source: code/trunk/morty.go@ 119

package main

import (
        "bytes"
        "crypto/hmac"
        "crypto/sha256"
        "encoding/base64"
        "encoding/hex"
        "errors"
        "flag"
        "fmt"
        "io"
        "log"
        "mime"
        "net/url"
        "os"
        "path/filepath"
        "regexp"
        "strings"
        "time"
        "unicode/utf8"

        "github.com/valyala/fasthttp"
        "github.com/valyala/fasthttp/fasthttpproxy"
        "golang.org/x/net/html"
        "golang.org/x/net/html/charset"
        "golang.org/x/text/encoding"

        "github.com/asciimoo/morty/contenttype"
)

const (
        STATE_DEFAULT     int = 0
        STATE_IN_STYLE    int = 1
        STATE_IN_NOSCRIPT int = 2
)

const VERSION = "v0.2.0"

var DEBUG = os.Getenv("DEBUG") != "false"

var CLIENT *fasthttp.Client = &fasthttp.Client{
        MaxResponseBodySize: 10 * 1024 * 1024, // 10M
        ReadBufferSize:      16 * 1024,        // 16K
}

var CSS_URL_REGEXP *regexp.Regexp = regexp.MustCompile("url\\((['\"]?)[ \\t\\f]*([\u0009\u0021\u0023-\u0026\u0028\u002a-\u007E]+)(['\"]?)\\)?")

var ALLOWED_CONTENTTYPE_FILTER contenttype.Filter = contenttype.NewFilterOr([]contenttype.Filter{
        // html
        contenttype.NewFilterEquals("text", "html", ""),
        contenttype.NewFilterEquals("application", "xhtml", "xml"),
        // css
        contenttype.NewFilterEquals("text", "css", ""),
        // images
        contenttype.NewFilterEquals("image", "gif", ""),
        contenttype.NewFilterEquals("image", "png", ""),
        contenttype.NewFilterEquals("image", "jpeg", ""),
        contenttype.NewFilterEquals("image", "pjpeg", ""),
        contenttype.NewFilterEquals("image", "webp", ""),
        contenttype.NewFilterEquals("image", "tiff", ""),
        contenttype.NewFilterEquals("image", "vnd.microsoft.icon", ""),
        contenttype.NewFilterEquals("image", "bmp", ""),
        contenttype.NewFilterEquals("image", "x-ms-bmp", ""),
        contenttype.NewFilterEquals("image", "x-icon", ""),
        // fonts
        contenttype.NewFilterEquals("application", "font-otf", ""),
        contenttype.NewFilterEquals("application", "font-ttf", ""),
        contenttype.NewFilterEquals("application", "font-woff", ""),
        contenttype.NewFilterEquals("application", "vnd.ms-fontobject", ""),
})

var ALLOWED_CONTENTTYPE_ATTACHMENT_FILTER contenttype.Filter = contenttype.NewFilterOr([]contenttype.Filter{
        // texts
        contenttype.NewFilterEquals("text", "csv", ""),
        contenttype.NewFilterEquals("text", "tab-separated-values", ""),
        contenttype.NewFilterEquals("text", "plain", ""),
        // API
        contenttype.NewFilterEquals("application", "json", ""),
        // Documents
        contenttype.NewFilterEquals("application", "x-latex", ""),
        contenttype.NewFilterEquals("application", "pdf", ""),
        contenttype.NewFilterEquals("application", "vnd.oasis.opendocument.text", ""),
        contenttype.NewFilterEquals("application", "vnd.oasis.opendocument.spreadsheet", ""),
        contenttype.NewFilterEquals("application", "vnd.oasis.opendocument.presentation", ""),
        contenttype.NewFilterEquals("application", "vnd.oasis.opendocument.graphics", ""),
        // Compressed archives
        contenttype.NewFilterEquals("application", "zip", ""),
        contenttype.NewFilterEquals("application", "gzip", ""),
        contenttype.NewFilterEquals("application", "x-compressed", ""),
        contenttype.NewFilterEquals("application", "x-gtar", ""),
        contenttype.NewFilterEquals("application", "x-compress", ""),
        // Generic binary
        contenttype.NewFilterEquals("application", "octet-stream", ""),
})

var ALLOWED_CONTENTTYPE_PARAMETERS map[string]bool = map[string]bool{
        "charset": true,
}

var UNSAFE_ELEMENTS [][]byte = [][]byte{
        []byte("applet"),
        []byte("canvas"),
        []byte("embed"),
        //[]byte("iframe"),
        []byte("math"),
        []byte("script"),
        []byte("svg"),
}

