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<article>

<title>The Linux-PAM System Administrators' Guide
<author>Andrew G. Morgan, <tt>morgan@kernel.org</tt>
<date>DRAFT v0.76 2001/12/08
<abstract>
This manual documents what a system-administrator needs to know about
the <bf>Linux-PAM</bf> library. It covers the correct syntax of the
PAM configuration file and discusses strategies for maintaining a
secure system.
</abstract>

<!-- Table of contents -->
<toc>

<!-- Begin the document -->

<sect>Introduction

<p><bf/Linux-PAM/ (Pluggable Authentication Modules for Linux) is a
suite of shared libraries that enable the local system administrator
to choose how applications authenticate users.

<p>In other words, without (rewriting and) recompiling a PAM-aware
application, it is possible to switch between the authentication
mechanism(s) it uses. Indeed, one may entirely upgrade the local
authentication system without touching the applications themselves.

<p>Historically an application that has required a given user to be
authenticated, has had to be compiled to use a specific authentication
mechanism.  For example, in the case of traditional UN*X systems, the
identity of the user is verified by the user entering a correct
password.  This password, after being prefixed by a two character
``salt'', is encrypted (with crypt(3)). The user is then authenticated
if this encrypted password is identical to the second field of the
user's entry in the system password database (the <tt>/etc/passwd</tt>
file).  On such systems, most if not all forms of privileges are
granted based on this single authentication scheme. Privilege comes in
the form of a personal user-identifier (<tt/uid/) and membership of
various groups. Services and applications are available based on the
personal and group identity of the user. Traditionally, group
membership has been assigned based on entries in the
<tt>/etc/group</tt> file.

<p>
Unfortunately, increases in the speed of computers and the
widespread introduction of network based computing, have made once
secure authentication mechanisms, such as this, vulnerable to
attack. In the light of such realities, new methods of authentication
are continuously being developed.

<p>
It is the purpose of the <bf/Linux-PAM/ project to separate the
development of privilege granting software from the development of
secure and appropriate authentication schemes.  This is accomplished
by providing a library of functions that an application may use to
request that a user be authenticated. This PAM library is configured
locally with a system file, <tt>/etc/pam.conf</tt> (or a series of
configuration files located in <tt>/etc/pam.d/</tt>) to authenticate a
user request via the locally available authentication modules. The
modules themselves will usually be located in the directory
<tt>/lib/security</tt> and take the form of dynamically loadable
object files (see <tt/dlopen(3)/).

<sect>Some comments on the text<label id="text-conventions">

<p>
Before proceeding to read the rest of this document, it should be
noted that the text assumes that certain files are placed in certain
directories.  Where they have been specified, the conventions we adopt
here for locating these files are those of the relevant RFC (RFC-86.0,
see <ref id="see-also-sec" name="bibliography">).  If you are using a
distribution of Linux (or some other operating system) that supports
PAM but chooses to distribute these files in a diferent way (Red Hat
is one such distribution), you should be careful when copying examples
directly from the text.

<p>
As an example of the above, where it is explicit, the text assumes
that PAM loadable object files (the <em/modules/) are to be located in
the following directory: <tt>/lib/security/</tt>. This is generally
the location that seems to be compatible with the Linux File System
Standard (the FSSTND). On Solaris, which has its own licensed version
of PAM, and some other implementations of UN*X, these files can be
found in <tt>/usr/lib/security</tt>. Please be careful to perform the
necessary transcription when using the examples from the text.

<sect>Overview<label id="overview-section">

<p>
For the uninitiated, we begin by considering an example.  We take an
application that grants some service to users; <em/login/ is one such
program. <em/Login/ does two things, it first establishes that the
requesting user is whom they claim to be and second provides them with
the requested service: in the case of <em/login/ the service is a
command shell (<em>bash, tcsh, zsh, etc.</em>) running with the
identity of the user.

<p>
Traditionally, the former step is achieved by the <em/login/
application prompting the user for a password and then verifying that
it agrees with that located on the system; hence verifying that
as far as the system is concerned the user is who they claim to be.
This is the task that is delegated to <bf/Linux-PAM/.

<p>
From the perspective of the application programmer (in this case the
person that wrote the <em/login/ application), <bf/Linux-PAM/ takes
care of this authentication task -- verifying the identity of the user.

