path: root/doc/man/pam.8.xml

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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
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<refentry id='pam'>
<refmeta>
<refentrytitle>pam</refentrytitle>
<manvolnum>8</manvolnum>
<refmiscinfo class='setdesc'>Linux-PAM Manual</refmiscinfo>
</refmeta>
<refnamediv id='name'>
<refname>pam</refname>
<refpurpose>Pluggable Authentication Modules for Linux</refpurpose>
</refnamediv>
<!-- body begins here -->

<refsect1 id='description'><title>DESCRIPTION</title>
<para>This manual is intended to offer a quick introduction to
<emphasis remap='B'>Linux-PAM</emphasis>. 
For more information the reader is directed to the
<emphasis remap='B'>Linux-PAM system administrators' guide</emphasis>.</para>


<para><emphasis remap='B'>Linux-PAM</emphasis>
Is a system of libraries that handle the authentication tasks of
applications (services) on the system.  The library provides a stable
general interface (Application Programming Interface - API) that
privilege granting programs (such as
<citerefentry><refentrytitle>login</refentrytitle><manvolnum>1</manvolnum></citerefentry> 
and
<citerefentry><refentrytitle>su</refentrytitle><manvolnum>1</manvolnum></citerefentry>) 
defer to to perform standard authentication tasks.</para>


<para>The principal feature of the PAM approach is that the nature of the
authentication is dynamically configurable.  In other words, the
system administrator is free to choose how individual
service-providing applications will authenticate users. This dynamic
configuration is set by the contents of the single
<emphasis remap='B'>Linux-PAM</emphasis>
configuration file
<filename>/etc/pam.conf</filename>.
Alternatively, the configuration can be set by individual
configuration files located in the
<filename>/etc/pam.d/</filename>
directory.
<emphasis remap='I'>The presence of this directory will cause </emphasis><emphasis remap='B'>Linux-PAM</emphasis><emphasis remap='I'> to ignore</emphasis>
<filename>/etc/pam.conf</filename><literal>.</literal></para>


<para>From the point of view of the system administrator, for whom this
manual is provided, it is not of primary importance to understand the
internal behavior of the
<emphasis remap='B'>Linux-PAM</emphasis>
library.  The important point to recognize is that the configuration
file(s)
<emphasis remap='I'>define</emphasis>
the connection between applications
<emphasis remap='B'></emphasis>(<emphasis remap='B'>services</emphasis>)
and the pluggable authentication modules
<emphasis remap='B'></emphasis>(<emphasis remap='B'>PAM</emphasis>s)
that perform the actual authentication tasks.</para>


<para><emphasis remap='B'>Linux-PAM</emphasis>
separates the tasks of
<emphasis remap='I'>authentication</emphasis>
into four independent management groups:
<emphasis remap='B'>account</emphasis> management; 
<emphasis remap='B'>auth</emphasis>entication management; 
<emphasis remap='B'>password</emphasis> management; 
and
<emphasis remap='B'>session</emphasis> management.
(We highlight the abbreviations used for these groups in the
configuration file.)</para>


<para>Simply put, these groups take care of different aspects of a typical
user's request for a restricted service:</para>


<para><emphasis remap='B'>account</emphasis> - 
provide account verification types of service: has the user's password
expired?; is this user permitted access to the requested service?</para>

<!-- .br -->
<para><emphasis remap='B'>auth</emphasis>entication - 
authenticate a user and set up user credentials. Typically this is via
some challenge-response request that the user must satisfy: if you are
who you claim to be please enter your password. Not all authentications
are of this type, there exist hardware based authentication schemes
(such as the use of smart-cards and biometric devices), with suitable
modules, these may be substituted seamlessly for more standard
approaches to authentication - such is the flexibility of
<emphasis remap='B'>Linux-PAM</emphasis>.</para>

<!-- .br -->
<para><emphasis remap='B'>password</emphasis> - 
this group's responsibility is the task of updating authentication
mechanisms. Typically, such services are strongly coupled to those of
the
<emphasis remap='B'>auth</emphasis>
group. Some authentication mechanisms lend themselves well to being
updated with such a function. Standard UN*X password-based access is
the obvious example: please enter a replacement password.</para>

<!-- .br -->
<para><emphasis remap='B'>session</emphasis> - 
this group of tasks cover things that should be done prior to a
service being given and after it is withdrawn. Such tasks include the
maintenance of audit trails and the mounting of the user's home
directory. The
<emphasis remap='B'>session</emphasis>
management group is important as it provides both an opening and
closing hook for modules to affect the services available to a user.</para>

</refsect1>

<refsect1 id='the_configuration_files'><title>The configuration file(s)</title>
<para>When a
<emphasis remap='B'>Linux-PAM</emphasis>
aware privilege granting application is started, it activates its
attachment to the PAM-API.  This activation performs a number of
tasks, the most important being the reading of the configuration file(s):
<filename>/etc/pam.conf</filename>.
Alternatively, this may be the contents of the
<filename>/etc/pam.d/</filename>
directory.</para>