var SAFE_ATTRIBUTES [][]byte = [][]byte{
        []byte("abbr"),
        []byte("accesskey"),
        []byte("align"),
        []byte("alt"),
        []byte("as"),
        []byte("autocomplete"),
        []byte("charset"),
        []byte("checked"),
        []byte("class"),
        []byte("content"),
        []byte("contenteditable"),
        []byte("contextmenu"),
        []byte("dir"),
        []byte("for"),
        []byte("height"),
        []byte("hidden"),
        []byte("hreflang"),
        []byte("id"),
        []byte("lang"),
        []byte("media"),
        []byte("method"),
        []byte("name"),
        []byte("nowrap"),
        []byte("placeholder"),
        []byte("property"),
        []byte("rel"),
        []byte("spellcheck"),
        []byte("tabindex"),
        []byte("target"),
        []byte("title"),
        []byte("translate"),
        []byte("type"),
        []byte("value"),
        []byte("width"),
}

var LINK_REL_SAFE_VALUES [][]byte = [][]byte{
        []byte("alternate"),
        []byte("archives"),
        []byte("author"),
        []byte("copyright"),
        []byte("first"),
        []byte("help"),
        []byte("icon"),
        []byte("index"),
        []byte("last"),
        []byte("license"),
        []byte("manifest"),
        []byte("next"),
        []byte("pingback"),
        []byte("prev"),
        []byte("publisher"),
        []byte("search"),
        []byte("shortcut icon"),
        []byte("stylesheet"),
        []byte("up"),
}

var LINK_HTTP_EQUIV_SAFE_VALUES [][]byte = [][]byte{
        // X-UA-Compatible will be added automaticaly, so it can be skipped
        []byte("date"),
        []byte("last-modified"),
        []byte("refresh"), // URL rewrite
        // []byte("location"), TODO URL rewrite
        []byte("content-language"),
}

type Proxy struct {
        Key            []byte
        RequestTimeout time.Duration
}

type RequestConfig struct {
        Key     []byte
        BaseURL *url.URL
}

var HTML_FORM_EXTENSION string = `<input type="hidden" name="mortyurl" value="%s" /><input type="hidden" name="mortyhash" value="%s" />`

var HTML_BODY_EXTENSION string = `
<input type="checkbox" id="mortytoggle" autocomplete="off" />
<div id="mortyheader">
  <form method="get">
    <label for="mortytoggle">hide</label>
    <span><a href="/">Morty Proxy</a></span>
    <input type="url" value="%s" name="mortyurl" readonly="true" />
    This is a <a href="https://github.com/asciimoo/morty">proxified and sanitized</a> view of the page, visit <a href="%s" rel="noreferrer">original site</a>.
  </form>
</div>
<style>
body{ position: absolute !important; top: 42px !important; left: 0 !important; right: 0 !important; bottom: 0 !important; }
#mortyheader { position: fixed; margin: 0; box-sizing: border-box; -webkit-box-sizing: border-box; top: 0; left: 0; right: 0; z-index: 2147483647 !important; font-size: 12px; line-height: normal; border-width: 0px 0px 2px 0; border-style: solid; border-color: #AAAAAA; background: #FFF; padding: 4px; color: #444; height: 42px; }
#mortyheader p { padding: 0 0 0.7em 0; display: block; }
#mortyheader a { color: #3498db; font-weight: bold; display: inline; }
#mortyheader label { text-align: right; cursor: pointer; position: fixed; right: 4px; top: 4px; display: block; color: #444; }
#mortyheader > form > span { font-size: 24px; font-weight: bold; margin-right: 20px; margin-left: 20px; }
input[type=checkbox]#mortytoggle { display: none; }
input[type=checkbox]#mortytoggle:checked ~ div { display: none; visibility: hidden; }
#mortyheader input[type=url] { width: 50%%; padding: 4px; font-size: 16px; }
</style>
`

var HTML_HEAD_CONTENT_TYPE string = `<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="referrer" content="no-referrer">
`

var MORTY_HTML_PAGE_START string = `<!doctype html>
<html>
<head>
<title>MortyProxy</title>
<meta name="viewport" content="width=device-width, initial-scale=1 , maximum-scale=1.0, user-scalable=1" />
<style>
html { height: 100%; }
body { min-height : 100%; display: flex; flex-direction:column; font-family: 'Garamond', 'Georgia', serif; text-align: center; color: #444; background: #FAFAFA; margin: 0; padding: 0; font-size: 1.1em; }
input { border: 1px solid #888; padding: 0.3em; color: #444; background: #FFF; font-size: 1.1em; }
input[placeholder] { width:80%; }
a { text-decoration: none; #2980b9; }
h1, h2 { font-weight: 200; margin-bottom: 2rem; }
h1 { font-size: 3em; }
.container { flex:1; min-height: 100%; margin-bottom: 1em; }
.footer { margin: 1em; }
.footer p { font-size: 0.8em; }
</style>
</head>
<body>
        <div class="container">
                <h1>MortyProxy</h1>
`

