src/net/http/httputil/reverseproxy.go - go - Git at Google

 // Copyright 2011 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.

 // HTTP reverse proxy handler

 package httputil

 import (
 	"context"
 	"fmt"
 	"io"
 	"log"
 	"net"
 	"net/http"
 	"net/textproto"
 	"net/url"
 	"strings"
 	"sync"
 	"time"

 	"golang.org/x/net/http/httpguts"
 )

 // ReverseProxy is an HTTP Handler that takes an incoming request and
 // sends it to another server, proxying the response back to the
 // client.
 //
 // ReverseProxy by default sets the client IP as the value of the
 // X-Forwarded-For header.
 //
 // If an X-Forwarded-For header already exists, the client IP is
 // appended to the existing values. As a special case, if the header
 // exists in the Request.Header map but has a nil value (such as when
 // set by the Director func), the X-Forwarded-For header is
 // not modified.
 //
 // To prevent IP spoofing, be sure to delete any pre-existing
 // X-Forwarded-For header coming from the client or
 // an untrusted proxy.
 type ReverseProxy struct {
 	// Director must be a function which modifies
 	// the request into a new request to be sent
 	// using Transport. Its response is then copied
 	// back to the original client unmodified.
 	// Director must not access the provided Request
 	// after returning.
 	Director func(*http.Request)

 	// The transport used to perform proxy requests.
 	// If nil, http.DefaultTransport is used.
 	Transport http.RoundTripper

 	// FlushInterval specifies the flush interval
 	// to flush to the client while copying the
 	// response body.
 	// If zero, no periodic flushing is done.
 	// A negative value means to flush immediately
 	// after each write to the client.
 	// The FlushInterval is ignored when ReverseProxy
 	// recognizes a response as a streaming response, or
 	// if its ContentLength is -1; for such responses, writes
 	// are flushed to the client immediately.
 	FlushInterval time.Duration

 	// ErrorLog specifies an optional logger for errors
 	// that occur when attempting to proxy the request.
 	// If nil, logging is done via the log package's standard logger.
 	ErrorLog *log.Logger

 	// BufferPool optionally specifies a buffer pool to
 	// get byte slices for use by io.CopyBuffer when
 	// copying HTTP response bodies.
 	BufferPool BufferPool

 	// ModifyResponse is an optional function that modifies the
 	// Response from the backend. It is called if the backend
 	// returns a response at all, with any HTTP status code.
 	// If the backend is unreachable, the optional ErrorHandler is
 	// called without any call to ModifyResponse.
 	//
 	// If ModifyResponse returns an error, ErrorHandler is called
 	// with its error value. If ErrorHandler is nil, its default
 	// implementation is used.
 	ModifyResponse func(*http.Response) error

 	// ErrorHandler is an optional function that handles errors
 	// reaching the backend or errors from ModifyResponse.
 	//
 	// If nil, the default is to log the provided error and return
 	// a 502 Status Bad Gateway response.
 	ErrorHandler func(http.ResponseWriter, *http.Request, error)
 }

 // A BufferPool is an interface for getting and returning temporary
 // byte slices for use by io.CopyBuffer.
 type BufferPool interface {
 	Get() []byte
 	Put([]byte)
 }

 func singleJoiningSlash(a, b string) string {
 	aslash := strings.HasSuffix(a, "/")
 	bslash := strings.HasPrefix(b, "/")
 	switch {
 	case aslash && bslash:
 		return a + b[1:]
 	case !aslash && !bslash:
 		return a + "/" + b
 	}
 	return a + b
 }

 func joinURLPath(a, b *url.URL) (path, rawpath string) {
 	if a.RawPath == "" && b.RawPath == "" {
 		return singleJoiningSlash(a.Path, b.Path), ""
 	}
 	// Same as singleJoiningSlash, but uses EscapedPath to determine
 	// whether a slash should be added
 	apath := a.EscapedPath()
 	bpath := b.EscapedPath()

 	aslash := strings.HasSuffix(apath, "/")
 	bslash := strings.HasPrefix(bpath, "/")

 	switch {
 	case aslash && bslash:
 		return a.Path + b.Path[1:], apath + bpath[1:]
 	case !aslash && !bslash:
 		return a.Path + "/" + b.Path, apath + "/" + bpath
 	}
 	return a.Path + b.Path, apath + bpath
 }

 // NewSingleHostReverseProxy returns a new ReverseProxy that routes
 // URLs to the scheme, host, and base path provided in target. If the
 // target's path is "/base" and the incoming request was for "/dir",
 // the target request will be for /base/dir.
 // NewSingleHostReverseProxy does not rewrite the Host header.
 // To rewrite Host headers, use ReverseProxy directly with a custom
 // Director policy.
 func NewSingleHostReverseProxy(target *url.URL) *ReverseProxy {
 	targetQuery := target.RawQuery
 	director := func(req *http.Request) {
 		req.URL.Scheme = target.Scheme
 		req.URL.Host = target.Host
 		req.URL.Path, req.URL.RawPath = joinURLPath(target, req.URL)
 		if targetQuery == "" || req.URL.RawQuery == "" {
 			req.URL.RawQuery = targetQuery + req.URL.RawQuery
 		} else {
 			req.URL.RawQuery = targetQuery + "&" + req.URL.RawQuery
 		}
 		if _, ok := req.Header["User-Agent"]; !ok {
 			// explicitly disable User-Agent so it's not set to default value
 			req.Header.Set("User-Agent", "")
 		}
 	}
 	return &ReverseProxy{Director: director}
 }

 func copyHeader(dst, src http.Header) {
 	for k, vv := range src {
 		for _, v := range vv {
 			dst.Add(k, v)
 		}
 	}
 }

