Source file src/net/http/httputil/reverseproxy.go

     1	// Copyright 2011 The Go Authors. All rights reserved.
     2	// Use of this source code is governed by a BSD-style
     3	// license that can be found in the LICENSE file.
     4	
     5	// HTTP reverse proxy handler
     6	
     7	package httputil
     8	
     9	import (
    10		"context"
    11		"fmt"
    12		"io"
    13		"log"
    14		"net"
    15		"net/http"
    16		"net/url"
    17		"strings"
    18		"sync"
    19		"time"
    20	
    21		"golang.org/x/net/http/httpguts"
    22	)
    23	
    24	// ReverseProxy is an HTTP Handler that takes an incoming request and
    25	// sends it to another server, proxying the response back to the
    26	// client.
    27	type ReverseProxy struct {
    28		// Director must be a function which modifies
    29		// the request into a new request to be sent
    30		// using Transport. Its response is then copied
    31		// back to the original client unmodified.
    32		// Director must not access the provided Request
    33		// after returning.
    34		Director func(*http.Request)
    35	
    36		// The transport used to perform proxy requests.
    37		// If nil, http.DefaultTransport is used.
    38		Transport http.RoundTripper
    39	
    40		// FlushInterval specifies the flush interval
    41		// to flush to the client while copying the
    42		// response body.
    43		// If zero, no periodic flushing is done.
    44		// A negative value means to flush immediately
    45		// after each write to the client.
    46		// The FlushInterval is ignored when ReverseProxy
    47		// recognizes a response as a streaming response;
    48		// for such responses, writes are flushed to the client
    49		// immediately.
    50		FlushInterval time.Duration
    51	
    52		// ErrorLog specifies an optional logger for errors
    53		// that occur when attempting to proxy the request.
    54		// If nil, logging is done via the log package's standard logger.
    55		ErrorLog *log.Logger
    56	
    57		// BufferPool optionally specifies a buffer pool to
    58		// get byte slices for use by io.CopyBuffer when
    59		// copying HTTP response bodies.
    60		BufferPool BufferPool
    61	
    62		// ModifyResponse is an optional function that modifies the
    63		// Response from the backend. It is called if the backend
    64		// returns a response at all, with any HTTP status code.
    65		// If the backend is unreachable, the optional ErrorHandler is
    66		// called without any call to ModifyResponse.
    67		//
    68		// If ModifyResponse returns an error, ErrorHandler is called
    69		// with its error value. If ErrorHandler is nil, its default
    70		// implementation is used.
    71		ModifyResponse func(*http.Response) error
    72	
    73		// ErrorHandler is an optional function that handles errors
    74		// reaching the backend or errors from ModifyResponse.
    75		//
    76		// If nil, the default is to log the provided error and return
    77		// a 502 Status Bad Gateway response.
    78		ErrorHandler func(http.ResponseWriter, *http.Request, error)
    79	}
    80	
    81	// A BufferPool is an interface for getting and returning temporary
    82	// byte slices for use by io.CopyBuffer.
    83	type BufferPool interface {
    84		Get() []byte
    85		Put([]byte)
    86	}
    87	
    88	func singleJoiningSlash(a, b string) string {
    89		aslash := strings.HasSuffix(a, "/")
    90		bslash := strings.HasPrefix(b, "/")
    91		switch {
    92		case aslash && bslash:
    93			return a + b[1:]
    94		case !aslash && !bslash:
    95			return a + "/" + b
    96		}
    97		return a + b
    98	}
    99	
   100	// NewSingleHostReverseProxy returns a new ReverseProxy that routes
   101	// URLs to the scheme, host, and base path provided in target. If the
   102	// target's path is "/base" and the incoming request was for "/dir",
   103	// the target request will be for /base/dir.
   104	// NewSingleHostReverseProxy does not rewrite the Host header.
   105	// To rewrite Host headers, use ReverseProxy directly with a custom
   106	// Director policy.