var MORTY_HTML_PAGE_END string = `
        </div>
        <div class="footer">
                <p>Morty rewrites web pages to exclude malicious HTML tags and CSS/HTML attributes. It also replaces external resource references to prevent third-party information leaks.<br />
                <a href="https://github.com/asciimoo/morty">view on github</a>
                </p>
        </div>
</body>
</html>`

var FAVICON_BYTES []byte

func init() {
        FaviconBase64 := "iVBORw0KGgoAAAANSUhEUgAAABAAAAAQEAYAAABPYyMiAAAABmJLR0T///////8JWPfcAAAACXBIWXMAAABIAAAASABGyWs+AAAAF0lEQVRIx2NgGAWjYBSMglEwCkbBSAcACBAAAeaR9cIAAAAASUVORK5CYII"

        FAVICON_BYTES, _ = base64.StdEncoding.DecodeString(FaviconBase64)
}

func (p *Proxy) RequestHandler(ctx *fasthttp.RequestCtx) {

        if appRequestHandler(ctx) {
                return
        }

        requestHash := popRequestParam(ctx, []byte("mortyhash"))

        requestURI := popRequestParam(ctx, []byte("mortyurl"))

        if requestURI == nil {
                p.serveMainPage(ctx, 200, nil)
                return
        }

        if p.Key != nil {
                if !verifyRequestURI(requestURI, requestHash, p.Key) {
                        // HTTP status code 403 : Forbidden
                        p.serveMainPage(ctx, 403, errors.New(`invalid "mortyhash" parameter`))
                        return
                }
        }

        requestURIQuery := ctx.QueryArgs().QueryString()
        if len(requestURIQuery) > 0 {
                requestURI = append(requestURI, '?')
                requestURI = append(requestURI, requestURIQuery...)
        }

        parsedURI, err := url.Parse(string(requestURI))

        if err != nil {
                // HTTP status code 500 : Internal Server Error
                p.serveMainPage(ctx, 500, err)
                return
        }

        // Serve an intermediate page for protocols other than HTTP(S)
        if (parsedURI.Scheme != "http" && parsedURI.Scheme != "https") || strings.HasSuffix(parsedURI.Host, ".onion") {
                p.serveExitMortyPage(ctx, parsedURI)
                return
        }

        req := fasthttp.AcquireRequest()
        defer fasthttp.ReleaseRequest(req)
        req.SetConnectionClose()

        requestURIStr := string(requestURI)

        if DEBUG {
                log.Println("getting", requestURIStr)
        }

        req.SetRequestURI(requestURIStr)
        req.Header.SetUserAgentBytes([]byte("Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Firefox/78.0"))

        resp := fasthttp.AcquireResponse()
        defer fasthttp.ReleaseResponse(resp)

        req.Header.SetMethodBytes(ctx.Method())
        if ctx.IsPost() || ctx.IsPut() {
                req.SetBody(ctx.PostBody())
        }

        err = CLIENT.DoTimeout(req, resp, p.RequestTimeout)

        if err != nil {
                if err == fasthttp.ErrTimeout {
                        // HTTP status code 504 : Gateway Time-Out
                        p.serveMainPage(ctx, 504, err)
                } else {
                        // HTTP status code 500 : Internal Server Error
                        p.serveMainPage(ctx, 500, err)
                }
                return
        }

        if resp.StatusCode() != 200 {
                switch resp.StatusCode() {
                case 301, 302, 303, 307, 308:
                        loc := resp.Header.Peek("Location")
                        if loc != nil {
                                rc := &RequestConfig{Key: p.Key, BaseURL: parsedURI}
                                url, err := rc.ProxifyURI(loc)
                                if err == nil {
                                        ctx.SetStatusCode(resp.StatusCode())
                                        ctx.Response.Header.Add("Location", url)
                                        if DEBUG {
                                                log.Println("redirect to", string(loc))
                                        }
                                        return
                                }
                        }
                }
                error_message := fmt.Sprintf("invalid response: %d (%s)", resp.StatusCode(), requestURIStr)
                p.serveMainPage(ctx, resp.StatusCode(), errors.New(error_message))
                return
        }

        contentTypeBytes := resp.Header.Peek("Content-Type")

        if contentTypeBytes == nil {
                // HTTP status code 503 : Service Unavailable
                p.serveMainPage(ctx, 503, errors.New("invalid content type"))
                return
        }

        contentTypeString := string(contentTypeBytes)

