diff --git a/staticlibs/testutils/devicetests/com/android/testutils/NatExternalPacketForwarder.kt b/staticlibs/testutils/devicetests/com/android/testutils/NatExternalPacketForwarder.kt
new file mode 100644
index 0000000..d7961a0
--- /dev/null
+++ b/staticlibs/testutils/devicetests/com/android/testutils/NatExternalPacketForwarder.kt
@@ -0,0 +1,81 @@
+/*
+ * Copyright (C) 2023 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.testutils
+
+import java.io.FileDescriptor
+import java.net.InetAddress
+
+/**
+ * A class that forwards packets from the external {@link TestNetworkInterface} to the internal
+ * {@link TestNetworkInterface} with NAT. See {@link NatPacketForwarderBase} for detail.
+ */
+class NatExternalPacketForwarder(
+    srcFd: FileDescriptor,
+    mtu: Int,
+    dstFd: FileDescriptor,
+    extAddr: InetAddress,
+    natMap: PacketBridge.NatMap
+) : NatPacketForwarderBase(srcFd, mtu, dstFd, extAddr, natMap) {
+
+    /**
+     * Rewrite addresses, ports and fix up checksums for packets received on the external
+     * interface.
+     *
+     * Incoming response from external interface which is being forwarded to the internal
+     * interface with translated address, e.g. 1.2.3.4:80 -> 8.8.8.8:1234
+     * will be translated into 8.8.8.8:80 -> 192.168.1.1:5678.
+     *
+     * For packets that are not an incoming response, do not forward them to the
+     * internal interface.
+     */
+    override fun preparePacketForForwarding(buf: ByteArray, len: Int, version: Int, proto: Int) {
+        val (addrPos, addrLen) = getAddressPositionAndLength(version)
+
+        // TODO: support one external address per ip version.
+        val extAddrBuf = mExtAddr.address
+        if (addrLen != extAddrBuf.size) throw IllegalStateException("Packet IP version mismatch")
+
+        // Get internal address by port.
+        val transportOffset =
+            if (version == 4) PacketReflector.IPV4_HEADER_LENGTH
+            else PacketReflector.IPV6_HEADER_LENGTH
+        val dstPort = getPortAt(buf, transportOffset + DESTINATION_PORT_OFFSET)
+        val intAddrInfo = synchronized(mNatMap) { mNatMap.fromExternalPort(dstPort) }
+        // No mapping, skip. This usually happens if the connection is initiated directly on
+        // the external interface, e.g. DNS64 resolution, network validation, etc.
+        if (intAddrInfo == null) return
+
+        val intAddrBuf = intAddrInfo.address.address
+        val intPort = intAddrInfo.port
+
+        // Copy the original destination to into the source address.
+        for (i in 0 until addrLen) {
+            buf[addrPos + i] = buf[addrPos + addrLen + i]
+        }
+
+        // Copy the internal address into the destination address.
+        for (i in 0 until addrLen) {
+            buf[addrPos + addrLen + i] = intAddrBuf[i]
+        }
+
+        // Copy the internal port into the destination port.
+        setPortAt(intPort, buf, transportOffset + DESTINATION_PORT_OFFSET)
+
+        // Fix IP and Transport layer checksum.
+        fixPacketChecksum(buf, len, version, proto.toByte())
+    }
+}
diff --git a/staticlibs/testutils/devicetests/com/android/testutils/NatInternalPacketForwarder.kt b/staticlibs/testutils/devicetests/com/android/testutils/NatInternalPacketForwarder.kt
new file mode 100644
index 0000000..fa39d19
--- /dev/null
+++ b/staticlibs/testutils/devicetests/com/android/testutils/NatInternalPacketForwarder.kt
@@ -0,0 +1,78 @@
+/*
+ * Copyright (C) 2023 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.testutils
+
+import java.io.FileDescriptor
