Provided by: libpam-runtime_0.79-3ubuntu14_all
Linux-PAM - Pluggable Authentication Modules for Linux
This manual is intended to offer a quick introduction to Linux-PAM.
For more information the reader is directed to the Linux-PAM system
Linux-PAM 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 login(1) and su(1)) defer to to
perform standard authentication tasks.
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 Linux-PAM configuration file
/etc/pam.conf. Alternatively, the configuration can be set by
individual configuration files located in the /etc/pam.d/ directory.
The presence of this directory will cause Linux-PAM to ignore
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 Linux-PAM library. The important point to
recognize is that the configuration file(s) define the connection
between applications (services) and the pluggable authentication
modules (PAMs) that perform the actual authentication tasks.
Linux-PAM separates the tasks of authentication into four independent
management groups: account management; authentication management;
password management; and session management. (We highlight the
abbreviations used for these groups in the configuration file.)
Simply put, these groups take care of different aspects of a typical
userâ€™s request for a restricted service:
account - provide account verification types of service: has the userâ€™s
password expired?; is this user permitted access to the requested
authentication - establish the user is who they claim to be. 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 Linux-PAM.
password - this groupâ€™s responsibility is the task of updating
authentication mechanisms. Typically, such services are strongly
coupled to those of the auth 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
session - 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 userâ€™s home
directory. The session management group is important as it provides
both an opening and closing hook for modules to affect the services
available to a user.
The configuration file(s)
When a Linux-PAM 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): /etc/pam.conf. Alternatively, this may be the
contents of the /etc/pam.d/ directory.
These files list the PAMs that will do the authentication tasks
required by this service, and the appropriate behavior of the PAM-API
in the event that individual PAMs fail.
The syntax of the /etc/pam.conf 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: â€˜\<LF>â€™.
Comments are preceded with â€˜#â€™ marks and extend to the next end of
The format of each rule is a space separated collection of tokens, the
first three being case-insensitive:
service type control module-path module-arguments
The syntax of files contained in the /etc/pam.d/ directory, are
identical except for the absence of any service field. In this case,
the service is the name of the file in the /etc/pam.d/ directory. This
filename must be in lower case.
An important feature of Linux-PAM, is that a number of rules may be
stacked to combine the services of a number of PAMs for a given
The service is typically the familiar name of the corresponding
application: login and su are good examples. The service-name, other,
is reserved for giving default rules. Only lines that mention the
current service (or in the absence of such, the other entries) will be
associated with the given service-application.
The type 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: account; auth; password; and
session. The meaning of each of these tokens was explained above.
The third field, control, 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 value=action pairs.
For the simple (historical) syntax valid control values are: requisite
- failure of such a PAM results in the immediate termination of the
authentication process; required - failure of such a PAM will
ultimately lead to the PAM-API returning failure but only after the
remaining stacked modules (for this service and type) have been
invoked; sufficient - success of such a module is enough to satisfy the
authentication requirements of the stack of modules (if a prior
required module has failed the success of this one is ignored);
optional - the success or failure of this module is only important if
it is the only module in the stack associated with this service+type.
For the more complicated syntax valid control values have the following
Where valueN 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: success; open_err; symbol_err; service_err; system_err; buf_err;
perm_denied; auth_err; cred_insufficient; authinfo_unavail;
user_unknown; maxtries; new_authtok_reqd; acct_expired; session_err;
cred_unavail; cred_expired; cred_err; no_module_data; conv_err;
authtok_err; authtok_recover_err; authtok_lock_busy;
authtok_disable_aging; try_again; ignore; abort; authtok_expired;
module_unknown; bad_item; and default. The last of these, default,
implies â€™all valueNâ€™s not mentioned explicitly. Note, the full list of
PAM errors is available in /usr/include/security/_pam_types.h . The
actionN can be: an unsigned integer, J, signifying an action of â€™jump
over the next J modules in the stackâ€™; or take one of the following
ignore - when used with a stack of modules, the moduleâ€™s return status
will not contribute to the return code the application obtains;
bad - 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
die - equivalent to bad with the side effect of terminating the module
stack and PAM immediately returning to the application.
ok - 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 PAM_SUCCESS, the moduleâ€™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 â€™okâ€™ value will not be used to
override that value.
done - equivalent to ok with the side effect of terminating the module
stack and PAM immediately returning to the application.
reset - clear all memory of the state of the module stack and start
again with the next stacked module.
module-path - this is either the full filename of the PAM to be used by
the application (it begins with a â€™/â€™), or a relative pathname from the
default module location: /lib/security/.
module-arguments - 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.
/etc/pam.conf - the configuration file
/etc/pam.d/ - the Linux-PAM configuration directory. Generally, if this
directory is present, the /etc/pam.conf file is ignored.
/lib/libpam.so.X - the dynamic library
/lib/security/*.so - the PAMs
Typically errors generated by the Linux-PAM system of libraries, will
be written to syslog(3).
DCE-RFC 86.0, October 1995.
Contains additional features, but remains backwardly compatible with
The three Linux-PAM Guides, for system administrators, module
developers, and application developers.