reverseproxy.go

Documentation: net/http/httputil

     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  	"io"
    12  	"log"
    13  	"net"
    14  	"net/http"
    15  	"net/url"
    16  	"strings"
    17  	"sync"
    18  	"time"
    19  )
    20  
    21  // onExitFlushLoop is a callback set by tests to detect the state of the
    22  // flushLoop() goroutine.
    23  var onExitFlushLoop func()
    24  
    25  // ReverseProxy is an HTTP Handler that takes an incoming request and
    26  // sends it to another server, proxying the response back to the
    27  // client.
    28  type ReverseProxy struct {
    29  	// Director must be a function which modifies
    30  	// the request into a new request to be sent
    31  	// using Transport. Its response is then copied
    32  	// back to the original client unmodified.
    33  	// Director must not access the provided Request
    34  	// after returning.
    35  	Director func(*http.Request)
    36  
    37  	// The transport used to perform proxy requests.
    38  	// If nil, http.DefaultTransport is used.
    39  	Transport http.RoundTripper
    40  
    41  	// FlushInterval specifies the flush interval
    42  	// to flush to the client while copying the
    43  	// response body.
    44  	// If zero, no periodic flushing is done.
    45  	FlushInterval time.Duration
    46  
    47  	// ErrorLog specifies an optional logger for errors
    48  	// that occur when attempting to proxy the request.
    49  	// If nil, logging goes to os.Stderr via the log package's
    50  	// standard logger.
    51  	ErrorLog *log.Logger
    52  
    53  	// BufferPool optionally specifies a buffer pool to
    54  	// get byte slices for use by io.CopyBuffer when
    55  	// copying HTTP response bodies.
    56  	BufferPool BufferPool
    57  
    58  	// ModifyResponse is an optional function that
    59  	// modifies the Response from the backend.
    60  	// If it returns an error, the proxy returns a StatusBadGateway error.
    61  	ModifyResponse func(*http.Response) error
    62  }
    63  
    64  // A BufferPool is an interface for getting and returning temporary
    65  // byte slices for use by io.CopyBuffer.
    66  type BufferPool interface {
    67  	Get() []byte
    68  	Put([]byte)
    69  }
    70  
    71  func singleJoiningSlash(a, b string) string {
    72  	aslash := strings.HasSuffix(a, "/")
    73  	bslash := strings.HasPrefix(b, "/")
    74  	switch {
    75  	case aslash && bslash:
    76  		return a + b[1:]
    77  	case !aslash && !bslash:
    78  		return a + "/" + b
    79  	}
    80  	return a + b
    81  }
    82  
    83  // NewSingleHostReverseProxy returns a new ReverseProxy that routes
    84  // URLs to the scheme, host, and base path provided in target. If the
    85  // target's path is "/base" and the incoming request was for "/dir",
    86  // the target request will be for /base/dir.
    87  // NewSingleHostReverseProxy does not rewrite the Host header.
    88  // To rewrite Host headers, use ReverseProxy directly with a custom
    89  // Director policy.
    90  func NewSingleHostReverseProxy(target *url.URL) *ReverseProxy {
    91  	targetQuery := target.RawQuery
    92  	director := func(req *http.Request) {
    93  		req.URL.Scheme = target.Scheme
    94  		req.URL.Host = target.Host
    95  		req.URL.Path = singleJoiningSlash(target.Path, req.URL.Path)
    96  		if targetQuery == "" || req.URL.RawQuery == "" {
    97  			req.URL.RawQuery = targetQuery + req.URL.RawQuery
    98  		} else {
    99  			req.URL.RawQuery = targetQuery + "&" + req.URL.RawQuery
   100  		}
   101  		if _, ok := req.Header["User-Agent"]; !ok {
   102  			// explicitly disable User-Agent so it's not set to default value
   103  			req.Header.Set("User-Agent", "")
   104  		}
   105  	}
   106  	return &ReverseProxy{Director: director}
   107  }
   108  
   109  func copyHeader(dst, src http.Header) {
   110  	for k, vv := range src {
   111  		for _, v := range vv {
   112  			dst.Add(k, v)
   113  		}
   114  	}
   115  }
   116  
   117  func cloneHeader(h http.Header) http.Header {
   118  	h2 := make(http.Header, len(h))
   119  	for k, vv := range h {
   120  		vv2 := make([]string, len(vv))
   121  		copy(vv2, vv)
   122  		h2[k] = vv2
   123  	}
   124  	return h2
   125  }
   126  
   127  // Hop-by-hop headers. These are removed when sent to the backend.