        // decode Content-Type header
        contentType, error := contenttype.ParseContentType(contentTypeString)
        if error != nil {
                // HTTP status code 503 : Service Unavailable
                p.serveMainPage(ctx, 503, errors.New("invalid content type"))
                return
        }

        // content-disposition
        contentDispositionBytes := ctx.Request.Header.Peek("Content-Disposition")

        // check content type
        if !ALLOWED_CONTENTTYPE_FILTER(contentType) {
                // it is not a usual content type
                if ALLOWED_CONTENTTYPE_ATTACHMENT_FILTER(contentType) {
                        // force attachment for allowed content type
                        contentDispositionBytes = contentDispositionForceAttachment(contentDispositionBytes, parsedURI)
                } else {
                        // deny access to forbidden content type
                        // HTTP status code 403 : Forbidden
                        p.serveMainPage(ctx, 403, errors.New("forbidden content type"))
                        return
                }
        }

        // HACK : replace */xhtml by text/html
        if contentType.SubType == "xhtml" {
                contentType.TopLevelType = "text"
                contentType.SubType = "html"
                contentType.Suffix = ""
        }

        // conversion to UTF-8
        var responseBody []byte

        if contentType.TopLevelType == "text" {
                e, ename, _ := charset.DetermineEncoding(resp.Body(), contentTypeString)
                if (e != encoding.Nop) && (!strings.EqualFold("utf-8", ename)) {
                        responseBody, err = e.NewDecoder().Bytes(resp.Body())
                        if err != nil {
                                // HTTP status code 503 : Service Unavailable
                                p.serveMainPage(ctx, 503, err)
                                return
                        }
                } else {
                        responseBody = resp.Body()
                }
                // update the charset or specify it
                contentType.Parameters["charset"] = "UTF-8"
        } else {
                responseBody = resp.Body()
        }

        //
        contentType.FilterParameters(ALLOWED_CONTENTTYPE_PARAMETERS)

        // set the content type
        ctx.SetContentType(contentType.String())

        // output according to MIME type
        switch {
        case contentType.SubType == "css" && contentType.Suffix == "":
                sanitizeCSS(&RequestConfig{Key: p.Key, BaseURL: parsedURI}, ctx, responseBody)
        case contentType.SubType == "html" && contentType.Suffix == "":
                sanitizeHTML(&RequestConfig{Key: p.Key, BaseURL: parsedURI}, ctx, responseBody)
        default:
                if contentDispositionBytes != nil {
                        ctx.Response.Header.AddBytesV("Content-Disposition", contentDispositionBytes)
                }
                ctx.Write(responseBody)
        }
}

// force content-disposition to attachment
func contentDispositionForceAttachment(contentDispositionBytes []byte, url *url.URL) []byte {
        var contentDispositionParams map[string]string

        if contentDispositionBytes != nil {
                var err error
                _, contentDispositionParams, err = mime.ParseMediaType(string(contentDispositionBytes))
                if err != nil {
                        contentDispositionParams = make(map[string]string)
                }
        } else {
                contentDispositionParams = make(map[string]string)
        }

        _, fileNameDefined := contentDispositionParams["filename"]
        if !fileNameDefined {
                // TODO : sanitize filename
                contentDispositionParams["fileName"] = filepath.Base(url.Path)
        }

        return []byte(mime.FormatMediaType("attachment", contentDispositionParams))
}

func appRequestHandler(ctx *fasthttp.RequestCtx) bool {
        // serve robots.txt
        if bytes.Equal(ctx.Path(), []byte("/robots.txt")) {
                ctx.SetContentType("text/plain")
                ctx.Write([]byte("User-Agent: *\nDisallow: /\n"))
                return true
        }

        // server favicon.ico
        if bytes.Equal(ctx.Path(), []byte("/favicon.ico")) {
                ctx.SetContentType("image/png")
                ctx.Write(FAVICON_BYTES)
                return true
        }

        return false
}

func popRequestParam(ctx *fasthttp.RequestCtx, paramName []byte) []byte {
        param := ctx.QueryArgs().PeekBytes(paramName)

        if param == nil {
                param = ctx.PostArgs().PeekBytes(paramName)
                if param != nil {
                        ctx.PostArgs().DelBytes(paramName)
                }
        } else {
                ctx.QueryArgs().DelBytes(paramName)
        }