+import java.net.InetAddress
+
+/**
+ * A class that forwards packets from the internal {@link TestNetworkInterface} to the external
+ * {@link TestNetworkInterface} with NAT. See {@link NatPacketForwarderBase} for detail.
+ */
+class NatInternalPacketForwarder(
+    srcFd: FileDescriptor,
+    mtu: Int,
+    dstFd: FileDescriptor,
+    extAddr: InetAddress,
+    natMap: PacketBridge.NatMap
+) : NatPacketForwarderBase(srcFd, mtu, dstFd, extAddr, natMap) {
+
+    /**
+     * Rewrite addresses, ports and fix up checksums for packets received on the internal
+     * interface.
+     *
+     * Outgoing packet from the internal interface which is being forwarded to the
+     * external interface with translated address, e.g. 192.168.1.1:5678 -> 8.8.8.8:80
+     * will be translated into 8.8.8.8:1234 -> 1.2.3.4:80.
+     *
+     * The external port, e.g. 1234 in the above example, is the port number assigned by
+     * the forwarder when creating the mapping to identify the source address and port when
+     * the response is coming from the external interface. See {@link PacketBridge.NatMap}
+     * for detail.
+     */
+    override fun preparePacketForForwarding(buf: ByteArray, len: Int, version: Int, proto: Int) {
+        val (addrPos, addrLen) = getAddressPositionAndLength(version)
+
+        // TODO: support one external address per ip version.
+        val extAddrBuf = mExtAddr.address
+        if (addrLen != extAddrBuf.size) throw IllegalStateException("Packet IP version mismatch")
+
+        val srcAddr = getInetAddressAt(buf, addrPos, addrLen)
+
+        // Copy the original destination to into the source address.
+        for (i in 0 until addrLen) {
+            buf[addrPos + i] = buf[addrPos + addrLen + i]
+        }
+
+        // Copy the external address into the destination address.
+        for (i in 0 until addrLen) {
+            buf[addrPos + addrLen + i] = extAddrBuf[i]
+        }
+
+        // Add an entry to NAT mapping table.
+        val transportOffset =
+            if (version == 4) PacketReflector.IPV4_HEADER_LENGTH
+            else PacketReflector.IPV6_HEADER_LENGTH
+        val srcPort = getPortAt(buf, transportOffset)
+        val extPort = synchronized(mNatMap) { mNatMap.toExternalPort(srcAddr, srcPort, proto) }
+        // Copy the external port to into the source port.
+        setPortAt(extPort, buf, transportOffset)
+
+        // Fix IP and Transport layer checksum.
+        fixPacketChecksum(buf, len, version, proto.toByte())
+    }
+}
diff --git a/staticlibs/testutils/devicetests/com/android/testutils/NatPacketForwarderBase.java b/staticlibs/testutils/devicetests/com/android/testutils/NatPacketForwarderBase.java
new file mode 100644
index 0000000..85c6493
--- /dev/null
+++ b/staticlibs/testutils/devicetests/com/android/testutils/NatPacketForwarderBase.java
@@ -0,0 +1,206 @@
+/*
+ * Copyright (C) 2023 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.testutils;
+
+import static com.android.testutils.PacketReflector.IPPROTO_TCP;
+import static com.android.testutils.PacketReflector.IPPROTO_UDP;
+import static com.android.testutils.PacketReflector.IPV4_HEADER_LENGTH;
+import static com.android.testutils.PacketReflector.IPV6_HEADER_LENGTH;
+import static com.android.testutils.PacketReflector.IPV6_PROTO_OFFSET;
+import static com.android.testutils.PacketReflector.TCP_HEADER_LENGTH;
+import static com.android.testutils.PacketReflector.UDP_HEADER_LENGTH;
+
+import android.annotation.NonNull;
+import android.net.TestNetworkInterface;
+import android.system.ErrnoException;
