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<html><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8"><title>4.1. Configuration file syntax</title><meta name="generator" content="DocBook XSL Stylesheets V1.78.1"><link rel="home" href="Linux-PAM_SAG.html" title="The Linux-PAM System Administrators' Guide"><link rel="up" href="sag-configuration.html" title="Chapter 4. The Linux-PAM configuration file"><link rel="prev" href="sag-configuration.html" title="Chapter 4. The Linux-PAM configuration file"><link rel="next" href="sag-configuration-directory.html" title="4.2. Directory based configuration"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">4.1. Configuration file syntax</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="sag-configuration.html">Prev</a> </td><th width="60%" align="center">Chapter 4. The Linux-PAM configuration file</th><td width="20%" align="right"> <a accesskey="n" href="sag-configuration-directory.html">Next</a></td></tr></table><hr></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="sag-configuration-file"></a>4.1. Configuration file syntax</h2></div></div></div><p>
The syntax of the <code class="filename">/etc/pam.conf</code>
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: `\<LF>'.
Comments are preceded with `#' marks and extend to the next end of
line.
</p><p>
The format of each rule is a space separated collection of tokens,
the first three being case-insensitive:
</p><p>
<span class="emphasis"><em> service type control module-path module-arguments</em></span>
</p><p>
The syntax of files contained in the <code class="filename">/etc/pam.d/</code>
directory, are identical except for the absence of any
<span class="emphasis"><em>service</em></span> field. In this case, the
<span class="emphasis"><em>service</em></span> is the name of the file in the
<code class="filename">/etc/pam.d/</code> directory. This filename must be
in lower case.
</p><p>
An important feature of <span class="emphasis"><em>PAM</em></span>, is that a
number of rules may be <span class="emphasis"><em>stacked</em></span> to combine
the services of a number of PAMs for a given authentication task.
</p><p>
The <span class="emphasis"><em>service</em></span> is typically the familiar name of
the corresponding application: <span class="emphasis"><em>login</em></span> and
<span class="emphasis"><em>su</em></span> are good examples. The
<span class="emphasis"><em>service</em></span>-name, <span class="emphasis"><em>other</em></span>,
is reserved for giving <span class="emphasis"><em>default</em></span> rules.
Only lines that mention the current service (or in the absence
of such, the <span class="emphasis"><em>other</em></span> entries) will be associated
with the given service-application.
</p><p>
The <span class="emphasis"><em>type</em></span> 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:
</p><div class="variablelist"><dl class="variablelist"><dt><span class="term">account</span></dt><dd><p>
this module type performs non-authentication based account
management. It is typically used to restrict/permit access
to a service based on the time of day, currently available
system resources (maximum number of users) or perhaps the
location of the applicant user -- 'root' login only on the
console.
</p></dd><dt><span class="term">auth</span></dt><dd><p>
this module type provides two aspects of authenticating
the user. Firstly, it establishes that the user is who they
claim to be, by instructing the application to prompt the user
for a password or other means of identification. Secondly, the
module can grant group membership or other privileges through
its credential granting properties.
</p></dd><dt><span class="term">password</span></dt><dd><p>
this module type is required for updating the authentication
token associated with the user. Typically, there is one module
for each 'challenge/response' based authentication (auth) type.
</p></dd><dt><span class="term">session</span></dt><dd><p>
this module type is associated with doing things that need to
be done for the user before/after they can be given service.
Such things include the logging of information concerning the
opening/closing of some data exchange with a user, mounting
directories, etc.
</p></dd></dl></div><p>
If the <span class="emphasis"><em>type</em></span> value from the list above is prepended
with a <span class="emphasis"><em>-</em></span> character the PAM library will not log to
the system log if it is not possible to load the module because it is
missing in the system. This can be useful especially for modules which
are not always installed on the system and are not required for correct
authentication and authorization of the login session.
</p><p>
The third field, <span class="emphasis"><em>control</em></span>, 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
<span class="emphasis"><em>value=action</em></span> pairs.