        return param
}

func sanitizeCSS(rc *RequestConfig, out io.Writer, css []byte) {
        // TODO

        urlSlices := CSS_URL_REGEXP.FindAllSubmatchIndex(css, -1)

        if urlSlices == nil {
                out.Write(css)
                return
        }

        startIndex := 0

        for _, s := range urlSlices {
                urlStart := s[4]
                urlEnd := s[5]

                if uri, err := rc.ProxifyURI(css[urlStart:urlEnd]); err == nil {
                        out.Write(css[startIndex:urlStart])
                        out.Write([]byte(uri))
                        startIndex = urlEnd
                } else if DEBUG {
                        log.Println("cannot proxify css uri:", string(css[urlStart:urlEnd]))
                }
        }
        if startIndex < len(css) {
                out.Write(css[startIndex:len(css)])
        }
}

func sanitizeHTML(rc *RequestConfig, out io.Writer, htmlDoc []byte) {
        r := bytes.NewReader(htmlDoc)
        decoder := html.NewTokenizer(r)
        decoder.AllowCDATA(true)

        unsafeElements := make([][]byte, 0, 8)
        state := STATE_DEFAULT
        for {
                token := decoder.Next()
                if token == html.ErrorToken {
                        err := decoder.Err()
                        if err != io.EOF {
                                log.Println("failed to parse HTML")
                        }
                        break
                }

                if len(unsafeElements) == 0 {

                        switch token {
                        case html.StartTagToken, html.SelfClosingTagToken:
                                tag, hasAttrs := decoder.TagName()
                                safe := !inArray(tag, UNSAFE_ELEMENTS)
                                if !safe {
                                        if token != html.SelfClosingTagToken {
                                                var unsafeTag []byte = make([]byte, len(tag))
                                                copy(unsafeTag, tag)
                                                unsafeElements = append(unsafeElements, unsafeTag)
                                        }
                                        break
                                }
                                if bytes.Equal(tag, []byte("base")) {
                                        for {
                                                attrName, attrValue, moreAttr := decoder.TagAttr()
                                                if bytes.Equal(attrName, []byte("href")) {
                                                        parsedURI, err := url.Parse(string(attrValue))
                                                        if err == nil {
                                                                rc.BaseURL = parsedURI
                                                        }
                                                }
                                                if !moreAttr {
                                                        break
                                                }
                                        }
                                        break
                                }
                                if bytes.Equal(tag, []byte("noscript")) {
                                        state = STATE_IN_NOSCRIPT
                                        break
                                }
                                var attrs [][][]byte
                                if hasAttrs {
                                        for {
                                                attrName, attrValue, moreAttr := decoder.TagAttr()
                                                attrs = append(attrs, [][]byte{
                                                        attrName,
                                                        attrValue,
                                                        []byte(html.EscapeString(string(attrValue))),
                                                })
                                                if !moreAttr {
                                                        break
                                                }
                                        }
                                }
                                if bytes.Equal(tag, []byte("link")) {
                                        sanitizeLinkTag(rc, out, attrs)
                                        break
                                }

                                if bytes.Equal(tag, []byte("meta")) {
                                        sanitizeMetaTag(rc, out, attrs)
                                        break
                                }

                                fmt.Fprintf(out, "<%s", tag)

                                if hasAttrs {
                                        sanitizeAttrs(rc, out, attrs)
                                }

                                if token == html.SelfClosingTagToken {
                                        fmt.Fprintf(out, " />")
                                } else {
                                        fmt.Fprintf(out, ">")
                                        if bytes.Equal(tag, []byte("style")) {
                                                state = STATE_IN_STYLE
                                        }
                                }

                                if bytes.Equal(tag, []byte("head")) {
                                        fmt.Fprintf(out, HTML_HEAD_CONTENT_TYPE)
                                }

                                if bytes.Equal(tag, []byte("form")) {
                                        var formURL *url.URL
                                        for _, attr := range attrs {
                                                if bytes.Equal(attr[0], []byte("action")) {
                                                        formURL, _ = url.Parse(string(attr[1]))
                                                        formURL = mergeURIs(rc.BaseURL, formURL)
                                                        break
                                                }
                                        }
                                        if formURL == nil {
                                                formURL = rc.BaseURL
                                        }
                                        urlStr := formURL.String()
                                        var key string
                                        if rc.Key != nil {
                                                key = hash(urlStr, rc.Key)
                                        }
                                        fmt.Fprintf(out, HTML_FORM_EXTENSION, urlStr, key)

