1 /*
2  * Copyright (C) 2014 The Android Open Source Project
3  *
4  * Licensed under the Apache License, Version 2.0 (the "License");
5  * you may not use this file except in compliance with the License.
6  * You may obtain a copy of the License at
7  *
8  *      http://www.apache.org/licenses/LICENSE-2.0
9  *
10  * Unless required by applicable law or agreed to in writing, software
11  * distributed under the License is distributed on an "AS IS" BASIS,
12  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13  * See the License for the specific language governing permissions and
14  * limitations under the License.
15  */
16 
17 package com.android.testutils;
18 
19 import static android.system.OsConstants.ICMP6_ECHO_REPLY;
20 import static android.system.OsConstants.ICMP6_ECHO_REQUEST;
21 
22 import android.annotation.NonNull;
23 import android.net.TestNetworkInterface;
24 import android.system.ErrnoException;
25 import android.system.Os;
26 import android.util.Log;
27 
28 import java.io.FileDescriptor;
29 import java.io.IOException;
30 import java.util.Objects;
31 
32 /**
33  * A class that echoes packets received on a {@link TestNetworkInterface} back to itself.
34  *
35  * For testing purposes, sometimes a mocked environment to simulate a simple echo from the
36  * server side is needed. This is particularly useful if the test, e.g. VpnTest, is
37  * heavily relying on the outside world.
38  *
39  * This class reads packets from the {@link FileDescriptor} of a {@link TestNetworkInterface}, and:
40  *   1. For TCP and UDP packets, simply swaps the source address and the destination
41  *      address, then send it back to the {@link FileDescriptor}.
42  *   2. For ICMP ping packets, composes a ping reply and sends it back to the sender.
43  *   3. Ignore all other packets.
44  */
45 public class PacketReflector extends Thread {
46 
47     static final int IPV4_HEADER_LENGTH = 20;
48     static final int IPV6_HEADER_LENGTH = 40;
49 
50     static final int IPV4_ADDR_OFFSET = 12;
51     static final int IPV6_ADDR_OFFSET = 8;
52     static final int IPV4_ADDR_LENGTH = 4;
53     static final int IPV6_ADDR_LENGTH = 16;
54 
55     static final int IPV4_PROTO_OFFSET = 9;
56     static final int IPV6_PROTO_OFFSET = 6;
57 
58     static final byte IPPROTO_ICMP = 1;
59     static final byte IPPROTO_TCP = 6;
60     static final byte IPPROTO_UDP = 17;
61     private static final byte IPPROTO_ICMPV6 = 58;
62 
63     private static final int ICMP_HEADER_LENGTH = 8;
64     static final int TCP_HEADER_LENGTH = 20;
65     static final int UDP_HEADER_LENGTH = 8;
66 
67     private static final byte ICMP_ECHO = 8;
68     private static final byte ICMP_ECHOREPLY = 0;
69 
70     private static String TAG = "PacketReflector";
71 
72     @NonNull
73     private final FileDescriptor mFd;
74     @NonNull
75     private final byte[] mBuf;
76 
77     /**
78      * Construct a {@link PacketReflector} from the given {@code fd} of
79      * a {@link TestNetworkInterface}.
80      *
81      * @param fd {@link FileDescriptor} to read/write packets.
82      * @param mtu MTU of the test network.
83      */
PacketReflector(@onNull FileDescriptor fd, int mtu)84     public PacketReflector(@NonNull FileDescriptor fd, int mtu) {
85         super("PacketReflector");
86         mFd = Objects.requireNonNull(fd);
87         mBuf = new byte[mtu];
88     }
89 
90     // Reflect TCP packets: swap the source and destination addresses, but don't change the ports.
91     // This is used by the test to "connect to itself" through the VPN.
processTcpPacket(@onNull byte[] buf, int version, int len, int hdrLen)92     private void processTcpPacket(@NonNull byte[] buf, int version, int len, int hdrLen) {
93         if (len < hdrLen + TCP_HEADER_LENGTH) {
94             return;
95         }
96 
97         // Swap src and dst IP addresses.
98         PacketReflectorUtil.swapAddresses(buf, version);
99 
100         // Send the packet back.
101         writePacket(buf, len);
102     }
103 
104     // Echo UDP packets: swap source and destination addresses, and source and destination ports.
105     // This is used by the test to check that the bytes it sends are echoed back.
processUdpPacket(@onNull byte[] buf, int version, int len, int hdrLen)106     private void processUdpPacket(@NonNull byte[] buf, int version, int len, int hdrLen) {
107         if (len < hdrLen + UDP_HEADER_LENGTH) {
108             return;
109         }
110 
111         // Swap src and dst IP addresses.
112         PacketReflectorUtil.swapAddresses(buf, version);
113 
114         // Swap dst and src ports.
115         int portOffset = hdrLen;
116         PacketReflectorUtil.swapBytes(buf, portOffset, portOffset + 2, 2);
117 
118         // Send the packet back.