<p>
The flexibility of <bf/Linux-PAM/ is that <em/you/, the system
administrator, have the freedom to stipulate which authentication
scheme is to be used.  You have the freedom to set the scheme for
any/all PAM-aware applications on your Linux system.  That is, you can
authenticate from anything as naive as <em/simple trust/
(<tt/pam_permit/) to something as paranoid as a combination of a
retinal scan, a voice print and a one-time password!

<p>
To illustrate the flexibility you face, consider the following
situation: a system administrator (parent) wishes to improve the
mathematical ability of her users (children). She can configure their
favorite ``Shoot 'em up game'' (PAM-aware of course) to authenticate
them with a request for the product of a couple of random numbers less
than 12. It is clear that if the game is any good they will soon learn
their <em/multiplication tables/.   As they mature, the authentication
can be upgraded to include (long) division!

<p>
<bf/Linux-PAM/ deals with four separate types of (management)
task. These are: <em/authentication management/; <em/account
management/; <em/session management/; and <em/password management/.
The association of the preferred management scheme with the behavior
of an application is made with entries in the relevant <bf/Linux-PAM/
configuration file.  The management functions are performed by
<em/modules/ specified in the configuration file. The syntax for this
file is discussed in the section <ref id="configuration"
name="below">.

<p>
Here is a figure that describes the overall organization of
<bf/Linux-PAM/.
<tscreen>
<verb>
         +----------------+
         | application: X |
         +----------------+       /  +----------+     +================+
         | authentication-[---->--\--] Linux-   |--<--| PAM config file|
         |       +        [----<--/--]   PAM    |     |================|
         |[conversation()][--+    \  |          |     | X auth .. a.so |
         +----------------+  |    /  +-n--n-----+     | X auth .. b.so |
         |                |  |       __|  |           |           _____/
         |  service user  |  A      |     |           |____,-----' 
         |                |  |      V     A                        
         +----------------+  +------|-----|---------+ -----+------+
                                +---u-----u----+    |      |      |
                                |   auth....   |--[ a ]--[ b ]--[ c ]
                                +--------------+
                                |   acct....   |--[ b ]--[ d ]
                                +--------------+
                                |   password   |--[ b ]--[ c ]
                                +--------------+
                                |   session    |--[ e ]--[ c ]
                                +--------------+
</verb>
</tscreen>
By way of explanation, the left of the figure represents the
application; application X.  Such an application interfaces with the
<bf/Linux-PAM/ library and knows none of the specifics of its
configured authentication method.  The <bf/Linux-PAM/ library (in the
center) consults the contents of the PAM configuration file and loads
the modules that are appropriate for application-X. These modules fall
into one of four management groups (lower-center) and are stacked in
the order they appear in the configuration file. These modules, when
called by <bf/Linux-PAM/, perform the various authentication tasks for
the application. Textual information, required from/or offered to the
user, can be exchanged through the use of the application-supplied
<em/conversation/ function.

<sect1>Getting started

<p>
The following text was contributed by Seth Chaiklin:
<tscreen>
<verb>
To this point, we have described how PAM should work in an
ideal world, in which all applications are coded properly.
However, at the present time (October 1998), this is far
from the case.  Therefore, here are some practical considerations
in trying to use PAM in your system.

Why bother, is it really worth all the trouble?  

If you running Linux as a single user system, or in an
environment where all the users are trusted, then there 
is no real advantage for using PAM.
</verb>
</tscreen>

<p>
<BF>Ed:</BF> there is actually an advantage since you can <em/dummy
down/ the authentication to the point where you don't have
any... Almost like Win95.
<p>
In a networked environment, it is clear that you need to think a
little more about how users etc., are authenticated:]

<p>
<tscreen>
<verb>
If you are running Linux as a server, where several different
services are being provided (e.g., WWW with areas restricted by
password control, PPP), then there can be some real and interesting
value for PAM.  In particular, through the use of modules, PAM can
enable a program to search through several different password
databases, even if that program is not explicitly coded for
that particular database.  Here are some examples of the possibilities
that this enables.

