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[apache] / docs / manual / mod / mod_session.xml

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<modulesynopsis metafile="mod_session.xml.meta">

<name>mod_session</name>
<description>Session support</description>
<status>Extension</status>
<sourcefile>mod_session.c</sourcefile>
<identifier>session_module</identifier>
<compatibility>Available in Apache 2.3 and later</compatibility>

<summary>
    <note type="warning"><title>Warning</title>
      <p>The session modules make use of HTTP cookies, and as such can fall
      victim to Cross Site Scripting attacks, or expose potentially private
      information to clients. Please ensure that the relevant risks have
      been taken into account before enabling the session functionality on
      your server.</p>
    </note>

    <p>This module provides support for a server wide per user session
    interface. Sessions can be used for keeping track of whether a user
    has been logged in, or for other per user information that should
    be kept available across requests.</p>
    
    <p>Sessions may be stored on the server, or may be stored on the
    browser. Sessions may also be optionally encrypted for added security.
    These features are divided into several modules in addition to
    <module>mod_session</module>; <module>mod_session_crypto</module>,
    <module>mod_session_cookie</module> and <module>mod_session_dbd</module>.
    Depending on the server requirements, load the appropriate modules
    into the server (either statically at compile time or dynamically
    via the <directive module="mod_so">LoadModule</directive> directive).</p>

    <p>Sessions may be manipulated from other modules that depend on the
    session, or the session may be read from and written to using
    environment variables and HTTP headers, as appropriate.</p>
    
</summary>
<seealso><module>mod_session_cookie</module></seealso>
<seealso><module>mod_session_crypto</module></seealso>
<seealso><module>mod_session_dbd</module></seealso>

    <section id="whatisasession"><title>What is a session?</title>
      <p>At the core of the session interface is a table of key and value pairs
      that are made accessible across browser requests.</p>
      
      <p>These pairs can be set to any valid string, as needed by the
      application making use of the session.</p>
      
    </section>
    <section id="whocanuseasession"><title>Who can use a session?</title>
      <p>The session interface is primarily developed for the use by other
      server modules, such as <module>mod_auth_form</module>, however CGI
      based applications can optionally be granted access to the contents
      of the session via the HTTP_SESSION environment variable. Sessions
      have the option to be modified and/or updated by inserting an HTTP
      response header containing the new session parameters.</p>

    </section>
    <section id="serversession"><title>Keeping sessions on the server</title>
      <p>Apache can be configured to keep track of per user sessions stored
      on a particular server or group of servers. This functionality is
      similar to the sessions available in typical application servers.</p>
      
      <p>If configured, sessions are tracked through the use of a session ID that
      is stored inside a cookie, or extracted from the parameters embedded
      within the URL query string, as found in a typical GET request.</p>
      
      <p>As the contents of the session are stored exclusively on the server,
      there is an expectation of privacy of the contents of the session. This
      does have performance and resource implications should a large number
      of sessions be present, or where a large number of webservers have to
      share sessions with one another.</p>
      
      <p>The <module>mod_session_dbd</module> module allows the storage of user
      sessions within a SQL database via <module>mod_dbd</module>.</p>

    </section> <!-- /serversession -->
    
    <section id="browsersession"><title>Keeping sessions on the browser</title>
      <p>Where keeping track of a session on a server is too resource
      intensive or inconvenient, the option exists to store the contents
      of the session within a cookie on the client browser instead.</p>
      
      <p>This has the advantage that minimal resources are required on the
      server to keep track of sessions, and multiple servers within a server
      farm have no need to share session information.</p>
      
      <p>The contents of the session however are exposed to the client, with a
      corresponding risk of a loss of privacy. The
      <module>mod_session_crypto</module> module can be configured to encrypt the
      contents of the session before writing the session to the client.</p>

      <p>The <module>mod_session_cookie</module> allows the storage of user
      sessions on the browser within an HTTP cookie.</p>

    </section> <!-- /browsersession -->

    <section id="basicexamples"><title>Basic Examples</title>
    
      <p>Creating a session is as simple as turning the session on, and deciding
      where the session will be stored. In this example, the session will be
      stored on the browser, in a cookie called <code>session</code>.</p>
      
      <example><title>Browser based session</title>
        Session On<br />
        SessionCookieName session path=/<br />
      </example>

      <p>The session is not useful unless it can be written to or read from. The
      following example shows how values can be injected into the session through
      the use of a predetermined HTTP response header called
      <code>X-Replace-Session</code>.</p>
      