   107	func NewSingleHostReverseProxy(target *url.URL) *ReverseProxy {
   108		targetQuery := target.RawQuery
   109		director := func(req *http.Request) {
   110			req.URL.Scheme = target.Scheme
   111			req.URL.Host = target.Host
   112			req.URL.Path = singleJoiningSlash(target.Path, req.URL.Path)
   113			if targetQuery == "" || req.URL.RawQuery == "" {
   114				req.URL.RawQuery = targetQuery + req.URL.RawQuery
   115			} else {
   116				req.URL.RawQuery = targetQuery + "&" + req.URL.RawQuery
   117			}
   118			if _, ok := req.Header["User-Agent"]; !ok {
   119				// explicitly disable User-Agent so it's not set to default value
   120				req.Header.Set("User-Agent", "")
   121			}
   122		}
   123		return &ReverseProxy{Director: director}
   124	}
   125	
   126	func copyHeader(dst, src http.Header) {
   127		for k, vv := range src {
   128			for _, v := range vv {
   129				dst.Add(k, v)
   130			}
   131		}
   132	}
   133	
   134	// Hop-by-hop headers. These are removed when sent to the backend.
   135	// As of RFC 7230, hop-by-hop headers are required to appear in the
   136	// Connection header field. These are the headers defined by the
   137	// obsoleted RFC 2616 (section 13.5.1) and are used for backward
   138	// compatibility.
   139	var hopHeaders = []string{
   140		"Connection",
   141		"Proxy-Connection", // non-standard but still sent by libcurl and rejected by e.g. google
   142		"Keep-Alive",
   143		"Proxy-Authenticate",
   144		"Proxy-Authorization",
   145		"Te",      // canonicalized version of "TE"
   146		"Trailer", // not Trailers per URL above; https://www.rfc-editor.org/errata_search.php?eid=4522
   147		"Transfer-Encoding",
   148		"Upgrade",
   149	}
   150	
   151	func (p *ReverseProxy) defaultErrorHandler(rw http.ResponseWriter, req *http.Request, err error) {
   152		p.logf("http: proxy error: %v", err)
   153		rw.WriteHeader(http.StatusBadGateway)
   154	}
   155	
   156	func (p *ReverseProxy) getErrorHandler() func(http.ResponseWriter, *http.Request, error) {
   157		if p.ErrorHandler != nil {
   158			return p.ErrorHandler
   159		}
   160		return p.defaultErrorHandler
   161	}
   162	
   163	// modifyResponse conditionally runs the optional ModifyResponse hook
   164	// and reports whether the request should proceed.
   165	func (p *ReverseProxy) modifyResponse(rw http.ResponseWriter, res *http.Response, req *http.Request) bool {
   166		if p.ModifyResponse == nil {
   167			return true
   168		}
   169		if err := p.ModifyResponse(res); err != nil {
   170			res.Body.Close()
   171			p.getErrorHandler()(rw, req, err)
   172			return false
   173		}
   174		return true
   175	}
   176	
   177	func (p *ReverseProxy) ServeHTTP(rw http.ResponseWriter, req *http.Request) {
   178		transport := p.Transport
   179		if transport == nil {
   180			transport = http.DefaultTransport
   181		}
   182	
   183		ctx := req.Context()
   184		if cn, ok := rw.(http.CloseNotifier); ok {
   185			var cancel context.CancelFunc
   186			ctx, cancel = context.WithCancel(ctx)
   187			defer cancel()
   188			notifyChan := cn.CloseNotify()
   189			go func() {
   190				select {
   191				case <-notifyChan:
   192					cancel()
   193				case <-ctx.Done():
   194				}
   195			}()
   196		}
   197	
   198		outreq := req.Clone(ctx)
   199		if req.ContentLength == 0 {
   200			outreq.Body = nil // Issue 16036: nil Body for http.Transport retries
   201		}
   202		if outreq.Header == nil {
   203			outreq.Header = make(http.Header) // Issue 33142: historical behavior was to always allocate
   204		}
   205	
   206		p.Director(outreq)
   207		outreq.Close = false
   208	
   209		reqUpType := upgradeType(outreq.Header)
   210		removeConnectionHeaders(outreq.Header)
   211	
   212		// Remove hop-by-hop headers to the backend. Especially
   213		// important is "Connection" because we want a persistent
   214		// connection, regardless of what the client sent to us.