 // Hop-by-hop headers. These are removed when sent to the backend.
 // As of RFC 7230, hop-by-hop headers are required to appear in the
 // Connection header field. These are the headers defined by the
 // obsoleted RFC 2616 (section 13.5.1) and are used for backward
 // compatibility.
 var hopHeaders = []string{
 	"Connection",
 	"Proxy-Connection", // non-standard but still sent by libcurl and rejected by e.g. google
 	"Keep-Alive",
 	"Proxy-Authenticate",
 	"Proxy-Authorization",
 	"Te",      // canonicalized version of "TE"
 	"Trailer", // not Trailers per URL above; https://www.rfc-editor.org/errata_search.php?eid=4522
 	"Transfer-Encoding",
 	"Upgrade",
 }

 func (p *ReverseProxy) defaultErrorHandler(rw http.ResponseWriter, req *http.Request, err error) {
 	p.logf("http: proxy error: %v", err)
 	rw.WriteHeader(http.StatusBadGateway)
 }

 func (p *ReverseProxy) getErrorHandler() func(http.ResponseWriter, *http.Request, error) {
 	if p.ErrorHandler != nil {
 		return p.ErrorHandler
 	}
 	return p.defaultErrorHandler
 }

 // modifyResponse conditionally runs the optional ModifyResponse hook
 // and reports whether the request should proceed.
 func (p *ReverseProxy) modifyResponse(rw http.ResponseWriter, res *http.Response, req *http.Request) bool {
 	if p.ModifyResponse == nil {
 		return true
 	}
 	if err := p.ModifyResponse(res); err != nil {
 		res.Body.Close()
 		p.getErrorHandler()(rw, req, err)
 		return false
 	}
 	return true
 }

 func (p *ReverseProxy) ServeHTTP(rw http.ResponseWriter, req *http.Request) {
 	transport := p.Transport
 	if transport == nil {
 		transport = http.DefaultTransport
 	}

 	ctx := req.Context()
 	if cn, ok := rw.(http.CloseNotifier); ok {
 		var cancel context.CancelFunc
 		ctx, cancel = context.WithCancel(ctx)
 		defer cancel()
 		notifyChan := cn.CloseNotify()
 		go func() {
 			select {
 			case <-notifyChan:
 				cancel()
 			case <-ctx.Done():
 			}
 		}()
 	}

 	outreq := req.Clone(ctx)
 	if req.ContentLength == 0 {
 		outreq.Body = nil // Issue 16036: nil Body for http.Transport retries
 	}
 	if outreq.Body != nil {
 		// Reading from the request body after returning from a handler is not
 		// allowed, and the RoundTrip goroutine that reads the Body can outlive
 		// this handler. This can lead to a crash if the handler panics (see
 		// Issue 46866). Although calling Close doesn't guarantee there isn't
 		// any Read in flight after the handle returns, in practice it's safe to
 		// read after closing it.
 		defer outreq.Body.Close()
 	}
 	if outreq.Header == nil {
 		outreq.Header = make(http.Header) // Issue 33142: historical behavior was to always allocate
 	}

 	p.Director(outreq)
 	outreq.Close = false

 	reqUpType := upgradeType(outreq.Header)
 	removeConnectionHeaders(outreq.Header)

 	// Remove hop-by-hop headers to the backend. Especially
 	// important is "Connection" because we want a persistent
 	// connection, regardless of what the client sent to us.
 	for _, h := range hopHeaders {
 		outreq.Header.Del(h)
 	}

 	// Issue 21096: tell backend applications that care about trailer support
 	// that we support trailers. (We do, but we don't go out of our way to
 	// advertise that unless the incoming client request thought it was worth
 	// mentioning.) Note that we look at req.Header, not outreq.Header, since
 	// the latter has passed through removeConnectionHeaders.
 	if httpguts.HeaderValuesContainsToken(req.Header["Te"], "trailers") {
 		outreq.Header.Set("Te", "trailers")
 	}

 	// After stripping all the hop-by-hop connection headers above, add back any
 	// necessary for protocol upgrades, such as for websockets.
 	if reqUpType != "" {
 		outreq.Header.Set("Connection", "Upgrade")
 		outreq.Header.Set("Upgrade", reqUpType)
 	}

 	if clientIP, _, err := net.SplitHostPort(req.RemoteAddr); err == nil {
 		// If we aren't the first proxy retain prior
 		// X-Forwarded-For information as a comma+space
 		// separated list and fold multiple headers into one.
 		prior, ok := outreq.Header["X-Forwarded-For"]
 		omit := ok && prior == nil // Issue 38079: nil now means don't populate the header
 		if len(prior) > 0 {
 			clientIP = strings.Join(prior, ", ") + ", " + clientIP
 		}
 		if !omit {
 			outreq.Header.Set("X-Forwarded-For", clientIP)
 		}
 	}

 	res, err := transport.RoundTrip(outreq)
 	if err != nil {
 		p.getErrorHandler()(rw, outreq, err)
 		return
 	}