                                }

                        case html.EndTagToken:
                                tag, _ := decoder.TagName()
                                writeEndTag := true
                                switch string(tag) {
                                case "body":
                                        fmt.Fprintf(out, HTML_BODY_EXTENSION, rc.BaseURL.String(), rc.BaseURL.String())
                                case "style":
                                        state = STATE_DEFAULT
                                case "noscript":
                                        state = STATE_DEFAULT
                                        writeEndTag = false
                                }
                                // skip noscript tags - only the tag, not the content, because javascript is sanitized
                                if writeEndTag {
                                        fmt.Fprintf(out, "</%s>", tag)
                                }

                        case html.TextToken:
                                switch state {
                                case STATE_DEFAULT:
                                        fmt.Fprintf(out, "%s", decoder.Raw())
                                case STATE_IN_STYLE:
                                        sanitizeCSS(rc, out, decoder.Raw())
                                case STATE_IN_NOSCRIPT:
                                        sanitizeHTML(rc, out, decoder.Raw())
                                }

                        case html.CommentToken:
                                // ignore comment. TODO : parse IE conditional comment

                        case html.DoctypeToken:
                                out.Write(decoder.Raw())
                        }
                } else {
                        switch token {
                        case html.StartTagToken, html.SelfClosingTagToken:
                                tag, _ := decoder.TagName()
                                if inArray(tag, UNSAFE_ELEMENTS) {
                                        unsafeElements = append(unsafeElements, tag)
                                }

                        case html.EndTagToken:
                                tag, _ := decoder.TagName()
                                if bytes.Equal(unsafeElements[len(unsafeElements)-1], tag) {
                                        unsafeElements = unsafeElements[:len(unsafeElements)-1]
                                }
                        }
                }
        }
}

func sanitizeLinkTag(rc *RequestConfig, out io.Writer, attrs [][][]byte) {
        exclude := false
        for _, attr := range attrs {
                attrName := attr[0]
                attrValue := attr[1]
                if bytes.Equal(attrName, []byte("rel")) {
                        if !inArray(attrValue, LINK_REL_SAFE_VALUES) {
                                exclude = true
                                break
                        }
                }
                if bytes.Equal(attrName, []byte("as")) {
                        if bytes.Equal(attrValue, []byte("script")) {
                                exclude = true
                                break
                        }
                }
        }

        if !exclude {
                out.Write([]byte("<link"))
                for _, attr := range attrs {
                        sanitizeAttr(rc, out, attr[0], attr[1], attr[2])
                }
                out.Write([]byte(">"))
        }
}

func sanitizeMetaTag(rc *RequestConfig, out io.Writer, attrs [][][]byte) {
        var http_equiv []byte
        var content []byte

        for _, attr := range attrs {
                attrName := attr[0]
                attrValue := attr[1]
                if bytes.Equal(attrName, []byte("http-equiv")) {
                        http_equiv = bytes.ToLower(attrValue)
                        // exclude some <meta http-equiv="..." ..>
                        if !inArray(http_equiv, LINK_HTTP_EQUIV_SAFE_VALUES) {
                                return
                        }
                }
                if bytes.Equal(attrName, []byte("content")) {
                        content = attrValue
                }
                if bytes.Equal(attrName, []byte("charset")) {
                        // exclude <meta charset="...">
                        return
                }
        }

        out.Write([]byte("<meta"))
        urlIndex := bytes.Index(bytes.ToLower(content), []byte("url="))
        if bytes.Equal(http_equiv, []byte("refresh")) && urlIndex != -1 {
                contentUrl := content[urlIndex+4:]
                // special case of <meta http-equiv="refresh" content="0; url='example.com/url.with.quote.outside'">
                if len(contentUrl) >= 2 && (contentUrl[0] == byte('\'') || contentUrl[0] == byte('"')) {
                        if contentUrl[0] == contentUrl[len(contentUrl)-1] {
                                contentUrl = contentUrl[1 : len(contentUrl)-1]
                        }
                }
                // output proxify result
                if uri, err := rc.ProxifyURI(contentUrl); err == nil {
                        fmt.Fprintf(out, ` http-equiv="refresh" content="%surl=%s"`, content[:urlIndex], uri)
                }
        } else {
                if len(http_equiv) > 0 {
                        fmt.Fprintf(out, ` http-equiv="%s"`, http_equiv)
                }
                sanitizeAttrs(rc, out, attrs)
        }
        out.Write([]byte(">"))
}

func sanitizeAttrs(rc *RequestConfig, out io.Writer, attrs [][][]byte) {
        for _, attr := range attrs {
                sanitizeAttr(rc, out, attr[0], attr[1], attr[2])
        }
}

