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// 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 }