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<ol class="chapter"><li class="chapter-item expanded affix "><li class="part-title">Introduction</li><li class="chapter-item expanded "><a href="../welcome_and_overview.html">Welcome and Overview</a></li><li class="chapter-item expanded affix "><li class="part-title">Setup</li><li class="chapter-item expanded "><a href="../setup/installation.html">Installation</a></li><li class="chapter-item expanded "><a href="../postgres.html">Using Postgres</a></li><li class="chapter-item expanded "><a href="../reverse_proxy.html">Configuring a Reverse Proxy</a></li><li class="chapter-item expanded "><a href="../setup/forward_proxy.html">Configuring a Forward/Outbound Proxy</a></li><li class="chapter-item expanded "><a href="../turn-howto.html">Configuring a Turn Server</a></li><li class="chapter-item expanded "><a href="../delegate.html">Delegation</a></li><li class="chapter-item expanded affix "><li class="part-title">Upgrading</li><li class="chapter-item expanded "><a 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Notices</a></li><li class="chapter-item expanded "><a href="../consent_tracking.html">Consent Tracking</a></li><li class="chapter-item expanded "><a href="../development/url_previews.html" class="active">URL Previews</a></li><li class="chapter-item expanded "><a href="../user_directory.html">User Directory</a></li><li class="chapter-item expanded "><a href="../message_retention_policies.html">Message Retention Policies</a></li><li class="chapter-item expanded "><a href="../modules/index.html">Pluggable Modules</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../modules/writing_a_module.html">Writing a module</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../modules/spam_checker_callbacks.html">Spam checker callbacks</a></li><li class="chapter-item expanded "><a href="../modules/third_party_rules_callbacks.html">Third-party rules callbacks</a></li><li class="chapter-item expanded "><a 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<h1 id="url-previews"><a class="header" href="#url-previews">URL Previews</a></h1>
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<p>The <code>GET /_matrix/media/r0/preview_url</code> endpoint provides a generic preview API
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for URLs which outputs <a href="https://ogp.me/">Open Graph</a> responses (with some Matrix
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specific additions).</p>
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<p>This does have trade-offs compared to other designs:</p>
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<ul>
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<li>Pros:
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<ul>
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<li>Simple and flexible; can be used by any clients at any point</li>
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</ul>
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</li>
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<li>Cons:
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<ul>
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<li>If each homeserver provides one of these independently, all the HSes in a
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room may needlessly DoS the target URI</li>
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<li>The URL metadata must be stored somewhere, rather than just using Matrix
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itself to store the media.</li>
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<li>Matrix cannot be used to distribute the metadata between homeservers.</li>
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</ul>
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</li>
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</ul>
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<p>When Synapse is asked to preview a URL it does the following:</p>
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<ol>
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<li>Checks against a URL blacklist (defined as <code>url_preview_url_blacklist</code> in the
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config).</li>
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<li>Checks the in-memory cache by URLs and returns the result if it exists. (This
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is also used to de-duplicate processing of multiple in-flight requests at once.)</li>
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<li>Kicks off a background process to generate a preview:
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<ol>
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<li>Checks the database cache by URL and timestamp and returns the result if it
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has not expired and was successful (a 2xx return code).</li>
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<li>Checks if the URL matches an oEmbed pattern. If it does, fetch the oEmbed
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response. If this is an image, replace the URL to fetch and continue. If
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if it is HTML content, use the HTML as the document and continue.</li>
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<li>If it doesn't match an oEmbed pattern, downloads the URL and stores it
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into a file via the media storage provider and saves the local media
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metadata.</li>
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<li>If the media is an image:
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<ol>
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<li>Generates thumbnails.</li>
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<li>Generates an Open Graph response based on image properties.</li>
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</ol>
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</li>
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<li>If the media is HTML:
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<ol>
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<li>Decodes the HTML via the stored file.</li>
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<li>Generates an Open Graph response from the HTML.</li>
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<li>If an image exists in the Open Graph response:
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<ol>
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<li>Downloads the URL and stores it into a file via the media storage
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provider and saves the local media metadata.</li>
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<li>Generates thumbnails.</li>
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<li>Updates the Open Graph response based on image properties.</li>
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</ol>
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</li>
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</ol>
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</li>
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<li>Stores the result in the database cache.</li>
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</ol>
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</li>
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<li>Returns the result.</li>
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</ol>
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<p>The in-memory cache expires after 1 hour.</p>
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<p>Expired entries in the database cache (and their associated media files) are
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deleted every 10 seconds. The default expiration time is 1 hour from download.</p>
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