This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.
此份代码仅用作学术交流用途，其他行为带来的后果本人概不负责
This copy of code is for study of python only
10、udprelay.py，实现udp的转达，用于local端处理local和 客户器端的SOCKS5协议通信，用于local端和远程端Shadowsocks协议的通信；用于远程端与local端Shadowsocks协议的通信，用于远程端和dest端(destination)的通信
代码质量相当的高，感觉都能达到重用的级别。而且由于作者设计的思想是，一个配置文件，同一段程序，在本地和远程通用，所以其中的代码，常常能够达到一个函数，在本地和服务器有不同的功能这样的效果。
===============================================================
eventloop使用select、epoll、kqueue等IO复用实现异步处理。优先级为epoll>kqueue>select。Eventloop将三种复用机制的add，remove，poll，add_handler，remve_handler接口统一起来，程序员只需要使用这些函数即可，不需要处理底层细节。
后三个文件分别实现用来处理udp的请求，tcp的请求，dns的查询请求，并且将三种请求的处理包装成handler。对于tcp，udp的handler，它们bind到特定的端口，并且将socket交给eventloop，并且将自己的处理函数加到eventloop的handlers；对于dns的handler，它接受来自udp handler和tcp handler的dns查询请求，并且向远程dns服务器发出udp请求；
当eventloop监测到socket的数据，程序就将所有监测到的socket和事件交给所有handler去处理，每个handler通过socket和事件判断自己是否要处理该事件，并进行相对的处理：
+----+------+------+----------+----------+----------+ |RSV | FRAG | ATYP | DST.ADDR | DST.PORT | DATA | +----+------+------+----------+----------+----------+ | 2 | 1 | 1 | Variable | 2 | Variable | +----+------+------+----------+----------+----------+
trim->
+------+----------+----------+----------+ | ATYP | DST.ADDR | DST.PORT | DATA | +------+----------+----------+----------+ | 1 | Variable | 2 | Variable | +------+----------+----------+----------+
->encrypt
+-------+--------------+ | IV | PAYLOAD | +-------+--------------+ | Fixed | Variable | +-------+--------------+
+-------+--------------+ | IV | PAYLOAD | +-------+--------------+ | Fixed | Variable | +-------+--------------+
->decrypt
+------+----------+----------+----------+ | ATYP | DST.ADDR | DST.PORT | DATA | +------+----------+----------+----------+ | 1 | Variable | 2 | Variable | +------+----------+----------+----------+
->add
+----+------+------+----------+----------+----------+ |RSV | FRAG | ATYP | DST.ADDR | DST.PORT | DATA | +----+------+------+----------+----------+----------+ | 2 | 1 | 1 | Variable | 2 | Variable | +----+------+------+----------+----------+----------+
+-------+--------------+ | IV | PAYLOAD | +-------+--------------+ | Fixed | Variable | +-------+--------------+
->decrypt
+------+----------+----------+----------+ | ATYP | DST.ADDR | DST.PORT | DATA | +------+----------+----------+----------+ | 1 | Variable | 2 | Variable | +------+----------+----------+----------+
->trim
+----------+ | DATA | +----------+ | Variable | +----------+
->getaddrinfo->tcp/udp ->send to dest server via tcp/udp
+----------+ | DATA | +----------+ | Variable | +----------+
->add
+------+----------+----------+----------+ | ATYP | DST.ADDR | DST.PORT | DATA | +------+----------+----------+----------+ | 1 | Variable | 2 | Variable | +------+----------+----------+----------+
->encrypt
+-------+--------------+ | IV | PAYLOAD | +-------+--------------+ | Fixed | Variable | +-------+--------------+
->send to local
在handler函数里面的基本逻辑就是：
if sock == self._server_socket: self._handle_server() elif sock and (fd in self._sockets): self._handle_client(sock)
协议解析和构建用的struct.pack()和struct.unpack()
===============================================================
1.1、读取/etc/hosts和/etc/resolv.conf文件，如果没有设置，就设置dns服务器为8.8.8.8和8.8.4.4 1.2、收到tcp handler和udp handler的dns请求之后，建立socket并且向远程服务器发送请求，并把（hostname：callback）加入_hostname_to_cb 1.3、收到响应之后触发callback _hostname_to_cbhostname
# 请求 # 1 1 1 1 1 1 # 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 # +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ # | ID | # +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ # |QR| Opcode |AA|TC|RD|RA| Z | RCODE | # +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ # | QDCOUNT | # +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ # | ANCOUNT | # +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ # | NSCOUNT | # +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ # | ARCOUNT | # +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
响应：
1 1 1 1 1 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | | / / / NAME / | | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | TYPE | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | CLASS | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | TTL | | | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | RDLENGTH | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--| / RDATA / / / +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
===============================================================
self._store = self._time_to_keys = collections.defaultdict(list) self._keys_to_last_time = self._last_visits = collections.deque()