tun2socks/proxy/socks/socks.go

package socks

import (
	"bytes"
	"errors"
	"io"
	"net"
	"strconv"
)

const Version = 5

// Error represents a SOCKS error
type Error byte

func (err Error) Error() string {
	return "SOCKS5 error: " + strconv.Itoa(int(err))
}

// Command is request commands as defined in RFC 1928 section 4.
type Command = uint8

// SOCKS request commands as defined in RFC 1928 section 4.
const (
	CmdConnect      Command = 1
	CmdBind         Command = 2
	CmdUDPAssociate Command = 3
)

// SOCKS address types as defined in RFC 1928 section 5.
const (
	AtypIPv4       = 1
	AtypDomainName = 3
	AtypIPv6       = 4
)

// MaxAddrLen is the maximum size of SOCKS address in bytes.
const MaxAddrLen = 1 + 1 + 255 + 2

// MaxAuthLen is the maximum size of user/password field in SOCKS5 Auth
const MaxAuthLen = 255

// Addr represents a SOCKS address as defined in RFC 1928 section 5.
type Addr []byte

func (a Addr) String() string {
	var host, port string

	switch a[0] {
	case AtypDomainName:
		hostLen := uint16(a[1])
		host = string(a[2 : 2+hostLen])
		port = strconv.Itoa((int(a[2+hostLen]) << 8) | int(a[2+hostLen+1]))
	case AtypIPv4:
		host = net.IP(a[1 : 1+net.IPv4len]).String()
		port = strconv.Itoa((int(a[1+net.IPv4len]) << 8) | int(a[1+net.IPv4len+1]))
	case AtypIPv6:
		host = net.IP(a[1 : 1+net.IPv6len]).String()
		port = strconv.Itoa((int(a[1+net.IPv6len]) << 8) | int(a[1+net.IPv6len+1]))
	}

	return net.JoinHostPort(host, port)
}

// SOCKS errors as defined in RFC 1928 section 6.
const (
	ErrGeneralFailure       = Error(1)
	ErrConnectionNotAllowed = Error(2)
	ErrNetworkUnreachable   = Error(3)
	ErrHostUnreachable      = Error(4)
	ErrConnectionRefused    = Error(5)
	ErrTTLExpired           = Error(6)
	ErrCommandNotSupported  = Error(7)
	ErrAddressNotSupported  = Error(8)
)

// Auth errors used to return a specific "Auth failed" error
var ErrAuth = errors.New("auth failed")

// ClientHandshake fast-tracks SOCKS initialization to get target address to connect on client side.
func ClientHandshake(rw io.ReadWriter, addr Addr, command Command) (Addr, error) {
	buf := make([]byte, MaxAddrLen)
	var err error

	// VER, NMETHODS, METHODS
	_, err = rw.Write([]byte{Version, 1, 0})

	if err != nil {
		return nil, err
	}

	// VER, METHOD
	if _, err := io.ReadFull(rw, buf[:2]); err != nil {
		return nil, err
	}

	if buf[0] != 5 {
		return nil, errors.New("SOCKS version error")
	}

	if buf[1] != 0 {
		return nil, errors.New("SOCKS need auth")
	}

	// VER, CMD, RSV, ADDR
	if _, err := rw.Write(bytes.Join([][]byte{{5, command, 0}, addr}, []byte{})); err != nil {
		return nil, err
	}

	// VER, REP, RSV
	if _, err := io.ReadFull(rw, buf[:3]); err != nil {
		return nil, err
	}

	return readAddr(rw, buf)
}

func readAddr(r io.Reader, b []byte) (Addr, error) {
	if len(b) < MaxAddrLen {
		return nil, io.ErrShortBuffer
	}
	_, err := io.ReadFull(r, b[:1]) // read 1st byte for address type
	if err != nil {
		return nil, err
	}

	switch b[0] {
	case AtypDomainName:
		_, err = io.ReadFull(r, b[1:2]) // read 2nd byte for domain length
		if err != nil {
			return nil, err
		}
		domainLength := uint16(b[1])
		_, err = io.ReadFull(r, b[2:2+domainLength+2])
		return b[:1+1+domainLength+2], err
	case AtypIPv4:
		_, err = io.ReadFull(r, b[1:1+net.IPv4len+2])
		return b[:1+net.IPv4len+2], err
	case AtypIPv6:
		_, err = io.ReadFull(r, b[1:1+net.IPv6len+2])
		return b[:1+net.IPv6len+2], err
	}

	return nil, ErrAddressNotSupported
}

// SplitAddr slices a SOCKS address from beginning of b. Returns nil if failed.
func SplitAddr(b []byte) Addr {
	addrLen := 1
	if len(b) < addrLen {
		return nil
	}

	switch b[0] {
	case AtypDomainName:
		if len(b) < 2 {
			return nil
		}
		addrLen = 1 + 1 + int(b[1]) + 2
	case AtypIPv4:
		addrLen = 1 + net.IPv4len + 2
	case AtypIPv6:
		addrLen = 1 + net.IPv6len + 2
	default:
		return nil

	}

	if len(b) < addrLen {
		return nil
	}

	return b[:addrLen]
}

// ParseAddr parses the address in string s. Returns nil if failed.
func ParseAddr(s string) Addr {
	var addr Addr
	host, port, err := net.SplitHostPort(s)
	if err != nil {
		return nil
	}
	if ip := net.ParseIP(host); ip != nil {
		if ip4 := ip.To4(); ip4 != nil {
			addr = make([]byte, 1+net.IPv4len+2)
			addr[0] = AtypIPv4
			copy(addr[1:], ip4)
		} else {
			addr = make([]byte, 1+net.IPv6len+2)
			addr[0] = AtypIPv6
			copy(addr[1:], ip)
		}
	} else {
		if len(host) > 255 {
			return nil
		}
		addr = make([]byte, 1+1+len(host)+2)
		addr[0] = AtypDomainName
		addr[1] = byte(len(host))
		copy(addr[2:], host)
	}

	portUint, err := strconv.ParseUint(port, 10, 16)
	if err != nil {
		return nil
	}

	addr[len(addr)-2], addr[len(addr)-1] = byte(portUint>>8), byte(portUint)

	return addr
}

func DecodeUDPPacket(packet []byte) (addr Addr, payload []byte, err error) {
	if len(packet) < 5 {
		err = errors.New("insufficient length of packet")
		return
	}

	// packet[0] and packet[1] are reserved
	if !bytes.Equal(packet[:2], []byte{0, 0}) {
		err = errors.New("reserved fields should be zero")
		return
	}

	if packet[2] != 0 /* fragments */ {
		err = errors.New("discarding fragmented payload")
		return
	}

	addr = SplitAddr(packet[3:])
	if addr == nil {
		err = errors.New("failed to read UDP header")
	}

	payload = make([]byte, len(packet[3+len(addr):]))
	copy(payload, packet[3+len(addr):])
	return
}

func EncodeUDPPacket(addr Addr, payload []byte) (packet []byte, err error) {
	if addr == nil {
		err = errors.New("address is invalid")
		return
	}
	packet = bytes.Join([][]byte{{0, 0, 0}, addr, payload}, []byte{})
	return
}