<para>These files list the
<emphasis remap='B'>PAM</emphasis>s
that will do the authentication tasks required by this service, and
the appropriate behavior of the PAM-API in the event that individual
<emphasis remap='B'>PAM</emphasis>s 
fail.</para>


<para>The syntax of the
<filename>/etc/pam.conf</filename>
configuration file is as follows. The file is made
up of a list of rules, each rule is typically placed on a single line,
but may be extended with an escaped end of line: `\&lt;LF&gt;'. Comments
are preceded with `#' marks and extend to the next end of line.</para>


<para>The format of each rule is a space separated collection of tokens, the
first three being case-insensitive:</para>


<!-- .br -->
<para><emphasis remap='B'>   service  type  control  module-path  module-arguments</emphasis></para>


<para>The syntax of files contained in the
<filename>/etc/pam.d/</filename>
directory, are identical except for the absence of any
<emphasis remap='I'>service</emphasis>
field. In this case, the
<emphasis remap='I'>service</emphasis>
is the name of the file in the
<filename>/etc/pam.d/</filename>
directory. This filename must be in lower case.</para>


<para>An important feature of
<emphasis remap='B'>Linux-PAM</emphasis>, 
is that a number of rules may be
<emphasis remap='I'>stacked</emphasis>
to combine the services of a number of PAMs for a given authentication
task.</para>


<para>The
<emphasis remap='B'>service</emphasis>
is typically the familiar name of the corresponding application:
<emphasis remap='B'>login</emphasis>
and
<emphasis remap='B'>su</emphasis>
are good examples. The
<emphasis remap='B'>service</emphasis>-name, <emphasis remap='B'>other</emphasis>, 
is reserved for giving
<emphasis remap='I'>default</emphasis>
rules.  Only lines that mention the current service (or in the absence
of such, the
<emphasis remap='B'>other</emphasis>
entries) will be associated with the given service-application.</para>


<para>The
<emphasis remap='B'>type</emphasis>
is the management group that the rule corresponds to. It is used to
specify which of the management groups the subsequent module is to
be associated with. Valid entries are:
<emphasis remap='B'>account</emphasis>; 
<emphasis remap='B'>auth</emphasis>; 
<emphasis remap='B'>password</emphasis>; 
and
<emphasis remap='B'>session</emphasis>.
The meaning of each of these tokens was explained above.</para>


<para>The third field,
<emphasis remap='B'>control</emphasis>, 
indicates the behavior of the PAM-API should the module fail to
succeed in its authentication task. There are two types of syntax for
this control field: the simple one has a single simple keyword; the
more complicated one involves a square-bracketed selection of
<emphasis remap='B'>value=action</emphasis>
pairs.</para>


<para>For the simple (historical) syntax valid
<emphasis remap='B'>control</emphasis>
values are:
<emphasis remap='B'>requisite</emphasis>
- failure of such a PAM results in the immediate termination of the
authentication process;
<emphasis remap='B'>required</emphasis>
- failure of such a PAM will ultimately lead to the PAM-API returning
failure but only after the remaining
<emphasis remap='I'>stacked</emphasis>
modules (for this
<emphasis remap='B'>service</emphasis>
and
<emphasis remap='B'>type</emphasis>)
have been invoked;
<emphasis remap='B'>sufficient</emphasis>
- success of such a module is enough to satisfy the authentication
requirements of the stack of modules (if a prior
<emphasis remap='B'>required</emphasis>
module has failed the success of this one is
<emphasis remap='I'>ignored</emphasis>); 
<emphasis remap='B'>optional</emphasis>
- the success or failure of this module is only important if it is the
only module in the stack associated with this
<emphasis remap='B'>service</emphasis>+<emphasis remap='B'>type</emphasis>.</para>


<para>New control directive first introduced in ALT Linux is
<emphasis remap='B'>include</emphasis>
- include all lines of given type from the configuration
file specified as an argument to this control.</para>


<para>For the more complicated syntax valid
<emphasis remap='B'>control</emphasis>
values have the following form:</para>

