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 sta-
ble 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 appli-
cations 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 /etc/pam.conf.
From the point of view of the system administrator, for whom this man-
ual 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 mod-
ules (PAMs) that perform the actual authentication tasks.
Linux-PAM separates the tasks of authentication into four independent
management groups: account management; authentication management; pass-
word management; and session management. (We highlight the abbrevia-
tions used for these groups in the configuration file.)
Simply put, these groups take care of different aspects of a typical
user's request for a restricted service:
account - provide account verification types of service: has the user's
password expired?; is this user permitted access to the requested ser-
authentication - establish the user is who they claim to be. Typically
tication 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 replacement pass-
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a service being given and after it is withdrawn.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 user's home direc-
tory. 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 configura-
tion 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: `\<LF>'.
Comments are preceded with `#' 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 identi-
cal except for the absence of any service field. In this case, the ser-
vice is the name of the file in the /etc/pam.d/ directory. This file-
name 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 authen-
The service is typically the familiar name of the corresponding appli-
cation: 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 associ-
ated with the given service-application.
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 ulti-
mately lead to the PAM-API returning failure but only after the remain-
ing stacked modules (for this service and type) have been invoked; suf-
ficient - success of such a module is enough to satisfy the authentica-
tion 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; auth-
tok_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 'all valueN'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 'jump over the next J modules in
the stack'; or take one of the following forms:
ignore - when used with a stack of modules, the module'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 mod-
ules. In other words, if the former state of the stack would lead to a
return of PAM_SUCCESS, 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.
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
/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 develop-
ers, and application developers.
Linux-pam 0.74 2001 Jan 20 pam(8)