 	// Deal with 101 Switching Protocols responses: (WebSocket, h2c, etc)
 	if res.StatusCode == http.StatusSwitchingProtocols {
 		if !p.modifyResponse(rw, res, outreq) {
 			return
 		}
 		p.handleUpgradeResponse(rw, outreq, res)
 		return
 	}

 	removeConnectionHeaders(res.Header)

 	for _, h := range hopHeaders {
 		res.Header.Del(h)
 	}

 	if !p.modifyResponse(rw, res, outreq) {
 		return
 	}

 	copyHeader(rw.Header(), res.Header)

 	// The "Trailer" header isn't included in the Transport's response,
 	// at least for *http.Transport. Build it up from Trailer.
 	announcedTrailers := len(res.Trailer)
 	if announcedTrailers > 0 {
 		trailerKeys := make([]string, 0, len(res.Trailer))
 		for k := range res.Trailer {
 			trailerKeys = append(trailerKeys, k)
 		}
 		rw.Header().Add("Trailer", strings.Join(trailerKeys, ", "))
 	}

 	rw.WriteHeader(res.StatusCode)

 	err = p.copyResponse(rw, res.Body, p.flushInterval(res))
 	if err != nil {
 		defer res.Body.Close()
 		// Since we're streaming the response, if we run into an error all we can do
 		// is abort the request. Issue 23643: ReverseProxy should use ErrAbortHandler
 		// on read error while copying body.
 		if !shouldPanicOnCopyError(req) {
 			p.logf("suppressing panic for copyResponse error in test; copy error: %v", err)
 			return
 		}
 		panic(http.ErrAbortHandler)
 	}
 	res.Body.Close() // close now, instead of defer, to populate res.Trailer

 	if len(res.Trailer) > 0 {
 		// Force chunking if we saw a response trailer.
 		// This prevents net/http from calculating the length for short
 		// bodies and adding a Content-Length.
 		if fl, ok := rw.(http.Flusher); ok {
 			fl.Flush()
 		}
 	}

 	if len(res.Trailer) == announcedTrailers {
 		copyHeader(rw.Header(), res.Trailer)
 		return
 	}

 	for k, vv := range res.Trailer {
 		k = http.TrailerPrefix + k
 		for _, v := range vv {
 			rw.Header().Add(k, v)
 		}
 	}
 }

 var inOurTests bool // whether we're in our own tests

 // shouldPanicOnCopyError reports whether the reverse proxy should
 // panic with http.ErrAbortHandler. This is the right thing to do by
 // default, but Go 1.10 and earlier did not, so existing unit tests
 // weren't expecting panics. Only panic in our own tests, or when
 // running under the HTTP server.
 func shouldPanicOnCopyError(req *http.Request) bool {
 	if inOurTests {
 		// Our tests know to handle this panic.
 		return true
 	}
 	if req.Context().Value(http.ServerContextKey) != nil {
 		// We seem to be running under an HTTP server, so
 		// it'll recover the panic.
 		return true
 	}
 	// Otherwise act like Go 1.10 and earlier to not break
 	// existing tests.
 	return false
 }

 // removeConnectionHeaders removes hop-by-hop headers listed in the "Connection" header of h.
 // See RFC 7230, section 6.1
 func removeConnectionHeaders(h http.Header) {
 	for _, f := range h["Connection"] {
 		for _, sf := range strings.Split(f, ",") {
 			if sf = textproto.TrimString(sf); sf != "" {
 				h.Del(sf)
 			}
 		}
 	}
 }

 // flushInterval returns the p.FlushInterval value, conditionally
 // overriding its value for a specific request/response.
 func (p *ReverseProxy) flushInterval(res *http.Response) time.Duration {
 	resCT := res.Header.Get("Content-Type")

 	// For Server-Sent Events responses, flush immediately.
 	// The MIME type is defined in https://www.w3.org/TR/eventsource/#text-event-stream
 	if resCT == "text/event-stream" {
 		return -1 // negative means immediately
 	}

 	// We might have the case of streaming for which Content-Length might be unset.
 	if res.ContentLength == -1 {
 		return -1
 	}

 	return p.FlushInterval
 }

 func (p *ReverseProxy) copyResponse(dst io.Writer, src io.Reader, flushInterval time.Duration) error {
 	if flushInterval != 0 {
 		if wf, ok := dst.(writeFlusher); ok {
 			mlw := &maxLatencyWriter{
 				dst:     wf,
 				latency: flushInterval,
 			}
 			defer mlw.stop()

 			// set up initial timer so headers get flushed even if body writes are delayed
 			mlw.flushPending = true
 			mlw.t = time.AfterFunc(flushInterval, mlw.delayedFlush)

 			dst = mlw
 		}
 	}

 	var buf []byte
 	if p.BufferPool != nil {
 		buf = p.BufferPool.Get()
 		defer p.BufferPool.Put(buf)
 	}
 	_, err := p.copyBuffer(dst, src, buf)
 	return err
 }

