net/socks5: create SOCKS5 package for proxy server in userspace networking

Updates #707
Updates #504

Signed-off-by: Naman Sood <mail@nsood.in>

net/socks5/socks5.go

@@ -0,0 +1,356 @@
+// Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package socks5 is a SOCKS5 server implementation
+// for userspace networking in Tailscale.
+package socks5
+
+import (
+	"context"
+	"encoding/binary"
+	"fmt"
+	"io"
+	"log"
+	"net"
+	"strconv"
+	"time"
+
+	"tailscale.com/types/logger"
+)
+
+const (
+	noAuthRequired   byte = 0
+	noAcceptableAuth byte = 255
+
+	// socks5Version is the byte that represents the SOCKS version
+	// in requests.
+	socks5Version byte = 5
+)
+
+// commandType are the bytes sent in SOCKS5 packets
+// that represent the kind of connection the client needs.
+type commandType byte
+
+// The set of valid SOCKS5 commans as described in RFC 1928.
+const (
+	connect      commandType = 1
+	bind         commandType = 2
+	udpAssociate commandType = 3
+)
+
+// addrType are the bytes sent in SOCKS5 packets
+// that represent particular address types.
+type addrType byte
+
+// The set of valid SOCKS5 address types as defined in RFC 1928.
+const (
+	ipv4       addrType = 1
+	domainName addrType = 3
+	ipv6       addrType = 4
+)
+
+// replyCode are the bytes sent in SOCKS5 packets
+// that represent replies from the server to a client
+// request.
+type replyCode byte
+
+// The set of valid SOCKS5 reply types as per the RFC 1928.
+const (
+	success              replyCode = 0
+	generalFailure       replyCode = 1
+	connectionNotAllowed replyCode = 2
+	networkUnreachable   replyCode = 3
+	hostUnreachable      replyCode = 4
+	connectionRefused    replyCode = 5
+	ttlExpired           replyCode = 6
+	commandNotSupported  replyCode = 7
+	addrTypeNotSupported replyCode = 8
+)
+
+// Server is a SOCKS5 proxy server.
+type Server struct {
+	// Logf optionally specifies the logger to use.
+	// If nil, the standard logger is used.
+	Logf logger.Logf
+
+	// Dialer optionally specifies the dialer to use for outgoing connections.
+	// If nil, the net package's standard dialer is used.
+	Dialer func(ctx context.Context, network, addr string) (net.Conn, error)
+}
+
+func (s *Server) dial(ctx context.Context, network, addr string) (net.Conn, error) {
+	dial := s.Dialer
+	if dial == nil {
+		dialer := &net.Dialer{}
+		dial = dialer.DialContext
+	}
+	return dial(ctx, network, addr)
+}
+
+func (s *Server) logf(format string, args ...interface{}) {
+	logf := s.Logf
+	if logf == nil {
+		logf = log.Printf
+	}
+	logf(format, args...)
+}
+
+// Serve accepts and handles incoming connections on the given listener.
+func (s *Server) Serve(l net.Listener) error {
+	defer l.Close()
+	for {
+		c, err := l.Accept()
+		if err != nil {
+			return err
+		}
+		go func() {
+			conn := &Conn{clientConn: c, srv: s}
+			err := conn.Run()
+			if err != nil {
+				s.logf("socks5: client connection failed: %v", err)
+				conn.clientConn.Close()
+			}
+		}()
+	}
+}
+
+// Conn is a SOCKS5 connection for client to reach
+// server.
+type Conn struct {
+	// The struct is filled by each of the internal
+	// methods in turn as the transaction progresses.
+
+	srv        *Server
+	clientConn net.Conn
+	serverConn net.Conn
+	request    *request
+}
+
+// Run starts the new connection.
+func (c *Conn) Run() error {
+	err := parseClientGreeting(c.clientConn)
+	if err != nil {
+		c.clientConn.Write([]byte{socks5Version, noAcceptableAuth})
+		return err
+	}
+	c.clientConn.Write([]byte{socks5Version, noAuthRequired})
+	return c.handleRequest()
+}
+
+func (c *Conn) handleRequest() error {
+	req, err := parseClientRequest(c.clientConn)
+	if err != nil {
+		res := &response{reply: generalFailure}
+		buf, _ := res.marshal()
+		c.clientConn.Write(buf)
+		return err
+	}
+	if req.command != connect {
+		res := &response{reply: commandNotSupported}
+		buf, _ := res.marshal()
+		c.clientConn.Write(buf)
+		return fmt.Errorf("unsupported command %v", req.command)
+	}
+	c.request = req
+	return c.createReply()
+}
+
+func (c *Conn) createReply() error {
+	var err error
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+	srv, err := c.srv.dial(
+		ctx,
+		"tcp",
+		net.JoinHostPort(c.request.destination, strconv.Itoa(int(c.request.port))),
+	)
+	if err != nil {
+		res := &response{reply: generalFailure}
+		buf, _ := res.marshal()
+		c.clientConn.Write(buf)
+		return err
+	}
+	c.serverConn = srv
+	serverAddr, serverPortStr, err := net.SplitHostPort(c.serverConn.LocalAddr().String())
+	if err != nil {
+		return err
+	}
+	serverPort, _ := strconv.Atoi(serverPortStr)