</p><p>
For the simple (historical) syntax valid <span class="emphasis"><em>control</em></span>
values are:
</p><div class="variablelist"><dl class="variablelist"><dt><span class="term">required</span></dt><dd><p>
failure of such a PAM will ultimately lead to the PAM-API
returning failure but only after the remaining
<span class="emphasis"><em>stacked</em></span> modules (for this
<span class="emphasis"><em>service</em></span> and <span class="emphasis"><em>type</em></span>)
have been invoked.
</p></dd><dt><span class="term">requisite</span></dt><dd><p>
like <span class="emphasis"><em>required</em></span>, however, in the case that
such a module returns a failure, control is directly returned
to the application or to the superior PAM stack.
The return value is that associated with
the first required or requisite module to fail. Note, this flag
can be used to protect against the possibility of a user getting
the opportunity to enter a password over an unsafe medium. It is
conceivable that such behavior might inform an attacker of valid
accounts on a system. This possibility should be weighed against
the not insignificant concerns of exposing a sensitive password
in a hostile environment.
</p></dd><dt><span class="term">sufficient</span></dt><dd><p>
if such a module succeeds and no prior <span class="emphasis"><em>required</em></span>
module has failed the PAM framework returns success to
the application or to the superior PAM stack immediately without
calling any further modules in the stack. A failure of a
<span class="emphasis"><em>sufficient</em></span> module is ignored and processing
of the PAM module stack continues unaffected.
</p></dd><dt><span class="term">optional</span></dt><dd><p>
the success or failure of this module is only important if
it is the only module in the stack associated with this
<span class="emphasis"><em>service</em></span>+<span class="emphasis"><em>type</em></span>.
</p></dd><dt><span class="term">include</span></dt><dd><p>
include all lines of given type from the configuration
file specified as an argument to this control.
</p></dd><dt><span class="term">substack</span></dt><dd><p>
include all lines of given type from the configuration
file specified as an argument to this control. This differs from
<span class="emphasis"><em>include</em></span> in that evaluation of the
<span class="emphasis"><em>done</em></span> and <span class="emphasis"><em>die</em></span> actions
in a substack does not cause skipping the rest of the complete
module stack, but only of the substack. Jumps in a substack
also can not make evaluation jump out of it, and the whole substack
is counted as one module when the jump is done in a parent stack.
The <span class="emphasis"><em>reset</em></span> action will reset the state of a
module stack to the state it was in as of beginning of the substack
evaluation.
</p></dd></dl></div><p>
For the more complicated syntax valid <span class="emphasis"><em>control</em></span>
values have the following form:
</p><pre class="programlisting">
[value1=action1 value2=action2 ...]
</pre><p>
Where <span class="emphasis"><em>valueN</em></span> 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:
<span class="emphasis"><em>success</em></span>, <span class="emphasis"><em>open_err</em></span>,
<span class="emphasis"><em>symbol_err</em></span>, <span class="emphasis"><em>service_err</em></span>,
<span class="emphasis"><em>system_err</em></span>, <span class="emphasis"><em>buf_err</em></span>,
<span class="emphasis"><em>perm_denied</em></span>, <span class="emphasis"><em>auth_err</em></span>,
<span class="emphasis"><em>cred_insufficient</em></span>,
<span class="emphasis"><em>authinfo_unavail</em></span>,
<span class="emphasis"><em>user_unknown</em></span>, <span class="emphasis"><em>maxtries</em></span>,
<span class="emphasis"><em>new_authtok_reqd</em></span>,
<span class="emphasis"><em>acct_expired</em></span>, <span class="emphasis"><em>session_err</em></span>,
<span class="emphasis"><em>cred_unavail</em></span>, <span class="emphasis"><em>cred_expired</em></span>,
<span class="emphasis"><em>cred_err</em></span>, <span class="emphasis"><em>no_module_data</em></span>,
<span class="emphasis"><em>conv_err</em></span>, <span class="emphasis"><em>authtok_err</em></span>,
<span class="emphasis"><em>authtok_recover_err</em></span>,
<span class="emphasis"><em>authtok_lock_busy</em></span>,
<span class="emphasis"><em>authtok_disable_aging</em></span>,
<span class="emphasis"><em>try_again</em></span>, <span class="emphasis"><em>ignore</em></span>,
<span class="emphasis"><em>abort</em></span>, <span class="emphasis"><em>authtok_expired</em></span>,
<span class="emphasis"><em>module_unknown</em></span>, <span class="emphasis"><em>bad_item</em></span>,
<span class="emphasis"><em>conv_again</em></span>, <span class="emphasis"><em>incomplete</em></span>,
and <span class="emphasis"><em>default</em></span>.