 // copyBuffer returns any write errors or non-EOF read errors, and the amount
 // of bytes written.
 func (p *ReverseProxy) copyBuffer(dst io.Writer, src io.Reader, buf []byte) (int64, error) {
 	if len(buf) == 0 {
 		buf = make([]byte, 32*1024)
 	}
 	var written int64
 	for {
 		nr, rerr := src.Read(buf)
 		if rerr != nil && rerr != io.EOF && rerr != context.Canceled {
 			p.logf("httputil: ReverseProxy read error during body copy: %v", rerr)
 		}
 		if nr > 0 {
 			nw, werr := dst.Write(buf[:nr])
 			if nw > 0 {
 				written += int64(nw)
 			}
 			if werr != nil {
 				return written, werr
 			}
 			if nr != nw {
 				return written, io.ErrShortWrite
 			}
 		}
 		if rerr != nil {
 			if rerr == io.EOF {
 				rerr = nil
 			}
 			return written, rerr
 		}
 	}
 }

 func (p *ReverseProxy) logf(format string, args ...interface{}) {
 	if p.ErrorLog != nil {
 		p.ErrorLog.Printf(format, args...)
 	} else {
 		log.Printf(format, args...)
 	}
 }

 type writeFlusher interface {
 	io.Writer
 	http.Flusher
 }

 type maxLatencyWriter struct {
 	dst     writeFlusher
 	latency time.Duration // non-zero; negative means to flush immediately

 	mu           sync.Mutex // protects t, flushPending, and dst.Flush
 	t            *time.Timer
 	flushPending bool
 }

 func (m *maxLatencyWriter) Write(p []byte) (n int, err error) {
 	m.mu.Lock()
 	defer m.mu.Unlock()
 	n, err = m.dst.Write(p)
 	if m.latency < 0 {
 		m.dst.Flush()
 		return
 	}
 	if m.flushPending {
 		return
 	}
 	if m.t == nil {
 		m.t = time.AfterFunc(m.latency, m.delayedFlush)
 	} else {
 		m.t.Reset(m.latency)
 	}
 	m.flushPending = true
 	return
 }

 func (m *maxLatencyWriter) delayedFlush() {
 	m.mu.Lock()
 	defer m.mu.Unlock()
 	if !m.flushPending { // if stop was called but AfterFunc already started this goroutine
 		return
 	}
 	m.dst.Flush()
 	m.flushPending = false
 }

 func (m *maxLatencyWriter) stop() {
 	m.mu.Lock()
 	defer m.mu.Unlock()
 	m.flushPending = false
 	if m.t != nil {
 		m.t.Stop()
 	}
 }

 func upgradeType(h http.Header) string {
 	if !httpguts.HeaderValuesContainsToken(h["Connection"], "Upgrade") {
 		return ""
 	}
 	return strings.ToLower(h.Get("Upgrade"))
 }

 func (p *ReverseProxy) handleUpgradeResponse(rw http.ResponseWriter, req *http.Request, res *http.Response) {
 	reqUpType := upgradeType(req.Header)
 	resUpType := upgradeType(res.Header)
 	if reqUpType != resUpType {
 		p.getErrorHandler()(rw, req, fmt.Errorf("backend tried to switch protocol %q when %q was requested", resUpType, reqUpType))
 		return
 	}

 	hj, ok := rw.(http.Hijacker)
 	if !ok {
 		p.getErrorHandler()(rw, req, fmt.Errorf("can't switch protocols using non-Hijacker ResponseWriter type %T", rw))
 		return
 	}
 	backConn, ok := res.Body.(io.ReadWriteCloser)
 	if !ok {
 		p.getErrorHandler()(rw, req, fmt.Errorf("internal error: 101 switching protocols response with non-writable body"))
 		return
 	}

 	backConnCloseCh := make(chan bool)
 	go func() {
 		// Ensure that the cancelation of a request closes the backend.
 		// See issue https://golang.org/issue/35559.
 		select {
 		case <-req.Context().Done():
 		case <-backConnCloseCh:
 		}
 		backConn.Close()
 	}()

 	defer close(backConnCloseCh)

 	conn, brw, err := hj.Hijack()
 	if err != nil {
 		p.getErrorHandler()(rw, req, fmt.Errorf("Hijack failed on protocol switch: %v", err))
 		return
 	}
 	defer conn.Close()

 	copyHeader(rw.Header(), res.Header)

 	res.Header = rw.Header()
 	res.Body = nil // so res.Write only writes the headers; we have res.Body in backConn above
 	if err := res.Write(brw); err != nil {
 		p.getErrorHandler()(rw, req, fmt.Errorf("response write: %v", err))
 		return
 	}
 	if err := brw.Flush(); err != nil {
 		p.getErrorHandler()(rw, req, fmt.Errorf("response flush: %v", err))
 		return
 	}
 	errc := make(chan error, 1)
 	spc := switchProtocolCopier{user: conn, backend: backConn}
 	go spc.copyToBackend(errc)
 	go spc.copyFromBackend(errc)
 	<-errc
 	return
 }

 // switchProtocolCopier exists so goroutines proxying data back and
 // forth have nice names in stacks.
 type switchProtocolCopier struct {
 	user, backend io.ReadWriter
 }

 func (c switchProtocolCopier) copyFromBackend(errc chan<- error) {
 	_, err := io.Copy(c.user, c.backend)
 	errc <- err
 }

 func (c switchProtocolCopier) copyToBackend(errc chan<- error) {
 	_, err := io.Copy(c.backend, c.user)
 	errc <- err
 }
	// Copyright 2011 The Go Authors. All rights reserved.
	// Use of this source code is governed by a BSD-style
	// license that can be found in the LICENSE file.

