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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 href="upgrade.html">Upgrading between Synapse Versions</a></li><li class="chapter-item expanded "><a href="MSC1711_certificates_FAQ.html">Upgrading from pre-Synapse 1.0</a></li><li class="chapter-item expanded affix "><li class="part-title">Usage</li><li class="chapter-item expanded "><a href="federate.html">Federation</a></li><li class="chapter-item expanded "><a href="usage/configuration/index.html">Configuration</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="usage/configuration/homeserver_sample_config.html">Homeserver Sample Config File</a></li><li class="chapter-item expanded "><a href="usage/configuration/logging_sample_config.html">Logging Sample Config File</a></li><li class="chapter-item expanded "><a href="structured_logging.html">Structured Logging</a></li><li class="chapter-item expanded "><a href="templates.html">Templates</a></li><li class="chapter-item expanded "><a href="usage/configuration/user_authentication/index.html">User Authentication</a></li><li><ol 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@@ -7261,6 +7261,128 @@ eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9.eyJzdWIiOiJ0ZXN0LXVzZXIifQ.Ag71GT8v01UO3w80
 </li>
 </ol>
 <p>You should now be able to use the returned access token to query the client API.</p>
+<div id="chapter_begin" style="break-before: page; page-break-before: always;"></div><h1 id="refresh-tokens"><a class="header" href="#refresh-tokens">Refresh Tokens</a></h1>
+<p>Synapse supports refresh tokens since version 1.49 (some earlier versions had support for an earlier, experimental draft of <a href="https://github.com/matrix-org/matrix-doc/blob/main/proposals/2918-refreshtokens.md#msc2918-refresh-tokens">MSC2918</a> which is not compatible).</p>
+<h2 id="background-and-motivation"><a class="header" href="#background-and-motivation">Background and motivation</a></h2>
+<p>Synapse users' sessions are identified by <strong>access tokens</strong>; access tokens are
+issued to users on login. Each session gets a unique access token which identifies
+it; the access token must be kept secret as it grants access to the user's account.</p>
+<p>Traditionally, these access tokens were eternally valid (at least until the user
+explicitly chose to log out).</p>
+<p>In some cases, it may be desirable for these access tokens to expire so that the
+potential damage caused by leaking an access token is reduced.
+On the other hand, forcing a user to re-authenticate (log in again) often might
+be too much of an inconvenience.</p>
+<p><strong>Refresh tokens</strong> are a mechanism to avoid some of this inconvenience whilst
+still getting most of the benefits of short access token lifetimes.
+Refresh tokens are also a concept present in OAuth 2 — further reading is available
+<a href="https://datatracker.ietf.org/doc/html/rfc6749#section-1.5">here</a>.</p>
+<p>When refresh tokens are in use, both an access token and a refresh token will be
+issued to users on login. The access token will expire after a predetermined amount
+of time, but otherwise works in the same way as before. When the access token is
+close to expiring (or has expired), the user's client should present the homeserver
+(Synapse) with the refresh token.</p>
+<p>The homeserver will then generate a new access token and refresh token for the user
+and return them. The old refresh token is invalidated and can not be used again*.</p>
+<p>Finally, refresh tokens also make it possible for sessions to be logged out if they
+are inactive for too long, before the session naturally ends; see the configuration
+guide below.</p>
+<p>*To prevent issues if clients lose connection half-way through refreshing a token,
+the refresh token is only invalidated once the new access token has been used at
+least once. For all intents and purposes, the above simplification is sufficient.</p>
+<h2 id="caveats"><a class="header" href="#caveats">Caveats</a></h2>
+<p>There are some caveats:</p>
+<ul>
+<li>If a third party gets both your access token and refresh token, they will be able to
+continue to enjoy access to your session.
+<ul>
+<li>This is still an improvement because you (the user) will notice when <em>your</em>
+session expires and you're not able to use your refresh token.
+That would be a giveaway that someone else has compromised your session.
+You would be able to log in again and terminate that session.
+Previously (with long-lived access tokens), a third party that has your access
+token could go undetected for a very long time.</li>
+</ul>
+</li>
+<li>Clients need to implement support for refresh tokens in order for them to be a
+useful mechanism.
+<ul>
+<li>It is up to homeserver administrators if they want to issue long-lived access
+tokens to clients not implementing refresh tokens.
+<ul>
+<li>For compatibility, it is likely that they should, at least until client support
+is widespread.
