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synapse
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                        <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 &amp; 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: &quot;Generate your own URL previews if none are available?&quot;</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
{
    &quot;og:type&quot;        : &quot;article&quot;
    &quot;og:url&quot;         : &quot;https://twitter.com/matrixdotorg/status/684074366691356672&quot;
    &quot;og:title&quot;       : &quot;Matrix on Twitter&quot;
    &quot;og:image&quot;       : &quot;https://pbs.twimg.com/profile_images/500400952029888512/yI0qtFi7_400x400.png&quot;
    &quot;og:description&quot; : &quot;“Synapse 0.12 is out! Lots of polishing, performance &amp;amp;amp; bugfixes: /sync API, /r0 prefix, fulltext search, 3PID invites https://t.co/5alhXLLEGP”&quot;
    &quot;og:site_name&quot;   : &quot;Twitter&quot;
}
</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>

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