   215		for _, h := range hopHeaders {
   216			hv := outreq.Header.Get(h)
   217			if hv == "" {
   218				continue
   219			}
   220			if h == "Te" && hv == "trailers" {
   221				// Issue 21096: tell backend applications that
   222				// care about trailer support that we support
   223				// trailers. (We do, but we don't go out of
   224				// our way to advertise that unless the
   225				// incoming client request thought it was
   226				// worth mentioning)
   227				continue
   228			}
   229			outreq.Header.Del(h)
   230		}
   231	
   232		// After stripping all the hop-by-hop connection headers above, add back any
   233		// necessary for protocol upgrades, such as for websockets.
   234		if reqUpType != "" {
   235			outreq.Header.Set("Connection", "Upgrade")
   236			outreq.Header.Set("Upgrade", reqUpType)
   237		}
   238	
   239		if clientIP, _, err := net.SplitHostPort(req.RemoteAddr); err == nil {
   240			// If we aren't the first proxy retain prior
   241			// X-Forwarded-For information as a comma+space
   242			// separated list and fold multiple headers into one.
   243			if prior, ok := outreq.Header["X-Forwarded-For"]; ok {
   244				clientIP = strings.Join(prior, ", ") + ", " + clientIP
   245			}
   246			outreq.Header.Set("X-Forwarded-For", clientIP)
   247		}
   248	
   249		res, err := transport.RoundTrip(outreq)
   250		if err != nil {
   251			p.getErrorHandler()(rw, outreq, err)
   252			return
   253		}
   254	
   255		// Deal with 101 Switching Protocols responses: (WebSocket, h2c, etc)
   256		if res.StatusCode == http.StatusSwitchingProtocols {
   257			if !p.modifyResponse(rw, res, outreq) {
   258				return
   259			}
   260			p.handleUpgradeResponse(rw, outreq, res)
   261			return
   262		}
   263	
   264		removeConnectionHeaders(res.Header)
   265	
   266		for _, h := range hopHeaders {
   267			res.Header.Del(h)
   268		}
   269	
   270		if !p.modifyResponse(rw, res, outreq) {
   271			return
   272		}
   273	
   274		copyHeader(rw.Header(), res.Header)
   275	
   276		// The "Trailer" header isn't included in the Transport's response,
   277		// at least for *http.Transport. Build it up from Trailer.
   278		announcedTrailers := len(res.Trailer)
   279		if announcedTrailers > 0 {
   280			trailerKeys := make([]string, 0, len(res.Trailer))
   281			for k := range res.Trailer {
   282				trailerKeys = append(trailerKeys, k)
   283			}
   284			rw.Header().Add("Trailer", strings.Join(trailerKeys, ", "))
   285		}
   286	
   287		rw.WriteHeader(res.StatusCode)
   288	
   289		err = p.copyResponse(rw, res.Body, p.flushInterval(req, res))
   290		if err != nil {
   291			defer res.Body.Close()
   292			// Since we're streaming the response, if we run into an error all we can do
   293			// is abort the request. Issue 23643: ReverseProxy should use ErrAbortHandler
   294			// on read error while copying body.
   295			if !shouldPanicOnCopyError(req) {
   296				p.logf("suppressing panic for copyResponse error in test; copy error: %v", err)
   297				return
   298			}
   299			panic(http.ErrAbortHandler)
   300		}
   301		res.Body.Close() // close now, instead of defer, to populate res.Trailer
   302	
   303		if len(res.Trailer) > 0 {
   304			// Force chunking if we saw a response trailer.
   305			// This prevents net/http from calculating the length for short
   306			// bodies and adding a Content-Length.
   307			if fl, ok := rw.(http.Flusher); ok {
   308				fl.Flush()
   309			}
   310		}
   311	
   312		if len(res.Trailer) == announcedTrailers {
   313			copyHeader(rw.Header(), res.Trailer)
   314			return
   315		}
   316	
   317		for k, vv := range res.Trailer {
   318			k = http.TrailerPrefix + k
   319			for _, v := range vv {
   320				rw.Header().Add(k, v)
   321			}
   322		}
   323	}
   324	
   325	var inOurTests bool // whether we're in our own tests
   326	
   327	// shouldPanicOnCopyError reports whether the reverse proxy should
   328	// panic with http.ErrAbortHandler. This is the right thing to do by
   329	// default, but Go 1.10 and earlier did not, so existing unit tests
   330	// weren't expecting panics. Only panic in our own tests, or when
   331	// running under the HTTP server.