	// HTTP reverse proxy handler

	package httputil

	import (
	"context"
	"fmt"
	"io"
	"log"
	"net"
	"net/http"
	"net/textproto"
	"net/url"
	"strings"
	"sync"
	"time"

	"golang.org/x/net/http/httpguts"
	)

	// ReverseProxy is an HTTP Handler that takes an incoming request and
	// sends it to another server, proxying the response back to the
	// client.
	//
	// ReverseProxy by default sets the client IP as the value of the
	// X-Forwarded-For header.
	//
	// If an X-Forwarded-For header already exists, the client IP is
	// appended to the existing values. As a special case, if the header
	// exists in the Request.Header map but has a nil value (such as when
	// set by the Director func), the X-Forwarded-For header is
	// not modified.
	//
	// To prevent IP spoofing, be sure to delete any pre-existing
	// X-Forwarded-For header coming from the client or
	// an untrusted proxy.
	type ReverseProxy struct {
	// Director must be a function which modifies
	// the request into a new request to be sent
	// using Transport. Its response is then copied
	// back to the original client unmodified.
	// Director must not access the provided Request
	// after returning.
	Director func(*http.Request)

	// The transport used to perform proxy requests.
	// If nil, http.DefaultTransport is used.
	Transport http.RoundTripper

	// FlushInterval specifies the flush interval
	// to flush to the client while copying the
	// response body.
	// If zero, no periodic flushing is done.
	// A negative value means to flush immediately
	// after each write to the client.
	// The FlushInterval is ignored when ReverseProxy
	// recognizes a response as a streaming response, or
	// if its ContentLength is -1; for such responses, writes
	// are flushed to the client immediately.
	FlushInterval time.Duration

	// ErrorLog specifies an optional logger for errors
	// that occur when attempting to proxy the request.
	// If nil, logging is done via the log package's standard logger.
	ErrorLog *log.Logger

	// BufferPool optionally specifies a buffer pool to
	// get byte slices for use by io.CopyBuffer when
	// copying HTTP response bodies.
	BufferPool BufferPool

	// ModifyResponse is an optional function that modifies the
	// Response from the backend. It is called if the backend
	// returns a response at all, with any HTTP status code.
	// If the backend is unreachable, the optional ErrorHandler is
	// called without any call to ModifyResponse.
	//
	// If ModifyResponse returns an error, ErrorHandler is called
	// with its error value. If ErrorHandler is nil, its default
	// implementation is used.
	ModifyResponse func(*http.Response) error

	// ErrorHandler is an optional function that handles errors
	// reaching the backend or errors from ModifyResponse.
	//
	// If nil, the default is to log the provided error and return
	// a 502 Status Bad Gateway response.
	ErrorHandler func(http.ResponseWriter, *http.Request, error)
	}

	// A BufferPool is an interface for getting and returning temporary
	// byte slices for use by io.CopyBuffer.
	type BufferPool interface {
	Get() []byte
	Put([]byte)
	}

	func singleJoiningSlash(a, b string) string {
	aslash := strings.HasSuffix(a, "/")
	bslash := strings.HasPrefix(b, "/")
	switch {
	case aslash && bslash:
	return a + b[1:]
	case !aslash && !bslash:
	return a + "/" + b
	}
	return a + b
	}

	func joinURLPath(a, b *url.URL) (path, rawpath string) {
	if a.RawPath == "" && b.RawPath == "" {
	return singleJoiningSlash(a.Path, b.Path), ""
	}
	// Same as singleJoiningSlash, but uses EscapedPath to determine
	// whether a slash should be added
	apath := a.EscapedPath()
	bpath := b.EscapedPath()

	aslash := strings.HasSuffix(apath, "/")
	bslash := strings.HasPrefix(bpath, "/")

	switch {
	case aslash && bslash:
	return a.Path + b.Path[1:], apath + bpath[1:]
	case !aslash && !bslash:
	return a.Path + "/" + b.Path, apath + "/" + bpath
	}
	return a.Path + b.Path, apath + bpath
	}