119         writePacket(buf, len);
120     }
121 
processIcmpPacket(@onNull byte[] buf, int version, int len, int hdrLen)122     private void processIcmpPacket(@NonNull byte[] buf, int version, int len, int hdrLen) {
123         if (len < hdrLen + ICMP_HEADER_LENGTH) {
124             return;
125         }
126 
127         byte type = buf[hdrLen];
128         if (!(version == 4 && type == ICMP_ECHO) &&
129                 !(version == 6 && type == (byte) ICMP6_ECHO_REQUEST)) {
130             return;
131         }
132 
133         // Save the ping packet we received.
134         byte[] request = buf.clone();
135 
136         // Swap src and dst IP addresses, and send the packet back.
137         // This effectively pings the device to see if it replies.
138         PacketReflectorUtil.swapAddresses(buf, version);
139         writePacket(buf, len);
140 
141         // The device should have replied, and buf should now contain a ping response.
142         int received = PacketReflectorUtil.readPacket(mFd, buf);
143         if (received != len) {
144             Log.i(TAG, "Reflecting ping did not result in ping response: " +
145                     "read=" + received + " expected=" + len);
146             return;
147         }
148 
149         byte replyType = buf[hdrLen];
150         if ((type == ICMP_ECHO && replyType != ICMP_ECHOREPLY)
151                 || (type == (byte) ICMP6_ECHO_REQUEST && replyType != (byte) ICMP6_ECHO_REPLY)) {
152             Log.i(TAG, "Received unexpected ICMP reply: original " + type
153                     + ", reply " + replyType);
154             return;
155         }
156 
157         // Compare the response we got with the original packet.
158         // The only thing that should have changed are addresses, type and checksum.
159         // Overwrite them with the received bytes and see if the packet is otherwise identical.
160         request[hdrLen] = buf[hdrLen];          // Type
161         request[hdrLen + 2] = buf[hdrLen + 2];  // Checksum byte 1.
162         request[hdrLen + 3] = buf[hdrLen + 3];  // Checksum byte 2.
163 
164         // Since Linux kernel 4.2, net.ipv6.auto_flowlabels is set by default, and therefore
165         // the request and reply may have different IPv6 flow label: ignore that as well.
166         if (version == 6) {
167             request[1] = (byte) (request[1] & 0xf0 | buf[1] & 0x0f);
168             request[2] = buf[2];
169             request[3] = buf[3];
170         }
171 
172         for (int i = 0; i < len; i++) {
173             if (buf[i] != request[i]) {
174                 Log.i(TAG, "Received non-matching packet when expecting ping response.");
175                 return;
176             }
177         }
178 
179         // Now swap the addresses again and reflect the packet. This sends a ping reply.
180         PacketReflectorUtil.swapAddresses(buf, version);
181         writePacket(buf, len);
182     }
183 
writePacket(@onNull byte[] buf, int len)184     private void writePacket(@NonNull byte[] buf, int len) {
185         try {
186             Os.write(mFd, buf, 0, len);
187         } catch (ErrnoException | IOException e) {
188             Log.e(TAG, "Error writing packet: " + e.getMessage());
189         }
190     }
191 
192     // Reads one packet from our mFd, and possibly writes the packet back.
processPacket()193     private void processPacket() {
194         int len = PacketReflectorUtil.readPacket(mFd, mBuf);
195         if (len < 1) {
196             // Usually happens when socket read is being interrupted, e.g. stopping PacketReflector.
197             return;
198         }
199 
200         int version = mBuf[0] >> 4;
201         int protoPos, hdrLen;
202         if (version == 4) {
203             hdrLen = IPV4_HEADER_LENGTH;
204             protoPos = IPV4_PROTO_OFFSET;
205         } else if (version == 6) {
206             hdrLen = IPV6_HEADER_LENGTH;
207             protoPos = IPV6_PROTO_OFFSET;
208         } else {
209             throw new IllegalStateException("Unexpected version: " + version);
210         }
211 
212         if (len < hdrLen) {
213             throw new IllegalStateException("Unexpected buffer length: " + len);
214         }
215 
216         byte proto = mBuf[protoPos];
217         switch (proto) {
218             case IPPROTO_ICMP:
219                 // fall through
220             case IPPROTO_ICMPV6:
221                 processIcmpPacket(mBuf, version, len, hdrLen);
222                 break;
223             case IPPROTO_TCP:
224                 processTcpPacket(mBuf, version, len, hdrLen);
225                 break;
226             case IPPROTO_UDP:
227                 processUdpPacket(mBuf, version, len, hdrLen);
228                 break;
229         }
230     }
231 
run()232     public void run() {
233         Log.i(TAG, "starting fd=" + mFd + " valid=" + mFd.valid());
234         while (!interrupted() && mFd.valid()) {
235             processPacket();
236         }
237         Log.i(TAG, "exiting fd=" + mFd + " valid=" + mFd.valid());
238     }
239 }
240