   332	func shouldPanicOnCopyError(req *http.Request) bool {
   333		if inOurTests {
   334			// Our tests know to handle this panic.
   335			return true
   336		}
   337		if req.Context().Value(http.ServerContextKey) != nil {
   338			// We seem to be running under an HTTP server, so
   339			// it'll recover the panic.
   340			return true
   341		}
   342		// Otherwise act like Go 1.10 and earlier to not break
   343		// existing tests.
   344		return false
   345	}
   346	
   347	// removeConnectionHeaders removes hop-by-hop headers listed in the "Connection" header of h.
   348	// See RFC 7230, section 6.1
   349	func removeConnectionHeaders(h http.Header) {
   350		for _, f := range h["Connection"] {
   351			for _, sf := range strings.Split(f, ",") {
   352				if sf = strings.TrimSpace(sf); sf != "" {
   353					h.Del(sf)
   354				}
   355			}
   356		}
   357	}
   358	
   359	// flushInterval returns the p.FlushInterval value, conditionally
   360	// overriding its value for a specific request/response.
   361	func (p *ReverseProxy) flushInterval(req *http.Request, res *http.Response) time.Duration {
   362		resCT := res.Header.Get("Content-Type")
   363	
   364		// For Server-Sent Events responses, flush immediately.
   365		// The MIME type is defined in https://www.w3.org/TR/eventsource/#text-event-stream
   366		if resCT == "text/event-stream" {
   367			return -1 // negative means immediately
   368		}
   369	
   370		// TODO: more specific cases? e.g. res.ContentLength == -1?
   371		return p.FlushInterval
   372	}
   373	
   374	func (p *ReverseProxy) copyResponse(dst io.Writer, src io.Reader, flushInterval time.Duration) error {
   375		if flushInterval != 0 {
   376			if wf, ok := dst.(writeFlusher); ok {
   377				mlw := &maxLatencyWriter{
   378					dst:     wf,
   379					latency: flushInterval,
   380				}
   381				defer mlw.stop()
   382	
   383				// set up initial timer so headers get flushed even if body writes are delayed
   384				mlw.flushPending = true
   385				mlw.t = time.AfterFunc(flushInterval, mlw.delayedFlush)
   386	
   387				dst = mlw
   388			}
   389		}
   390	
   391		var buf []byte
   392		if p.BufferPool != nil {
   393			buf = p.BufferPool.Get()
   394			defer p.BufferPool.Put(buf)
   395		}
   396		_, err := p.copyBuffer(dst, src, buf)
   397		return err
   398	}
   399	
   400	// copyBuffer returns any write errors or non-EOF read errors, and the amount
   401	// of bytes written.
   402	func (p *ReverseProxy) copyBuffer(dst io.Writer, src io.Reader, buf []byte) (int64, error) {
   403		if len(buf) == 0 {
   404			buf = make([]byte, 32*1024)
   405		}
   406		var written int64
   407		for {
   408			nr, rerr := src.Read(buf)
   409			if rerr != nil && rerr != io.EOF && rerr != context.Canceled {
   410				p.logf("httputil: ReverseProxy read error during body copy: %v", rerr)
   411			}
   412			if nr > 0 {
   413				nw, werr := dst.Write(buf[:nr])
   414				if nw > 0 {
   415					written += int64(nw)
   416				}
   417				if werr != nil {
   418					return written, werr
   419				}
   420				if nr != nw {
   421					return written, io.ErrShortWrite
   422				}
   423			}
   424			if rerr != nil {
   425				if rerr == io.EOF {
   426					rerr = nil
   427				}
   428				return written, rerr
   429			}
   430		}
   431	}
   432	
   433	func (p *ReverseProxy) logf(format string, args ...interface{}) {
   434		if p.ErrorLog != nil {
   435			p.ErrorLog.Printf(format, args...)
   436		} else {
   437			log.Printf(format, args...)
