func readAddrSpec(r io.Reader) (*AddrSpec, error) {
	d := &AddrSpec{}
	addrType := []byte{0}
	if _, err := r.Read(addrType); err != nil {
		return nil, err
	}
	switch addrType[0] {
	case ipv4Address:
		addr := make([]byte, 4)
		if _, err := io.ReadAtLeast(r, addr, len(addr)); err != nil {
			return nil, err
		}
		d.IP = net.IP(addr)
	case ipv6Address:
		addr := make([]byte, 16)
		if _, err := io.ReadAtLeast(r, addr, len(addr)); err != nil {
			return nil, err
		}
		d.IP = net.IP(addr)
	case fqdnAddress:
		if _, err := r.Read(addrType); err != nil {
			return nil, err
		}
		addrLen := int(addrType[0])
		fqdn := make([]byte, addrLen)
		if _, err := io.ReadAtLeast(r, fqdn, addrLen); err != nil {
			return nil, err
		}
		d.FQDN = string(fqdn)
	default:
		return nil, unrecognizedAddrType
	}
	port := []byte{0, 0}
	if _, err := io.ReadAtLeast(r, port, 2); err != nil {
		return nil, err
	}
	d.Port = (int(port[0]) << 8) | int(port[1])
	return d, nil
}
readAddrSpec is used to read AddrSpec.
Expects an address type byte, follwed by the address and port
package socks5
import (
	"fmt"
	"io"
	"net"
	"strconv"
	"strings"
	"golang.org/x/net/context"
)
const (
	ConnectCommand   = uint8(1)
	BindCommand      = uint8(2)
	AssociateCommand = uint8(3)
	ipv4Address      = uint8(1)
	fqdnAddress      = uint8(3)
	ipv6Address      = uint8(4)
)
const (
	successReply uint8 = iota
	serverFailure
	ruleFailure
	networkUnreachable
	hostUnreachable
	connectionRefused
	ttlExpired
	commandNotSupported
	addrTypeNotSupported
)
var (
	unrecognizedAddrType = fmt.Errorf("Unrecognized address type")
)
type AddressRewriter interface {
	Rewrite(ctx context.Context, request *Request) (context.Context, *AddrSpec)
}
type AddrSpec struct {
	FQDN string
	IP   net.IP
	Port int
}
func (a *AddrSpec) String() string {
	if a.FQDN != "" {
		return fmt.Sprintf("%s (%s):%d", a.FQDN, a.IP, a.Port)
	}
	return fmt.Sprintf("%s:%d", a.IP, a.Port)
}
func (a AddrSpec) Address() string {
	if 0 != len(a.IP) {
		return net.JoinHostPort(a.IP.String(), strconv.Itoa(a.Port))
	}
	return net.JoinHostPort(a.FQDN, strconv.Itoa(a.Port))
}
type Request struct {
	Version uint8
	Command uint8
	AuthContext *AuthContext
	RemoteAddr *AddrSpec
	DestAddr *AddrSpec
	realDestAddr *AddrSpec
	bufConn      io.Reader
}
type conn interface {
	Write([]byte) (int, error)
	RemoteAddr() net.Addr
}
func NewRequest(bufConn io.Reader) (*Request, error) {
	header := []byte{0, 0, 0}
	if _, err := io.ReadAtLeast(bufConn, header, 3); err != nil {
		return nil, fmt.Errorf("Failed to get command version: %v", err)
	}
	if header[0] != socks5Version {
		return nil, fmt.Errorf("Unsupported command version: %v", header[0])
	}
	dest, err := readAddrSpec(bufConn)
	if err != nil {
		return nil, err
	}
	request := &Request{
		Version:  socks5Version,
		Command:  header[1],
		DestAddr: dest,
		bufConn:  bufConn,
	}
	return request, nil
}
func (s *Server) handleRequest(req *Request, conn conn) error {
	ctx := context.Background()
	dest := req.DestAddr
	if dest.FQDN != "" {
		ctx_, addr, err := s.config.Resolver.Resolve(ctx, dest.FQDN)
		if err != nil {
