package main
import (
  "bufio"
  "context"
  "encoding/binary"
  "errors"
  "fmt"
  "io"
  "log"
  "net"
)
const socks5Ver = 0x05
const cmdBind = 0x01
const atypIPV4 = 0x01
const atypeHOST = 0x03
const atypeIPV6 = 0x04
func main() {
  server, err := net.Listen("tcp", "127.0.0.1:1080")
  if err != nil {
    panic(err)
  }
  for {
    client, err := server.Accept()
    if err != nil {
      log.Printf("Accept failed %v", err)
      continue
    }
    go process(client)
  }
}
func process(conn net.Conn) {
  defer conn.Close()
  reader := bufio.NewReader(conn)
  err := auth(reader, conn)
  if err != nil {
    log.Printf("client %v auth failed:%v", conn.RemoteAddr(), err)
    return
  }
  err = connect(reader, conn)
  if err != nil {
    log.Printf("client %v auth failed:%v", conn.RemoteAddr(), err)
    return
  }
}
func auth(reader *bufio.Reader, conn net.Conn) (err error) {
  // +----+----------+----------+
  // |VER | NMETHODS | METHODS  |
  // +----+----------+----------+
  // | 1  |    1     | 1 to 255 |
  // +----+----------+----------+
  // VER: 协议版本，socks5为0x05
  // NMETHODS: 支持认证的方法数量
  // METHODS: 对应NMETHODS，NMETHODS的值为多少，METHODS就有多少个字节。RFC预定义了一些值的含义，内容如下:
  // X’00’ NO AUTHENTICATION REQUIRED
  // X’02’ USERNAME/PASSWORD
  ver, err := reader.ReadByte()
  if err != nil {
    return fmt.Errorf("read ver failed:%w", err)
  }
  if ver != socks5Ver {
    return fmt.Errorf("not supported ver:%v", ver)
  }
  methodSize, err := reader.ReadByte()
  if err != nil {
    return fmt.Errorf("read methodSize failed:%w", err)
  }
  method := make([]byte, methodSize)
  _, err = io.ReadFull(reader, method)
  if err != nil {
    return fmt.Errorf("read method failed:%w", err)
  }
  // +----+--------+
  // |VER | METHOD |
  // +----+--------+
  // | 1  |   1    |
  // +----+--------+
  _, err = conn.Write([]byte{socks5Ver, 0x00})
  if err != nil {
    return fmt.Errorf("write failed:%w", err)
  }
  return nil
}
func connect(reader *bufio.Reader, conn net.Conn) (err error) {
  // +----+-----+-------+------+----------+----------+
  // |VER | CMD |  RSV  | ATYP | DST.ADDR | DST.PORT |
  // +----+-----+-------+------+----------+----------+
  // | 1  |  1  | X'00' |  1   | Variable |    2     |
  // +----+-----+-------+------+----------+----------+
  // VER 版本号，socks5的值为0x05
  // CMD 0x01表示CONNECT请求
  // RSV 保留字段，值为0x00
  // ATYP 目标地址类型，DST.ADDR的数据对应这个字段的类型。
  //   0x01表示IPv4地址，DST.ADDR为4个字节
  //   0x03表示域名，DST.ADDR是一个可变长度的域名
  // DST.ADDR 一个可变长度的值
  // DST.PORT 目标端口，固定2个字节
  buf := make([]byte, 4)
  _, err = io.ReadFull(reader, buf)
  if err != nil {
    return fmt.Errorf("read header failed:%w", err)
  }
  ver, cmd, atyp := buf[0], buf[1], buf[3]
  if ver != socks5Ver {
    return fmt.Errorf("not supported ver:%v", ver)
  }
  if cmd != cmdBind {
    return fmt.Errorf("not supported cmd:%v", ver)
  }
  addr := ""
  switch atyp {
  case atypIPV4:
    _, err = io.ReadFull(reader, buf)
    if err != nil {
      return fmt.Errorf("read atyp failed:%w", err)
    }
    addr = fmt.Sprintf("%d.%d.%d.%d", buf[0], buf[1], buf[2], buf[3])
  case atypeHOST:
    hostSize, err := reader.ReadByte()
    if err != nil {
      return fmt.Errorf("read hostSize failed:%w", err)
    }
    host := make([]byte, hostSize)
    _, err = io.ReadFull(reader, host)
    if err != nil {
      return fmt.Errorf("read host failed:%w", err)
    }
    addr = string(host)
  case atypeIPV6:
    return errors.New("IPv6: no supported yet")
  default:
    return errors.New("invalid atyp")
  }
  _, err = io.ReadFull(reader, buf[:2])
  if err != nil {
    return fmt.Errorf("read port failed:%w", err)
  }
  port := binary.BigEndian.Uint16(buf[:2])
  dest, err := net.Dial("tcp", fmt.Sprintf("%v:%v", addr, port))
  if err != nil {
    return fmt.Errorf("dial dst failed:%w", err)
  }
  defer dest.Close()
  log.Println("dial", addr, port)
  // +----+-----+-------+------+----------+----------+
  // |VER | REP |  RSV  | ATYP | BND.ADDR | BND.PORT |
  // +----+-----+-------+------+----------+----------+
  // | 1  |  1  | X'00' |  1   | Variable |    2     |
  // +----+-----+-------+------+----------+----------+
  // VER socks版本，这里为0x05
  // REP Relay field,内容取值如下 X’00’ succeeded
  // RSV 保留字段
  // ATYPE 地址类型
  // BND.ADDR 服务绑定的地址
  // BND.PORT 服务绑定的端口DST.PORT
  _, err = conn.Write([]byte{0x05, 0x00, 0x00, 0x01, 0, 0, 0, 0, 0, 0})
  if err != nil {
    return fmt.Errorf("write failed: %w", err)
  }
  ctx, cancel := context.WithCancel(context.Background())
  defer cancel()
  go func() {
    _, _ = io.Copy(dest, reader)
    cancel()
  }()
  go func() {
    _, _ = io.Copy(conn, dest)
    cancel()
  }()
  <-ctx.Done()
  return nil
}