      <example><title>Writing to a session</title>
        Session On<br />
        SessionCookieName session path=/<br />
        SessionHeader X-Replace-Session<br />
      </example>

      <p>The header should contain name value pairs expressed in the same format
      as a query string in a URL, as in the example below. Setting a key to the
      empty string has the effect of removing that key from the session.</p>
      
      <example><title>CGI to write to a session</title>
        #!/bin/bash<br />
        echo "Content-Type: text/plain"<br />
        echo "X-Replace-Session: key1=foo&amp;key2=&amp;key3=bar"<br />
        echo<br />
        env<br />
      </example>

      <p>If configured, the session can be read back from the HTTP_SESSION
      environment variable. By default, the session is kept private, so this
      has to be explicitly turned on with the
      <directive module="mod_session">SessionEnv</directive> directive.</p>
      
      <example><title>Read from a session</title>
        Session On<br />
        SessionEnv On<br />
        SessionCookieName session path=/<br />
        SessionHeader X-Replace-Session<br />
      </example>

      <p>Once read, the CGI variable <code>HTTP_SESSION</code> should contain
      the value <code>key1=foo&amp;key3=bar</code>.</p>

    </section>
    <section id="sessionprivacy"><title>Session Privacy</title>
    
      <p>Using the "show cookies" feature of your browser, you would have seen
      a clear text representation of the session. This could potentially be a
      problem should the end user need to be kept unaware of the contents of
      the session, or where a third party could gain unauthorised access to the
      data within the session.</p>
      
      <p>The contents of the session can be optionally encrypted before being
      placed on the browser using the <module>mod_session_crypto</module>
      module.</p>
      
      <example><title>Browser based encrypted session</title>
        Session On<br />
        SessionCryptoPassphrase secret<br />
        SessionCookieName session path=/<br />
      </example>
      
      <p>The session will be automatically decrypted on load, and encrypted on
      save by Apache, the underlying application using the session need have
      no knowledge that encryption is taking place.</p>
      
      <p>Sessions stored on the server rather than on the browser can also be
      encrypted as needed, offering privacy where potentially sensitive
      information is being shared between webservers in a server farm using
      the <module>mod_session_dbd</module> module.</p>
      
    </section>
    <section id="cookieprivacy"><title>Cookie Privacy</title>

      <p>The HTTP cookie mechanism also offers privacy features, such as the
      ability to restrict cookie transport to SSL protected pages only, or
      to prevent browser based javascript from gaining access to the contents
      of the cookie.</p>
      
      <note type="warning"><title>Warning</title>
      <p>Some of the HTTP cookie privacy features are either non-standard, or
      are not implemented consistently across browsers. The session modules
      allow you to set cookie parameters, but it makes no guarantee that privacy
      will be respected by the browser. If security is a concern, use the
      <module>mod_session_crypto</module> to encrypt the contents of the session,
      or store the session on the server using the <module>mod_session_dbd</module>
      module.</p>
      </note>

      <p>Standard cookie parameters can be specified after the name of the cookie,
      as in the example below.</p>
      
      <example><title>Setting cookie parameters</title>
        Session On<br />
        SessionCryptoPassphrase secret<br />
        SessionCookieName session path=/private;domain=example.com;httponly;secure;<br />
      </example>
      
      <p>In cases where the Apache server forms the frontend for backend origin servers,
      it is possible to have the session cookies removed from the incoming HTTP headers using
      the <directive module="mod_session_cookie">SessionCookieRemove</directive> directive.
      This keeps the contents of the session cookies from becoming accessible from the
      backend server.
      </p>

    </section>
    <section id="authentication"><title>Session Support for Authentication</title>

      <p>As is possible within many application servers, authentication modules can use
      a session for storing the username and password after login. The
      <module>mod_auth_form</module> saves the user's login name and password within
      the session.</p>

      <example><title>Form based authentication</title>
        Session On<br />
        SessionCryptoPassphrase secret<br />
        SessionCookieName session path=/<br />
        AuthFormProvider file<br />
        AuthUserFile conf/passwd<br />
        AuthType form<br />
        AuthName realm<br />
        ...<br />
      </example>
      
      <p>See the <module>mod_auth_form</module> module for documentation and complete
      examples.</p>

    </section>

<directivesynopsis>
<name>Session</name>
<description>Enables a session for the current directory or location</description>
<syntax>Session On|Off</syntax>
<default>Session Off</default>
<contextlist><context>server config</context>
<context>virtual host</context>
<context>directory</context>
<context>.htaccess</context>
</contextlist>