+import android.system.Os;
+import android.util.Log;
+
+import androidx.annotation.GuardedBy;
+
+import java.io.FileDescriptor;
+import java.io.IOException;
+import java.net.InetAddress;
+import java.util.Objects;
+
+/**
+ * A class that forwards packets from a {@link TestNetworkInterface} to another
+ * {@link TestNetworkInterface} with NAT.
+ *
+ * For testing purposes, a {@link TestNetworkInterface} provides a {@link FileDescriptor}
+ * which allows content injection on the test network. However, this could be hard to use
+ * because the callers need to compose IP packets in order to inject content to the
+ * test network.
+ *
+ * In order to remove the need of composing the IP packets, this class forwards IP packets to
+ * the {@link FileDescriptor} of another {@link TestNetworkInterface} instance. Thus,
+ * the TCP/IP headers could be parsed/composed automatically by the protocol stack of this
+ * additional {@link TestNetworkInterface}, while the payload is supplied by the
+ * servers run on the interface.
+ *
+ * To make it work, an internal interface and an external interface are defined, where
+ * the client might send packets from the internal interface which are originated from
+ * multiple addresses to a server that listens on the external address.
+ *
+ * When forwarding the outgoing packet on the internal interface, a simple NAT mechanism
+ * is implemented during forwarding, which will swap the source and destination,
+ * but replacing the source address with the external address,
+ * e.g. 192.168.1.1:1234 -> 8.8.8.8:80 will be translated into 8.8.8.8:1234 -> 1.2.3.4:80.
+ *
+ * For the above example, a client who sends http request will have a hallucination that
+ * it is talking to a remote server at 8.8.8.8. Also, the server listens on 1.2.3.4 will
+ * have a different hallucination that the request is sent from a remote client at 8.8.8.8,
+ * to a local address 1.2.3.4.
+ *
+ * And a NAT mapping is created at the time when the outgoing packet is forwarded.
+ * With a different internal source port, the instance learned that when a response with the
+ * destination port 1234, it should forward the packet to the internal address 192.168.1.1.
+ *
+ * For the incoming packet received from external interface, for example a http response sent
+ * from the http server, the same mechanism is applied but in a different direction,
+ * where the source and destination will be swapped, and the source address will be replaced
+ * with the internal address, which is obtained from the NAT mapping described above.
+ */
+public abstract class NatPacketForwarderBase extends Thread {
+    private static final String TAG = "NatPacketForwarder";
+    static final int DESTINATION_PORT_OFFSET = 2;
+
+    // The source fd to read packets from.
+    @NonNull
+    final FileDescriptor mSrcFd;
+    // The buffer to temporarily hold the entire packet after receiving.
+    @NonNull
+    final byte[] mBuf;
+    // The destination fd to write packets to.
+    @NonNull
+    final FileDescriptor mDstFd;
+    // The NAT mapping table shared between two NatPacketForwarder instances to map from
+    // the source port to the associated internal address. The map can be read/write from two
+    // different threads on any given time whenever receiving packets on the
+    // {@link TestNetworkInterface}. Thus, synchronize on the object when reading/writing is needed.
+    @GuardedBy("mNatMap")
+    @NonNull
+    final PacketBridge.NatMap mNatMap;
+    // The address of the external interface. See {@link NatPacketForwarder}.
