最近在做的项目，需要自己搭建一个socks代理。netty4.0附带了一个socks代理的样例，但是3.x就没有这个东西了，碰巧使用的又是3.7，就只能自己摸索并实现一遍，也算是对netty和socks协议的一个熟悉。 socks代理涉及到协议解析、server、client等功能，是一个比较复杂的网络程序，对于学习netty的使用也是非常好的例子。
socks是在传输层之上的一层协议，主要功能是提供代理认证等功能。socks协议虽然是应用层协议(在TCP/IP4层协议栈里)，本身可以理解为一个信道，可以传输任何TCP/UDP内容。例如著名的科学上网软件就是基于socks协议，对通信内容进行加密实现的。
TCP/IP协议栈的结构中，下层协议总会在上层协议内容之前加上自己的头。而socks协议稍微不同，其实它对比TCP协议，仅仅是多了验证部分，验证之后，完全是使用TCP来进行传输，而没有socks报文头。socks协议的具体内容可以参考rfc1928。这一点来说，其实将socks理解成与其他应用层协议平级也没什么问题。
一个最基本的socks连接流程是这样的：
那么我们开始netty之旅吧。
首先我们需要建立一个server：
public void run() {
// 新建线程池
Executor executor = Executors.newCachedThreadPool();
Executor executorWorker = Executors.newCachedThreadPool();
ServerBootstrap sb = new ServerBootstrap(
new NioServerSocketChannelFactory(executor, executorWorker));
// 初始化代理部分使用的client
ClientSocketChannelFactory cf =
new NioClientSocketChannelFactory(executor, executorWorker);
//设置处理逻辑
sb.setPipelineFactory(
new SocksProxyPipelineFactory(cf));
// Start up the server.
sb.bind(new InetSocketAddress(1080));
}如你所见，主要的处理逻辑以SocksProxyPipelineFactory的形式提供。SocksProxyPipelineFactory的代码包括几部分：
public class SocksProxyPipelineFactory implements ChannelPipelineFactory {
private final ClientSocketChannelFactory cf;
public SocksProxyPipelineFactory(ClientSocketChannelFactory cf) {
this.cf = cf;
}
@Override
public ChannelPipeline getPipeline() throws Exception {
ChannelPipeline pipeline = Channels.pipeline();
pipeline.addLast(SocksInitRequestDecoder.getName(),new SocksInitRequestDecoder());
pipeline.addLast(SocksMessageEncoder.getName(),new SocksMessageEncoder());
pipeline.addLast(SocksServerHandler.getName(),new SocksServerHandler(cf));
return pipeline;
}
}这里要详细解释一下几个handler的作用：
ChannelUpstreamHandler用于接收之后的处理，而ChannelDownstreamHandler则相反，用于写入数据之后的处理。这两个都可以附加到ChannelPipeline中。偷个懒，直接附上netty的ChannelPipeline中的一段很有爱的javadoc：
I/O Request
via {@link Channel} or
{@link ChannelHandlerContext}
|
+----------------------------------------+---------------+
| ChannelPipeline | |
| \|/ |
| +----------------------+ +-----------+------------+ |
| | Upstream Handler N | | Downstream Handler 1 | |
| +----------+-----------+ +-----------+------------+ |
| /|\ | |
| | \|/ |
| +----------+-----------+ +-----------+------------+ |
| | Upstream Handler N-1 | | Downstream Handler 2 | |
| +----------+-----------+ +-----------+------------+ |
| /|\ . |
| . . |
| [ sendUpstream() ] [ sendDownstream() ] |
| [ + INBOUND data ] [ + OUTBOUND data ] |
| . . |
| . \|/ |
| +----------+-----------+ +-----------+------------+ |
| | Upstream Handler 2 | | Downstream Handler M-1 | |
| +----------+-----------+ +-----------+------------+ |
| /|\ | |
| | \|/ |
| +----------+-----------+ +-----------+------------+ |
| | Upstream Handler 1 | | Downstream Handler M | |
| +----------+-----------+ +-----------+------------+ |
| /|\ | |
+-------------+--------------------------+---------------+
| \|/
+-------------+--------------------------+---------------+
| | | |
| [ Socket.read() ] [ Socket.write() ] |
| |
| Netty Internal I/O Threads (Transport Implementation) |
+--------------------------------------------------------+
SocksInitRequestDecoder用于对socks的请求进行解码。你可能会说，为什么没有SocksCmdRequest的解码？别急，netty的handler是可以动态添加的，这里我们先解码一个初始化的请求。SocksInitRequestDecoder是一个ChannelUpstreamHandler，即接收流的处理器。
SocksMessageEncoder是一个ChannelDownstreamHandler，即输出时的编码器，有了它，我们可以很开心的在channel.write()里直接传入一个对象，而无需自己去写buffer了。
SocksServerHandler是处理的重头。这里会根据请求的不同类型，做不同的处理。
public void messageReceived(ChannelHandlerContext ctx, MessageEvent e) throws Exception {
SocksRequest socksRequest = (SocksRequest) e.getMessage();
switch (socksRequest.getSocksRequestType()) {
case INIT:
//添加cmd解码器
ctx.getPipeline().addFirst(SocksCmdRequestDecoder.getName(), new SocksCmdRequestDecoder());
//简单起见，无需认证
ctx.getChannel().write(new SocksInitResponse(SocksMessage.AuthScheme.NO_AUTH));
break;
case AUTH:
ctx.getPipeline().addFirst(SocksCmdRequestDecoder.getName(), new SocksCmdRequestDecoder());
//直接成功
ctx.getChannel().write(new SocksAuthResponse(SocksMessage.AuthStatus.SUCCESS));
break;
case CMD:
SocksCmdRequest req = (SocksCmdRequest) socksRequest;
if (req.getCmdType() == SocksMessage.CmdType.CONNECT) {
//添加处理连接的handler
ctx.getPipeline().addLast(SocksServerConnectHandler.getName(), new SocksServerConnectHandler(cf));
ctx.getPipeline().remove(this);
} else {
ctx.getChannel().close();
}
break;
case UNKNOWN:
break;
}
super.messageReceived(ctx, e);
}前面两种INIT和AUTH就不做赘述了，后面当CMD为Connect时，添加一个处理连接的SocksServerConnectHandler，它会起到client与外部server的桥梁作用。
这里我们先实现一个纯转发的handler-OutboundHandler:
private class OutboundHandler extends SimpleChannelUpstreamHandler {
private final Channel inboundChannel;
OutboundHandler(Channel inboundChannel) {
this.inboundChannel = inboundChannel;
}
@Override
public void messageReceived(ChannelHandlerContext ctx, final MessageEvent e) throws Exception {
final ChannelBuffer msg = (ChannelBuffer) e.getMessage();
synchronized (trafficLock) {
inboundChannel.write(msg);
}
}
} 它会把收到的内容，写入到inboundChannel中，其他转发的作用。最后就是我们的SocksServerConnectHandler了:
public class SocksServerConnectHandler extends SimpleChannelUpstreamHandler {
private final ClientSocketChannelFactory cf;
private volatile Channel outboundChannel;
final Object trafficLock = new Object();
public SocksServerConnectHandler(ClientSocketChannelFactory cf) {
this.cf = cf;
}
@Override
public void messageReceived(ChannelHandlerContext ctx, MessageEvent e) throws Exception {
final SocksCmdRequest socksCmdRequest = (SocksCmdRequest) e.getMessage();
final Channel inboundChannel = e.getChannel();
inboundChannel.setReadable(false);
// Start the connection attempt.
final ClientBootstrap cb = new ClientBootstrap(cf);
cb.setOption("keepAlive", true);
cb.setOption("tcpNoDelay", true);
cb.setPipelineFactory(new ChannelPipelineFactory() {
@Override
public ChannelPipeline getPipeline() throws Exception {
ChannelPipeline pipeline = Channels.pipeline();
// 外部server数据转发到client
pipeline.addLast("outboundChannel", new OutboundHandler(inboundChannel, "out"));
return pipeline;
}
});
ChannelFuture f = cb.connect(new InetSocketAddress(socksCmdRequest.getHost(), socksCmdRequest.getPort()));
outboundChannel = f.getChannel();
ctx.getPipeline().remove(getName());
f.addListener(new ChannelFutureListener() {
public void operationComplete(ChannelFuture future) throws Exception {
if (future.isSuccess()) {
// client数据转发到外部server
inboundChannel.getPipeline().addLast("inboundChannel", new OutboundHandler(outboundChannel, "in"));
inboundChannel.write(new SocksCmdResponse(SocksMessage.CmdStatus.SUCCESS, socksCmdRequest
.getAddressType()));
inboundChannel.setReadable(true);
} else {
inboundChannel.write(new SocksCmdResponse(SocksMessage.CmdStatus.FAILURE, socksCmdRequest
.getAddressType()));
inboundChannel.close();
}
}
});
}
}好了，完工！输入curl --socks5 127.0.0.1:1080 http://www.oschina.net/测试一下吧？但是测试时发现，怎么老是无法接收到响应？
使用wiredshark抓包之后，发现对外请求完全正常，但是对客户端的响应，则完全没有http响应部分？
一步步debug下去，才发现SocksMessageEncoder出了问题！
@Override
protected Object encode(ChannelHandlerContext ctx, Channel channel, Object msg) throws Exception {
ChannelBuffer buffer = null;
if (msg instanceof SocksMessage) {
buffer = ChannelBuffers.buffer(DEFAULT_ENCODER_BUFFER_SIZE);
((SocksMessage) msg).encodeAsByteBuf(buffer);
}
return buffer;
}这里只有SocksMessage才会被处理，其他的message全部被丢掉了！于是我们加上一行:
@Override
protected Object encode(ChannelHandlerContext ctx, Channel channel, Object msg) throws Exception {
ChannelBuffer buffer = null;
if (msg instanceof SocksMessage) {
buffer = ChannelBuffers.buffer(DEFAULT_ENCODER_BUFFER_SIZE);
((SocksMessage) msg).encodeAsByteBuf(buffer);
} else if (msg instanceof ChannelBuffer) {
//直接转发是ChannelBuffer类型
buffer = (ChannelBuffer) msg;
}
return buffer;
}至此，一个代理完成！点这里查看代码：https://github.com/code4craft/netty-learning/tree/master/learning-src/socksproxy