   o  Apache has a module that provides PAM services.  Now
   authentication
      to use particular directories can be conducted by PAM, which
      means that the range of modules that are available to PAM can
      be used, including RADIUS, NIS, NCP (which means that Novell
      password databases can be used).

   o  pppd has a PAMified version (available from RedHat)  Now it is
      possible to use a series of databases to authenticate ppp users.
      In addition to the normal Linux-based password databases (such
      as /etc/passwd and /etc/shadow), you can use PAM modules to 
      authenticate against Novell password databases or NT-based 
      password databases. 

   o  The preceding two examples can be combined.  Imagaine that the
      persons in your office/department are already registered with a
      username and password in a Novell or NT LAN.  If you wanted to
      use this database on your Linux server (for PPP access, for 
      web access, or even for normal shell access), you can use PAM
      to authenticate against this existing database, rather than
      maintain a separate database on both Linux and the LAN server.


Can I use PAM for any program that requires authentication?

Yes and no.   Yes, if you have access to the source code, and can
add the appropriate PAM functions.  No, if you do not have access 
to the source code, and the binary does not have the PAM functions
included.

In other words, if a program is going to use PAM, then it has to
have PAM functions explicitly coded into the program.  If they
are not, then it is not possible to use PAM. 

How can I tell whether a program has PAM coded into it or not?

A quick-and-dirty (but not always reliable) method is to ldd
<programname>
If libpam and libpam_misc are not among the libraries that the program
uses, then it is not going to work with PAM.  However, it is possible
that the libraries are included, but there are still problems, because
the PAM coding in the program does not work as it should.  So a
more reliable method is to make the follow tests.

In the /etc/pam.d directory, one needs to make a configuration file
for the program that one wants to run.  The exact name of the
configuration
file is hard-coded into the program.  Usually, it is the same name as
the
program, but not always.  For sake of illustration, let's assume that
the program is named "pamprog" and the name of the configuration file
is /etc/pam.d/pamprog.

In the /etc/pam.d/pamprog but the following two lines:

auth    required  pam_permit.so
auth    required  pam_warn.so


Now try to use pamprog.  The first line in the configuration file 
says that all users are permitted.  The second line will write a
warning to your syslog file (or whether you syslog is writing

messages).  If this test succeeds, then you know that you have
a program that can understand pam, and you can start the more
interesting work of deciding how to stack modules in your
/etc/pam.d/pamprog  file.
</verb>
</tscreen>

<sect>The Linux-PAM configuration file
<label id="configuration">

<p>
<bf/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 <tt/Linux-PAM/ is contained in one of
two places: either the single system file, <tt>/etc/pam.conf</tt>; or
the <tt>/etc/pam.d/</tt> directory.  In this section we discuss the
correct syntax of and generic options respected by entries to these
files.

<sect1>Configuration file syntax

<p>
The reader should note that the <bf/Linux-PAM/ specific tokens in this
file are case <em/insensitive/. The module paths, however, are case
sensitive since they indicate a file's <em/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.

<p>
In addition to the lines described below, there are two <em/special/
characters provided for the convenience of the system administrator:
comments are preceded by a `<tt/&num;/' and extend to the
next end-of-line; also, module specification lines may be extended
with a `<tt/&bsol;/' escaped newline.

<p>
A general configuration line of the <tt>/etc/pam.conf</tt> file has
the following form:
<tscreen>
<verb>
service-name   module-type   control-flag   module-path   arguments
</verb>
</tscreen>
Below, we explain the meaning of each of these tokens. The second (and
more recently adopted) way of configuring <bf/Linux-PAM/ is via the
contents of the <tt>/etc/pam.d/</tt> directory. Once we have explained
the meaning of the above tokens, we will describe this method.

<p>
<descrip>
<tag><tt/service-name/</tag>
The name of the service associated with this entry. Frequently the
service name is the conventional name of the given application. For
example, `<tt/ftpd/', `<tt/rlogind/' and `<tt/su/', <em/etc./ .

<p>
There is a special <tt/service-name/, reserved for defining a default
authentication mechanism. It has the name `<tt/OTHER/' and may be
specified in either lower or upper case characters. Note, when there
is a module specified for a named service, the `<tt/OTHER/' entries
are ignored.