	// NewSingleHostReverseProxy returns a new ReverseProxy that routes
	// URLs to the scheme, host, and base path provided in target. If the
	// target's path is "/base" and the incoming request was for "/dir",
	// the target request will be for /base/dir.
	// NewSingleHostReverseProxy does not rewrite the Host header.
	// To rewrite Host headers, use ReverseProxy directly with a custom
	// Director policy.
	func NewSingleHostReverseProxy(target url.URL) ReverseProxy {
	targetQuery := target.RawQuery
	director := func(req *http.Request) {
	req.URL.Scheme = target.Scheme
	req.URL.Host = target.Host
	req.URL.Path, req.URL.RawPath = joinURLPath(target, req.URL)
	if targetQuery == "" \|\| req.URL.RawQuery == "" {
	req.URL.RawQuery = targetQuery + req.URL.RawQuery
	} else {
	req.URL.RawQuery = targetQuery + "&" + req.URL.RawQuery
	}
	if _, ok := req.Header["User-Agent"]; !ok {
	// explicitly disable User-Agent so it's not set to default value
	req.Header.Set("User-Agent", "")
	}
	}
	return &ReverseProxy{Director: director}
	}

	func copyHeader(dst, src http.Header) {
	for k, vv := range src {
	for _, v := range vv {
	dst.Add(k, v)
	}
	}
	}

	// Hop-by-hop headers. These are removed when sent to the backend.
	// As of RFC 7230, hop-by-hop headers are required to appear in the
	// Connection header field. These are the headers defined by the
	// obsoleted RFC 2616 (section 13.5.1) and are used for backward
	// compatibility.
	var hopHeaders = []string{
	"Connection",
	"Proxy-Connection", // non-standard but still sent by libcurl and rejected by e.g. google
	"Keep-Alive",
	"Proxy-Authenticate",
	"Proxy-Authorization",
	"Te", // canonicalized version of "TE"
	"Trailer", // not Trailers per URL above; https://www.rfc-editor.org/errata_search.php?eid=4522
	"Transfer-Encoding",
	"Upgrade",
	}

	func (p ReverseProxy) defaultErrorHandler(rw http.ResponseWriter, req http.Request, err error) {
	p.logf("http: proxy error: %v", err)
	rw.WriteHeader(http.StatusBadGateway)
	}

	func (p ReverseProxy) getErrorHandler() func(http.ResponseWriter, http.Request, error) {
	if p.ErrorHandler != nil {
	return p.ErrorHandler
	}
	return p.defaultErrorHandler
	}

	// modifyResponse conditionally runs the optional ModifyResponse hook
	// and reports whether the request should proceed.
	func (p ReverseProxy) modifyResponse(rw http.ResponseWriter, res http.Response, req *http.Request) bool {
	if p.ModifyResponse == nil {
	return true
	}
	if err := p.ModifyResponse(res); err != nil {
	res.Body.Close()
	p.getErrorHandler()(rw, req, err)
	return false
	}
	return true
	}

	func (p ReverseProxy) ServeHTTP(rw http.ResponseWriter, req http.Request) {
	transport := p.Transport
	if transport == nil {
	transport = http.DefaultTransport
	}

	ctx := req.Context()
	if cn, ok := rw.(http.CloseNotifier); ok {
	var cancel context.CancelFunc
	ctx, cancel = context.WithCancel(ctx)
	defer cancel()
	notifyChan := cn.CloseNotify()
	go func() {
	select {
	case <-notifyChan:
	cancel()
	case <-ctx.Done():
	}
	}()
	}

	outreq := req.Clone(ctx)
	if req.ContentLength == 0 {
	outreq.Body = nil // Issue 16036: nil Body for http.Transport retries
	}
	if outreq.Body != nil {
	// Reading from the request body after returning from a handler is not
	// allowed, and the RoundTrip goroutine that reads the Body can outlive
	// this handler. This can lead to a crash if the handler panics (see
	// Issue 46866). Although calling Close doesn't guarantee there isn't
	// any Read in flight after the handle returns, in practice it's safe to
	// read after closing it.
	defer outreq.Body.Close()
	}
	if outreq.Header == nil {
	outreq.Header = make(http.Header) // Issue 33142: historical behavior was to always allocate
	}

	p.Director(outreq)
	outreq.Close = false

	reqUpType := upgradeType(outreq.Header)
	removeConnectionHeaders(outreq.Header)

	// Remove hop-by-hop headers to the backend. Especially
	// important is "Connection" because we want a persistent
	// connection, regardless of what the client sent to us.
	for _, h := range hopHeaders {
	outreq.Header.Del(h)
	}

	// Issue 21096: tell backend applications that care about trailer support
	// that we support trailers. (We do, but we don't go out of our way to
	// advertise that unless the incoming client request thought it was worth
	// mentioning.) Note that we look at req.Header, not outreq.Header, since
	// the latter has passed through removeConnectionHeaders.
	if httpguts.HeaderValuesContainsToken(req.Header["Te"], "trailers") {
	outreq.Header.Set("Te", "trailers")
	}

	// After stripping all the hop-by-hop connection headers above, add back any
	// necessary for protocol upgrades, such as for websockets.
	if reqUpType != "" {
	outreq.Header.Set("Connection", "Upgrade")
	outreq.Header.Set("Upgrade", reqUpType)
	}

	if clientIP, _, err := net.SplitHostPort(req.RemoteAddr); err == nil {
	// If we aren't the first proxy retain prior
	// X-Forwarded-For information as a comma+space
	// separated list and fold multiple headers into one.
	prior, ok := outreq.Header["X-Forwarded-For"]
	omit := ok && prior == nil // Issue 38079: nil now means don't populate the header
	if len(prior) > 0 {
	clientIP = strings.Join(prior, ", ") + ", " + clientIP
	}
	if !omit {
	outreq.Header.Set("X-Forwarded-For", clientIP)
	}
	}

	res, err := transport.RoundTrip(outreq)
	if err != nil {
	p.getErrorHandler()(rw, outreq, err)
	return
	}

