今天打算写一个FtpServer玩一下的，需要看看ftp软件常用命令形式（完整实现所有ftp命令太麻烦），最开始打算通过抓包看cuteftp是如何访问ftpserver的，但要把其中的命令保存下来还得一条条复制，太麻烦，便通过proxy模式写了一个代理程序，来获取其交互的命令，写了一个简单的同步模式下的端口映射程序后，发现比常用的异步proxy要简单的多，便把这段代码贴出来，以备日后查询：

class Program
{
    static void Main(string[] args)
    {
        TcpListener listener = new TcpListener(new IPEndPoint(IPAddress.Loopback, 8000));
        listener.Start();
        while (true)
        {
            var client = listener.AcceptTcpClient();

Console.WriteLine("connected");
            var proxy = new TcpClient();
            Console.WriteLine("remote connected");
            proxy.Connect(new IPEndPoint(IPAddress.Loopback, 21));

new SyncProxy("client->remote",proxy.GetStream(), client.GetStream());
            new SyncProxy("remote->client",client.GetStream(), proxy.GetStream());
        }
    }
}

class SyncProxy
{
    NetworkStream read;
    NetworkStream write;
    string name;

public SyncProxy(string name, NetworkStream read,NetworkStream write)
    {
        this.name = name;
        this.read = read;
        this.write = write;

System.Threading.ThreadPool.QueueUserWorkItem(PipeStream);
    }

void PipeStream(object state)
    {
        byte[] buffer = new byte[1500];
        int count = 0;
        while (true)
        {
            try
            {
                count = read.Read(buffer, 0, buffer.Length);
            }
            catch (Exception)
            {
                count = 0;
            }

if (count == 0)
            {
                Console.WriteLine(name+" closed");
                write.Close();
                break;
            }

Console.Write(name + ": "+ Encoding.Default.GetString(buffer, 0, count));
            write.Write(buffer, 0, count);
        }
    }
}

通过它获取到的cuteFtp交互命令如下：
connected
remote connected
client->remote: 220 Serv-U FTP Server v6.0 for WinSock ready...
remote->client: USER 1
client->remote: 331 User name okay, need password.
remote->client: PASS 1
client->remote: 230 User logged in, proceed.
remote->client: PWD
client->remote: 257 "/" is current directory.
remote->client: FEAT
client->remote: 211-Extension supported
client->remote: CLNT
MDTM
MDTM YYYYMMDDHHMMSS[+-TZ];filename
SIZE
SITE PSWD;EXEC;SET;INDEX;ZONE;CHMOD;MSG
REST STREAM
XCRC filename;start;end
MODE Z
211 End
remote->client: REST 0
client->remote: 350 Restarting at 0. Send STORE or RETRIEVE.
remote->client: PASV
client->remote: 227 Entering Passive Mode (127,0,0,1,29,18)
remote->client: LIST
client->remote: 150 Opening ASCII mode data connection for /bin/ls.
client->remote: 226 Transfer complete.