<tag><tt/module-type/</tag>
One of (currently) four types of module. The four types are as
follows:
<itemize>
<item> <tt/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 <tt/group/ membership (independently of the
<tt>/etc/groups</tt> file discussed above) or other privileges through
its <em/credential/ granting properties.

<item> <tt/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---`<tt/root/' login only on the console.

<item> <tt/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. .

<item> <tt/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 (<tt/auth/)
module-type.

</itemize>

<tag><tt/control-flag/</tag>

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 <em/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
`<tt>/etc/pam.conf</tt>' file.  Instead, it receives a summary
<em/success/ or <em/fail/ response from the <bf/Linux-PAM/ library.
The order of execution of these modules is that of the entries in the
<tt>/etc/pam.conf</tt> file; earlier entries are executed before later
ones.  As of Linux-PAM v0.60, this <em/control-flag/ can be defined
with one of two syntaxes.

<p>
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
keywords: <tt/required/, <tt/requisite/, <tt/sufficient/ and
<tt/optional/.

<p>
The Linux-PAM library interprets these keywords in the following
manner:

<itemize>

<item> <tt/required/; this indicates that the success of the module is
required for the <tt/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 <tt/module-type/) have been executed.

<item> <tt/requisite/; like <tt/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 <em/first/
<tt/required/ or <tt/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.

<item> <tt/sufficient/; the success of this module is deemed
`<em/sufficient/' to satisfy the <bf/Linux-PAM/ library that this
module-type has succeeded in its purpose. In the event that no
previous <tt/required/ module has failed, no more `<em/stacked/'
modules of this type are invoked. (Note, in this case subsequent
<tt/required/ modules are <bf/not/ invoked.). A failure of this module
is not deemed as fatal to satisfying the application that this
<tt/module-type/ has succeeded.

<item> <tt/optional/; as its name suggests, this <tt/control-flag/
marks the module as not being critical to the success or failure of
the user's application for service.  In general, <bf/Linux-PAM/
ignores such a module when determining if the module stack will
succeed or fail.  However, in the absence of any definite successes or
failures of previous or subsequent stacked modules this module will
determine the nature of the response to the application.  One example
of this latter case, is when the other modules return something like
<tt/PAM_IGNORE/.

</itemize>

<p>
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 <tt/value=action/ tokens:
<tscreen>
<verb>
    [value1=action1 value2=action2 ...]
</verb>
</tscreen>

<p>
Here, <tt/valueI/ is one of the following <em/return values/:
<tt/success/; <tt/open_err/; <tt/symbol_err/; <tt/service_err/;
<tt/system_err/; <tt/buf_err/; <tt/perm_denied/; <tt/auth_err/;
<tt/cred_insufficient/; <tt/authinfo_unavail/; <tt/user_unknown/;
<tt/maxtries/; <tt/new_authtok_reqd/; <tt/acct_expired/;
<tt/session_err/; <tt/cred_unavail/; <tt/cred_expired/; <tt/cred_err/;
<tt/no_module_data/; <tt/conv_err/; <tt/authtok_err/;
<tt/authtok_recover_err/; <tt/authtok_lock_busy/;
<tt/authtok_disable_aging/; <tt/try_again/; <tt/ignore/; <tt/abort/;
<tt/authtok_expired/; <tt/module_unknown/; <tt/bad_item/; and
<tt/default/.  The last of these (<tt/default/) can be used to set the
action for those return values that are not explicitly defined.

<p>
The <tt/actionI/ can be a positive integer or one of the following
tokens: <tt/ignore/; <tt/ok/; <tt/done/; <tt/bad/; <tt/die/; and
<tt/reset/.  A positive integer, <tt/J/, when specified as the action,
can be used to indicate that the next <em/J/ modules of the current
module-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.

