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 one of
two places: either the single system file,
/etc/pam.d/ directory. In this section we discuss the
correct syntax of and generic options respected by entries to these
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 `
\' escaped newline.
A general configuration line of the
/etc/pam.conf file has
the following form:
Below, we explain the meaning of each of these tokens. The second (and more recently adopted) way of configuring Linux-PAM is via the contents of the
service-name module-type control-flag module-path arguments
/etc/pam.d/directory. Once we have explained the meaning of the above tokens, we will describe this method.
The name of the service associated with this entry. Frequently the
service name is the conventional name of the given application. For
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 `
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
groupmembership (independently of the
/etc/groupsfile 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 (
The control-flag is used to indicate how the PAM library will react to
the success or failure of the module it is associated with. Since
modules can be stacked (modules of the same type execute in
series, one after another), the control-flags determine the relative
importance of each module. 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. As of Linux-PAM v0.60, this control-flag can be defined
with one of two syntaxes.
The simpler (and historical) syntax for the control-flag is a single
keyword defined to indicate the severity of concern associated with
the success or failure of a specific module. There are four such
The Linux-PAM library interprets these keywords in the following manner:
required; this indicates that the success of the module is required for the
module-typefacility 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.
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
requisitemodule 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
requiredmodule has failed, no more `stacked' modules of this type are invoked. (Note, in this case subsequent
requiredmodules are not invoked.). A failure of this module is not deemed as fatal to satisfying the application that this
optional; as its name suggests, this
control-flagmarks 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.
The more elaborate (newer) syntax is much more specific and gives the
administrator a great deal of control over how the user is
authenticated. This form of the control flag is delimeted with square
brackets and consists of a series of
[value1=action1 value2=action2 ...]
valueI is one of the following return values:
default. The last of these (
default) can be used to set the
action for those return values that are not set explicitly.
actionI can be a positive integer or one of the following
reset. A positive integer,
J, when specified as the action
can be used to indicate that the next J modules of the current
type will be skipped. In this way, the administrator can develop a
moderately sophisticated stack of modules with a number of different
paths of execution. Which path is taken can be determined by the
reactions of individual modules.
Note, at time of writing, this newer syntax is so new that I don't want to write too much about it. Please play with this. Report all the bugs and make suggestions for new actions (etc.).
The path-name of the dynamically loadable object file; the
pluggable module itself. If the first character of the module path is
/', it is assumed to be a complete path. If this is not the
case, the given module path is appended to the default module path:
/usr/lib/security (but see the notes
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
syslog(3). For a list of generic options see the
Any line in (one of) the confiuration file(s), that is not formatted
correctly, will generally tend (erring on the side of caution) to make
the authentication process fail. A corresponding error is written to
the system log files with a call to
More flexible than the single configuration file, as of version 0.56,
it is possible to configure
libpam via the contents of the
/etc/pam.d/ directory. In this case the directory is filled
with files each of which has a filename equal to a service-name (in
lower-case): it is the personal configuration file for the named
Linux-PAM can be compiled in one of two modes. The preferred
mode uses either
configuration but not both. That is to say, if there is a
/etc/pam.d/ directory then libpam only uses the files
contained in this directory. However, in the absence of the
/etc/pam.d/ directory the
/etc/pam.conf file is
used. The other mode (and the one currently supported by Red Hat 4.2)
is to use both
sequence. In this mode, entries in
The syntax of each file in
/etc/pam.d/ is similar to that of
/etc/pam.conf file and is made up of lines of the
The only difference being that the
module-type control-flag module-path arguments
service-nameis not present. The service-name is of course the name of the given configuration file. For example,
/etc/pam.d/logincontains the configuration for the login service.
This method of configuration has a number of advantages over the single file approach. We list them here to assist the reader in deciding which scheme to adopt:
The following are optional arguments which are likely to be understood by any module. Arguments (including these) are in general optional.
syslog(3) call to log debugging information to the system
Instruct module to not give warning messages to the application.
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
password modules only).
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).
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 section bibliography for a pointer to this document.
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
In 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!):
Whilst fundamentally a secure default, this is not very sympathetic to a misconfigured system. For example, such a system is vulnerable to locking everyone out should the rest of the file become badly written.
# # 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
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.
Having two ``
# # 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 system that uses the
/etc/pam.d/ configuration, the
corresponding default setup would be achieved with the following file:
This is the only explicit example we give for an
# # default configuration: /etc/pam.d/other # auth required /usr/lib/security/pam_warn.so auth required /usr/lib/security/pam_deny.so account required /usr/lib/security/pam_deny.so password required /usr/lib/security/pam_warn.so password required /usr/lib/security/pam_deny.so session required /usr/lib/security/pam_deny.so
/etc/pam.d/file. In general, it should be clear how to transpose the remaining examples to this configuration scheme.
On a less sensitive computer, one on which the system administrator
wishes to remain ignorant of much of the power of
following selection of lines (in
/etc/pam.conf) is likely to
mimic the historically familiar Linux setup.
In general this will provide a starting place for most applications. Unfortunately, most is not all. One application that might require additional lines is ftpd if you wish to enable anonymous-ftp.
# # 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_session.so
To enable anonymous-ftp, the following lines might be used to replace
the default (
OTHER) ones. (*WARNING* as of 1996/12/28 this
does not work correctly with any ftpd. Consequently, this description
may be subject to change or the application will be fixed.)
Note, the second line is necessary since the default entries are ignored by a service application (here ftpd) if there are any entries in
# # ftpd; 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.conffor that specified service. Again, this is an example of authentication module stacking. Note the use of the
sufficientcontrol-flag. It says that ``if this module authenticates the user, ignore the subsequent
authmodules''. 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
libc' user database functions (see for example,
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_pwdb. See the section below for a more complete