Add SOCKS5 support

- Bundle the golang.org/x/net/proxy package to x_net_proxy.go. The
package contains a SOCKS5 proxy. The package is bundled to avoid adding
a dependency from the weboscket package to golang.org/x/net.
- Restructure the existing HTTP proxy code so the code can be used as a
dialer with the proxy package.
- Modify Dialer.Dial to use proxy.FromURL.
- Improve tests (avoid modifying package-level data, use timeouts in
tests, use correct proxy URLs in tests).

Fixes #297.

client.go

@@ -5,10 +5,8 @@
 package websocket
 
 import (
-	"bufio"
 	"bytes"
 	"crypto/tls"
-	"encoding/base64"
 	"errors"
 	"io"
 	"io/ioutil"
@@ -106,7 +104,7 @@ func hostPortNoPort(u *url.URL) (hostPort, hostNoPort string) {
 	return hostPort, hostNoPort
 }
 
-// DefaultDialer is a dialer with all fields set to the default zero values.
+// DefaultDialer is a dialer with all fields set to the default values.
 var DefaultDialer = &Dialer{
 	Proxy: http.ProxyFromEnvironment,
 }
@@ -202,36 +200,52 @@ func (d *Dialer) Dial(urlStr string, requestHeader http.Header) (*Conn, *http.Re
 		req.Header.Set("Sec-Websocket-Extensions", "permessage-deflate; server_no_context_takeover; client_no_context_takeover")
 	}
 
-	hostPort, hostNoPort := hostPortNoPort(u)
-
-	var proxyURL *url.URL
-	// Check wether the proxy method has been configured
-	if d.Proxy != nil {
-		proxyURL, err = d.Proxy(req)
-	}
-	if err != nil {
-		return nil, nil, err
-	}
-
-	var targetHostPort string
-	if proxyURL != nil {
-		targetHostPort, _ = hostPortNoPort(proxyURL)
-	} else {
-		targetHostPort = hostPort
-	}
-
 	var deadline time.Time
 	if d.HandshakeTimeout != 0 {
 		deadline = time.Now().Add(d.HandshakeTimeout)
 	}
 
+	// Get network dial function.
 	netDial := d.NetDial
 	if netDial == nil {
 		netDialer := &net.Dialer{Deadline: deadline}
 		netDial = netDialer.Dial
 	}
 
-	netConn, err := netDial("tcp", targetHostPort)
+	// If needed, wrap the dial function to set the connection deadline.
+	if !deadline.Equal(time.Time{}) {
+		forwardDial := netDial
+		netDial = func(network, addr string) (net.Conn, error) {
+			c, err := forwardDial(network, addr)
+			if err != nil {
+				return nil, err
+			}
+			err = c.SetDeadline(deadline)
+			if err != nil {
+				c.Close()
+				return nil, err
+			}
+			return c, nil
+		}
+	}
+
+	// If needed, wrap the dial function to connect through a proxy.
+	if d.Proxy != nil {
+		proxyURL, err := d.Proxy(req)
+		if err != nil {
+			return nil, nil, err
+		}
+		if proxyURL != nil {
+			dialer, err := proxy_FromURL(proxyURL, netDialerFunc(netDial))
+			if err != nil {
+				return nil, nil, err
+			}
+			netDial = dialer.Dial
+		}
+	}
+
+	hostPort, hostNoPort := hostPortNoPort(u)
+	netConn, err := netDial("tcp", hostPort)
 	if err != nil {
 		return nil, nil, err
 	}
@@ -242,42 +256,6 @@ func (d *Dialer) Dial(urlStr string, requestHeader http.Header) (*Conn, *http.Re
 		}
 	}()
 
