Add utilities to test tap interface networks

Add an ArpResponder based on a generic PacketResponder class, and a
TestHttpServer based on NanoHttpd.

Test: tests based on these utilities
Bug: 160617623
Bug: 160656765
Change-Id: I50b872a8b23e8df997e8f62f0adc7c0256c4d74d
diff --git a/staticlibs/devicetests/com/android/testutils/ArpResponder.kt b/staticlibs/devicetests/com/android/testutils/ArpResponder.kt
new file mode 100644
index 0000000..86631c3
--- /dev/null
+++ b/staticlibs/devicetests/com/android/testutils/ArpResponder.kt
@@ -0,0 +1,60 @@
+/*
+ * 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.testutils
+
+import android.net.MacAddress
+import java.net.Inet4Address
+import java.net.InetAddress
+import java.nio.ByteBuffer
+
+private val TYPE_ARP = byteArrayOf(0x08, 0x06)
+// Arp reply header for IPv4 over ethernet
+private val ARP_REPLY_IPV4 = byteArrayOf(0x00, 0x01, 0x08, 0x00, 0x06, 0x04, 0x00, 0x02)
+
+/**
+ * A class that can be used to reply to ARP packets on a [TapPacketReader].
+ */
+class ArpResponder(
+    reader: TapPacketReader,
+    table: Map<Inet4Address, MacAddress>,
+    name: String = ArpResponder::class.java.simpleName
+) : PacketResponder(reader, ArpRequestFilter(), name) {
+    // Copy the map if not already immutable (toMap) to make sure it is not modified
+    private val table = table.toMap()
+
+    override fun replyToPacket(packet: ByteArray, reader: TapPacketReader) {
+        val targetIp = InetAddress.getByAddress(
+                packet.copyFromIndexWithLength(ARP_TARGET_IPADDR_OFFSET, 4))
+                as Inet4Address
+
+        val macAddr = table[targetIp]?.toByteArray() ?: return
+        val senderMac = packet.copyFromIndexWithLength(ARP_SENDER_MAC_OFFSET, 6)
+        reader.sendResponse(ByteBuffer.wrap(
+                // Ethernet header
+                senderMac + macAddr + TYPE_ARP +
+                        // ARP message
+                        ARP_REPLY_IPV4 +
+                        macAddr /* sender MAC */ +
+                        targetIp.address /* sender IP addr */ +
+                        macAddr /* target mac */ +
+                        targetIp.address /* target IP addr */
+        ))
+    }
+}
+
+private fun ByteArray.copyFromIndexWithLength(start: Int, len: Int) =
+        copyOfRange(start, start + len)