<p>
<itemize>
<item><tt/ignore/ - when used with a stack of modules, the module's
  return status will not contribute to the return code the application
  obtains.
<item><tt/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.
<item><tt/die/ - equivalent to <tt/bad/ with the side effect of
  terminating the module stack and PAM immediately returning to the
  application.
<item><tt/ok/ - this tells <bf/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 <tt/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.
<item><tt/done/ - equivalent to <tt/ok/ with the side effect of
  terminating the module stack and PAM immediately returning to the
  application.
<item><tt/reset/ - clear all memory of the state of the module stack and
  start again with the next stacked module.
</itemize>

<p>
Just to get a feel for the power of this new syntax, here is a taste
of what you can do with it.  With <bf/Linux-PAM-0.63/, the notion of
client plug-in agents was introduced.  This is something that makes it
possible for PAM to support machine-machine authentication using the
transport protocol inherent to the client/server application.  With
the ``<tt/[ ... value=action ... ]/'' control syntax, it is possible
for an application to be configured to support binary prompts with
compliant clients, but to gracefully fall over into an alternative
authentication mode for older, legacy, applications.

<tag> <tt/module-path/</tag>

The path-name of the dynamically loadable object file; <em/the
pluggable module/ itself. If the first character of the module path is
`<tt>/</tt>', 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:
<tt>/lib/security</tt> (but see the notes <ref id="text-conventions"
name="above">).

<tag> <tt/args/</tag>

The <tt/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 <tt/syslog(3)/. For a list of <em/generic/ options see the
next section.

</descrip>

<p>
Any line in (one of) the configuration 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 <tt/syslog(3)/.

<sect1>Directory based configuration

<p>
More flexible than the single configuration file, as of version 0.56,
it is possible to configure <tt>libpam</tt> via the contents of the
<tt>/etc/pam.d/</tt> 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
service.

<p>
<bf/Linux-PAM/ can be compiled in one of two modes.  The preferred
mode uses either <tt>/etc/pam.d/</tt> or <tt>/etc/pam.conf</tt>
configuration but not both.  That is to say, if there is a
<tt>/etc/pam.d/</tt> directory then libpam only uses the files
contained in this directory.  However, in the absence of the
<tt>/etc/pam.d/</tt> directory the <tt>/etc/pam.conf</tt> file is
used.  The other mode (and the one currently supported by Red Hat 4.2
and higher) is to use both <tt>/etc/pam.d/</tt> and
<tt>/etc/pam.conf</tt> in sequence.  In this mode, entries in
<tt>/etc/pam.d/</tt> override those of <tt>/etc/pam.conf</tt>.

The syntax of each file in <tt>/etc/pam.d/</tt> is similar to that of
the <tt>/etc/pam.conf</tt> file and is made up of lines of the
following form:
<tscreen>
<verb>
module-type   control-flag   module-path   arguments
</verb>
</tscreen>
The only difference being that the <tt>service-name</tt> is not
present.   The service-name is of course the name of the given
configuration file.  For example, <tt>/etc/pam.d/login</tt> contains
the configuration for the <em>login</em> service.

<p>
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:

<p>
<itemize>

<item>A lower chance of misconfiguring an application. There is one
less field to mis-type when editing the configuration files by hand.

<item>Easier to maintain. One application may be reconfigured without
risk of interfering with other applications on the system.

<item>It is possible to symbolically link different services
configuration files to a single file. This makes it easier to keep the
system policy for access consistent across different applications.
(It should be noted, to conserve space, it is equally possible to
<em>hard</em> link a number of configuration files.  However, care
should be taken when administering this arrangement as editing a hard
linked file is likely to break the link.)

<item>A potential for quicker configuration file parsing. Only the
relevant entries are parsed when a service gets bound to its modules.

<item>It is possible to limit read access to individual <bf/Linux-PAM/
configuration files using the file protections of the filesystem.

<item>Package management becomes simpler.  Every time a new
application is installed, it can be accompanied by an
<tt>/etc/pam.d/</tt><em>xxxxxx</em> file.

</itemize>

<sect1>Generic optional arguments

<p>
The following are optional arguments which are likely to be understood
by any module. Arguments (including these) are in general
<em/optional/.

<p>
<descrip>
<tag><tt/debug/</tag>

Use the <tt/syslog(3)/ call to log debugging information to the system
log files.

<tag> <tt/no_warn/</tag>

Instruct module to not give warning messages to the application.

<tag> <tt/use_first_pass/</tag>

The module should not prompt the user for a password. Instead, it
should obtain the previously typed password (from the preceding
<tt/auth/ module), and use that. If that doesn't work, then the user
will not be authenticated. (This option is intended for <tt/auth/
and <tt/password/ modules only).