-	if err := netConn.SetDeadline(deadline); err != nil {
-		return nil, nil, err
-	}
-
-	if proxyURL != nil {
-		connectHeader := make(http.Header)
-		if user := proxyURL.User; user != nil {
-			proxyUser := user.Username()
-			if proxyPassword, passwordSet := user.Password(); passwordSet {
-				credential := base64.StdEncoding.EncodeToString([]byte(proxyUser + ":" + proxyPassword))
-				connectHeader.Set("Proxy-Authorization", "Basic "+credential)
-			}
-		}
-		connectReq := &http.Request{
-			Method: "CONNECT",
-			URL:    &url.URL{Opaque: hostPort},
-			Host:   hostPort,
-			Header: connectHeader,
-		}
-
-		connectReq.Write(netConn)
-
-		// Read response.
-		// Okay to use and discard buffered reader here, because
-		// TLS server will not speak until spoken to.
-		br := bufio.NewReader(netConn)
-		resp, err := http.ReadResponse(br, connectReq)
-		if err != nil {
-			return nil, nil, err
-		}
-		if resp.StatusCode != 200 {
-			f := strings.SplitN(resp.Status, " ", 2)
-			return nil, nil, errors.New(f[1])
-		}
-	}
-
 	if u.Scheme == "https" {
 		cfg := cloneTLSConfig(d.TLSClientConfig)
 		if cfg.ServerName == "" {

client_server_test.go

@@ -5,11 +5,14 @@
 package websocket
 
 import (
+	"bytes"
 	"crypto/tls"
 	"crypto/x509"
 	"encoding/base64"
+	"encoding/binary"
 	"io"
 	"io/ioutil"
+	"net"
 	"net/http"
 	"net/http/cookiejar"
 	"net/http/httptest"
@@ -34,6 +37,7 @@ var cstDialer = Dialer{
 	Subprotocols:     []string{"p1", "p2"},
 	ReadBufferSize:   1024,
 	WriteBufferSize:  1024,
+	HandshakeTimeout: 30 * time.Second,
 }
 
 type cstHandler struct{ *testing.T }
@@ -143,8 +147,9 @@ func TestProxyDial(t *testing.T) {
 	s := newServer(t)
 	defer s.Close()
 
-	surl, _ := url.Parse(s.URL)
+	surl, _ := url.Parse(s.Server.URL)
 
+	cstDialer := cstDialer // make local copy for modification on next line.
 	cstDialer.Proxy = http.ProxyURL(surl)
 
 	connect := false
@@ -173,16 +178,16 @@ func TestProxyDial(t *testing.T) {
 	}
 	defer ws.Close()
 	sendRecv(t, ws)
-
-	cstDialer.Proxy = http.ProxyFromEnvironment
 }
 
 func TestProxyAuthorizationDial(t *testing.T) {
 	s := newServer(t)
 	defer s.Close()
 
-	surl, _ := url.Parse(s.URL)
+	surl, _ := url.Parse(s.Server.URL)
 	surl.User = url.UserPassword("username", "password")
+
+	cstDialer := cstDialer // make local copy for modification on next line.
 	cstDialer.Proxy = http.ProxyURL(surl)
 
 	connect := false
@@ -213,8 +218,6 @@ func TestProxyAuthorizationDial(t *testing.T) {
 	}
 	defer ws.Close()
 	sendRecv(t, ws)
-
-	cstDialer.Proxy = http.ProxyFromEnvironment
 }
 