<usage>
    <p>The <directive>Session</directive> directive enables a session for the
    directory or location container. Further directives control where the
    session will be stored and how privacy is maintained.</p>
</usage>
</directivesynopsis>

<directivesynopsis>
<name>SessionMaxAge</name>
<description>Define a maximum age in seconds for a session</description>
<syntax>SessionMaxAge <var>maxage</var></syntax>
<default>SessionMaxAge 0</default>
<contextlist><context>server config</context>
<context>virtual host</context>
<context>directory</context>
<context>.htaccess</context>
</contextlist>

<usage>
    <p>The <directive>SessionMaxAge</directive> directive defines a time limit
    for which a session will remain valid. When a session is saved, this time
    limit is reset and an existing session can be continued. If a session
    becomes older than this limit without a request to the server to refresh
    the session, the session will time out and be removed. Where a session is
    used to stored user login details, this has the effect of logging the user
    out automatically after the given time.</p>
    
    <p>Setting the maxage to zero disables session expiry.</p>
</usage>
</directivesynopsis>

<directivesynopsis>
<name>SessionEnv</name>
<description>Control whether the contents of the session are written to the
<var>HTTP_SESSION</var> environment variable</description>
<syntax>SessionEnv On|Off</syntax>
<default>SessionEnv Off</default>
<contextlist><context>server config</context>
<context>virtual host</context>
<context>directory</context>
<context>.htaccess</context>
</contextlist>

<usage>
    <p>If set to <var>On</var>, the <directive>SessionEnv</directive> directive
    causes the contents of the session to be written to a CGI environment
    variable called <var>HTTP_SESSION</var>.</p>
    
    <p>The string is written in the URL query format, for example:</p>

    <example>
      <code>key1=foo&amp;key3=bar</code>
    </example>

</usage>
</directivesynopsis>

<directivesynopsis>
<name>SessionHeader</name>
<description>Import session updates from a given HTTP response header</description>
<syntax>SessionHeader <var>header</var></syntax>
<default>none</default>
<contextlist><context>server config</context>
<context>virtual host</context>
<context>directory</context>
<context>.htaccess</context>
</contextlist>

<usage>
    <p>The <directive>SessionHeader</directive> directive defines the name of an
    HTTP response header which, if present, will be parsed and written to the
    current session.</p>
    
    <p>The header value is expected to be in the URL query format, for example:</p>

    <example>
      <code>key1=foo&amp;key2=&amp;key3=bar</code>
    </example>
    
    <p>Where a key is set to the empty string, that key will be removed from the
    session.</p>

</usage>
</directivesynopsis>

<directivesynopsis>
<name>SessionInclude</name>
<description>Define URL prefixes for which a session is valid</description>
<syntax>SessionInclude <var>path</var></syntax>
<default>all URLs</default>
<contextlist><context>server config</context>
<context>virtual host</context>
<context>directory</context>
<context>.htaccess</context>
</contextlist>

<usage>
    <p>The <directive>SessionInclude</directive> directive allows sessions to
    be made valid for specific URL prefixes only. This can be used to make a
    website more efficient, by targeting a more precise URL space for which
    a session should be maintained. By default, all URLs within the directory
    or location are included in the session.</p>
    
    <note type="warning"><title>Warning</title>
    <p>This directive has a similar purpose to the <var>path</var> attribute
    in HTTP cookies, but should not be confused with this attribute. This
    directive does not set the <var>path</var> attribute, which must be
    configured separately.</p></note>
</usage>
</directivesynopsis>

<directivesynopsis>
<name>SessionExclude</name>
<description>Define URL prefixes for which a session is ignored</description>
<syntax>SessionExclude <var>path</var></syntax>
<default>none</default>
<contextlist><context>server config</context>
<context>virtual host</context>
<context>directory</context>
<context>.htaccess</context>
</contextlist>

<usage>
    <p>The <directive>SessionExclude</directive> directive allows sessions to
    be disabled relative to URL prefixes only. This can be used to make a
    website more efficient, by targeting a more precise URL space for which
    a session should be maintained. By default, all URLs within the directory
    or location are included in the session. The
    <directive module="mod_session">SessionExclude</directive> directive takes
    precedence over the
    <directive module="mod_session">SessionInclude</directive> directive.</p>

    <note type="warning"><title>Warning</title>
    <p>This directive has a similar purpose to the <var>path</var> attribute
    in HTTP cookies, but should not be confused with this attribute. This
    directive does not set the <var>path</var> attribute, which must be
    configured separately.</p></note>
</usage>
</directivesynopsis>

</modulesynopsis>