<tag> <tt/try_first_pass/</tag>

The module should attempt authentication with the previously typed
password (from the preceding <tt/auth/ module). If that doesn't work,
then the user is prompted for a password. (This option is intended for
<tt/auth/ modules only).

<tag> <tt/use_mapped_pass/</tag>

This argument is not currently supported by any of the modules in the
<bf/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 <bf/DCE-RFC 86.0/, see section <ref
id="see-also-sec" name="bibliography"> for a pointer to this document.

<p>
The <tt/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 <tt/auth/ and <tt/password/ module
types only.

<tag><tt/expose_account/</tag>

<p>
In general the leakage of some information about user accounts is not
a secure policy for modules to adopt. Sometimes information such as
users names or home directories, or preferred shell, can be used to
attack a user's account. In some circumstances, however, this sort of
information is not deemed a threat: displaying a user's full name when
asking them for a password in a secured environment could also be
called being 'friendly'. The <tt/expose_account/ argument is a
standard module argument to encourage a module to be less discrete
about account information as it is deemed appropriate by the local
administrator.

</descrip>

<sect1>Example configuration file entries

<p>
In this section, we give some examples of entries that can be present
in the <bf/Linux-PAM/ configuration file. As a first attempt at
configuring your system you could do worse than to implement these.

<sect2>Default policy

<p>
If a system is to be considered secure, it had better have a
reasonably secure `<tt/OTHER/' entry. The following is a paranoid
setting (which is not a bad place to start!):
<tscreen>
<verb>
#
# default; deny access
#
OTHER   auth     required       pam_deny.so
OTHER   account  required       pam_deny.so
OTHER   password required       pam_deny.so
OTHER   session  required       pam_deny.so
</verb>
</tscreen>
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.

<p>
The module <tt/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.

<p>
The addition of the following line before those in the above example
would provide a suitable warning to the administrator.
<tscreen>
<verb>
#
# default; wake up! This application is not configured
#
OTHER   auth     required       pam_warn.so
OTHER   password required       pam_warn.so
</verb>
</tscreen>
Having two ``<tt/OTHER auth/'' lines is an example of stacking.

<p>
On a system that uses the <tt>/etc/pam.d/</tt> configuration, the
corresponding default setup would be achieved with the following file:
<tscreen>
<verb>
#
# default configuration: /etc/pam.d/other
#
auth     required       pam_warn.so
auth     required       pam_deny.so
account  required       pam_deny.so
password required       pam_warn.so
password required       pam_deny.so
session  required       pam_deny.so
</verb>
</tscreen>
This is the only explicit example we give for an <tt>/etc/pam.d/</tt>
file. In general, it should be clear how to transpose the remaining
examples to this configuration scheme.

<p>
On a less sensitive computer, one on which the system administrator
wishes to remain ignorant of much of the power of <tt/Linux-PAM/, the
following selection of lines (in <tt>/etc/pam.conf</tt>) is likely to
mimic the historically familiar Linux setup.
<tscreen>
<verb>
#
# default; standard UN*X access
#
OTHER   auth     required       pam_unix.so
OTHER   account  required       pam_unix.so
OTHER   password required       pam_unix.so
OTHER   session  required       pam_unix.so
</verb>
</tscreen>
In general this will provide a starting place for most applications.
Unfortunately, most is not all. One application that might require
additional lines is <em/ftpd/ if you wish to enable
<em/anonymous-ftp/.

<p>
To enable anonymous-ftp, the following lines might be used to replace
the default (<tt/OTHER/) ones. (<bf/*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.)
<tscreen>
<verb>
#
# ftpd; add ftp-specifics. These lines enable anonymous ftp over
#       standard UN*X access (the listfile entry blocks access to
#       users listed in /etc/ftpusers)
#
ftpd    auth    sufficient  pam_ftp.so
ftpd    auth    required    pam_unix_auth.so use_first_pass
ftpd    auth    required    pam_listfile.so \
        onerr=succeed item=user sense=deny file=/etc/ftpusers
</verb>
</tscreen>
Note, the second line is necessary since the default entries are
ignored by a service application (here <em/ftpd/) if there are
<em/any/ entries in <tt>/etc/pam.conf</tt> for that specified service.
Again, this is an example of authentication module stacking.  Note the
use of the <tt/sufficient/ control-flag. It says that ``if this module
authenticates the user, ignore the subsequent <tt/auth/
modules''. Also note the use of the ``<tt/use_first_pass/''
module-argument, this instructs the UN*X authentication module that it
is not to prompt for a password but rely on one already having been
obtained by the <tt/pam_ftp/ module.