			if err := sendReply(conn, hostUnreachable, nil); err != nil {
				return fmt.Errorf("Failed to send reply: %v", err)
			}
			return fmt.Errorf("Failed to resolve destination '%v': %v", dest.FQDN, err)
		}
		ctx = ctx_
		dest.IP = addr
	}
	req.realDestAddr = req.DestAddr
	if s.config.Rewriter != nil {
		ctx, req.realDestAddr = s.config.Rewriter.Rewrite(ctx, req)
	}
	switch req.Command {
	case ConnectCommand:
		return s.handleConnect(ctx, conn, req)
	case BindCommand:
		return s.handleBind(ctx, conn, req)
	case AssociateCommand:
		return s.handleAssociate(ctx, conn, req)
	default:
		if err := sendReply(conn, commandNotSupported, nil); err != nil {
			return fmt.Errorf("Failed to send reply: %v", err)
		}
		return fmt.Errorf("Unsupported command: %v", req.Command)
	}
}
func (s *Server) handleConnect(ctx context.Context, conn conn, req *Request) error {
	if ctx_, ok := s.config.Rules.Allow(ctx, req); !ok {
		if err := sendReply(conn, ruleFailure, nil); err != nil {
			return fmt.Errorf("Failed to send reply: %v", err)
		}
		return fmt.Errorf("Connect to %v blocked by rules", req.DestAddr)
	} else {
		ctx = ctx_
	}
	dial := s.config.Dial
	if dial == nil {
		dial = func(ctx context.Context, net_, addr string) (net.Conn, error) {
			return net.Dial(net_, addr)
		}
	}
	target, err := dial(ctx, "tcp", req.realDestAddr.Address())
	if err != nil {
		msg := err.Error()
		resp := hostUnreachable
		if strings.Contains(msg, "refused") {
			resp = connectionRefused
		} else if strings.Contains(msg, "network is unreachable") {
			resp = networkUnreachable
		}
		if err := sendReply(conn, resp, nil); err != nil {
			return fmt.Errorf("Failed to send reply: %v", err)
		}
		return fmt.Errorf("Connect to %v failed: %v", req.DestAddr, err)
	}
	defer target.Close()
	local := target.LocalAddr().(*net.TCPAddr)
	bind := AddrSpec{IP: local.IP, Port: local.Port}
	if err := sendReply(conn, successReply, &bind); err != nil {
		return fmt.Errorf("Failed to send reply: %v", err)
	}
	errCh := make(chan error, 2)
	go proxy(target, req.bufConn, errCh)
	go proxy(conn, target, errCh)
	for i := 0; i < 2; i++ {
		e := <-errCh
		if e != nil {
			return e
		}
	}
	return nil
}
func (s *Server) handleBind(ctx context.Context, conn conn, req *Request) error {
	if ctx_, ok := s.config.Rules.Allow(ctx, req); !ok {
		if err := sendReply(conn, ruleFailure, nil); err != nil {
			return fmt.Errorf("Failed to send reply: %v", err)
		}
		return fmt.Errorf("Bind to %v blocked by rules", req.DestAddr)
	} else {
		ctx = ctx_
	}
	if err := sendReply(conn, commandNotSupported, nil); err != nil {
		return fmt.Errorf("Failed to send reply: %v", err)
	}
	return nil
}
func (s *Server) handleAssociate(ctx context.Context, conn conn, req *Request) error {
	if ctx_, ok := s.config.Rules.Allow(ctx, req); !ok {
		if err := sendReply(conn, ruleFailure, nil); err != nil {
			return fmt.Errorf("Failed to send reply: %v", err)
		}
		return fmt.Errorf("Associate to %v blocked by rules", req.DestAddr)
	} else {
		ctx = ctx_
	}
	if err := sendReply(conn, commandNotSupported, nil); err != nil {
		return fmt.Errorf("Failed to send reply: %v", err)
	}
	return nil
}