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PAM(8) Linux-PAM Manual PAM(8)
NAME
Linux-PAM - Pluggable Authentication Modules for Linux
SYNOPSIS
/etc/pam.conf
DESCRIPTION
This manual is intended to offer a quick introduction to Linux-PAM. For more
information the reader is directed to the Linux-PAM system administratorsâ€â€™ guide.
Linux-PAM Is a system of libraries that handle the authentication tasks of applica-
tions (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 con-
figuration files located in the /etc/pam.d/ directory. The presence of this direc-
tory will cause Linux-PAM to ignore /etc/pam.conf.
From the point of view of the system administrator, for whom this manual is pro-
vided, 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 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 service?
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 group’s responsibility is the task of updating authentication mech-
anisms. 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 password.
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 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. Alterna-
tively, 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 line.
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 com-
bine the services of a number of PAMs for a given authentication task.
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 spec-
ify 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.
New control directive first introduced in ALT Linux is include - include all lines
of given type from the configuration file specified as an argument to this control.
For the more complicated syntax valid control values have the following form:
[value1=action1value2=action2...]
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_insuffi-
cient; 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 stack.
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 con-
tribute 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 mod-
ule’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 the appli-
cation (it begins with a ’/’), or a relative pathname from the default module loca-
tion: /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.
FILES
/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
ERRORS
Typically errors generated by the Linux-PAM system of libraries, will be written to
syslog(3).
CONFORMING TO
DCE-RFC 86.0, October 1995.
Contains additional features, but remains backwardly compatible with this RFC.
BUGS
None known.
SEE ALSO
The three Linux-PAM Guides, for system administrators, module developers, and
application developers.
Linux-PAM 0.74 2001 Jan 20 PAM(8)
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On Apache/1.3.41 (Unix) PHP/5.2.5 mod_perl/1.30 mod_gzip/1.3.26.1a
Under GNU General Public License
2008-12-02 01:12 @38.103.63.58 CrawledBy CCBot/1.0 (+http://www.commoncrawl.org/bot.html)