PAM(8) Linux-PAM Manual PAM(8)
NAME
Linux-PAM - Pluggable Authentication Modules for Linux
SYNOPSIS
/etc/pam.conf
DESCRIPTION
This is a port of Linux-PAM to the Mac OS X operating system.
This manual is intended to offer a quick introduction to Linux-PAM. For more information the reader
is directed to the Linux-PAM system administrators' guide.
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) InterfaceAPI)
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-pro-viding service-providing
viding 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 /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 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 man-agement; management;
agement; 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 challengeresponse
response request that the user must satisfy: if you are who you claim to be please enter your pass-word. password.
word. 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 substi-tuted substituted
tuted seamlessly for more standard approaches to authentication - such is the flexibility of Linux-PAM. LinuxPAM.
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 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: `\<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-insen-sitive: case-insensitive:
sitive:
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 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 ser-vice-application. service-application.
vice-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; sys-tem_err; system_err;
tem_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 '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 mod-ule. module.
ule.
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/lib/pam/.
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.
/usr/lib/libpam.dylib - the dynamic library
/usr/lib/pam/*.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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