reverseproxy.go

Documentation: net/http/httputil

  // 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"
  	"io"
  	"log"
  	"net"
  	"net/http"
  	"net/url"
  	"strings"
  	"sync"
  	"time"
  )
  
  // onExitFlushLoop is a callback set by tests to detect the state of the
  // flushLoop() goroutine.
  var onExitFlushLoop func()
  
  // ReverseProxy is an HTTP Handler that takes an incoming request and
  // sends it to another server, proxying the response back to the
  // client.
  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.
  	FlushInterval time.Duration
  
  	// ErrorLog specifies an optional logger for errors
  	// that occur when attempting to proxy the request.
  	// If nil, logging goes to os.Stderr 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.
  	// If it returns an error, the proxy returns a StatusBadGateway error.
  	ModifyResponse func(*http.Response) 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
  }
  
  // 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 = singleJoiningSlash(target.Path, req.URL.Path)
  		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.
  // http://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html
  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; http://www.rfc-editor.org/errata_search.php?eid=4522
  	"Transfer-Encoding",
  	"Upgrade",
  }
  
  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 := new(http.Request)
  	*outreq = *req // includes shallow copies of maps, but okay
  	if req.ContentLength == 0 {
  		outreq.Body = nil // Issue 16036: nil Body for http.Transport retries
  	}
  	outreq = outreq.WithContext(ctx)
  
  	p.Director(outreq)
  	outreq.Close = false
  
  	// We are modifying the same underlying map from req (shallow
  	// copied above) so we only copy it if necessary.
  	copiedHeaders := false
  
  	// Remove hop-by-hop headers listed in the "Connection" header.
  	// See RFC 2616, section 14.10.
  	if c := outreq.Header.Get("Connection"); c != "" {
  		for _, f := range strings.Split(c, ",") {
  			if f = strings.TrimSpace(f); f != "" {
  				if !copiedHeaders {
  					outreq.Header = make(http.Header)
  					copyHeader(outreq.Header, req.Header)
  					copiedHeaders = true
  				}
  				outreq.Header.Del(f)
  			}
  		}
  	}
  
  	// 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 {
  		if outreq.Header.Get(h) != "" {
  			if !copiedHeaders {
  				outreq.Header = make(http.Header)
  				copyHeader(outreq.Header, req.Header)
  				copiedHeaders = true
  			}
  			outreq.Header.Del(h)
  		}
  	}
  
  	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.
  		if prior, ok := outreq.Header["X-Forwarded-For"]; ok {
  			clientIP = strings.Join(prior, ", ") + ", " + clientIP
  		}
  		outreq.Header.Set("X-Forwarded-For", clientIP)
  	}
  
  	res, err := transport.RoundTrip(outreq)
  	if err != nil {
  		p.logf("http: proxy error: %v", err)
  		rw.WriteHeader(http.StatusBadGateway)
  		return
  	}
  
  	// Remove hop-by-hop headers listed in the
  	// "Connection" header of the response.
  	if c := res.Header.Get("Connection"); c != "" {
  		for _, f := range strings.Split(c, ",") {
  			if f = strings.TrimSpace(f); f != "" {
  				res.Header.Del(f)
  			}
  		}
  	}
  
  	for _, h := range hopHeaders {
  		res.Header.Del(h)
  	}
  
  	if p.ModifyResponse != nil {
  		if err := p.ModifyResponse(res); err != nil {
  			p.logf("http: proxy error: %v", err)
  			rw.WriteHeader(http.StatusBadGateway)
  			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.
  	if len(res.Trailer) > 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)
  	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()
  		}
  	}
  	p.copyResponse(rw, res.Body)
  	res.Body.Close() // close now, instead of defer, to populate res.Trailer
  	copyHeader(rw.Header(), res.Trailer)
  }
  
  func (p *ReverseProxy) copyResponse(dst io.Writer, src io.Reader) {
  	if p.FlushInterval != 0 {
  		if wf, ok := dst.(writeFlusher); ok {
  			mlw := &maxLatencyWriter{
  				dst:     wf,
  				latency: p.FlushInterval,
  				done:    make(chan bool),
  			}
  			go mlw.flushLoop()
  			defer mlw.stop()
  			dst = mlw
  		}
  	}
  
  	var buf []byte
  	if p.BufferPool != nil {
  		buf = p.BufferPool.Get()
  	}
  	p.copyBuffer(dst, src, buf)
  	if p.BufferPool != nil {
  		p.BufferPool.Put(buf)
  	}
  }
  
  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 {
  			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 {
  			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
  
  	mu   sync.Mutex // protects Write + Flush
  	done chan bool
  }
  
  func (m *maxLatencyWriter) Write(p []byte) (int, error) {
  	m.mu.Lock()
  	defer m.mu.Unlock()
  	return m.dst.Write(p)
  }
  
  func (m *maxLatencyWriter) flushLoop() {
  	t := time.NewTicker(m.latency)
  	defer t.Stop()
  	for {
  		select {
  		case <-m.done:
  			if onExitFlushLoop != nil {
  				onExitFlushLoop()
  			}
  			return
  		case <-t.C:
  			m.mu.Lock()
  			m.dst.Flush()
  			m.mu.Unlock()
  		}
  	}
  }
  
  func (m *maxLatencyWriter) stop() { m.done <- true }
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