transport.go

/*
Copyright 2014 Google Inc. All rights reserved.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
*/

package proxy

import (
	"bytes"
	"compress/gzip"
	"fmt"
	"io"
	"io/ioutil"
	"net/http"
	"net/url"
	"path"
	"strings"

	"github.com/golang/glog"
	"golang.org/x/net/html"

	"github.com/GoogleCloudPlatform/kubernetes/pkg/util"
)

// tagsToAttrs states which attributes of which tags require URL substitution.
// Sources: http://www.w3.org/TR/REC-html40/index/attributes.html
//          http://www.w3.org/html/wg/drafts/html/master/index.html#attributes-1
var tagsToAttrs = map[string]util.StringSet{
	"a":          util.NewStringSet("href"),
	"applet":     util.NewStringSet("codebase"),
	"area":       util.NewStringSet("href"),
	"audio":      util.NewStringSet("src"),
	"base":       util.NewStringSet("href"),
	"blockquote": util.NewStringSet("cite"),
	"body":       util.NewStringSet("background"),
	"button":     util.NewStringSet("formaction"),
	"command":    util.NewStringSet("icon"),
	"del":        util.NewStringSet("cite"),
	"embed":      util.NewStringSet("src"),
	"form":       util.NewStringSet("action"),
	"frame":      util.NewStringSet("longdesc", "src"),
	"head":       util.NewStringSet("profile"),
	"html":       util.NewStringSet("manifest"),
	"iframe":     util.NewStringSet("longdesc", "src"),
	"img":        util.NewStringSet("longdesc", "src", "usemap"),
	"input":      util.NewStringSet("src", "usemap", "formaction"),
	"ins":        util.NewStringSet("cite"),
	"link":       util.NewStringSet("href"),
	"object":     util.NewStringSet("classid", "codebase", "data", "usemap"),
	"q":          util.NewStringSet("cite"),
	"script":     util.NewStringSet("src"),
	"source":     util.NewStringSet("src"),
	"video":      util.NewStringSet("poster", "src"),

	// TODO: css URLs hidden in style elements.
}

// Transport is a transport for text/html content that replaces URLs in html
// content with the prefix of the proxy server
type Transport struct {
	Scheme      string
	Host        string
	PathPrepend string
}

// RoundTrip implements the http.RoundTripper interface
func (t *Transport) RoundTrip(req *http.Request) (*http.Response, error) {
	// Add reverse proxy headers.
	req.Header.Set("X-Forwarded-Uri", t.PathPrepend+req.URL.Path)
	req.Header.Set("X-Forwarded-Host", t.Host)
	req.Header.Set("X-Forwarded-Proto", t.Scheme)

	resp, err := http.DefaultTransport.RoundTrip(req)

	if err != nil {
		message := fmt.Sprintf("Error: '%s'\nTrying to reach: '%v'", err.Error(), req.URL.String())
		resp = &http.Response{
			StatusCode: http.StatusServiceUnavailable,
			Body:       ioutil.NopCloser(strings.NewReader(message)),
		}
		return resp, nil
	}

	if redirect := resp.Header.Get("Location"); redirect != "" {
		resp.Header.Set("Location", t.rewriteURL(redirect, req.URL))
		return resp, nil
	}

	cType := resp.Header.Get("Content-Type")
	cType = strings.TrimSpace(strings.SplitN(cType, ";", 2)[0])
	if cType != "text/html" {
		// Do nothing, simply pass through
		return resp, nil
	}

	return t.fixLinks(req, resp)
}

// rewriteURL rewrites a single URL to go through the proxy, if the URL refers
// to the same host as sourceURL, which is the page on which the target URL
// occurred. If any error occurs (e.g. parsing), it returns targetURL.
func (t *Transport) rewriteURL(targetURL string, sourceURL *url.URL) string {
	url, err := url.Parse(targetURL)
	if err != nil {
		return targetURL
	}
	if url.Host != "" && url.Host != sourceURL.Host {
		return targetURL
	}

	url.Scheme = t.Scheme
	url.Host = t.Host
	origPath := url.Path

	if strings.HasPrefix(url.Path, "/") {
		// The path is rooted at the host. Just add proxy prepend.
		url.Path = path.Join(t.PathPrepend, url.Path)
	} else {
		// The path is relative to sourceURL.
		url.Path = path.Join(t.PathPrepend, path.Dir(sourceURL.Path), url.Path)
	}

	if strings.HasSuffix(origPath, "/") {
		// Add back the trailing slash, which was stripped by path.Join().
		url.Path += "/"
	}

	return url.String()
}

// updateURLs checks and updates any of n's attributes that are listed in tagsToAttrs.
// Any URLs found are, if they're relative, updated with the necessary changes to make
// a visit to that URL also go through the proxy.
// sourceURL is the URL of the page which we're currently on; it's required to make
// relative links work.
func (t *Transport) updateURLs(n *html.Node, sourceURL *url.URL) {
	if n.Type != html.ElementNode {
		return
	}
	attrs, ok := tagsToAttrs[n.Data]
	if !ok {
		return
	}
	for i, attr := range n.Attr {
		if !attrs.Has(attr.Key) {
			continue
		}
		n.Attr[i].Val = t.rewriteURL(attr.Val, sourceURL)
	}
}

// scan recursively calls f for every n and every subnode of n.
func (t *Transport) scan(n *html.Node, f func(*html.Node)) {
	f(n)
	for c := n.FirstChild; c != nil; c = c.NextSibling {
		t.scan(c, f)
	}
}

// fixLinks modifies links in an HTML file such that they will be redirected through the proxy if needed.
func (t *Transport) fixLinks(req *http.Request, resp *http.Response) (*http.Response, error) {
	origBody := resp.Body
	defer origBody.Close()

	newContent := &bytes.Buffer{}
	var reader io.Reader = origBody
	var writer io.Writer = newContent
	encoding := resp.Header.Get("Content-Encoding")
	switch encoding {
	case "gzip":
		var err error
		reader, err = gzip.NewReader(reader)
		if err != nil {
			return nil, fmt.Errorf("errorf making gzip reader: %v", err)
		}
		gzw := gzip.NewWriter(writer)
		defer gzw.Close()
		writer = gzw
	// TODO: support flate, other encodings.
	case "":
		// This is fine
	default:
		// Some encoding we don't understand-- don't try to parse this
		glog.Errorf("Proxy encountered encoding %v for text/html; can't understand this so not fixing links.", encoding)
		return resp, nil
	}

	doc, err := html.Parse(reader)
	if err != nil {
		glog.Errorf("Parse failed: %v", err)
		return resp, err
	}

	t.scan(doc, func(n *html.Node) { t.updateURLs(n, req.URL) })
	if err := html.Render(writer, doc); err != nil {
		glog.Errorf("Failed to render: %v", err)
	}

	resp.Body = ioutil.NopCloser(newContent)
	// Update header node with new content-length
	// TODO: Remove any hash/signature headers here?
	resp.Header.Del("Content-Length")
	resp.ContentLength = int64(newContent.Len())

	return resp, err
}