	// Deal with 101 Switching Protocols responses: (WebSocket, h2c, etc)
	if res.StatusCode == http.StatusSwitchingProtocols {
	if !p.modifyResponse(rw, res, outreq) {
	return
	}
	p.handleUpgradeResponse(rw, outreq, res)
	return
	}

	removeConnectionHeaders(res.Header)

	for _, h := range hopHeaders {
	res.Header.Del(h)
	}

	if !p.modifyResponse(rw, res, outreq) {
	return
	}

	copyHeader(rw.Header(), res.Header)

	// The "Trailer" header isn't included in the Transport's response,
	// at least for *http.Transport. Build it up from Trailer.
	announcedTrailers := len(res.Trailer)
	if announcedTrailers > 0 {
	trailerKeys := make([]string, 0, len(res.Trailer))
	for k := range res.Trailer {
	trailerKeys = append(trailerKeys, k)
	}
	rw.Header().Add("Trailer", strings.Join(trailerKeys, ", "))
	}

	rw.WriteHeader(res.StatusCode)

	err = p.copyResponse(rw, res.Body, p.flushInterval(res))
	if err != nil {
	defer res.Body.Close()
	// Since we're streaming the response, if we run into an error all we can do
	// is abort the request. Issue 23643: ReverseProxy should use ErrAbortHandler
	// on read error while copying body.
	if !shouldPanicOnCopyError(req) {
	p.logf("suppressing panic for copyResponse error in test; copy error: %v", err)
	return
	}
	panic(http.ErrAbortHandler)
	}
	res.Body.Close() // close now, instead of defer, to populate res.Trailer

	if len(res.Trailer) > 0 {
	// Force chunking if we saw a response trailer.
	// This prevents net/http from calculating the length for short
	// bodies and adding a Content-Length.
	if fl, ok := rw.(http.Flusher); ok {
	fl.Flush()
	}
	}

	if len(res.Trailer) == announcedTrailers {
	copyHeader(rw.Header(), res.Trailer)
	return
	}

	for k, vv := range res.Trailer {
	k = http.TrailerPrefix + k
	for _, v := range vv {
	rw.Header().Add(k, v)
	}
	}
	}

	var inOurTests bool // whether we're in our own tests

	// shouldPanicOnCopyError reports whether the reverse proxy should
	// panic with http.ErrAbortHandler. This is the right thing to do by
	// default, but Go 1.10 and earlier did not, so existing unit tests
	// weren't expecting panics. Only panic in our own tests, or when
	// running under the HTTP server.
	func shouldPanicOnCopyError(req *http.Request) bool {
	if inOurTests {
	// Our tests know to handle this panic.
	return true
	}
	if req.Context().Value(http.ServerContextKey) != nil {
	// We seem to be running under an HTTP server, so
	// it'll recover the panic.
	return true
	}
	// Otherwise act like Go 1.10 and earlier to not break
	// existing tests.
	return false
	}

	// removeConnectionHeaders removes hop-by-hop headers listed in the "Connection" header of h.
	// See RFC 7230, section 6.1
	func removeConnectionHeaders(h http.Header) {
	for _, f := range h["Connection"] {
	for _, sf := range strings.Split(f, ",") {
	if sf = textproto.TrimString(sf); sf != "" {
	h.Del(sf)
	}
	}
	}
	}

	// flushInterval returns the p.FlushInterval value, conditionally
	// overriding its value for a specific request/response.
	func (p ReverseProxy) flushInterval(res http.Response) time.Duration {
	resCT := res.Header.Get("Content-Type")

	// For Server-Sent Events responses, flush immediately.
	// The MIME type is defined in https://www.w3.org/TR/eventsource/#text-event-stream
	if resCT == "text/event-stream" {
	return -1 // negative means immediately
	}

	// We might have the case of streaming for which Content-Length might be unset.
	if res.ContentLength == -1 {
	return -1
	}

	return p.FlushInterval
	}

	func (p *ReverseProxy) copyResponse(dst io.Writer, src io.Reader, flushInterval time.Duration) error {
	if flushInterval != 0 {
	if wf, ok := dst.(writeFlusher); ok {
	mlw := &maxLatencyWriter{
	dst: wf,
	latency: flushInterval,
	}
	defer mlw.stop()

	// set up initial timer so headers get flushed even if body writes are delayed
	mlw.flushPending = true
	mlw.t = time.AfterFunc(flushInterval, mlw.delayedFlush)

	dst = mlw
	}
	}

	var buf []byte
	if p.BufferPool != nil {
	buf = p.BufferPool.Get()
	defer p.BufferPool.Put(buf)
	}
	_, err := p.copyBuffer(dst, src, buf)
	return err
	}