+	go io.Copy(c.clientConn, c.serverConn)
+	go io.Copy(c.serverConn, c.clientConn)
+
+	var bindAddrType addrType
+	if ip := net.ParseIP(serverAddr); ip != nil {
+		if ip.To4() != nil {
+			bindAddrType = ipv4
+		} else {
+			bindAddrType = ipv6
+		}
+	} else {
+		bindAddrType = domainName
+	}
+
+	res := &response{
+		reply:        success,
+		bindAddrType: bindAddrType,
+		bindAddr:     serverAddr,
+		bindPort:     uint16(serverPort),
+	}
+	buf, err := res.marshal()
+	if err != nil {
+		res = &response{reply: generalFailure}
+		buf, _ = res.marshal()
+	}
+	c.clientConn.Write(buf)
+	return err
+}
+
+// parseClientGreeting parses a request initiation packet
+// and returns a slice that contains the acceptable auth methods
+// for the client.
+func parseClientGreeting(r io.Reader) error {
+	var hdr [2]byte
+	_, err := io.ReadFull(r, hdr[:])
+	if err != nil {
+		return fmt.Errorf("could not read packet header")
+	}
+	if hdr[0] != socks5Version {
+		return fmt.Errorf("incompatible SOCKS version")
+	}
+	count := int(hdr[1])
+	methods := make([]byte, count)
+	_, err = io.ReadFull(r, methods)
+	if err != nil {
+		return fmt.Errorf("could not read methods")
+	}
+	for _, m := range methods {
+		if m == noAuthRequired {
+			return nil
+		}
+	}
+	return fmt.Errorf("no acceptable auth methods")
+}
+
+// request represents data contained within a SOCKS5
+// connection request packet.
+type request struct {
+	command      commandType
+	destination  string
+	port         uint16
+	destAddrType addrType
+}
+
+// parseClientRequest converts raw packet bytes into a
+// SOCKS5Request struct.
+func parseClientRequest(r io.Reader) (*request, error) {
+	var hdr [4]byte
+	_, err := io.ReadFull(r, hdr[:])
+	if err != nil {
+		return nil, fmt.Errorf("could not read packet header")
+	}
+	cmd := hdr[1]
+	destAddrType := addrType(hdr[3])
+
+	var destination string
+	var port uint16
+
+	if destAddrType == ipv4 {
+		var ip [4]byte
+		_, err = io.ReadFull(r, ip[:])
+		if err != nil {
+			return nil, fmt.Errorf("could not read IPv4 address")
+		}
+		destination = net.IP(ip[:]).String()
+	} else if destAddrType == domainName {
+		var dstSizeByte [1]byte
+		_, err = io.ReadFull(r, dstSizeByte[:])
+		if err != nil {
+			return nil, fmt.Errorf("could not read domain name size")
+		}
+		dstSize := int(dstSizeByte[0])
+		domainName := make([]byte, dstSize)
+		_, err = io.ReadFull(r, domainName)
+		if err != nil {
+			return nil, fmt.Errorf("could not read domain name")
+		}
+		destination = string(domainName)
+	} else if destAddrType == ipv6 {
+		var ip [16]byte
+		_, err = io.ReadFull(r, ip[:])
+		if err != nil {
+			return nil, fmt.Errorf("could not read IPv6 address")
+		}
+		destination = net.IP(ip[:]).String()
+	} else {
+		return nil, fmt.Errorf("unsupported address type")
+	}
+	var portBytes [2]byte
+	_, err = io.ReadFull(r, portBytes[:])
+	if err != nil {
+		return nil, fmt.Errorf("could not read port")
+	}
+	port = binary.BigEndian.Uint16(portBytes[:])
+
+	return &request{
+		command:      commandType(cmd),
+		destination:  destination,
+		port:         port,
+		destAddrType: destAddrType,
+	}, nil
+}
+
+// response contains the contents of
+// a response packet sent from the proxy
+// to the client.
+type response struct {
+	reply        replyCode
+	bindAddrType addrType
+	bindAddr     string
+	bindPort     uint16
+}
+
+// marshal converts a SOCKS5Response struct into
+// a packet. If res.reply == Success, it may throw an error on
+// receiving an invalid bind address. Otherwise, it will not throw.
+func (res *response) marshal() ([]byte, error) {
+	pkt := make([]byte, 4)
+	pkt[0] = socks5Version
+	pkt[1] = byte(res.reply)
+	pkt[2] = 0 // null reserved byte
+	pkt[3] = byte(res.bindAddrType)
+
+	if res.reply != success {
+		return pkt, nil
+	}
+
+	var addr []byte
+	switch res.bindAddrType {
+	case ipv4:
+		addr = net.ParseIP(res.bindAddr).To4()
+		if addr == nil {
+			return nil, fmt.Errorf("invalid IPv4 address for binding")
+		}
+	case domainName:
+		if len(res.bindAddr) > 255 {
+			return nil, fmt.Errorf("invalid domain name for binding")
+		}
+		addr = make([]byte, 0, len(res.bindAddr)+1)
+		addr = append(addr, byte(len(res.bindAddr)))
+		addr = append(addr, []byte(res.bindAddr)...)
+	case ipv6:
+		addr = net.ParseIP(res.bindAddr).To16()
+		if addr == nil {
+			return nil, fmt.Errorf("invalid IPv6 address for binding")
+		}
+	default:
+		return nil, fmt.Errorf("unsupported address type")
+	}
+
+	pkt = append(pkt, addr...)
+	port := make([]byte, 2)
+	binary.BigEndian.PutUint16(port, uint16(res.bindPort))
+	pkt = append(pkt, port...)
+
+	return pkt, nil
+}