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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. These pairs can be set

to any valid string, as needed by the application making use of the

session.</p>

<p>The "session" is a <strong>application/x-www-form-urlencoded</strong>

string containing these key value pairs, as defined by the

<a href="http://www.w3.org/TR/html4/">HTML specification</a>.</p>

<p>The session can optionally be encrypted and base64 encoded before

being written to the storage mechanism, as defined by the

administrator.</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>In high traffic environments 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>

<highlight language="config">

Session On

SessionCookieName session path=/

</highlight>

</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>

<highlight language="config">

Session On

SessionCookieName session path=/

SessionHeader X-Replace-Session

</highlight>

</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>

<highlight language="sh">

#!/bin/bash

echo "Content-Type: text/plain"

echo "X-Replace-Session: key1=foo&amp;key2=&amp;key3=bar"

echo

env

</highlight>

</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>

<highlight language="config">

Session On

SessionEnv On

SessionCookieName session path=/

SessionHeader X-Replace-Session

</highlight>

</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>

<highlight language="config">

Session On

SessionCryptoPassphrase secret

SessionCookieName session path=/

</highlight>

</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>

<highlight language="config">

Session On

SessionCryptoPassphrase secret

SessionCookieName session path=/private;domain=example.com;httponly;secure;

</highlight>

</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>

<highlight language="config">

Session On

SessionCryptoPassphrase secret

SessionCookieName session path=/

AuthFormProvider file

AuthUserFile "conf/passwd"

AuthType form

AuthName "realm"

#...

</highlight>

</example>

<p>See the <module>mod_auth_form</module> module for documentation and complete

examples.</p>

</section>

<section id="integration"><title>Integrating Sessions with External Applications</title>

<p>In order for sessions to be useful, it must be possible to share the contents

of a session with external applications, and it must be possible for an

external application to write a session of its own.</p>

<p> A typical example might be an application that changes a user's password set by

<module>mod_auth_form</module>. This application would need to read the current

username and password from the session, make the required changes to the user's

password, and then write the new password to the session in order to provide a

seamless transition to the new password.</p>

<p>A second example might involve an application that registers a new user for

the first time. When registration is complete, the username and password is

written to the session, providing a seamless transition to being logged in.</p>

<dl>

<dt>Apache modules</dt>

<dd>Modules within the server that need access to the session can use the

<strong>mod_session.h</strong> API in order to read from and write to the

session. This mechanism is used by modules like <module>mod_auth_form</module>.

</dd>

<dt>CGI programs and scripting languages</dt>

<dd>Applications that run within the webserver can optionally retrieve the

value of the session from the <strong>HTTP_SESSION</strong> environment

variable. The session should be encoded as a

<strong>application/x-www-form-urlencoded</strong> string as described by the

<a href="http://www.w3.org/TR/html4/">HTML specification</a>. The environment

variable is controlled by the setting of the

<directive module="mod_session">SessionEnv</directive> directive. The session

can be written to by the script by returning a

<strong>application/x-www-form-urlencoded</strong> response header with a name

set by the <directive module="mod_session">SessionHeader</directive>

directive. In both cases, any encryption or decryption, and the reading the

session from or writing the session to the chosen storage mechanism is handled

by the <module>mod_session</module> modules and corresponding configuration.

</dd>

<dt>Applications behind <module>mod_proxy</module></dt>

<dd>If the <directive module="mod_session">SessionHeader</directive>

directive is used to define an HTTP request header, the session, encoded as

a <strong>application/x-www-form-urlencoded</strong> string, will be made

available to the application. If the same header is provided in the response,

the value of this response header will be used to replace the session. As

above, any encryption or decryption, and the reading the session from or

writing the session to the chosen storage mechanism is handled by the

<module>mod_session</module> modules and corresponding configuration.</dd>

<dt>Standalone applications</dt>

<dd>Applications might choose to manipulate the session outside the control

of the Apache HTTP server. In this case, it is the responsibility of the

application to read the session from the chosen storage mechanism,

decrypt the session, update the session, encrypt the session and write

the session to the chosen storage mechanism, as appropriate.</dd>

</dl>

</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>

<override>AuthConfig</override>

<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>

<override>AuthConfig</override>

<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>

<override>AuthConfig</override>

<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>

<override>AuthConfig</override>

<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>

<override>AuthConfig</override>

<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>