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(Shortkey: s)" aria-label="Toggle Searchbar" aria-expanded="false" aria-keyshortcuts="S" aria-controls="searchbar"> + <i class="fa fa-search"></i> + </button> + + </div> + + <h1 class="menu-title">Synapse</h1> + + <div class="right-buttons"> + + <a href="print.html" title="Print this book" aria-label="Print this book"> + <i id="print-button" class="fa fa-print"></i> + </a> + + + <a href="https://github.com/matrix-org/synapse" title="Git repository" aria-label="Git repository"> + <i id="git-repository-button" class="fa fa-github"></i> + </a> + + + <a href="https://github.com/matrix-org/synapse/edit/develop/docs/url_previews.md" title="Suggest an edit" aria-label="Suggest an edit"> + <i id="git-edit-button" class="fa fa-edit"></i> + </a> + + + </div> + </div> + + + <div id="search-wrapper" class="hidden"> + <form id="searchbar-outer" class="searchbar-outer"> + <input type="search" id="searchbar" name="searchbar" placeholder="Search this book ..." aria-controls="searchresults-outer" aria-describedby="searchresults-header"> + </form> + <div id="searchresults-outer" class="searchresults-outer hidden"> + <div id="searchresults-header" class="searchresults-header"></div> + <ul id="searchresults"> + </ul> + </div> + </div> + + + <!-- Apply ARIA attributes after the sidebar and the sidebar toggle button are added to the DOM --> + <script type="text/javascript"> + document.getElementById('sidebar-toggle').setAttribute('aria-expanded', sidebar === 'visible'); + document.getElementById('sidebar').setAttribute('aria-hidden', sidebar !== 'visible'); + Array.from(document.querySelectorAll('#sidebar a')).forEach(function(link) { + link.setAttribute('tabIndex', sidebar === 'visible' ? 0 : -1); + }); + </script> + + <div id="content" class="content"> + <main> + <!-- Page table of contents --> + <div class="sidetoc"> + <nav class="pagetoc"></nav> + </div> + + <h1 id="url-previews"><a class="header" href="#url-previews">URL Previews</a></h1> +<p>Design notes on a URL previewing service for Matrix:</p> +<p>Options are:</p> +<ol> +<li>Have an AS which listens for URLs, downloads them, and inserts an event that describes their metadata.</li> +</ol> +<ul> +<li>Pros: +<ul> +<li>Decouples the implementation entirely from Synapse.</li> +<li>Uses existing Matrix events & content repo to store the metadata.</li> +</ul> +</li> +<li>Cons: +<ul> +<li>Which AS should provide this service for a room, and why should you trust it?</li> +<li>Doesn't work well with E2E; you'd have to cut the AS into every room</li> +<li>the AS would end up subscribing to every room anyway.</li> +</ul> +</li> +</ul> +<ol start="2"> +<li>Have a generic preview API (nothing to do with Matrix) that provides a previewing service:</li> +</ol> +<ul> +<li>Pros: +<ul> +<li>Simple and flexible; can be used by any clients at any point</li> +</ul> +</li> +<li>Cons: +<ul> +<li>If each HS provides one of these independently, all the HSes in a room may needlessly DoS the target URI</li> +<li>We need somewhere to store the URL metadata rather than just using Matrix itself</li> +<li>We can't piggyback on matrix to distribute the metadata between HSes.</li> +</ul> +</li> +</ul> +<ol start="3"> +<li>Make the synapse of the sending user responsible for spidering the URL and inserting an event asynchronously which describes the metadata.</li> +</ol> +<ul> +<li>Pros: +<ul> +<li>Works transparently for all clients</li> +<li>Piggy-backs nicely on using Matrix for distributing the metadata.</li> +<li>No confusion as to which AS</li> +</ul> +</li> +<li>Cons: +<ul> +<li>Doesn't work with E2E</li> +<li>We might want to decouple the implementation of the spider from the HS, given spider behaviour can be quite complicated and evolve much more rapidly than the HS. It's more like a bot than a core part of the server.</li> +</ul> +</li> +</ul> +<ol start="4"> +<li>Make the sending client use the preview API and insert the event itself when successful.</li> +</ol> +<ul> +<li>Pros: +<ul> +<li>Works well with E2E</li> +<li>No custom server functionality</li> +<li>Lets the client customise the preview that they send (like on FB)</li> +</ul> +</li> +<li>Cons: +<ul> +<li>Entirely specific to the sending client, whereas it'd be nice if /any/ URL was correctly previewed if clients support it.</li> +</ul> +</li> +</ul> +<ol start="5"> +<li>Have the option of specifying a shared (centralised) previewing service used by a room, to avoid all the different HSes in the room DoSing the target.