func sanitizeAttr(rc *RequestConfig, out io.Writer, attrName, attrValue, escapedAttrValue []byte) {
        if inArray(attrName, SAFE_ATTRIBUTES) {
                fmt.Fprintf(out, " %s=\"%s\"", attrName, escapedAttrValue)
                return
        }
        switch string(attrName) {
        case "src", "href", "action":
                if uri, err := rc.ProxifyURI(attrValue); err == nil {
                        fmt.Fprintf(out, " %s=\"%s\"", attrName, uri)
                } else if DEBUG {
                        log.Println("cannot proxify uri:", string(attrValue))
                }
        case "style":
                cssAttr := bytes.NewBuffer(nil)
                sanitizeCSS(rc, cssAttr, attrValue)
                fmt.Fprintf(out, " %s=\"%s\"", attrName, html.EscapeString(string(cssAttr.Bytes())))
        }
}

func mergeURIs(u1, u2 *url.URL) *url.URL {
        if u2 == nil {
                return u1
        }
        return u1.ResolveReference(u2)
}

// Sanitized URI : removes all runes bellow 32 (included) as the begining and end of URI, and lower case the scheme.
// avoid memory allocation (except for the scheme)
func sanitizeURI(uri []byte) ([]byte, string) {
        first_rune_index := 0
        first_rune_seen := false
        scheme_last_index := -1
        buffer := bytes.NewBuffer(make([]byte, 0, 10))

        // remove trailing space and special characters
        uri = bytes.TrimRight(uri, "\x00\x01\x02\x03\x04\x05\x06\x07\x08\x09\x0A\x0B\x0C\x0D\x0E\x0F\x10\x11\x12\x13\x14\x15\x16\x17\x18\x19\x1A\x1B\x1C\x1D\x1E\x1F\x20")

        // loop over byte by byte
        for i, c := range uri {
                // ignore special characters and space (c <= 32)
                if c > 32 {
                        // append to the lower case of the rune to buffer
                        if c < utf8.RuneSelf && 'A' <= c && c <= 'Z' {
                                c = c + 'a' - 'A'
                        }

                        buffer.WriteByte(c)

                        // update the first rune index that is not a special rune
                        if !first_rune_seen {
                                first_rune_index = i
                                first_rune_seen = true
                        }

                        if c == ':' {
                                // colon rune found, we have found the scheme
                                scheme_last_index = i
                                break
                        } else if c == '/' || c == '?' || c == '\\' || c == '#' {
                                // special case : most probably a relative URI
                                break
                        }
                }
        }

        if scheme_last_index != -1 {
                // scheme found
                // copy the "lower case without special runes scheme" before the ":" rune
                scheme_start_index := scheme_last_index - buffer.Len() + 1
                copy(uri[scheme_start_index:], buffer.Bytes())
                // and return the result
                return uri[scheme_start_index:], buffer.String()
        } else {
                // scheme NOT found
                return uri[first_rune_index:], ""
        }
}

func (rc *RequestConfig) ProxifyURI(uri []byte) (string, error) {
        // sanitize URI
        uri, scheme := sanitizeURI(uri)

        // remove javascript protocol
        if scheme == "javascript:" {
                return "", nil
        }

        // TODO check malicious data: - e.g. data:script
        if scheme == "data:" {
                if bytes.HasPrefix(uri, []byte("data:image/png")) ||
                        bytes.HasPrefix(uri, []byte("data:image/jpeg")) ||
                        bytes.HasPrefix(uri, []byte("data:image/pjpeg")) ||
                        bytes.HasPrefix(uri, []byte("data:image/gif")) ||
                        bytes.HasPrefix(uri, []byte("data:image/webp")) {
                        // should be safe
                        return string(uri), nil
                } else {
                        // unsafe data
                        return "", nil
                }
        }

        // parse the uri
        u, err := url.Parse(string(uri))
        if err != nil {
                return "", err
        }

        // get the fragment (with the prefix "#")
        fragment := ""
        if len(u.Fragment) > 0 {
                fragment = "#" + u.Fragment
        }

        // reset the fragment: it is not included in the mortyurl
        u.Fragment = ""

        // merge the URI with the document URI
        u = mergeURIs(rc.BaseURL, u)

        // simple internal link ?
        // some web pages describe the whole link https://same:auth@same.host/same.path?same.query#new.fragment
        if u.Scheme == rc.BaseURL.Scheme &&
                (rc.BaseURL.User == nil || (u.User != nil && u.User.String() == rc.BaseURL.User.String())) &&
                u.Host == rc.BaseURL.Host &&
                u.Path == rc.BaseURL.Path &&
                u.RawQuery == rc.BaseURL.RawQuery {
                // the fragment is the only difference between the document URI and the uri parameter
                return fragment, nil
        }

