Linux-PAM is designed to provide the system administrator with a
great deal of flexibility in configuring the privilege granting
applications of their system. The local configuration of those aspects
of system security controlled by Linux-PAM
is contained in the
system file, /etc/pam.conf
. In this section we discuss the
correct syntax of and generic options respected by entries to this
file.
The reader should note that the Linux-PAM specific tokens in this file are case insensitive. The module paths, however, are case sensitive since they indicate a file's name and reflect the case dependence of typical Linux file-systems. The case-sensitivity of the arguments to any given module is defined for each module in turn.
In addition to the lines described below, there are two special
characters provided for the convenience of the system administrator:
comments are preceded by a `#
', the comment extending to the
next end of line; also, module specification lines may be extended
with a `\
'.
A general configuration line the the /etc/pam.conf
file has
the following form:
service-name module-type control-flag module-path arguments
Here we explain the meaning of each of these tokens.
service-name
The name of the service associated with this entry. Frequently the
service name is the conventional name of the given application. For
example, `ftpd
', `rlogind
' and `su
', etc. .
There is a special service-name
, reserved for defining a default
authentication mechanism. It has the name `OTHER
' and may be
specified in either lower or upper case characters.
module-type
One of (currently) four types of module. The four types are as follows:
auth
; this module type provides two aspects of
authenticating the user. Firstly, it establishes that the user is who
they claim to be, by instructing the application to prompt the user
for a password or other means of identification. Secondly, the module
can grant group
membership (independently of the
/etc/groups
file discussed above) or other privileges through
its credential granting properties.
account
; this module performs non-authentication based
account management. It is typically used to restrict/permit access to
a service based on the time of day, currently available system
resources (maximum number of users) or perhaps the location of the
applicant user---`root
' login only on the console.
session
; primarily, this module is associated with logging
information concerning the opening/closing of some data exchange with
a user. It is intended for auditing purposes and is likely to be
indispensable for maintaining a secure Linux system.
password
; this last module type is required for updating the
authentication token associated with the user. Typically, there is one
module for each `challenge/response' based authentication (auth
)
module-type.
control-flag
One of (currently) three tokens that indicate the severity of
concern associated with the success or failure of a given module.
Linux-PAM provides for the stacking of similar modules,
providing a method of simultaneously exposing the user to more than
one authentication mechanism. The application is not made aware of
the individual success or failure of modules listed in the
`/etc/pam.conf
' file. Instead, it receives a summary
success or fail response from the Linux-PAM
library. The order of execution of these modules is that of the
entries in the /etc/pam.conf
file; earlier entries are
executed before later ones.
The policy for determining this response is based on these
three control-flag
s:
required
; this indicates that the success of the module is
required for the module-type
facility to succeed. Failure of this
module will not be apparent to the user until all of the remaining
modules (of the same module-type
) have been executed.
sufficient
; the success of this module is deemed
`sufficient' to satisfy the Linux-PAM library that this
module-type has succeeded in its purpose. In the event that no
previous required
module has failed, no more `stacked'
modules of this type are invoked. (Note, in this case subsequent
required
modules are not invoked.). A failure of this module
is not deemed as fatal to satisfying the application that this
module-type
has succeeded.
optional
; as its name suggests, this control-flag
marks the module as not being critical to the success or failure of
the user's application for service. However, in the absence of any
successes of previous or subsequent stacked modules this module will
determine the nature of the response to the application.
module-path
The path-name of the dynamically loadable object file; the pluggable module itself.
args
The args
are a list of tokens that are passed to the module when
it is invoked. Much like arguments to a typical Linux shell command.
Generally, valid arguments are optional and are specific to any given
module. Invalid arguments are ignored by a module, however, when
encountering an invalid argument, the module is required to write an
error to syslog(2)
. For a list of generic options see the
next section.
Any line, in /etc/pam.conf
, that is not formatted correctly
is ignored by Linux-PAM; however, a corresponding error is
written to the system log files with a call to syslog(2)
.
The following are optional arguments which are likely to be understood by any module. Arguments (including these) are in general optional.
debug
Use the syslog(2)
call to log debugging information to the system
log files.
no_warn
Instruct module to not give warning messages to the application.
use_first_pass
The module should not prompt the user for a password. Instead, it
should obtain the previously typed password (from the preceding
auth
module), and use that. If that doesn't work, then the user
will not be authenticated. (This option is intended for auth
and password
modules only).
try_first_pass
The module should attempt authentication with the previously typed
password (from the preceding auth
module). If that doesn't work,
then the user is prompted for a password. (This option is intended for
auth
modules only).
use_mapped_pass
This argument is not currently supported by any of the modules in the Linux-PAM distribution because of possible consequences associated with U.S. encryption exporting restrictions. Within the U.S., module developers are, of course, free to implement it (as are developers in other countries). For compatibility reasons we describe its use as suggested in the DCE-RFC 86.0, see the section bibliography for a pointer to this document.
The use_mapped_pass
argument instructs the module to take the
clear text authentication token entered by a previous module (that
requests such a token) and use it to generate an encryption/decryption
key with which to safely store/retrieve the authentication token
required for this module. In this way the user can enter a single
authentication token and be quietly authenticated by a number of
stacked modules. Obviously a convenient feature that necessarily
requires some reliably strong encryption to make it secure.
This argument is intended for the auth
and password
module
types only.
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