</p><p>
The last of these, <span class="emphasis"><em>default</em></span>, implies 'all
<span class="emphasis"><em>valueN</em></span>'s not mentioned explicitly. Note, the
full list of PAM errors is available in
<code class="filename">/usr/include/security/_pam_types.h</code>. The
<span class="emphasis"><em>actionN</em></span> can take one of the following forms:
</p><div class="variablelist"><dl class="variablelist"><dt><span class="term">ignore</span></dt><dd><p>
when used with a stack of modules, the module's return
status will not contribute to the return code the application
obtains.
</p></dd><dt><span class="term">bad</span></dt><dd><p>
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.
</p></dd><dt><span class="term">die</span></dt><dd><p>
equivalent to bad with the side effect of terminating the
module stack and PAM immediately returning to the application.
</p></dd><dt><span class="term">ok</span></dt><dd><p>
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 <span class="emphasis"><em>PAM_SUCCESS</em></span>,
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.
</p></dd><dt><span class="term">done</span></dt><dd><p>
equivalent to ok with the side effect of terminating the module
stack and PAM immediately returning to the application.
</p></dd><dt><span class="term">N (an unsigned integer)</span></dt><dd><p>
equivalent to ok with the side effect of jumping over the
next N modules in the stack. Note that N equal to 0 is not
allowed (and it would be identical to ok in such case).
</p></dd><dt><span class="term">reset</span></dt><dd><p>
clear all memory of the state of the module stack and
start again with the next stacked module.
</p></dd></dl></div><p>
Each of the four keywords: required; requisite; sufficient; and
optional, have an equivalent expression in terms of the [...]
syntax. They are as follows:
</p><div class="variablelist"><dl class="variablelist"><dt><span class="term">required</span></dt><dd><p>
[success=ok new_authtok_reqd=ok ignore=ignore default=bad]
</p></dd><dt><span class="term">requisite</span></dt><dd><p>
[success=ok new_authtok_reqd=ok ignore=ignore default=die]
</p></dd><dt><span class="term">sufficient</span></dt><dd><p>
[success=done new_authtok_reqd=done default=ignore]
</p></dd><dt><span class="term">optional</span></dt><dd><p>
[success=ok new_authtok_reqd=ok default=ignore]
</p></dd></dl></div><p>
<span class="emphasis"><em>module-path</em></span> 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:
<code class="filename">/lib/security/</code> or
<code class="filename">/lib64/security/</code>, depending on the architecture.
</p><p>
<span class="emphasis"><em>module-arguments</em></span> 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. Note, if you wish to include spaces in an argument, you
should surround that argument with square brackets.
</p><pre class="programlisting">
squid auth required pam_mysql.so user=passwd_query passwd=mada \
db=eminence [query=select user_name from internet_service \
where user_name='%u' and password=PASSWORD('%p') and \
service='web_proxy']
</pre><p>
When using this convention, you can include `[' characters
inside the string, and if you wish to include a `]' character
inside the string that will survive the argument parsing, you
should use `\]'. In other words:
</p><pre class="programlisting">
[..[..\]..] --> ..[..]..
</pre><p>
Any line in (one of) the configuration file(s), that is not formatted
correctly, will generally tend (erring on the side of caution) to make
the authentication process fail. A corresponding error is written to
the system log files with a call to
<span class="citerefentry"><span class="refentrytitle">syslog</span>(3)</span>.
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