200 行代码实现玩具版 FTP 服务

上个周，老张写了一篇文章《吃透FTP》（没看过的同学可以先点击浏览一下）。文章主要介绍了FTP的工作原理，写完之后觉得不过瘾，自己动手实现了一个玩具版的FTP服务。

当然，如果实现一个完整稳定的FTP服务，工作量还是相当庞大的。所以老张选择了利用Python实现一个玩具版来过过瘾，写完发现仅有200行代码。

所谓玩具版，就是说：

• 用户登录。使用预制的账号root，并没有使用系统账号

• 仅支持主动模式。

• 仅支持Binary模式。

• 仅支持文件的上传和下载。

• 单线程。

Talk is cheap，直接看代码。

#coding=utf-8

# FtpServer.py

# 一个玩具版的Ftp服务

# by 魔笛手CTO

import socket

import os

import six

END_FLAG = "\r\n"

ASCII_MODE = "II"

BINARY_MODE = "I"

def dump(string):

"""将字符串消息dump为网络序的字节"""

if six.PY2:

return string

return bytes(string, "utf-8")

def load(byte):

"""将字节消息load为字符串"""

if six.PY2:

return byte

return str(byte, "utf-8")

class FtpServer():

def __init__(self):

self.cmd_socket = None

self.ftp_users = {"root": "root"} # 允许登录的ftp账号密码

def __enter__(self):

return self

def __exit__(self, exc_type, exc_val, exc_tb):

"""确保服务关闭"""

try:

self.cmd_socket.close()

print("socket is closed")

except:

pass

def run(self):

"""启动服务，开启21端口监听"""

print("starting server on port 21...")

self.cmd_socket = socket.socket(socket.AF_INET, socket.SOCK_STREAM) # TCP

self.cmd_socket.bind(("0.0.0.0", 21))

self.cmd_socket.listen(1)

while True:

conn, addr = self.cmd_socket.accept()

self._handle(conn, addr)

def _close_conn(self, conn):

"""关闭指定连接"""

conn.close()

def _handle(self, conn, addr):

"""一旦同客户端建立连接，将有handle负责处理交互"""

user = None

password = None

authed = False

client_data_addr = None

self._say_hello(conn)

while True:

req = self._read_req(conn)

# 返回空字符串时，关闭连接，准备响应下一个连接

if req == "":

return self._close_conn(conn)

# 解析并响应客户端命令

cmd, arg = self._parse(req)

if cmd == "USER":

if not authed:

user = arg

resp = "331 Please specify the password"

else:

resp = "500 User has authed!"

elif cmd == "PASS":

if user and not authed:

password = arg

if self._auth(user, password):

authed = True

resp = "230 Login successful"

else:

resp = "500 Auth error"

else:

resp = "500 User is not specified or has login"

# binary模式和ascii模式, 当前仅支持binary模式

elif cmd == "TYPE":

if arg == ASCII_MODE:

resp = "500 Only support binary mode"

elif arg == BINARY_MODE:

resp = "200 Switching to binary mode"

# 主动模式下客户端的端口号

elif cmd == "PORT":

if not authed:

resp = "530 Not login"

else:

client_data_addr = self._parse_addr(arg)

resp = "200 PORT command successful"

# 上传文件

elif cmd == "STOR":

if not authed:

resp = "530 Not login"

else:

resp = "150 Ok to send data"

self._send_resp(conn, resp)

self._save_file(arg, client_data_addr)

resp = "226 Transfer complete"

# 下载文件

elif cmd == "RETR":

if not authed:

resp = "530 Not login"

else:

if not os.path.exists(arg):

resp = "550 File not exist"

else:

resp = "150 Ok to send data"

self._send_resp(conn, resp)

self._send_file(arg, client_data_addr)

resp = "226 Transfer complete"

else:

print("500 Unknown command")

# 发送响应

self._send_resp(conn, resp)

def _create_data_conn(self, host, port):

"""主动模式下建立数据通道"""

sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)

sock.bind(("0.0.0.0", 20))

sock.connect((host, port))

return sock

def _send_file(self, filename, client_data_add):

"""传输指定文件至客户端"""

conn = self._create_data_conn(*client_data_add)

with open(filename, "rb") as f:

conn.sendall(f.read())

conn.close()

def _save_file(self, filename, client_data_addr):

"""保存客户端上传的文件"""

conn = self._create_data_conn(*client_data_addr)

with open(filename, "wb") as f:

while True:

body = conn.recv(1)

if body == b'':

break

f.write(body)

conn.close()

def _read_req(self, conn):

"""读取请求消息"""

print("reading msg...")

msg = ""

while True:

body = load(conn.recv(1))

# 当客户端关闭连接时，body为空字符串

if body == "":

return body

msg += body

if msg.endswith(END_FLAG):

break

return msg

def _send_resp(self, conn, msg):

"""发送命令响应"""

print("ready to response:%s" % msg)

if not msg.endswith(END_FLAG):

msg += END_FLAG

conn.sendall(dump(msg))

def _auth(self, user, password):

"""登录用户认证"""

if user and self.ftp_users.get(user) and self.ftp_users.get(user) == password:

return True

return False

def _say_hello(self, conn):

"""发送欢迎语"""

self._send_resp(conn, "220 Hello!")

def _parse(self, msg):

"""解析客户端消息, 返回命令和参数"""

print("receive msg:%s" % msg)

msg = msg.strip()

args = msg.split(" ")

if len(args) == 2:

cmd, arg = args

return cmd, arg

return None, None

def _parse_addr(self, addr):

"""解析ip和端口号"""

args = addr.strip().split(",")

host = ".".join(args[:4])

port = int(args[4]) * 256 + int(args[5])

return host, port

if __name__ == "__main__":

with FtpServer() as server:

server.run()

然后为了验证程序是否能够正常工作，老张使用Python自带的ftplib来测试服务是否可用。

#coding=utf-8

import ftplib

# 登录FTP服务，使用主动模式

ftp = ftplib.FTP()

ftp.connect("127.0.0.1", 21)

ftp.login("root", "root")

ftp.set_pasv(False)

# 将本地文件上传至服务器

with open("client" , 'rb') as f:

ftp.storbinary("STOR upload_from_client", f, 1024)

# 下载服务器文件

with open("download_from_server", "wb") as f:

ftp.retrbinary("RETR server", f.write)

最后，如果有同学对老张的玩具版FTP感兴趣，可以点击文末的“阅读原文”，老张把代码上传到GitHub了。