
+            mIpv4Addr = (Inet4Address) dhcpResults.ipAddress.getAddress();
+        }
+    }
+
+    /** Simulate dhcp client to obtain ipv4 address. */
+    public DhcpResults runDhcp(byte[] clientMacAddr)
+            throws Exception {
+        // We have to retransmit DHCP requests because IpServer declares itself to be ready before
+        // its DhcpServer is actually started. TODO: fix this race and remove this loop.
+        DhcpPacket offerPacket = null;
+        for (int i = 0; i < DHCP_DISCOVER_ATTEMPTS; i++) {
+            Log.d(TAG, "Sending DHCP discover");
+            sendDhcpDiscover(clientMacAddr);
+            offerPacket = getNextDhcpPacket();
+            if (offerPacket instanceof DhcpOfferPacket) break;
+        }
+        if (!(offerPacket instanceof DhcpOfferPacket)) {
+            throw new TimeoutException("No DHCPOFFER received on interface within timeout");
+        }
+
+        sendDhcpRequest(offerPacket, clientMacAddr);
+        DhcpPacket ackPacket = getNextDhcpPacket();
+        if (!(ackPacket instanceof DhcpAckPacket)) {
+            throw new TimeoutException("No DHCPACK received on interface within timeout");
+        }
+
+        return ackPacket.toDhcpResults();
+    }
+
+    private void sendDhcpDiscover(byte[] macAddress) throws Exception {
+        ByteBuffer packet = DhcpPacket.buildDiscoverPacket(DhcpPacket.ENCAP_L2,
+                new Random().nextInt() /* transactionId */, (short) 0 /* secs */,
+                macAddress,  false /* unicast */, DHCP_REQUESTED_PARAMS,
+                false /* rapid commit */,  DHCP_HOSTNAME);
+        mDownstreamReader.sendResponse(packet);
+    }
+
+    private void sendDhcpRequest(DhcpPacket offerPacket, byte[] macAddress)
+            throws Exception {
+        DhcpResults results = offerPacket.toDhcpResults();
+        Inet4Address clientIp = (Inet4Address) results.ipAddress.getAddress();
+        Inet4Address serverIdentifier = results.serverAddress;
+        ByteBuffer packet = DhcpPacket.buildRequestPacket(DhcpPacket.ENCAP_L2,
+                0 /* transactionId */, (short) 0 /* secs */, DhcpPacket.INADDR_ANY /* clientIp */,
+                false /* broadcast */, macAddress, clientIp /* requestedIpAddress */,
+                serverIdentifier, DHCP_REQUESTED_PARAMS, DHCP_HOSTNAME);
+        mDownstreamReader.sendResponse(packet);
+    }
+
+    private DhcpPacket getNextDhcpPacket() {
+        return getNextMatchedPacket((p) -> {
+            try {
+                return DhcpPacket.decodeFullPacket(p, p.length, DhcpPacket.ENCAP_L2);
+            } catch (DhcpPacket.ParseException e) {
+                // Not a DHCP packet. Continue.
+            }
+
+            return null;
+        });
+    }
+
+    private <R> R getNextMatchedPacket(Function<byte[], R> match) {
+        byte[] packet;
+        R result;
+        while ((packet = mDownstreamReader.popPacket(PACKET_READ_TIMEOUT_MS)) != null) {
+            result = match.apply(packet);
+
+            if (result != null) return result;
+        }
+
+        return null;
+    }
+}
diff --git a/framework/src/android/net/ConnectivityManager.java b/framework/src/android/net/ConnectivityManager.java
index a35fc9c..4f10abc 100644
--- a/framework/src/android/net/ConnectivityManager.java
+++ b/framework/src/android/net/ConnectivityManager.java
@@ -2327,6 +2327,7 @@
         void onNetworkActive();
     }
 
+    @GuardedBy("mNetworkActivityListeners")
     private final ArrayMap<OnNetworkActiveListener, INetworkActivityListener>
             mNetworkActivityListeners = new ArrayMap<>();
 
@@ -2343,18 +2344,20 @@
      * @param l The listener to be told when the network is active.
      */
     public void addDefaultNetworkActiveListener(final OnNetworkActiveListener l) {
-        INetworkActivityListener rl = new INetworkActivityListener.Stub() {
+        final INetworkActivityListener rl = new INetworkActivityListener.Stub() {
             @Override
             public void onNetworkActive() throws RemoteException {
                 l.onNetworkActive();
             }
         };
 