 func TestDial(t *testing.T) {
@@ -518,3 +521,82 @@ func TestDialCompression(t *testing.T) {
 	defer ws.Close()
 	sendRecv(t, ws)
 }
+
+func TestSocksProxyDial(t *testing.T) {
+	s := newServer(t)
+	defer s.Close()
+
+	proxyListener, err := net.Listen("tcp", "127.0.0.1:0")
+	if err != nil {
+		t.Fatalf("listen failed: %v", err)
+	}
+	defer proxyListener.Close()
+	go func() {
+		c1, err := proxyListener.Accept()
+		if err != nil {
+			t.Errorf("proxy accept failed: %v", err)
+			return
+		}
+		defer c1.Close()
+
+		c1.SetDeadline(time.Now().Add(30 * time.Second))
+
+		buf := make([]byte, 32)
+		if _, err := io.ReadFull(c1, buf[:3]); err != nil {
+			t.Errorf("read failed: %v", err)
+			return
+		}
+		if want := []byte{5, 1, 0}; !bytes.Equal(want, buf[:len(want)]) {
+			t.Errorf("read %x, want %x", buf[:len(want)], want)
+		}
+		if _, err := c1.Write([]byte{5, 0}); err != nil {
+			t.Errorf("write failed: %v", err)
+			return
+		}
+		if _, err := io.ReadFull(c1, buf[:10]); err != nil {
+			t.Errorf("read failed: %v", err)
+			return
+		}
+		if want := []byte{5, 1, 0, 1}; !bytes.Equal(want, buf[:len(want)]) {
+			t.Errorf("read %x, want %x", buf[:len(want)], want)
+			return
+		}
+		buf[1] = 0
+		if _, err := c1.Write(buf[:10]); err != nil {
+			t.Errorf("write failed: %v", err)
+			return
+		}
+
+		ip := net.IP(buf[4:8])
+		port := binary.BigEndian.Uint16(buf[8:10])
+
+		c2, err := net.DialTCP("tcp", nil, &net.TCPAddr{IP: ip, Port: int(port)})
+		if err != nil {
+			t.Errorf("dial failed; %v", err)
+			return
+		}
+		defer c2.Close()
+		done := make(chan struct{})
+		go func() {
+			io.Copy(c1, c2)
+			close(done)
+		}()
+		io.Copy(c2, c1)
+		<-done
+	}()
+
+	purl, err := url.Parse("socks5://" + proxyListener.Addr().String())
+	if err != nil {
+		t.Fatalf("parse failed: %v", err)
+	}
+
+	cstDialer := cstDialer // make local copy for modification on next line.
+	cstDialer.Proxy = http.ProxyURL(purl)
+
+	ws, _, err := cstDialer.Dial(s.URL, nil)
+	if err != nil {
+		t.Fatalf("Dial: %v", err)
+	}
+	defer ws.Close()
+	sendRecv(t, ws)
+}

proxy.go

@@ -0,0 +1,77 @@
+// Copyright 2017 The Gorilla WebSocket 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 websocket
+
+import (
+	"bufio"
+	"encoding/base64"
+	"errors"
+	"net"
+	"net/http"
+	"net/url"
+	"strings"
+)
+
+type netDialerFunc func(netowrk, addr string) (net.Conn, error)
+
+func (fn netDialerFunc) Dial(network, addr string) (net.Conn, error) {
+	return fn(network, addr)
+}
+
+func init() {
+	proxy_RegisterDialerType("http", func(proxyURL *url.URL, forwardDialer proxy_Dialer) (proxy_Dialer, error) {
+		return &httpProxyDialer{proxyURL: proxyURL, fowardDial: forwardDialer.Dial}, nil
+	})
+}
+
+type httpProxyDialer struct {
+	proxyURL   *url.URL
+	fowardDial func(network, addr string) (net.Conn, error)
+}
+
+func (hpd *httpProxyDialer) Dial(network string, addr string) (net.Conn, error) {
+	hostPort, _ := hostPortNoPort(hpd.proxyURL)
+	conn, err := hpd.fowardDial(network, hostPort)
+	if err != nil {
+		return nil, err
+	}
+
+	connectHeader := make(http.Header)
+	if user := hpd.proxyURL.User; user != nil {
+		proxyUser := user.Username()
+		if proxyPassword, passwordSet := user.Password(); passwordSet {
+			credential := base64.StdEncoding.EncodeToString([]byte(proxyUser + ":" + proxyPassword))
+			connectHeader.Set("Proxy-Authorization", "Basic "+credential)
+		}
+	}
+
+	connectReq := &http.Request{
+		Method: "CONNECT",
+		URL:    &url.URL{Opaque: addr},
+		Host:   addr,
+		Header: connectHeader,
+	}
+
+	if err := connectReq.Write(conn); err != nil {
+		conn.Close()
+		return nil, err
+	}
+
+	// Read response. It's OK to use and discard buffered reader here becaue
+	// the remote server does not speak until spoken to.
+	br := bufio.NewReader(conn)
+	resp, err := http.ReadResponse(br, connectReq)
+	if err != nil {
+		conn.Close()
+		return nil, err
+	}
+
+	if resp.StatusCode != 200 {
+		conn.Close()
+		f := strings.SplitN(resp.Status, " ", 2)
+		return nil, errors.New(f[1])
+	}
+	return conn, nil
+}