+    @NonNull
+    final InetAddress mExtAddr;
+
+    /**
+     * Construct a {@link NatPacketForwarderBase}.
+     *
+     * This class reads packets from {@code srcFd} of a {@link TestNetworkInterface}, and
+     * forwards them to the {@code dstFd} of another {@link TestNetworkInterface} with
+     * NAT applied. See {@link NatPacketForwarderBase}.
+     *
+     * To apply NAT, the address of the external interface needs to be supplied through
+     * {@code extAddr} to identify the external interface. And a shared NAT mapping table,
+     * {@code natMap} is needed to be shared between these two instances.
+     *
+     * Note that this class is not useful if the instance is not managed by a
+     * {@link PacketBridge} to set up a two-way communication.
+     *
+     * @param srcFd   {@link FileDescriptor} to read packets from.
+     * @param mtu     MTU of the test network.
+     * @param dstFd   {@link FileDescriptor} to write packets to.
+     * @param extAddr the external address, which is the address of the external interface.
+     *                See {@link NatPacketForwarderBase}.
+     * @param natMap  the NAT mapping table shared between two {@link NatPacketForwarderBase}
+     *                instance.
+     */
+    public NatPacketForwarderBase(@NonNull FileDescriptor srcFd, int mtu,
+            @NonNull FileDescriptor dstFd, @NonNull InetAddress extAddr,
+            @NonNull PacketBridge.NatMap natMap) {
+        super(TAG);
+        mSrcFd = Objects.requireNonNull(srcFd);
+        mBuf = new byte[mtu];
+        mDstFd = Objects.requireNonNull(dstFd);
+        mExtAddr = Objects.requireNonNull(extAddr);
+        mNatMap = Objects.requireNonNull(natMap);
+    }
+
+    /**
+     * A method to prepare forwarding packets between two instances of {@link TestNetworkInterface},
+     * which includes re-write addresses, ports and fix up checksums.
+     * Subclasses should override this method to implement a simple NAT.
+     */
+    abstract void preparePacketForForwarding(@NonNull byte[] buf, int len, int version, int proto);
+
+    private void forwardPacket(@NonNull byte[] buf, int len) {
+        try {
+            Os.write(mDstFd, buf, 0, len);
+        } catch (ErrnoException | IOException e) {
+            Log.e(TAG, "Error writing packet: " + e.getMessage());
+        }
+    }
+
+    // Reads one packet from mSrcFd, and writes the packet to the mDstFd for supported protocols.
+    private void processPacket() {
+        final int len = PacketReflectorUtil.readPacket(mSrcFd, mBuf);
+        if (len < 1) {
+            throw new IllegalStateException("Unexpected buffer length: " + len);
+        }
+
+        final int version = mBuf[0] >>> 4;
+        final int protoPos, ipHdrLen;
+        switch (version) {
+            case 4:
+                ipHdrLen = IPV4_HEADER_LENGTH;
+                protoPos = PacketReflector.IPV4_PROTO_OFFSET;
+                break;
+            case 6:
+                ipHdrLen = IPV6_HEADER_LENGTH;
+                protoPos = IPV6_PROTO_OFFSET;
+                break;
+            default:
+                throw new IllegalStateException("Unexpected version: " + version);
+        }
+        if (len < ipHdrLen) {
+            throw new IllegalStateException("Unexpected buffer length: " + len);
+        }
+
+        final byte proto = mBuf[protoPos];
+        final int transportHdrLen;
+        switch (proto) {
+            case IPPROTO_TCP:
+                transportHdrLen = TCP_HEADER_LENGTH;
+                break;
+            case IPPROTO_UDP:
+                transportHdrLen = UDP_HEADER_LENGTH;
+                break;
+            // TODO: Support ICMP.
+            default:
+                return; // Unknown protocol, ignored.
+        }
+
+        if (len < ipHdrLen + transportHdrLen) {
+            throw new IllegalStateException("Unexpected buffer length: " + len);
+        }
+        // Re-write addresses, ports and fix up checksums.
+        preparePacketForForwarding(mBuf, len, version, proto);
+        // Send the packet to the destination fd.
+        forwardPacket(mBuf, len);
+    }
+
+    @Override
+    public void run() {
+        Log.i(TAG, "starting fd=" + mSrcFd + " valid=" + mSrcFd.valid());
+        while (!interrupted() && mSrcFd.valid()) {
+            processPacket();
+        }
+        Log.i(TAG, "exiting fd=" + mSrcFd + " valid=" + mSrcFd.valid());
+    }
+}
diff --git a/staticlibs/testutils/devicetests/com/android/testutils/PacketBridge.kt b/staticlibs/testutils/devicetests/com/android/testutils/PacketBridge.kt
new file mode 100644
index 0000000..da3508d
--- /dev/null
+++ b/staticlibs/testutils/devicetests/com/android/testutils/PacketBridge.kt
@@ -0,0 +1,173 @@
+/*
+ * Copyright (C) 2023 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.testutils
+
+import android.content.Context
+import android.net.ConnectivityManager
+import android.net.LinkAddress
+import android.net.LinkProperties
+import android.net.Network
+import android.net.NetworkCapabilities
+import android.net.NetworkRequest
+import android.net.TestNetworkInterface
+import android.net.TestNetworkManager
+import android.net.TestNetworkSpecifier
+import android.os.Binder
+import com.android.testutils.RecorderCallback.CallbackEntry.Available
+import java.net.InetAddress
+import libcore.io.IoUtils
+
+private const val MIN_PORT_NUMBER = 1025
+private const val MAX_PORT_NUMBER = 65535
+
+/**
+ * A class that set up two {@link TestNetworkInterface} with NAT, and forward packets between them.
+ *
+ * See {@link NatPacketForwarder} for more detailed information.
+ */
+class PacketBridge(
+    context: Context,
+    internalAddr: LinkAddress,
+    externalAddr: LinkAddress,
+    dnsAddr: InetAddress
+) {
+    private val natMap = NatMap()
+    private val binder = Binder()
+
+    private val cm = context.getSystemService(ConnectivityManager::class.java)
+    private val tnm = context.getSystemService(TestNetworkManager::class.java)
+
+    // Create test networks.
+    private val internalIface = tnm.createTunInterface(listOf(internalAddr))
+    private val externalIface = tnm.createTunInterface(listOf(externalAddr))
+
+    // Register test networks to ConnectivityService.
+    private val internalNetworkCallback: TestableNetworkCallback
+    private val externalNetworkCallback: TestableNetworkCallback
+    val internalNetwork: Network
+    val externalNetwork: Network
+    init {
+        val (inCb, inNet) = createTestNetwork(internalIface, internalAddr, dnsAddr)
+        val (exCb, exNet) = createTestNetwork(externalIface, externalAddr, dnsAddr)
+        internalNetworkCallback = inCb
+        externalNetworkCallback = exCb
+        internalNetwork = inNet
+        externalNetwork = exNet
+    }
+
+    // Setup the packet bridge.
+    private val internalFd = internalIface.fileDescriptor.fileDescriptor
+    private val externalFd = externalIface.fileDescriptor.fileDescriptor
+
+    private val pr1 = NatInternalPacketForwarder(
+        internalFd,
+        1500,
+        externalFd,
+        externalAddr.address,
+        natMap
+    )
+    private val pr2 = NatExternalPacketForwarder(