	// copyBuffer returns any write errors or non-EOF read errors, and the amount
	// of bytes written.
	func (p *ReverseProxy) copyBuffer(dst io.Writer, src io.Reader, buf []byte) (int64, error) {
	if len(buf) == 0 {
	buf = make([]byte, 32*1024)
	}
	var written int64
	for {
	nr, rerr := src.Read(buf)
	if rerr != nil && rerr != io.EOF && rerr != context.Canceled {
	p.logf("httputil: ReverseProxy read error during body copy: %v", rerr)
	}
	if nr > 0 {
	nw, werr := dst.Write(buf[:nr])
	if nw > 0 {
	written += int64(nw)
	}
	if werr != nil {
	return written, werr
	}
	if nr != nw {
	return written, io.ErrShortWrite
	}
	}
	if rerr != nil {
	if rerr == io.EOF {
	rerr = nil
	}
	return written, rerr
	}
	}
	}

	func (p *ReverseProxy) logf(format string, args ...interface{}) {
	if p.ErrorLog != nil {
	p.ErrorLog.Printf(format, args...)
	} else {
	log.Printf(format, args...)
	}
	}

	type writeFlusher interface {
	io.Writer
	http.Flusher
	}

	type maxLatencyWriter struct {
	dst writeFlusher
	latency time.Duration // non-zero; negative means to flush immediately

	mu sync.Mutex // protects t, flushPending, and dst.Flush
	t *time.Timer
	flushPending bool
	}

	func (m *maxLatencyWriter) Write(p []byte) (n int, err error) {
	m.mu.Lock()
	defer m.mu.Unlock()
	n, err = m.dst.Write(p)
	if m.latency < 0 {
	m.dst.Flush()
	return
	}
	if m.flushPending {
	return
	}
	if m.t == nil {
	m.t = time.AfterFunc(m.latency, m.delayedFlush)
	} else {
	m.t.Reset(m.latency)
	}
	m.flushPending = true
	return
	}

	func (m *maxLatencyWriter) delayedFlush() {
	m.mu.Lock()
	defer m.mu.Unlock()
	if !m.flushPending { // if stop was called but AfterFunc already started this goroutine
	return
	}
	m.dst.Flush()
	m.flushPending = false
	}

	func (m *maxLatencyWriter) stop() {
	m.mu.Lock()
	defer m.mu.Unlock()
	m.flushPending = false
	if m.t != nil {
	m.t.Stop()
	}
	}

	func upgradeType(h http.Header) string {
	if !httpguts.HeaderValuesContainsToken(h["Connection"], "Upgrade") {
	return ""
	}
	return strings.ToLower(h.Get("Upgrade"))
	}

	func (p ReverseProxy) handleUpgradeResponse(rw http.ResponseWriter, req http.Request, res *http.Response) {
	reqUpType := upgradeType(req.Header)
	resUpType := upgradeType(res.Header)
	if reqUpType != resUpType {
	p.getErrorHandler()(rw, req, fmt.Errorf("backend tried to switch protocol %q when %q was requested", resUpType, reqUpType))
	return
	}

	hj, ok := rw.(http.Hijacker)
	if !ok {
	p.getErrorHandler()(rw, req, fmt.Errorf("can't switch protocols using non-Hijacker ResponseWriter type %T", rw))
	return
	}
	backConn, ok := res.Body.(io.ReadWriteCloser)
	if !ok {
	p.getErrorHandler()(rw, req, fmt.Errorf("internal error: 101 switching protocols response with non-writable body"))
	return
	}

	backConnCloseCh := make(chan bool)
	go func() {
	// Ensure that the cancelation of a request closes the backend.
	// See issue https://golang.org/issue/35559.
	select {
	case <-req.Context().Done():
	case <-backConnCloseCh:
	}
	backConn.Close()
	}()

	defer close(backConnCloseCh)

	conn, brw, err := hj.Hijack()
	if err != nil {
	p.getErrorHandler()(rw, req, fmt.Errorf("Hijack failed on protocol switch: %v", err))
	return
	}
	defer conn.Close()

	copyHeader(rw.Header(), res.Header)

	res.Header = rw.Header()
	res.Body = nil // so res.Write only writes the headers; we have res.Body in backConn above
	if err := res.Write(brw); err != nil {
	p.getErrorHandler()(rw, req, fmt.Errorf("response write: %v", err))
	return
	}
	if err := brw.Flush(); err != nil {
	p.getErrorHandler()(rw, req, fmt.Errorf("response flush: %v", err))
	return
	}
	errc := make(chan error, 1)
	spc := switchProtocolCopier{user: conn, backend: backConn}
	go spc.copyToBackend(errc)
	go spc.copyFromBackend(errc)
	<-errc
	return
	}

	// switchProtocolCopier exists so goroutines proxying data back and
	// forth have nice names in stacks.
	type switchProtocolCopier struct {
	user, backend io.ReadWriter
	}

	func (c switchProtocolCopier) copyFromBackend(errc chan<- error) {
	_, err := io.Copy(c.user, c.backend)
	errc <- err
	}

	func (c switchProtocolCopier) copyToBackend(errc chan<- error) {
	_, err := io.Copy(c.backend, c.user)
	errc <- err
	}