   128  // http://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html
   129  var hopHeaders = []string{
   130  	"Connection",
   131  	"Proxy-Connection", // non-standard but still sent by libcurl and rejected by e.g. google
   132  	"Keep-Alive",
   133  	"Proxy-Authenticate",
   134  	"Proxy-Authorization",
   135  	"Te",      // canonicalized version of "TE"
   136  	"Trailer", // not Trailers per URL above; http://www.rfc-editor.org/errata_search.php?eid=4522
   137  	"Transfer-Encoding",
   138  	"Upgrade",
   139  }
   140  
   141  func (p *ReverseProxy) ServeHTTP(rw http.ResponseWriter, req *http.Request) {
   142  	transport := p.Transport
   143  	if transport == nil {
   144  		transport = http.DefaultTransport
   145  	}
   146  
   147  	ctx := req.Context()
   148  	if cn, ok := rw.(http.CloseNotifier); ok {
   149  		var cancel context.CancelFunc
   150  		ctx, cancel = context.WithCancel(ctx)
   151  		defer cancel()
   152  		notifyChan := cn.CloseNotify()
   153  		go func() {
   154  			select {
   155  			case <-notifyChan:
   156  				cancel()
   157  			case <-ctx.Done():
   158  			}
   159  		}()
   160  	}
   161  
   162  	outreq := req.WithContext(ctx) // includes shallow copies of maps, but okay
   163  	if req.ContentLength == 0 {
   164  		outreq.Body = nil // Issue 16036: nil Body for http.Transport retries
   165  	}
   166  
   167  	outreq.Header = cloneHeader(req.Header)
   168  
   169  	p.Director(outreq)
   170  	outreq.Close = false
   171  
   172  	removeConnectionHeaders(outreq.Header)
   173  
   174  	// Remove hop-by-hop headers to the backend. Especially
   175  	// important is "Connection" because we want a persistent
   176  	// connection, regardless of what the client sent to us.
   177  	for _, h := range hopHeaders {
   178  		if outreq.Header.Get(h) != "" {
   179  			outreq.Header.Del(h)
   180  		}
   181  	}
   182  
   183  	if clientIP, _, err := net.SplitHostPort(req.RemoteAddr); err == nil {
   184  		// If we aren't the first proxy retain prior
   185  		// X-Forwarded-For information as a comma+space
   186  		// separated list and fold multiple headers into one.
   187  		if prior, ok := outreq.Header["X-Forwarded-For"]; ok {
   188  			clientIP = strings.Join(prior, ", ") + ", " + clientIP
   189  		}
   190  		outreq.Header.Set("X-Forwarded-For", clientIP)
   191  	}
   192  
   193  	res, err := transport.RoundTrip(outreq)
   194  	if err != nil {
   195  		p.logf("http: proxy error: %v", err)
   196  		rw.WriteHeader(http.StatusBadGateway)
   197  		return
   198  	}
   199  
   200  	removeConnectionHeaders(res.Header)
   201  
   202  	for _, h := range hopHeaders {
   203  		res.Header.Del(h)
   204  	}
   205  
   206  	if p.ModifyResponse != nil {
   207  		if err := p.ModifyResponse(res); err != nil {
   208  			p.logf("http: proxy error: %v", err)
   209  			rw.WriteHeader(http.StatusBadGateway)
   210  			res.Body.Close()
   211  			return
   212  		}
   213  	}
   214  
   215  	copyHeader(rw.Header(), res.Header)
   216  
   217  	// The "Trailer" header isn't included in the Transport's response,
   218  	// at least for *http.Transport. Build it up from Trailer.
   219  	announcedTrailers := len(res.Trailer)
   220  	if announcedTrailers > 0 {
   221  		trailerKeys := make([]string, 0, len(res.Trailer))
   222  		for k := range res.Trailer {
   223  			trailerKeys = append(trailerKeys, k)
   224  		}
   225  		rw.Header().Add("Trailer", strings.Join(trailerKeys, ", "))
   226  	}
   227  
   228  	rw.WriteHeader(res.StatusCode)
   229  	if len(res.Trailer) > 0 {
   230  		// Force chunking if we saw a response trailer.
   231  		// This prevents net/http from calculating the length for short
   232  		// bodies and adding a Content-Length.