</li> +</ol> +<p>Best solution is probably a combination of both 2 and 4.</p> +<ul> +<li>Sending clients do their best to create and send a preview at the point of sending the message, perhaps delaying the message until the preview is computed? (This also lets the user validate the preview before sending)</li> +<li>Receiving clients have the option of going and creating their own preview if one doesn't arrive soon enough (or if the original sender didn't create one)</li> +</ul> +<p>This is a bit magical though in that the preview could come from two entirely different sources - the sending HS or your local one. However, this can always be exposed to users: "Generate your own URL previews if none are available?"</p> +<p>This is tantamount also to senders calculating their own thumbnails for sending in advance of the main content - we are trusting the sender not to lie about the content in the thumbnail. Whereas currently thumbnails are calculated by the receiving homeserver to avoid this attack.</p> +<p>However, this kind of phishing attack does exist whether we let senders pick their thumbnails or not, in that a malicious sender can send normal text messages around the attachment claiming it to be legitimate. We could rely on (future) reputation/abuse management to punish users who phish (be it with bogus metadata or bogus descriptions). Bogus metadata is particularly bad though, especially if it's avoidable.</p> +<p>As a first cut, let's do #2 and have the receiver hit the API to calculate its own previews (as it does currently for image thumbnails). We can then extend/optimise this to option 4 as a special extra if needed.</p> +<h2 id="api"><a class="header" href="#api">API</a></h2> +<pre><code>GET /_matrix/media/r0/preview_url?url=http://wherever.com +200 OK +{ + "og:type" : "article" + "og:url" : "https://twitter.com/matrixdotorg/status/684074366691356672" + "og:title" : "Matrix on Twitter" + "og:image" : "https://pbs.twimg.com/profile_images/500400952029888512/yI0qtFi7_400x400.png" + "og:description" : "“Synapse 0.12 is out! Lots of polishing, performance &amp;amp; bugfixes: /sync API, /r0 prefix, fulltext search, 3PID invites https://t.co/5alhXLLEGP”" + "og:site_name" : "Twitter" +} +</code></pre> +<ul> +<li>Downloads the URL +<ul> +<li>If HTML, just stores it in RAM and parses it for OG meta tags +<ul> +<li>Download any media OG meta tags to the media repo, and refer to them in the OG via mxc:// URIs.</li> +</ul> +</li> +<li>If a media filetype we know we can thumbnail: store it on disk, and hand it to the thumbnailer. Generate OG meta tags from the thumbnailer contents.</li> +<li>Otherwise, don't bother downloading further.</li> +</ul> +</li> +</ul> + + </main> + + <nav class="nav-wrapper" aria-label="Page navigation"> + <!-- Mobile navigation buttons --> + + <a rel="prev" href="consent_tracking.html" class="mobile-nav-chapters previous" title="Previous chapter" aria-label="Previous chapter" aria-keyshortcuts="Left"> + <i class="fa fa-angle-left"></i> + </a> + + + + <a rel="next" href="user_directory.html" class="mobile-nav-chapters next" title="Next chapter" aria-label="Next chapter" aria-keyshortcuts="Right"> + <i class="fa fa-angle-right"></i> + </a> + + + <div style="clear: both"></div> + </nav> + </div> + </div> + + <nav class="nav-wide-wrapper" aria-label="Page navigation"> + + <a rel="prev" href="consent_tracking.html" class="nav-chapters previous" title="Previous chapter" aria-label="Previous chapter" aria-keyshortcuts="Left"> + <i class="fa fa-angle-left"></i> + </a> + + + + <a rel="next" href="user_directory.html" class="nav-chapters next" title="Next chapter" aria-label="Next chapter" aria-keyshortcuts="Right"> + <i class="fa fa-angle-right"></i> + </a> + + </nav> + + </div> + + + + + + + + + <script type="text/javascript"> + window.playground_copyable = true; + </script> + + + + + + <script src="elasticlunr.min.js" type="text/javascript" charset="utf-8"></script> + <script src="mark.min.js" type="text/javascript" charset="utf-8"></script> + <script src="searcher.js" type="text/javascript" charset="utf-8"></script> + + + <script src="clipboard.min.js" type="text/javascript" charset="utf-8"></script> + <script src="highlight.js" type="text/javascript" charset="utf-8"></script> + <script src="book.js" type="text/javascript" charset="utf-8"></script> + + <!-- Custom JS scripts --> + + <script type="text/javascript" src="docs/website_files/table-of-contents.js"></script> + + + + + </body> +</html> \ No newline at end of file |