x_net_proxy.go

@@ -0,0 +1,473 @@
+// Code generated by golang.org/x/tools/cmd/bundle. DO NOT EDIT.
+//go:generate bundle -o x_net_proxy.go golang.org/x/net/proxy
+
+// Package proxy provides support for a variety of protocols to proxy network
+// data.
+//
+
+package websocket
+
+import (
+	"errors"
+	"io"
+	"net"
+	"net/url"
+	"os"
+	"strconv"
+	"strings"
+	"sync"
+)
+
+type proxy_direct struct{}
+
+// Direct is a direct proxy: one that makes network connections directly.
+var proxy_Direct = proxy_direct{}
+
+func (proxy_direct) Dial(network, addr string) (net.Conn, error) {
+	return net.Dial(network, addr)
+}
+
+// A PerHost directs connections to a default Dialer unless the host name
+// requested matches one of a number of exceptions.
+type proxy_PerHost struct {
+	def, bypass proxy_Dialer
+
+	bypassNetworks []*net.IPNet
+	bypassIPs      []net.IP
+	bypassZones    []string
+	bypassHosts    []string
+}
+
+// NewPerHost returns a PerHost Dialer that directs connections to either
+// defaultDialer or bypass, depending on whether the connection matches one of
+// the configured rules.
+func proxy_NewPerHost(defaultDialer, bypass proxy_Dialer) *proxy_PerHost {
+	return &proxy_PerHost{
+		def:    defaultDialer,
+		bypass: bypass,
+	}
+}
+
+// Dial connects to the address addr on the given network through either
+// defaultDialer or bypass.
+func (p *proxy_PerHost) Dial(network, addr string) (c net.Conn, err error) {
+	host, _, err := net.SplitHostPort(addr)
+	if err != nil {
+		return nil, err
+	}
+
+	return p.dialerForRequest(host).Dial(network, addr)
+}
+
+func (p *proxy_PerHost) dialerForRequest(host string) proxy_Dialer {
+	if ip := net.ParseIP(host); ip != nil {
+		for _, net := range p.bypassNetworks {
+			if net.Contains(ip) {
+				return p.bypass
+			}
+		}
+		for _, bypassIP := range p.bypassIPs {
+			if bypassIP.Equal(ip) {
+				return p.bypass
+			}
+		}
+		return p.def
+	}
+
+	for _, zone := range p.bypassZones {
+		if strings.HasSuffix(host, zone) {
+			return p.bypass
+		}
+		if host == zone[1:] {
+			// For a zone ".example.com", we match "example.com"
+			// too.
+			return p.bypass
+		}
+	}
+	for _, bypassHost := range p.bypassHosts {
+		if bypassHost == host {
+			return p.bypass
+		}
+	}
+	return p.def
+}
+
+// AddFromString parses a string that contains comma-separated values
+// specifying hosts that should use the bypass proxy. Each value is either an
+// IP address, a CIDR range, a zone (*.example.com) or a host name
+// (localhost). A best effort is made to parse the string and errors are
+// ignored.
+func (p *proxy_PerHost) AddFromString(s string) {
+	hosts := strings.Split(s, ",")
+	for _, host := range hosts {
+		host = strings.TrimSpace(host)
+		if len(host) == 0 {
+			continue
+		}
+		if strings.Contains(host, "/") {
+			// We assume that it's a CIDR address like 127.0.0.0/8
+			if _, net, err := net.ParseCIDR(host); err == nil {
+				p.AddNetwork(net)
+			}
+			continue
+		}
+		if ip := net.ParseIP(host); ip != nil {
+			p.AddIP(ip)
+			continue
+		}
+		if strings.HasPrefix(host, "*.") {
+			p.AddZone(host[1:])
+			continue
+		}
+		p.AddHost(host)
+	}
+}
+
+// AddIP specifies an IP address that will use the bypass proxy. Note that
+// this will only take effect if a literal IP address is dialed. A connection
+// to a named host will never match an IP.