<para>[value1=action1<emphasis remap='B'>value2=action2</emphasis>...]</para>

<para>Where
<emphasis remap='B'>valueN</emphasis>
corresponds to the return code from the function invoked in the module
for which the line is defined. It is selected from one of these:
<emphasis remap='B'>success</emphasis>;
<emphasis remap='B'>open_err</emphasis>;
<emphasis remap='B'>symbol_err</emphasis>;
<emphasis remap='B'>service_err</emphasis>;
<emphasis remap='B'>system_err</emphasis>;
<emphasis remap='B'>buf_err</emphasis>;
<emphasis remap='B'>perm_denied</emphasis>;
<emphasis remap='B'>auth_err</emphasis>;
<emphasis remap='B'>cred_insufficient</emphasis>;
<emphasis remap='B'>authinfo_unavail</emphasis>;
<emphasis remap='B'>user_unknown</emphasis>;
<emphasis remap='B'>maxtries</emphasis>;
<emphasis remap='B'>new_authtok_reqd</emphasis>;
<emphasis remap='B'>acct_expired</emphasis>;
<emphasis remap='B'>session_err</emphasis>;
<emphasis remap='B'>cred_unavail</emphasis>;
<emphasis remap='B'>cred_expired</emphasis>;
<emphasis remap='B'>cred_err</emphasis>;
<emphasis remap='B'>no_module_data</emphasis>;
<emphasis remap='B'>conv_err</emphasis>;
<emphasis remap='B'>authtok_err</emphasis>;
<emphasis remap='B'>authtok_recover_err</emphasis>;
<emphasis remap='B'>authtok_lock_busy</emphasis>;
<emphasis remap='B'>authtok_disable_aging</emphasis>;
<emphasis remap='B'>try_again</emphasis>;
<emphasis remap='B'>ignore</emphasis>;
<emphasis remap='B'>abort</emphasis>;
<emphasis remap='B'>authtok_expired</emphasis>;
<emphasis remap='B'>module_unknown</emphasis>;
<emphasis remap='B'>bad_item</emphasis>; and
<emphasis remap='B'>default</emphasis>.
The last of these,
<emphasis remap='B'>default</emphasis>,
implies 'all
<emphasis remap='B'>valueN</emphasis>'s
not mentioned explicitly. Note, the full list of PAM errors is
available in /usr/include/security/_pam_types.h . The
<emphasis remap='B'>actionN</emphasis>
can be: an unsigned integer,
<emphasis remap='B'>J</emphasis>,
signifying an action of 'jump over the next J modules in the stack';
or take one of the following forms:
<!-- .br -->
<emphasis remap='B'>ignore</emphasis>
- when used with a stack of modules, the module's return status will
not contribute to the return code the application obtains;
<!-- .br -->
<emphasis remap='B'>bad</emphasis>
- this action indicates that the return code should be thought of as
indicative of the module failing. If this module is the first in the
stack to fail, its status value will be used for that of the whole
stack.
<!-- .br -->
<emphasis remap='B'>die</emphasis>
- equivalent to bad with the side effect of terminating the module
stack and PAM immediately returning to the application.
<!-- .br -->
<emphasis remap='B'>ok</emphasis>
- this tells PAM that the administrator thinks this return code
should contribute directly to the return code of the full stack of
modules. In other words, if the former state of the stack would lead
to a return of
<emphasis remap='B'>PAM_SUCCESS</emphasis>,
the module's return code will override this value. Note, if the former
state of the stack holds some value that is indicative of a modules
failure, this 'ok' value will not be used to override that value.
<!-- .br -->
<emphasis remap='B'>done</emphasis>
- equivalent to ok with the side effect of terminating the module
stack and PAM immediately returning to the application.
<!-- .br -->
<emphasis remap='B'>reset</emphasis>
- clear all memory of the state of the module stack and start again
with the next stacked module.</para>


<para><emphasis remap='B'>module-path</emphasis>
- this is either the full filename of the PAM to be used by the
application (it begins with a '/'), or a relative pathname from the
default module location:
<filename>/lib/security/</filename>.</para>


<para><emphasis remap='B'>module-arguments</emphasis>
- these are a space separated list of tokens that can be used to
modify the specific behavior of the given PAM. Such arguments will be
documented for each individual module.</para>

</refsect1>

<refsect1 id='files'><title>FILES</title>
<para><filename>/etc/pam.conf</filename> - the configuration file
<!-- .br -->
<filename>/etc/pam.d/</filename> - the
<emphasis remap='B'>Linux-PAM</emphasis>
configuration directory. Generally, if this directory is present, the
<filename>/etc/pam.conf</filename>
file is ignored.
<!-- .br -->
<filename>/lib/libpam.so.X</filename> - the dynamic library
<!-- .br -->
<filename>/lib/security/*.so</filename> - the PAMs</para>

</refsect1>

<refsect1 id='errors'><title>ERRORS</title>
<para>Typically errors generated by the
<emphasis remap='B'>Linux-PAM</emphasis>
system of libraries, will be written to
<citerefentry><refentrytitle>syslog</refentrytitle><manvolnum>3</manvolnum></citerefentry>.</para>

</refsect1>

<refsect1 id='conforming_to'><title>CONFORMING TO</title>
<para>DCE-RFC 86.0, October 1995.
<!-- .br -->
Contains additional features, but remains backwardly compatible with
this RFC.</para>

</refsect1>

<refsect1 id='bugs'><title>BUGS</title>


<para>None known.</para>

</refsect1>

<refsect1 id='see_also'><title>SEE ALSO</title>
<para>The three
<emphasis remap='B'>Linux-PAM</emphasis>
Guides, for
<emphasis remap='B'>system administrators</emphasis>, 
<emphasis remap='B'>module developers</emphasis>, 
and
<emphasis remap='B'>application developers</emphasis>. </para>
</refsect1>
</refentry>