+        externalFd,
+        1500,
+        internalFd,
+        externalAddr.address,
+        natMap
+    )
+
+    fun start() {
+        IoUtils.setBlocking(internalFd, true /* blocking */)
+        IoUtils.setBlocking(externalFd, true /* blocking */)
+        pr1.start()
+        pr2.start()
+    }
+
+    fun stop() {
+        pr1.interrupt()
+        pr2.interrupt()
+        cm.unregisterNetworkCallback(internalNetworkCallback)
+        cm.unregisterNetworkCallback(externalNetworkCallback)
+    }
+
+    /**
+     * Creates a test network with given test TUN interface and addresses.
+     */
+    private fun createTestNetwork(
+        testIface: TestNetworkInterface,
+        addr: LinkAddress,
+        dnsAddr: InetAddress
+    ): Pair<TestableNetworkCallback, Network> {
+        // Make a network request to hold the test network
+        val nr = NetworkRequest.Builder()
+            .clearCapabilities()
+            .addTransportType(NetworkCapabilities.TRANSPORT_TEST)
+            .setNetworkSpecifier(TestNetworkSpecifier(testIface.interfaceName))
+            .build()
+        val testCb = TestableNetworkCallback()
+        cm.requestNetwork(nr, testCb)
+
+        val lp = LinkProperties().apply {
+            addLinkAddress(addr)
+            interfaceName = testIface.interfaceName
+            addDnsServer(dnsAddr)
+        }
+        tnm.setupTestNetwork(lp, true /* isMetered */, binder)
+
+        // Wait for available before return.
+        val network = testCb.expect<Available>().network
+        return testCb to network
+    }
+
+    /**
+     * A helper class to maintain the mappings between internal addresses/ports and external
+     * ports.
+     *
+     * This class assigns an unused external port number if the mapping between
+     * srcaddress:srcport:protocol and the external port does not exist yet.
+     *
+     * Note that this class is not thread-safe. The instance of the class needs to be
+     * synchronized in the callers when being used in multiple threads.
+     */
+    class NatMap {
+        data class AddressInfo(val address: InetAddress, val port: Int, val protocol: Int)
+
+        private val mToExternalPort = HashMap<AddressInfo, Int>()
+        private val mFromExternalPort = HashMap<Int, AddressInfo>()
+
+        // Skip well-known port 0~1024.
+        private var nextExternalPort = MIN_PORT_NUMBER
+
+        fun toExternalPort(addr: InetAddress, port: Int, protocol: Int): Int {
+            val info = AddressInfo(addr, port, protocol)
+            val extPort: Int
+            if (!mToExternalPort.containsKey(info)) {
+                extPort = nextExternalPort++
+                if (nextExternalPort > MAX_PORT_NUMBER) {
+                    throw IllegalStateException("Available ports are exhausted")
+                }
+                mToExternalPort[info] = extPort
+                mFromExternalPort[extPort] = info
+            } else {
+                extPort = mToExternalPort[info]!!
+            }
+            return extPort
+        }
+
+        fun fromExternalPort(port: Int): AddressInfo? {
+            return mFromExternalPort[port]
+        }
+    }
+}
diff --git a/staticlibs/testutils/devicetests/com/android/testutils/PacketReflector.java b/staticlibs/testutils/devicetests/com/android/testutils/PacketReflector.java
index 96bca62..a59750d 100644
--- a/staticlibs/testutils/devicetests/com/android/testutils/PacketReflector.java
+++ b/staticlibs/testutils/devicetests/com/android/testutils/PacketReflector.java
@@ -26,40 +26,43 @@
 