+func (p *proxy_PerHost) AddIP(ip net.IP) {
+	p.bypassIPs = append(p.bypassIPs, ip)
+}
+
+// AddNetwork specifies an IP range that will use the bypass proxy. Note that
+// this will only take effect if a literal IP address is dialed. A connection
+// to a named host will never match.
+func (p *proxy_PerHost) AddNetwork(net *net.IPNet) {
+	p.bypassNetworks = append(p.bypassNetworks, net)
+}
+
+// AddZone specifies a DNS suffix that will use the bypass proxy. A zone of
+// "example.com" matches "example.com" and all of its subdomains.
+func (p *proxy_PerHost) AddZone(zone string) {
+	if strings.HasSuffix(zone, ".") {
+		zone = zone[:len(zone)-1]
+	}
+	if !strings.HasPrefix(zone, ".") {
+		zone = "." + zone
+	}
+	p.bypassZones = append(p.bypassZones, zone)
+}
+
+// AddHost specifies a host name that will use the bypass proxy.
+func (p *proxy_PerHost) AddHost(host string) {
+	if strings.HasSuffix(host, ".") {
+		host = host[:len(host)-1]
+	}
+	p.bypassHosts = append(p.bypassHosts, host)
+}
+
+// A Dialer is a means to establish a connection.
+type proxy_Dialer interface {
+	// Dial connects to the given address via the proxy.
+	Dial(network, addr string) (c net.Conn, err error)
+}
+
+// Auth contains authentication parameters that specific Dialers may require.
+type proxy_Auth struct {
+	User, Password string
+}
+
+// FromEnvironment returns the dialer specified by the proxy related variables in
+// the environment.
+func proxy_FromEnvironment() proxy_Dialer {
+	allProxy := proxy_allProxyEnv.Get()
+	if len(allProxy) == 0 {
+		return proxy_Direct
+	}
+
+	proxyURL, err := url.Parse(allProxy)
+	if err != nil {
+		return proxy_Direct
+	}
+	proxy, err := proxy_FromURL(proxyURL, proxy_Direct)
+	if err != nil {
+		return proxy_Direct
+	}
+
+	noProxy := proxy_noProxyEnv.Get()
+	if len(noProxy) == 0 {
+		return proxy
+	}
+
+	perHost := proxy_NewPerHost(proxy, proxy_Direct)
+	perHost.AddFromString(noProxy)
+	return perHost
+}
+
+// proxySchemes is a map from URL schemes to a function that creates a Dialer
+// from a URL with such a scheme.
+var proxy_proxySchemes map[string]func(*url.URL, proxy_Dialer) (proxy_Dialer, error)
+
+// RegisterDialerType takes a URL scheme and a function to generate Dialers from
+// a URL with that scheme and a forwarding Dialer. Registered schemes are used
+// by FromURL.
+func proxy_RegisterDialerType(scheme string, f func(*url.URL, proxy_Dialer) (proxy_Dialer, error)) {
+	if proxy_proxySchemes == nil {
+		proxy_proxySchemes = make(map[string]func(*url.URL, proxy_Dialer) (proxy_Dialer, error))
+	}
+	proxy_proxySchemes[scheme] = f
+}
+
+// FromURL returns a Dialer given a URL specification and an underlying
+// Dialer for it to make network requests.
+func proxy_FromURL(u *url.URL, forward proxy_Dialer) (proxy_Dialer, error) {
+	var auth *proxy_Auth
+	if u.User != nil {
+		auth = new(proxy_Auth)
+		auth.User = u.User.Username()
+		if p, ok := u.User.Password(); ok {
+			auth.Password = p
+		}
+	}
+
+	switch u.Scheme {
+	case "socks5":
+		return proxy_SOCKS5("tcp", u.Host, auth, forward)
+	}
+
+	// If the scheme doesn't match any of the built-in schemes, see if it
+	// was registered by another package.
+	if proxy_proxySchemes != nil {
+		if f, ok := proxy_proxySchemes[u.Scheme]; ok {
+			return f(u, forward)
+		}
+	}
+
+	return nil, errors.New("proxy: unknown scheme: " + u.Scheme)
+}
+