<sect>Security issues of Linux-PAM

<p>
This section will discuss good practices for using PAM in a secure
manner.  <em>It is currently sadly lacking...suggestions are
welcome!</em>

<sect1>If something goes wrong

<p>
<bf/Linux-PAM/ has the potential to seriously change the security of
your system.  You can choose to have no security or absolute security
(no access permitted).  In general, <bf/Linux-PAM/ errs towards the
latter.  Any number of configuration errors can dissable access to
your system partially, or completely.

<p>
The most dramatic problem that is likely to be encountered when
configuring <bf/Linux-PAM/ is that of <em>deleting</em> the
configuration file(s): <tt>/etc/pam.d/*</tt> and/or
<tt>/etc/pam.conf</tt>.  This will lock you out of your own system!

<p>
To recover, your best bet is to reboot the system in single user mode
and set about correcting things from there.  The following has been
<em>adapted</em> from a life-saving email on the subject from David
Wood:
<verb>
> What the hell do I do now?

OK, don't panic. The first thing you have to realize is that
this happens to 50% of users who ever do anything with PAM.
It happened here, not once, not twice, but three times, all
different, and in the end, the solution was the same every
time.

First, I hope you installed LILO with a delay. If you can,
reboot, hit shift or tab or something and type:

    LILO boot: linux single

(Replace 'linux' with 'name-of-your-normal-linux-image').
This will let you in without logging in.  Ever wondered how
easy it is to break into a linux machine from the console?
Now you know.

If you can't do that, then get yourself a bootkernel floppy
and a root disk a-la slackware's rescue.gz.  (Red Hat's
installation disks can be used in this mode too.)

In either case, the point is to get back your root prompt.

Second, I'm going to assume that you haven't completely
nuked your pam installation - just your configuration files.
Here's how you make your configs nice again:

    cd /etc
    mv pam.conf pam.conf.orig
    mv pam.d pam.d.orig
    mkdir pam.d
    cd pam.d

and then use vi to create a file called "other" in this
directory.  It should contain the following four lines:

    auth     required       pam_unix.so
    account  required       pam_unix.so
    password required       pam_unix.so
    session  required       pam_unix.so

Now you have the simplest possible PAM configuration that
will work the way you're used to.  Everything should
magically start to work again.  Try it out by hitting ALT-F2
and logging in on another virtual console.  If it doesn't
work, you have bigger problems, or you've mistyped
something.  One of the wonders of this system (seriously,
perhaps) is that if you mistype anything in the conf files,
you usually get no error reporting of any kind on the
console - just some entries in the log file.  So look there!
(Try 'tail /var/log/messages'.)

From here you can go back and get a real configuration
going, hopefully after you've tested it first on a machine
you don't care about screwing up.  :/

Some pointers (to make everything "right" with Red Hat...):

    Install the newest pam, pamconfig, and pwdb from the
    redhat current directory, and do it all on the same
    command line with rpm...

        rpm -Uvh [maybe --force too] pam-* pamconfig-* pwdb-*

    Then make sure you install (or reinstall) the newest
    version of libc, util-linux, wuftp, and NetKit. For
    kicks you might try installing the newest versions of
    the affected x apps, like xlock, but I haven't gotten
    those to work at all yet.

</verb>

<sect1>Avoid having a weak `other' configuration

<p>
It is not a good thing to have a weak default (<tt/OTHER/) entry.
This service is the default configuration for all PAM aware
applications and if it is weak, your system is likely to be vulnerable
to attack.