   233  		if fl, ok := rw.(http.Flusher); ok {
   234  			fl.Flush()
   235  		}
   236  	}
   237  	p.copyResponse(rw, res.Body)
   238  	res.Body.Close() // close now, instead of defer, to populate res.Trailer
   239  
   240  	if len(res.Trailer) == announcedTrailers {
   241  		copyHeader(rw.Header(), res.Trailer)
   242  		return
   243  	}
   244  
   245  	for k, vv := range res.Trailer {
   246  		k = http.TrailerPrefix + k
   247  		for _, v := range vv {
   248  			rw.Header().Add(k, v)
   249  		}
   250  	}
   251  }
   252  
   253  // removeConnectionHeaders removes hop-by-hop headers listed in the "Connection" header of h.
   254  // See RFC 2616, section 14.10.
   255  func removeConnectionHeaders(h http.Header) {
   256  	if c := h.Get("Connection"); c != "" {
   257  		for _, f := range strings.Split(c, ",") {
   258  			if f = strings.TrimSpace(f); f != "" {
   259  				h.Del(f)
   260  			}
   261  		}
   262  	}
   263  }
   264  
   265  func (p *ReverseProxy) copyResponse(dst io.Writer, src io.Reader) {
   266  	if p.FlushInterval != 0 {
   267  		if wf, ok := dst.(writeFlusher); ok {
   268  			mlw := &maxLatencyWriter{
   269  				dst:     wf,
   270  				latency: p.FlushInterval,
   271  				done:    make(chan bool),
   272  			}
   273  			go mlw.flushLoop()
   274  			defer mlw.stop()
   275  			dst = mlw
   276  		}
   277  	}
   278  
   279  	var buf []byte
   280  	if p.BufferPool != nil {
   281  		buf = p.BufferPool.Get()
   282  	}
   283  	p.copyBuffer(dst, src, buf)
   284  	if p.BufferPool != nil {
   285  		p.BufferPool.Put(buf)
   286  	}
   287  }
   288  
   289  func (p *ReverseProxy) copyBuffer(dst io.Writer, src io.Reader, buf []byte) (int64, error) {
   290  	if len(buf) == 0 {
   291  		buf = make([]byte, 32*1024)
   292  	}
   293  	var written int64
   294  	for {
   295  		nr, rerr := src.Read(buf)
   296  		if rerr != nil && rerr != io.EOF && rerr != context.Canceled {
   297  			p.logf("httputil: ReverseProxy read error during body copy: %v", rerr)
   298  		}
   299  		if nr > 0 {
   300  			nw, werr := dst.Write(buf[:nr])
   301  			if nw > 0 {
   302  				written += int64(nw)
   303  			}
   304  			if werr != nil {
   305  				return written, werr
   306  			}
   307  			if nr != nw {
   308  				return written, io.ErrShortWrite
   309  			}
   310  		}
   311  		if rerr != nil {
   312  			return written, rerr
   313  		}
   314  	}
   315  }
   316  
   317  func (p *ReverseProxy) logf(format string, args ...interface{}) {
   318  	if p.ErrorLog != nil {
   319  		p.ErrorLog.Printf(format, args...)
   320  	} else {
   321  		log.Printf(format, args...)
   322  	}
   323  }
   324  
   325  type writeFlusher interface {
   326  	io.Writer
   327  	http.Flusher
   328  }
   329  
   330  type maxLatencyWriter struct {
   331  	dst     writeFlusher
   332  	latency time.Duration
   333  
   334  	mu   sync.Mutex // protects Write + Flush
   335  	done chan bool
   336  }
   337  
   338  func (m *maxLatencyWriter) Write(p []byte) (int, error) {
   339  	m.mu.Lock()
   340  	defer m.mu.Unlock()
   341  	return m.dst.Write(p)
   342  }
   343  
   344  func (m *maxLatencyWriter) flushLoop() {
   345  	t := time.NewTicker(m.latency)
   346  	defer t.Stop()
   347  	for {
   348  		select {
   349  		case <-m.done:
   350  			if onExitFlushLoop != nil {
   351  				onExitFlushLoop()
   352  			}
   353  			return
   354  		case <-t.C:
   355  			m.mu.Lock()
   356  			m.dst.Flush()
   357  			m.mu.Unlock()
   358  		}
   359  	}
   360  }
   361  
   362  func (m *maxLatencyWriter) stop() { m.done <- true }
   363
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