+<ul>
+<li>Users with clients that support refresh tokens will still benefit from the
+added security; it's not possible to downgrade a session to using long-lived
+access tokens so this effectively gives users the choice.</li>
+</ul>
+</li>
+<li>In a closed environment where all users use known clients, this may not be
+an issue as the homeserver administrator can know if the clients have refresh
+token support. In that case, the non-refreshable access token lifetime
+may be set to a short duration so that a similar level of security is provided.</li>
+</ul>
+</li>
+</ul>
+</li>
+</ul>
+<h2 id="configuration-guide"><a class="header" href="#configuration-guide">Configuration Guide</a></h2>
+<p>The following configuration options, in the <code>registration</code> section, are related:</p>
+<ul>
+<li><code>session_lifetime</code>: maximum length of a session, even if it's refreshed.
+In other words, the client must log in again after this time period.
+In most cases, this can be unset (infinite) or set to a long time (years or months).</li>
+<li><code>refreshable_access_token_lifetime</code>: lifetime of access tokens that are created
+by clients supporting refresh tokens.
+This should be short; a good value might be 5 minutes (<code>5m</code>).</li>
+<li><code>nonrefreshable_access_token_lifetime</code>: lifetime of access tokens that are created
+by clients which don't support refresh tokens.
+Make this short if you want to effectively force use of refresh tokens.
+Make this long if you don't want to inconvenience users of clients which don't
+support refresh tokens (by forcing them to frequently re-authenticate using
+login credentials).</li>
+<li><code>refresh_token_lifetime</code>: lifetime of refresh tokens.
+In other words, the client must refresh within this time period to maintain its session.
+Unless you want to log inactive sessions out, it is often fine to use a long
+value here or even leave it unset (infinite).
+Beware that making it too short will inconvenience clients that do not connect
+very often, including mobile clients and clients of infrequent users (by making
+it more difficult for them to refresh in time, which may force them to need to
+re-authenticate using login credentials).</li>
+</ul>
+<p><strong>Note:</strong> All four options above only apply when tokens are created (by logging in or refreshing).
+Changes to these settings do not apply retroactively.</p>
+<h3 id="using-refresh-token-expiry-to-log-out-inactive-sessions"><a class="header" href="#using-refresh-token-expiry-to-log-out-inactive-sessions">Using refresh token expiry to log out inactive sessions</a></h3>
+<p>If you'd like to force sessions to be logged out upon inactivity, you can enable
+refreshable access token expiry and refresh token expiry.</p>
+<p>This works because a client must refresh at least once within a period of
+<code>refresh_token_lifetime</code> in order to maintain valid credentials to access the
+account.</p>
+<p>(It's suggested that <code>refresh_token_lifetime</code> should be longer than
+<code>refreshable_access_token_lifetime</code> and this section assumes that to be the case
+for simplicity.)</p>
+<p>Note: this will only affect sessions using refresh tokens. You may wish to
+set a short <code>nonrefreshable_access_token_lifetime</code> to prevent this being bypassed
+by clients that do not support refresh tokens.</p>
+<h4 id="choosing-values-that-guarantee-permitting-some-inactivity"><a class="header" href="#choosing-values-that-guarantee-permitting-some-inactivity">Choosing values that guarantee permitting some inactivity</a></h4>
+<p>It may be desirable to permit some short periods of inactivity, for example to
+accommodate brief outages in client connectivity.</p>
+<p>The following model aims to provide guidance for choosing <code>refresh_token_lifetime</code>
+and <code>refreshable_access_token_lifetime</code> to satisfy requirements of the form:</p>
+<ol>
+<li>inactivity longer than <code>L</code> <strong>MUST</strong> cause the session to be logged out; and</li>
+<li>inactivity shorter than <code>S</code> <strong>MUST NOT</strong> cause the session to be logged out.</li>
+</ol>
+<p>This model makes the weakest assumption that all active clients will refresh as
+needed to maintain an active access token, but no sooner.
+<em>In reality, clients may refresh more often than this model assumes, but the
+above requirements will still hold.</em></p>
+<p>To satisfy the above model,</p>
+<ul>
+<li><code>refresh_token_lifetime</code> should be set to <code>L</code>; and</li>
+<li><code>refreshable_access_token_lifetime</code> should be set to <code>L - S</code>.</li>
+</ul>
 <div id="chapter_begin" style="break-before: page; page-break-before: always;"></div><h1 id="overview-2"><a class="header" href="#overview-2">Overview</a></h1>
 <p>A captcha can be enabled on your homeserver to help prevent bots from registering
 accounts. Synapse currently uses Google's reCAPTCHA service which requires API keys