 import java.io.FileDescriptor;
 import java.io.IOException;
+import java.util.Objects;
 
 public class PacketReflector extends Thread {
 
-    private static final int IPV4_HEADER_LENGTH = 20;
-    private static final int IPV6_HEADER_LENGTH = 40;
+    static final int IPV4_HEADER_LENGTH = 20;
+    static final int IPV6_HEADER_LENGTH = 40;
 
-    private static final int IPV4_ADDR_OFFSET = 12;
-    private static final int IPV6_ADDR_OFFSET = 8;
-    private static final int IPV4_ADDR_LENGTH = 4;
-    private static final int IPV6_ADDR_LENGTH = 16;
+    static final int IPV4_ADDR_OFFSET = 12;
+    static final int IPV6_ADDR_OFFSET = 8;
+    static final int IPV4_ADDR_LENGTH = 4;
+    static final int IPV6_ADDR_LENGTH = 16;
 
-    private static final int IPV4_PROTO_OFFSET = 9;
-    private static final int IPV6_PROTO_OFFSET = 6;
+    static final int IPV4_PROTO_OFFSET = 9;
+    static final int IPV6_PROTO_OFFSET = 6;
 
-    private static final byte IPPROTO_ICMP = 1;
-    private static final byte IPPROTO_TCP = 6;
-    private static final byte IPPROTO_UDP = 17;
+    static final byte IPPROTO_ICMP = 1;
+    static final byte IPPROTO_TCP = 6;
+    static final byte IPPROTO_UDP = 17;
     private static final byte IPPROTO_ICMPV6 = 58;
 
     private static final int ICMP_HEADER_LENGTH = 8;
-    private static final int TCP_HEADER_LENGTH = 20;
-    private static final int UDP_HEADER_LENGTH = 8;
+    static final int TCP_HEADER_LENGTH = 20;
+    static final int UDP_HEADER_LENGTH = 8;
 
     private static final byte ICMP_ECHO = 8;
     private static final byte ICMP_ECHOREPLY = 0;
 
     private static String TAG = "PacketReflector";
 
-    @NonNull private FileDescriptor mFd;
-    @NonNull private byte[] mBuf;
+    @NonNull
+    private final FileDescriptor mFd;
+    @NonNull
+    private final byte[] mBuf;
 
     public PacketReflector(@NonNull FileDescriptor fd, int mtu) {
         super("PacketReflector");
-        mFd = fd;
+        mFd = Objects.requireNonNull(fd);
         mBuf = new byte[mtu];
     }
 
@@ -140,7 +143,7 @@
         writePacket(buf, len);
 
         // The device should have replied, and buf should now contain a ping response.
-        int received = readPacket(buf);
+        int received = PacketReflectorUtil.readPacket(mFd, buf);
         if (received != len) {
             Log.i(TAG, "Reflecting ping did not result in ping response: " +
                     "read=" + received + " expected=" + len);
@@ -190,21 +193,11 @@
         }
     }
 
-    private int readPacket(@NonNull byte[] buf) {
-        int len;
-        try {
-            len = Os.read(mFd, buf, 0, buf.length);
-        } catch (ErrnoException | IOException e) {
-            Log.e(TAG, "Error reading packet: " + e.getMessage());
-            len = -1;
-        }
-        return len;
-    }
-
     // Reads one packet from our mFd, and possibly writes the packet back.
     private void processPacket() {
-        int len = readPacket(mBuf);
+        int len = PacketReflectorUtil.readPacket(mFd, mBuf);
         if (len < 1) {
+            // Usually happens when socket read is being interrupted, e.g. stopping PacketReflector.
             return;
         }
 
@@ -217,11 +210,11 @@
             hdrLen = IPV6_HEADER_LENGTH;
             protoPos = IPV6_PROTO_OFFSET;
         } else {
-            return;
+            throw new IllegalStateException("Unexpected version: " + version);
         }
 
         if (len < hdrLen) {
-            return;
+            throw new IllegalStateException("Unexpected buffer length: " + len);
         }
 