+var (
+	proxy_allProxyEnv = &proxy_envOnce{
+		names: []string{"ALL_PROXY", "all_proxy"},
+	}
+	proxy_noProxyEnv = &proxy_envOnce{
+		names: []string{"NO_PROXY", "no_proxy"},
+	}
+)
+
+// envOnce looks up an environment variable (optionally by multiple
+// names) once. It mitigates expensive lookups on some platforms
+// (e.g. Windows).
+// (Borrowed from net/http/transport.go)
+type proxy_envOnce struct {
+	names []string
+	once  sync.Once
+	val   string
+}
+
+func (e *proxy_envOnce) Get() string {
+	e.once.Do(e.init)
+	return e.val
+}
+
+func (e *proxy_envOnce) init() {
+	for _, n := range e.names {
+		e.val = os.Getenv(n)
+		if e.val != "" {
+			return
+		}
+	}
+}
+
+// SOCKS5 returns a Dialer that makes SOCKSv5 connections to the given address
+// with an optional username and password. See RFC 1928 and RFC 1929.
+func proxy_SOCKS5(network, addr string, auth *proxy_Auth, forward proxy_Dialer) (proxy_Dialer, error) {
+	s := &proxy_socks5{
+		network: network,
+		addr:    addr,
+		forward: forward,
+	}
+	if auth != nil {
+		s.user = auth.User
+		s.password = auth.Password
+	}
+
+	return s, nil
+}
+
+type proxy_socks5 struct {
+	user, password string
+	network, addr  string
+	forward        proxy_Dialer
+}
+
+const proxy_socks5Version = 5
+
+const (
+	proxy_socks5AuthNone     = 0
+	proxy_socks5AuthPassword = 2
+)
+
+const proxy_socks5Connect = 1
+
+const (
+	proxy_socks5IP4    = 1
+	proxy_socks5Domain = 3
+	proxy_socks5IP6    = 4
+)
+
+var proxy_socks5Errors = []string{
+	"",
+	"general failure",
+	"connection forbidden",
+	"network unreachable",
+	"host unreachable",
+	"connection refused",
+	"TTL expired",
+	"command not supported",
+	"address type not supported",
+}
+
+// Dial connects to the address addr on the given network via the SOCKS5 proxy.
+func (s *proxy_socks5) Dial(network, addr string) (net.Conn, error) {
+	switch network {
+	case "tcp", "tcp6", "tcp4":
+	default:
+		return nil, errors.New("proxy: no support for SOCKS5 proxy connections of type " + network)
+	}
+
+	conn, err := s.forward.Dial(s.network, s.addr)
+	if err != nil {
+		return nil, err
+	}
+	if err := s.connect(conn, addr); err != nil {
+		conn.Close()
+		return nil, err
+	}
+	return conn, nil
+}
+
+// connect takes an existing connection to a socks5 proxy server,
+// and commands the server to extend that connection to target,
+// which must be a canonical address with a host and port.
+func (s *proxy_socks5) connect(conn net.Conn, target string) error {
+	host, portStr, err := net.SplitHostPort(target)
+	if err != nil {
+		return err
+	}
+
+	port, err := strconv.Atoi(portStr)
+	if err != nil {
+		return errors.New("proxy: failed to parse port number: " + portStr)
+	}
+	if port < 1 || port > 0xffff {
+		return errors.New("proxy: port number out of range: " + portStr)
+	}
+
+	// the size here is just an estimate
+	buf := make([]byte, 0, 6+len(host))
+
+	buf = append(buf, proxy_socks5Version)
+	if len(s.user) > 0 && len(s.user) < 256 && len(s.password) < 256 {
+		buf = append(buf, 2 /* num auth methods */, proxy_socks5AuthNone, proxy_socks5AuthPassword)
+	} else {
+		buf = append(buf, 1 /* num auth methods */, proxy_socks5AuthNone)
+	}
+
+	if _, err := conn.Write(buf); err != nil {
+		return errors.New("proxy: failed to write greeting to SOCKS5 proxy at " + s.addr + ": " + err.Error())
+	}
+