<p>
Here is a sample "other" configuration file. The <em/pam_deny/ module will
deny access and the <em/pam_warn/ module will send a syslog message to
<tt/auth.notice/:

<p>
<tscreen>
<verb>
#
# The PAM configuration file for the `other' service 
# 
auth      required   pam_deny.so 
auth      required   pam_warn.so 
account   required   pam_deny.so 
account   required   pam_warn.so 
password  required   pam_deny.so 
password  required   pam_warn.so 
session   required   pam_deny.so 
session   required   pam_warn.so
</verb>
</tscreen>

<sect>A reference guide for available modules

<p>
Here, we collect together some descriptions of the various modules
available for <bf/Linux-PAM/.  In general these modules should be
freely available.  Where this is not the case, it will be indicated.

<p>
Also please note the comments contained in the section <ref 
id="text-conventions" name="on text conventions above"> when copying
the examples listed below.

<!-- insert-file MODULES-SGML -->

<sect>Files

<p><descrip>

<tag><tt>/lib/libpam.so.*</tt></tag>

the shared library providing applications with access to
<bf/Linux-PAM/.

<tag><tt>/etc/pam.conf</tt></tag>

the <bf/Linux-PAM/ configuration file.

<tag><tt>/lib/security/pam_*.so</tt></tag>

the primary location for <bf/Linux-PAM/ dynamically loadable object
files; the modules.

</descrip>

<sect>See also<label id="see-also-sec">

<p><itemize>

<item>The <bf/Linux-PAM/ Application Writers' Guide.

<item>The <bf/Linux-PAM/ Module Writers' Guide.

<item>The V. Samar and R. Schemers (SunSoft), ``UNIFIED LOGIN WITH
PLUGGABLE AUTHENTICATION MODULES'', Open Software Foundation Request
For Comments 86.0, October 1995. See this url:
<tt><htmlurl
url="http://www.kernel.org/pub/linux/libs/pam/pre/doc/rfc86.0.txt.gz"
name="http://www.kernel.org/pub/linux/libs/pam/pre/doc/rfc86.0.txt.gz"></tt>

</itemize>

<sect>Notes

<p>
I intend to put development comments here... like ``at the moment
this isn't actually supported''. At release time what ever is in
this section will be placed in the Bugs section below! :)

<p>
Are we going to be able to support the <tt/use_mapped_pass/ module
argument? Anyone know a cheap (free) good lawyer?!

<p>
<itemize>
<item>
This issue may go away, as Sun have investigated adding a new
management group for mappings. In this way, libpam would have mapping
modules that could securely store passwords using strong cryptography
and in such a way that they need not be distributed with Linux-PAM.
</itemize>

<sect>Author/acknowledgments

<p>
This document was written by Andrew G. Morgan (morgan@kernel.org)
with many contributions from
<!-- insert-file CREDITS -->

<p>
Thanks are also due to Sun Microsystems, especially to Vipin Samar and
Charlie Lai for their advice. At an early stage in the development of
<bf/Linux-PAM/, Sun graciously made the documentation for their
implementation of PAM available. This act greatly accelerated the
development of <bf/Linux-PAM/.

<sect>Bugs/omissions

<p>
More PAM modules are being developed all the time. It is unlikely that
this document will ever be truely up to date!

<p>
This manual is unfinished. Only a partial list of people is credited
for all the good work they have done.

<sect>Copyright information for this document

<p>
Copyright (c) Andrew G. Morgan 1996-9.  All rights reserved.
<newline>
Email: <tt>&lt;morgan@linux.kernel.org&gt;</tt>

<p>
Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions are
met:

<p>
<itemize>

<item>
1. Redistributions of source code must retain the above copyright
   notice, and the entire permission notice in its entirety,
   including the disclaimer of warranties.

<item>
2. Redistributions in binary form must reproduce the above copyright
   notice, this list of conditions and the following disclaimer in the
   documentation and/or other materials provided with the distribution.

<item>
3. The name of the author may not be used to endorse or promote
   products derived from this software without specific prior
   written permission.

</itemize>

<p>
<bf/Alternatively/, this product may be distributed under the terms of
the GNU General Public License (GPL), in which case the provisions of
the GNU GPL are required <bf/instead of/ the above restrictions.
(This clause is necessary due to a potential bad interaction between
the GNU GPL and the restrictions contained in a BSD-style copyright.)

<p>
THIS SOFTWARE IS PROVIDED ``AS IS'' AND ANY EXPRESS OR IMPLIED
WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR
TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE
USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH
DAMAGE.

<p>
<tt>$Id$</tt>

</article>