        // return full URI and fragment (if not empty)
        morty_uri := u.String()

        if rc.Key == nil {
                return fmt.Sprintf("./?mortyurl=%s%s", url.QueryEscape(morty_uri), fragment), nil
        }
        return fmt.Sprintf("./?mortyhash=%s&mortyurl=%s%s", hash(morty_uri, rc.Key), url.QueryEscape(morty_uri), fragment), nil
}

func inArray(b []byte, a [][]byte) bool {
        for _, b2 := range a {
                if bytes.Equal(b, b2) {
                        return true
                }
        }
        return false
}

func hash(msg string, key []byte) string {
        mac := hmac.New(sha256.New, key)
        mac.Write([]byte(msg))
        return hex.EncodeToString(mac.Sum(nil))
}

func verifyRequestURI(uri, hashMsg, key []byte) bool {
        h := make([]byte, hex.DecodedLen(len(hashMsg)))
        _, err := hex.Decode(h, hashMsg)
        if err != nil {
                if DEBUG {
                        log.Println("hmac error:", err)
                }
                return false
        }
        mac := hmac.New(sha256.New, key)
        mac.Write(uri)
        return hmac.Equal(h, mac.Sum(nil))
}

func (p *Proxy) serveExitMortyPage(ctx *fasthttp.RequestCtx, uri *url.URL) {
        ctx.SetContentType("text/html")
        ctx.SetStatusCode(403)
        ctx.Write([]byte(MORTY_HTML_PAGE_START))
        ctx.Write([]byte("<h2>You are about to exit MortyProxy</h2>"))
        ctx.Write([]byte("<p>Following</p><p><a href=\""))
        ctx.Write([]byte(html.EscapeString(uri.String())))
        ctx.Write([]byte("\" rel=\"noreferrer\">"))
        ctx.Write([]byte(html.EscapeString(uri.String())))
        ctx.Write([]byte("</a></p><p>the content of this URL will be <b>NOT</b> sanitized.</p>"))
        ctx.Write([]byte(MORTY_HTML_PAGE_END))
}

func (p *Proxy) serveMainPage(ctx *fasthttp.RequestCtx, statusCode int, err error) {
        ctx.SetContentType("text/html; charset=UTF-8")
        ctx.SetStatusCode(statusCode)
        ctx.Write([]byte(MORTY_HTML_PAGE_START))
        if err != nil {
                if DEBUG {
                        log.Println("error:", err)
                }
                ctx.Write([]byte("<h2>Error: "))
                ctx.Write([]byte(html.EscapeString(err.Error())))
                ctx.Write([]byte("</h2>"))
        }
        if p.Key == nil {
                ctx.Write([]byte(`
                <form action="post">
                Visit url: <input placeholder="https://url.." name="mortyurl" autofocus />
                <input type="submit" value="go" />
                </form>`))
        } else {
                ctx.Write([]byte(`<h3>Warning! This instance does not support direct URL opening.</h3>`))
        }
        ctx.Write([]byte(MORTY_HTML_PAGE_END))
}

func main() {
        default_listen_addr := os.Getenv("MORTY_ADDRESS")
        if default_listen_addr == "" {
                default_listen_addr = "127.0.0.1:3000"
        }
        default_key := os.Getenv("MORTY_KEY")
        listen := flag.String("listen", default_listen_addr, "Listen address")
        key := flag.String("key", default_key, "HMAC url validation key (base64 encoded) - leave blank to disable validation")
        ipv6 := flag.Bool("ipv6", false, "Allow IPv6 HTTP requests")
        version := flag.Bool("version", false, "Show version")
        requestTimeout := flag.Uint("timeout", 2, "Request timeout")
        socks5 := flag.String("socks5", "", "SOCKS5 proxy")
        flag.Parse()

        if *version {
                fmt.Println(VERSION)
                return
        }

        if *ipv6 {
                CLIENT.DialDualStack = true
        }

        if *socks5 != "" {
                // this disables CLIENT.DialDualStack
                CLIENT.Dial = fasthttpproxy.FasthttpSocksDialer(*socks5)
        }

        p := &Proxy{RequestTimeout: time.Duration(*requestTimeout) * time.Second}

        if *key != "" {
                var err error
                p.Key, err = base64.StdEncoding.DecodeString(*key)
                if err != nil {
                        log.Fatal("Error parsing -key", err.Error())
                        os.Exit(1)
                }
        }

        log.Println("listening on", *listen)

        if err := fasthttp.ListenAndServe(*listen, p.RequestHandler); err != nil {
                log.Fatal("Error in ListenAndServe:", err)
        }
}