PAM(8)PAM(8)NAME
PAM, pam.conf, pam.d - Pluggable Authentication Modules
SYNOPSIS
/etc/pam.conf
DESCRIPTION
This manual is intended to offer a quick introduction to PAM. For more
information the reader is directed to the PAM system administrators'
guide.
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 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 PAM to ignore /etc/pam.conf.
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 PAM library. The important point to recognize is that
the configuration file(s) define the connection between applications and
the pluggable authentication modules that perform the actual
authentication tasks.
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
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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
PAM.
password - this group'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 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 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: `\<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
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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 PAM, is that a number of rules may be stacked to
combine 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 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
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_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;
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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
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 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
the application (it begins with a '/'), or a relative pathname from the
default module location: /usr/lib32/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 PAM configuration directory. Generally, if this
directory is present, the /etc/pam.conf file is ignored.
/usr/lib32/libpam.so - the dynamic library
/usr/lib32/security/*.so - the PAMs
ERRORS
Typically errors generated by the PAM system of libraries, will be
written to syslog(3).
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PAM(8)PAM(8)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 PAM Guides, for system administrators, module developers, and
application developers.
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