-        try {
-            mService.registerNetworkActivityListener(rl);
-            mNetworkActivityListeners.put(l, rl);
-        } catch (RemoteException e) {
-            throw e.rethrowFromSystemServer();
+        synchronized (mNetworkActivityListeners) {
+            try {
+                mService.registerNetworkActivityListener(rl);
+                mNetworkActivityListeners.put(l, rl);
+            } catch (RemoteException e) {
+                throw e.rethrowFromSystemServer();
+            }
         }
     }
 
@@ -2365,14 +2368,17 @@
      * @param l Previously registered listener.
      */
     public void removeDefaultNetworkActiveListener(@NonNull OnNetworkActiveListener l) {
-        INetworkActivityListener rl = mNetworkActivityListeners.get(l);
-        if (rl == null) {
-            throw new IllegalArgumentException("Listener was not registered.");
-        }
-        try {
-            mService.unregisterNetworkActivityListener(rl);
-        } catch (RemoteException e) {
-            throw e.rethrowFromSystemServer();
+        synchronized (mNetworkActivityListeners) {
+            final INetworkActivityListener rl = mNetworkActivityListeners.get(l);
+            if (rl == null) {
+                throw new IllegalArgumentException("Listener was not registered.");
+            }
+            try {
+                mService.unregisterNetworkActivityListener(rl);
+                mNetworkActivityListeners.remove(l);
+            } catch (RemoteException e) {
+                throw e.rethrowFromSystemServer();
+            }
         }
     }
 
diff --git a/service/src/com/android/server/connectivity/PermissionMonitor.java b/service/src/com/android/server/connectivity/PermissionMonitor.java
index f596c4a..4c6b669 100755
--- a/service/src/com/android/server/connectivity/PermissionMonitor.java
+++ b/service/src/com/android/server/connectivity/PermissionMonitor.java
@@ -222,6 +222,10 @@
                 mIntentReceiver, intentFilter, null /* broadcastPermission */,
                 null /* scheduler */);
 
+        // Listen to EXTERNAL_APPLICATIONS_AVAILABLE is that an app becoming available means it may
+        // need to gain a permission. But an app that becomes unavailable can neither gain nor lose
+        // permissions on that account, it just can no longer run. Thus, doesn't need to listen to
+        // EXTERNAL_APPLICATIONS_UNAVAILABLE.
         final IntentFilter externalIntentFilter =
                 new IntentFilter(Intent.ACTION_EXTERNAL_APPLICATIONS_AVAILABLE);
         userAllContext.registerReceiver(
diff --git a/tests/cts/net/util/java/android/net/cts/util/CtsNetUtils.java b/tests/cts/net/util/java/android/net/cts/util/CtsNetUtils.java
index fd0cd18..7d9a24b 100644
--- a/tests/cts/net/util/java/android/net/cts/util/CtsNetUtils.java
+++ b/tests/cts/net/util/java/android/net/cts/util/CtsNetUtils.java
@@ -582,6 +582,9 @@
         }
 
         if (mOldPrivateDnsMode != ConnectivitySettingsUtils.PRIVATE_DNS_MODE_PROVIDER_HOSTNAME) {
+            // Also restore hostname even if the value is not used since private dns is not in
+            // the strict mode to prevent setting being changed after test.
+            ConnectivitySettingsUtils.setPrivateDnsHostname(mContext, mOldPrivateDnsSpecifier);
             ConnectivitySettingsUtils.setPrivateDnsMode(mContext, mOldPrivateDnsMode);
             return;
         }
diff --git a/tests/unit/java/android/net/ConnectivityManagerTest.java b/tests/unit/java/android/net/ConnectivityManagerTest.java
index 0914492..c475419 100644
--- a/tests/unit/java/android/net/ConnectivityManagerTest.java
+++ b/tests/unit/java/android/net/ConnectivityManagerTest.java
@@ -326,6 +326,8 @@
         verify(mService, times(1)).registerNetworkActivityListener(any());
         manager.removeDefaultNetworkActiveListener(listener);
         verify(mService, times(1)).unregisterNetworkActivityListener(any());
+        assertThrows(IllegalArgumentException.class,
+                () -> manager.removeDefaultNetworkActiveListener(listener));
     }
 
     @Test