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<title>Nine Nines: Using cookies</title>
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<h1 class="lined-header"><span>Using cookies</span></h1>
<div class="paragraph"><p>Cookies are a mechanism allowing applications to maintain
state on top of the stateless HTTP protocol.</p></div>
<div class="paragraph"><p>Cookies are a name/value store where the names and values are
stored in plain text. They expire either after a delay
or when the browser closes. They can be configured on a
specific domain name or path, and restricted to secure
resources (sent or downloaded over HTTPS), or restricted
to the server (disallowing access from client-side scripts).</p></div>
<div class="paragraph"><p>Cookie names are de facto case sensitive.</p></div>
<div class="paragraph"><p>Cookies are stored client-side and sent with every subsequent
request that matches the domain and path for which they were
stored, until they expire. This can create a non-negligible
cost.</p></div>
<div class="paragraph"><p>Cookies should not be considered secure. They are stored on
the user’s computer in plain text, and can be read by any
program. They can also be read by proxies when using clear
connections. Always validate the value before using it,
and never store any sensitive information inside it.</p></div>
<div class="paragraph"><p>Cookies set by the server are only available in requests
following the client reception of the response containing
them.</p></div>
<div class="paragraph"><p>Cookies may be sent repeatedly. This is often useful to
update the expiration time and avoid losing a cookie.</p></div>
<div class="sect1">
<h2 id="_setting_cookies">Setting cookies</h2>
<div class="sectionbody">
<div class="paragraph"><p>By default cookies are defined for the duration of the session:</p></div>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt><span style="color: #009900">SessionID</span> <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #000000">generate_session_id</span></span>(),
<span style="color: #009900">Req</span> <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #000000">cowboy_req:set_resp_cookie</span></span>(<span style="color: #990000"><<</span><span style="color: #FF0000">"sessionid"</span><span style="color: #990000">>></span>, <span style="color: #009900">SessionID</span>, <span style="color: #009900">Req0</span>)<span style="color: #990000">.</span></tt></pre></div></div>
<div class="paragraph"><p>They can also be set for a duration in seconds:</p></div>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt><span style="color: #009900">SessionID</span> <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #000000">generate_session_id</span></span>(),
<span style="color: #009900">Req</span> <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #000000">cowboy_req:set_resp_cookie</span></span>(<span style="color: #990000"><<</span><span style="color: #FF0000">"sessionid"</span><span style="color: #990000">>></span>, <span style="color: #009900">SessionID</span>, <span style="color: #009900">Req0</span>,
#{<span style="color: #0000FF">max_age</span> <span style="color: #990000">=></span> <span style="color: #993399">3600</span>})<span style="color: #990000">.</span></tt></pre></div></div>
<div class="paragraph"><p>To delete cookies, set <code>max_age</code> to 0:</p></div>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt><span style="color: #009900">SessionID</span> <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #000000">generate_session_id</span></span>(),
<span style="color: #009900">Req</span> <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #000000">cowboy_req:set_resp_cookie</span></span>(<span style="color: #990000"><<</span><span style="color: #FF0000">"sessionid"</span><span style="color: #990000">>></span>, <span style="color: #009900">SessionID</span>, <span style="color: #009900">Req0</span>,
#{<span style="color: #0000FF">max_age</span> <span style="color: #990000">=></span> <span style="color: #993399">0</span>})<span style="color: #990000">.</span></tt></pre></div></div>
<div class="paragraph"><p>To restrict cookies to a specific domain and path, the options
of the same name can be used:</p></div>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt><span style="color: #009900">Req</span> <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #000000">cowboy_req:set_resp_cookie</span></span>(<span style="color: #990000"><<</span><span style="color: #FF0000">"inaccount"</span><span style="color: #990000">>></span>, <span style="color: #990000"><<</span><span style="color: #FF0000">"1"</span><span style="color: #990000">>></span>, <span style="color: #009900">Req0</span>,
#{<span style="color: #0000FF">domain</span> <span style="color: #990000">=></span> <span style="color: #FF0000">"my.example.org"</span>, <span style="color: #0000FF">path</span> <span style="color: #990000">=></span> <span style="color: #FF0000">"/account"</span>})<span style="color: #990000">.</span></tt></pre></div></div>
<div class="paragraph"><p>Cookies will be sent with requests to this domain and all
its subdomains, and to resources on this path or deeper
in the path hierarchy.</p></div>
<div class="paragraph"><p>To restrict cookies to secure channels (typically resources
available over HTTPS):</p></div>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt><span style="color: #009900">SessionID</span> <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #000000">generate_session_id</span></span>(),
