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 `#' and extend to the
next end-of-line; also, module specification lines may be extended
with a `\'.
A general configuration line of the /etc/pam.conf file has
the following form:
service-name module-type control-flag module-path arguments
service-nameThe 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. Note, when there
is a module specified for a named service, the `OTHER' entries
are ignored.
module-typeOne 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 doing
things that need to be done for the user before/after they can be
given service. Such things include the logging of information
concerning the opening/closing of some data exchange with a user,
mounting directories, etc. .
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-flagOne of (currently) four 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 sequentially exposing the user to more than one
authentication mechanism per service application. 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-flags:
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.
requisite; like required, however, in the case that
such a module returns a failure, control is directly returned to the
application. The return value is that associated with the first
required or requisite module to fail. Note, this flag can be
used to protect against the possibility of a user getting the
opportunity to enter a password over an unsafe medium. It is
conceivable that such behavior might inform an attacker of valid
accounts on a system. This possibility should be weighed against the
not insignificant concerns of exposing a sensitive password in a
hostile environment.
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-pathThe path-name of the dynamically loadable object file; the pluggable module itself.
argsThe 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(3). 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(3).
The following are optional arguments which are likely to be understood by any module. Arguments (including these) are in general optional.
debugUse the syslog(3) call to log debugging information to the system
log files.
no_warnInstruct module to not give warning messages to the application.
use_first_passThe 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_passThe 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_passThis 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 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.
/etc/pam.conf fileIn this section, we give some examples of entries that can be present in the Linux-PAM configuration file. As a first attempt at configuring your system you could do worse than to implement these.
If a system is to be considered secure, it had better have a
reasonably secure `OTHER' entry. The following is a paranoid
setting (which is not a bad place to start!):
#
# default; deny access
#
OTHER auth required /usr/lib/security/pam_deny.so
OTHER account required /usr/lib/security/pam_deny.so
OTHER password required /usr/lib/security/pam_deny.so
OTHER session required /usr/lib/security/pam_deny.so
The module pam_deny (documented in a later section) is not very
sophisticated. For example, it logs no information when it is invoked
so unless the users of a system contact the administrator when failing
to execute a service application, the administrator may go for a long
while in ignorance of the fact that his system is misconfigured.
The addition of the following line before those in the above example would provide a suitable warning to the administrator.
#
# default; wake up! This application is not configured
#
OTHER auth required /usr/lib/security/pam_warn.so
OTHER password required /usr/lib/security/pam_warn.so
OTHER auth'' lines is an example of stacking.
On a less sensitive computer, one on which the system administrator
wishes to remain ignorant of the power of Linux-PAM, the
following selection of lines is likely to mimic the historically
familiar Linux setup.
#
# default; standard UNIX access
#
OTHER auth required /usr/lib/security/pam_unix_auth.so
OTHER account required /usr/lib/security/pam_unix_acct.so
OTHER password required /usr/lib/security/pam_unix_passwd.so
OTHER session required /usr/lib/security/pam_unix_sess.so
To enable anonymous-ftp, the following lines might be used to replace
the default (OTHER) ones. (*WARNING* as of 1996/11/14 this does
not work correctly with any ftpd. Consequently, this description may
be subject to change or the application will be fixed.)
#
# default; add ftp-specifics. These lines enable anonymous ftp over
# standard UNIX access (the listfile entry blocks access to
# users listed in /etc/ftpusers)
#
ftpd auth sufficient /usr/lib/security/pam_ftp.so
ftpd auth required /usr/lib/security/pam_unix_auth.so use_first_pass
ftpd auth required /usr/lib/security/pam_listfile.so \
onerr=succeed item=user sense=deny file=/etc/ftpusers
/etc/pam.conf for that specified service.
Again, this is an example of authentication module stacking. Note the
use of the sufficient control-flag. It says that ``if this module
authenticates the user, ignore the subsequent auth
modules''. Also note the use of the ``use_first_pass''
module-argument, this instructs the UNIX authentication module that it
is not to prompt for a password but rely one already having been
obtained by the ftp module.
The standard UNIX modules used above are strongly tied to using the
default `libc' user database functions (see for example, man
getpwent). It is the opinion of the author that these functions are
not sufficently flexible to make full use of the power of
Linux-PAM. For this reason, and as a small plug, I mention in
passing that there is a pluggable replacement for the pam_unix_..
modules; pam_pwdb. See the section below for a more complete
description.
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