+	if _, err := io.ReadFull(conn, buf[:2]); err != nil {
+		return errors.New("proxy: failed to read greeting from SOCKS5 proxy at " + s.addr + ": " + err.Error())
+	}
+	if buf[0] != 5 {
+		return errors.New("proxy: SOCKS5 proxy at " + s.addr + " has unexpected version " + strconv.Itoa(int(buf[0])))
+	}
+	if buf[1] == 0xff {
+		return errors.New("proxy: SOCKS5 proxy at " + s.addr + " requires authentication")
+	}
+
+	// See RFC 1929
+	if buf[1] == proxy_socks5AuthPassword {
+		buf = buf[:0]
+		buf = append(buf, 1 /* password protocol version */)
+		buf = append(buf, uint8(len(s.user)))
+		buf = append(buf, s.user...)
+		buf = append(buf, uint8(len(s.password)))
+		buf = append(buf, s.password...)
+
+		if _, err := conn.Write(buf); err != nil {
+			return errors.New("proxy: failed to write authentication request to SOCKS5 proxy at " + s.addr + ": " + err.Error())
+		}
+
+		if _, err := io.ReadFull(conn, buf[:2]); err != nil {
+			return errors.New("proxy: failed to read authentication reply from SOCKS5 proxy at " + s.addr + ": " + err.Error())
+		}
+
+		if buf[1] != 0 {
+			return errors.New("proxy: SOCKS5 proxy at " + s.addr + " rejected username/password")
+		}
+	}
+
+	buf = buf[:0]
+	buf = append(buf, proxy_socks5Version, proxy_socks5Connect, 0 /* reserved */)
+
+	if ip := net.ParseIP(host); ip != nil {
+		if ip4 := ip.To4(); ip4 != nil {
+			buf = append(buf, proxy_socks5IP4)
+			ip = ip4
+		} else {
+			buf = append(buf, proxy_socks5IP6)
+		}
+		buf = append(buf, ip...)
+	} else {
+		if len(host) > 255 {
+			return errors.New("proxy: destination host name too long: " + host)
+		}
+		buf = append(buf, proxy_socks5Domain)
+		buf = append(buf, byte(len(host)))
+		buf = append(buf, host...)
+	}
+	buf = append(buf, byte(port>>8), byte(port))
+
+	if _, err := conn.Write(buf); err != nil {
+		return errors.New("proxy: failed to write connect request to SOCKS5 proxy at " + s.addr + ": " + err.Error())
+	}
+
+	if _, err := io.ReadFull(conn, buf[:4]); err != nil {
+		return errors.New("proxy: failed to read connect reply from SOCKS5 proxy at " + s.addr + ": " + err.Error())
+	}
+
+	failure := "unknown error"
+	if int(buf[1]) < len(proxy_socks5Errors) {
+		failure = proxy_socks5Errors[buf[1]]
+	}
+
+	if len(failure) > 0 {
+		return errors.New("proxy: SOCKS5 proxy at " + s.addr + " failed to connect: " + failure)
+	}
+
+	bytesToDiscard := 0
+	switch buf[3] {
+	case proxy_socks5IP4:
+		bytesToDiscard = net.IPv4len
+	case proxy_socks5IP6:
+		bytesToDiscard = net.IPv6len
+	case proxy_socks5Domain:
+		_, err := io.ReadFull(conn, buf[:1])
+		if err != nil {
+			return errors.New("proxy: failed to read domain length from SOCKS5 proxy at " + s.addr + ": " + err.Error())
+		}
+		bytesToDiscard = int(buf[0])
+	default:
+		return errors.New("proxy: got unknown address type " + strconv.Itoa(int(buf[3])) + " from SOCKS5 proxy at " + s.addr)
+	}
+
+	if cap(buf) < bytesToDiscard {
+		buf = make([]byte, bytesToDiscard)
+	} else {
+		buf = buf[:bytesToDiscard]
+	}
+	if _, err := io.ReadFull(conn, buf); err != nil {
+		return errors.New("proxy: failed to read address from SOCKS5 proxy at " + s.addr + ": " + err.Error())
+	}
+
+	// Also need to discard the port number
+	if _, err := io.ReadFull(conn, buf[:2]); err != nil {
+		return errors.New("proxy: failed to read port from SOCKS5 proxy at " + s.addr + ": " + err.Error())
+	}
+
+	return nil
+}