<span style="color: #009900">Req</span> <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #000000">cowboy_req:set_resp_cookie</span></span>(<span style="color: #990000"><<</span><span style="color: #FF0000">"sessionid"</span><span style="color: #990000">>></span>, <span style="color: #009900">SessionID</span>, <span style="color: #009900">Req0</span>,
#{<span style="color: #0000FF">secure</span> <span style="color: #990000">=></span> <span style="color: #000080">true</span>})<span style="color: #990000">.</span></tt></pre></div></div>
<div class="paragraph"><p>To prevent client-side scripts from accessing a cookie:</p></div>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt><span style="color: #009900">SessionID</span> <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #000000">generate_session_id</span></span>(),
<span style="color: #009900">Req</span> <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #000000">cowboy_req:set_resp_cookie</span></span>(<span style="color: #990000"><<</span><span style="color: #FF0000">"sessionid"</span><span style="color: #990000">>></span>, <span style="color: #009900">SessionID</span>, <span style="color: #009900">Req0</span>,
#{<span style="color: #0000FF">http_only</span> <span style="color: #990000">=></span> <span style="color: #000080">true</span>})<span style="color: #990000">.</span></tt></pre></div></div>
<div class="paragraph"><p>Cookies may also be set client-side, for example using
Javascript.</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_reading_cookies">Reading cookies</h2>
<div class="sectionbody">
<div class="paragraph"><p>The client only ever sends back the cookie name and value.
All other options that can be set are never sent back.</p></div>
<div class="paragraph"><p>Cowboy provides two functions for reading cookies. Both
involve parsing the cookie header(s) and so should not
be called repeatedly.</p></div>
<div class="paragraph"><p>You can get all cookies as a key/value list:</p></div>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt><span style="color: #009900">Cookies</span> <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #000000">cowboy_req:parse_cookies</span></span>(<span style="color: #009900">Req</span>),
{<span style="color: #990000">_</span>, <span style="color: #009900">Lang</span>} <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #000000">lists:keyfind</span></span>(<span style="color: #990000"><<</span><span style="color: #FF0000">"lang"</span><span style="color: #990000">>></span>, <span style="color: #993399">1</span>, <span style="color: #009900">Cookies</span>)<span style="color: #990000">.</span></tt></pre></div></div>
<div class="paragraph"><p>Or you can perform a match against cookies and retrieve
only the ones you need, while at the same time doing
any required post processing using <a href="../constraints">constraints</a>.
This function returns a map:</p></div>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt>#{<span style="color: #FF6600">id</span> <span style="color: #990000">:=</span> <span style="color: #009900">ID</span>, <span style="color: #FF6600">lang</span> <span style="color: #990000">:=</span> <span style="color: #009900">Lang</span>} <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #000000">cowboy_req:match_cookies</span></span>([<span style="color: #FF6600">id</span>, <span style="color: #FF6600">lang</span>], <span style="color: #009900">Req</span>)<span style="color: #990000">.</span></tt></pre></div></div>
<div class="paragraph"><p>You can use constraints to validate the values while matching
them. The following snippet will crash if the <code>id</code> cookie is
not an integer number or if the <code>lang</code> cookie is empty. Additionally
the <code>id</code> cookie value will be converted to an integer term:</p></div>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt><span style="color: #009900">CookiesMap</span> <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #000000">cowboy_req:match_cookies</span></span>([{<span style="color: #FF6600">id</span>, <span style="color: #FF6600">int</span>}, {<span style="color: #FF6600">lang</span>, <span style="color: #FF6600">nonempty</span>}], <span style="color: #009900">Req</span>)<span style="color: #990000">.</span></tt></pre></div></div>
<div class="paragraph"><p>Note that if two cookies share the same name, then the map value
will be a list of the two cookie values.</p></div>
<div class="paragraph"><p>A default value can be provided. The default will be used
if the <code>lang</code> cookie is not found. It will not be used if
the cookie is found but has an empty value:</p></div>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt>#{<span style="color: #FF6600">lang</span> <span style="color: #990000">:=</span> <span style="color: #009900">Lang</span>} <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #000000">cowboy_req:match_cookies</span></span>([{<span style="color: #FF6600">lang</span>, [], <span style="color: #990000"><<</span><span style="color: #FF0000">"en-US"</span><span style="color: #990000">>></span>}], <span style="color: #009900">Req</span>)<span style="color: #990000">.</span></tt></pre></div></div>
<div class="paragraph"><p>If no default is provided and the value is missing, an
exception is thrown.</p></div>
</div>
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