   438		}
   439	}
   440	
   441	type writeFlusher interface {
   442		io.Writer
   443		http.Flusher
   444	}
   445	
   446	type maxLatencyWriter struct {
   447		dst     writeFlusher
   448		latency time.Duration // non-zero; negative means to flush immediately
   449	
   450		mu           sync.Mutex // protects t, flushPending, and dst.Flush
   451		t            *time.Timer
   452		flushPending bool
   453	}
   454	
   455	func (m *maxLatencyWriter) Write(p []byte) (n int, err error) {
   456		m.mu.Lock()
   457		defer m.mu.Unlock()
   458		n, err = m.dst.Write(p)
   459		if m.latency < 0 {
   460			m.dst.Flush()
   461			return
   462		}
   463		if m.flushPending {
   464			return
   465		}
   466		if m.t == nil {
   467			m.t = time.AfterFunc(m.latency, m.delayedFlush)
   468		} else {
   469			m.t.Reset(m.latency)
   470		}
   471		m.flushPending = true
   472		return
   473	}
   474	
   475	func (m *maxLatencyWriter) delayedFlush() {
   476		m.mu.Lock()
   477		defer m.mu.Unlock()
   478		if !m.flushPending { // if stop was called but AfterFunc already started this goroutine
   479			return
   480		}
   481		m.dst.Flush()
   482		m.flushPending = false
   483	}
   484	
   485	func (m *maxLatencyWriter) stop() {
   486		m.mu.Lock()
   487		defer m.mu.Unlock()
   488		m.flushPending = false
   489		if m.t != nil {
   490			m.t.Stop()
   491		}
   492	}
   493	
   494	func upgradeType(h http.Header) string {
   495		if !httpguts.HeaderValuesContainsToken(h["Connection"], "Upgrade") {
   496			return ""
   497		}
   498		return strings.ToLower(h.Get("Upgrade"))
   499	}
   500	
   501	func (p *ReverseProxy) handleUpgradeResponse(rw http.ResponseWriter, req *http.Request, res *http.Response) {
   502		reqUpType := upgradeType(req.Header)
   503		resUpType := upgradeType(res.Header)
   504		if reqUpType != resUpType {
   505			p.getErrorHandler()(rw, req, fmt.Errorf("backend tried to switch protocol %q when %q was requested", resUpType, reqUpType))
   506			return
   507		}
   508	
   509		copyHeader(res.Header, rw.Header())
   510	
   511		hj, ok := rw.(http.Hijacker)
   512		if !ok {
   513			p.getErrorHandler()(rw, req, fmt.Errorf("can't switch protocols using non-Hijacker ResponseWriter type %T", rw))
   514			return
   515		}
   516		backConn, ok := res.Body.(io.ReadWriteCloser)
   517		if !ok {
   518			p.getErrorHandler()(rw, req, fmt.Errorf("internal error: 101 switching protocols response with non-writable body"))
   519			return
   520		}
   521		defer backConn.Close()
   522		conn, brw, err := hj.Hijack()
   523		if err != nil {
   524			p.getErrorHandler()(rw, req, fmt.Errorf("Hijack failed on protocol switch: %v", err))
   525			return
   526		}
   527		defer conn.Close()
   528		res.Body = nil // so res.Write only writes the headers; we have res.Body in backConn above
   529		if err := res.Write(brw); err != nil {
   530			p.getErrorHandler()(rw, req, fmt.Errorf("response write: %v", err))
   531			return
   532		}
   533		if err := brw.Flush(); err != nil {
   534			p.getErrorHandler()(rw, req, fmt.Errorf("response flush: %v", err))
   535			return
   536		}
   537		errc := make(chan error, 1)
   538		spc := switchProtocolCopier{user: conn, backend: backConn}
   539		go spc.copyToBackend(errc)
   540		go spc.copyFromBackend(errc)
   541		<-errc
   542		return
   543	}
   544	
   545	// switchProtocolCopier exists so goroutines proxying data back and
   546	// forth have nice names in stacks.
   547	type switchProtocolCopier struct {
   548		user, backend io.ReadWriter
   549	}
   550	
   551	func (c switchProtocolCopier) copyFromBackend(errc chan<- error) {
   552		_, err := io.Copy(c.user, c.backend)
   553		errc <- err
   554	}
   555	
   556	func (c switchProtocolCopier) copyToBackend(errc chan<- error) {
   557		_, err := io.Copy(c.backend, c.user)
   558		errc <- err
   559	}
   560	

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