         byte proto = mBuf[protoPos];
@@ -241,10 +234,10 @@
     }
 
     public void run() {
-        Log.i(TAG, "PacketReflector starting fd=" + mFd + " valid=" + mFd.valid());
+        Log.i(TAG, "starting fd=" + mFd + " valid=" + mFd.valid());
         while (!interrupted() && mFd.valid()) {
             processPacket();
         }
-        Log.i(TAG, "PacketReflector exiting fd=" + mFd + " valid=" + mFd.valid());
+        Log.i(TAG, "exiting fd=" + mFd + " valid=" + mFd.valid());
     }
 }
diff --git a/staticlibs/testutils/devicetests/com/android/testutils/PacketReflectorUtil.kt b/staticlibs/testutils/devicetests/com/android/testutils/PacketReflectorUtil.kt
new file mode 100644
index 0000000..b028045
--- /dev/null
+++ b/staticlibs/testutils/devicetests/com/android/testutils/PacketReflectorUtil.kt
@@ -0,0 +1,106 @@
+/*
+ * Copyright (C) 2023 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.
+ */
+
+@file:JvmName("PacketReflectorUtil")
+
+package com.android.testutils
+
+import android.system.ErrnoException
+import android.system.Os
+import com.android.net.module.util.IpUtils
+import com.android.testutils.PacketReflector.IPV4_HEADER_LENGTH
+import com.android.testutils.PacketReflector.IPV6_HEADER_LENGTH
+import java.io.FileDescriptor
+import java.io.IOException
+import java.net.InetAddress
+import java.nio.ByteBuffer
+
+fun readPacket(fd: FileDescriptor, buf: ByteArray): Int {
+    return try {
+        Os.read(fd, buf, 0, buf.size)
+    } catch (e: ErrnoException) {
+        -1
+    } catch (e: IOException) {
+        -1
+    }
+}
+
+fun getInetAddressAt(buf: ByteArray, pos: Int, len: Int): InetAddress =
+    InetAddress.getByAddress(buf.copyOfRange(pos, pos + len))
+
+/**
+ * Reads a 16-bit unsigned int at pos in big endian, with no alignment requirements.
+ */
+fun getPortAt(buf: ByteArray, pos: Int): Int {
+    return (buf[pos].toInt() and 0xff shl 8) + (buf[pos + 1].toInt() and 0xff)
+}
+
+fun setPortAt(port: Int, buf: ByteArray, pos: Int) {
+    buf[pos] = (port ushr 8).toByte()
+    buf[pos + 1] = (port and 0xff).toByte()
+}
+
+fun getAddressPositionAndLength(version: Int) = when (version) {
+    4 -> PacketReflector.IPV4_ADDR_OFFSET to PacketReflector.IPV4_ADDR_LENGTH
+    6 -> PacketReflector.IPV6_ADDR_OFFSET to PacketReflector.IPV6_ADDR_LENGTH
+    else -> throw IllegalArgumentException("Unknown IP version $version")
+}
+
+private const val IPV4_CHKSUM_OFFSET = 10
+private const val UDP_CHECKSUM_OFFSET = 6
+private const val TCP_CHECKSUM_OFFSET = 16
+
+fun fixPacketChecksum(buf: ByteArray, len: Int, version: Int, protocol: Byte) {
+    // Fill Ip checksum for IPv4. IPv6 header doesn't have a checksum field.
+    if (version == 4) {
+        val checksum = IpUtils.ipChecksum(ByteBuffer.wrap(buf), 0)
+        // Place checksum in Big-endian order.
+        buf[IPV4_CHKSUM_OFFSET] = (checksum.toInt() ushr 8).toByte()
+        buf[IPV4_CHKSUM_OFFSET + 1] = (checksum.toInt() and 0xff).toByte()
+    }
+
+    // Fill transport layer checksum.
+    val transportOffset = if (version == 4) IPV4_HEADER_LENGTH else IPV6_HEADER_LENGTH
+    when (protocol) {
+        PacketReflector.IPPROTO_UDP -> {
+            val checksumPos = transportOffset + UDP_CHECKSUM_OFFSET
+            // Clear before calculate.
+            buf[checksumPos + 1] = 0x00
+            buf[checksumPos] = buf[checksumPos + 1]
+            val checksum = IpUtils.udpChecksum(
+                ByteBuffer.wrap(buf), 0,
+                transportOffset
+            )
+            buf[checksumPos] = (checksum.toInt() ushr 8).toByte()
+            buf[checksumPos + 1] = (checksum.toInt() and 0xff).toByte()
+        }
+        PacketReflector.IPPROTO_TCP -> {
+            val checksumPos = transportOffset + TCP_CHECKSUM_OFFSET
+            // Clear before calculate.
+            buf[checksumPos + 1] = 0x00
+            buf[checksumPos] = buf[checksumPos + 1]
+            val transportLen: Int = len - transportOffset
+            val checksum = IpUtils.tcpChecksum(
+                ByteBuffer.wrap(buf), 0, transportOffset,
+                transportLen
+            )
+            buf[checksumPos] = (checksum.toInt() ushr 8).toByte()
+            buf[checksumPos + 1] = (checksum.toInt() and 0xff).toByte()
+        }
+        // TODO: Support ICMP.
+        else -> throw IllegalArgumentException("Unsupported protocol: $protocol")
+    }
+}