# The Linux-PAM System Administrators' Guide

Andrew G. Morgan morgan@kernel.org
Thorsten Kukuk kukuk@thkukuk.de
Version 1.1.2, 31. August 2010

This manual documents what a system-administrator needs to know about the Linux-PAM library. It covers the correct syntax of the PAM configuration file and discusses strategies for maintaining a secure system.

## Introduction

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.

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.

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 `/etc/passwd` 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 (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 `/etc/group` file.

It is the purpose of the 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, `/etc/pam.conf` (or a series of configuration files located in `/etc/pam.d/`) to authenticate a user request via the locally available authentication modules. The modules themselves will usually be located in the directory `/lib/security` or `/lib64/security` and take the form of dynamically loadable object files (see dlopen 3 ).

## Some comments on the text

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 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 different way you should be careful when copying examples directly from the text.

As an example of the above, where it is explicit, the text assumes that PAM loadable object files (the modules) are to be located in the following directory: `/lib/security/` or `/lib64/security` depending on the architecture. This is generally the location that seems to be compatible with the Filesystem Hierarchy Standard (FHS). On Solaris, which has its own licensed version of PAM, and some other implementations of UN*X, these files can be found in `/usr/lib/security`. Please be careful to perform the necessary transcription when using the examples from the text.

## Overview

For the uninitiated, we begin by considering an example. We take an application that grants some service to users; `login` is one such program. `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 `login` the service is a command shell (bash, tcsh, zsh, etc.) running with the identity of the user.

Traditionally, the former step is achieved by the `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 Linux-PAM.

From the perspective of the application programmer (in this case the person that wrote the `login` application), Linux-PAM takes care of this authentication task -- verifying the identity of the user.

The flexibility of Linux-PAM is that 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 simple trust (`pam_permit`) to something as paranoid as a combination of a retinal scan, a voice print and a one-time password!

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 multiplication tables. As they mature, the authentication can be upgraded to include (long) division!

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

Here is a figure that describes the overall organization of Linux-PAM:

      +----------------+
      | 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 ]
                             +--------------+

By way of explanation, the left of the figure represents the application; application X. Such an application interfaces with the Linux-PAM library and knows none of the specifics of its configured authentication method. The 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 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 conversation function.

If a program is going to use PAM, then it has to have PAM functions explicitly coded into the program. If you have access to the source code you can add the appropriate PAM functions. If you do not have access to the source code, and the binary does not have the PAM functions included, then it is not possible to use PAM.

## The Linux-PAM configuration file

### Configuration file syntax

### Directory based configuration

### Example configuration file entries

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!):

    #
    # 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

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.

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       pam_warn.so
    other   password required       pam_warn.so

Having two '`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:

    #
    # 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

This is the only explicit example we give for an `/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 Linux-PAM, the following selection of lines (in `/etc/pam.d/other`) is likely to mimic the historically familiar Linux setup.

    #
    # default; standard UN*X access
    #
    auth     required       pam_unix.so
    account  required       pam_unix.so
    password required       pam_unix.so
    session  required       pam_unix.so

In general this will provide a starting place for most applications.

## Security issues

### If something goes wrong

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, Linux-PAM errs towards the latter. Any number of configuration errors can disable access to your system partially, or completely.

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

To recover, your best bet is to restore the system from a backup or boot the system into a rescue system and correct things from there.

### Avoid having a weak `other' configuration

It is not a good thing to have a weak default (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.

Here is a sample "other" configuration file. The `pam_deny` module will deny access and the `pam_warn` module will send a syslog message to auth.notice:

    #
    # 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

## A reference guide for available modules

Here, we collect together the descriptions of the various modules coming with Linux-PAM.

## See also

- The Linux-PAM Application Writers' Guide.

- The Linux-PAM Module Writers' Guide.

- The V. Samar and R. Schemers (SunSoft), ``UNIFIED LOGIN WITH PLUGGABLE AUTHENTICATION MODULES'', Open Software Foundation Request For Comments 86.0, October 1995.

## Author/acknowledgments

This document was written by Andrew G. Morgan (morgan@kernel.org) with many contributions from Chris Adams, Peter Allgeyer, Tim Baverstock, Tim Berger, Craig S. Bell, Derrick J. Brashear, Ben Buxton, Seth Chaiklin, Oliver Crow, Chris Dent, Marc Ewing, Cristian Gafton, Emmanuel Galanos, Brad M. Garcia, Eric Hester, Michel D'Hooge, Roger Hu, Eric Jacksch, Michael K. Johnson, David Kinchlea, Olaf Kirch, Marcin Korzonek, Thorsten Kukuk, Stephen Langasek, Nicolai Langfeldt, Elliot Lee, Luke Kenneth Casson Leighton, Al Longyear, Ingo Luetkebohle, Marek Michalkiewicz, Robert Milkowski, Aleph One, Martin Pool, Sean Reifschneider, Jan Rekorajski, Erik Troan, Theodore Ts'o, Jeff Uphoff, Myles Uyema, Savochkin Andrey Vladimirovich, Ronald Wahl, David Wood, John Wilmes, Joseph S. D. Yao and Alex O. Yuriev.

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 Linux-PAM, Sun graciously made the documentation for their implementation of PAM available. This act greatly accelerated the development of Linux-PAM.

## Copyright information for this document

    Copyright (c) 2006 Thorsten Kukuk <kukuk@thkukuk.de>
    Copyright (c) 1996-2002 Andrew G. Morgan <morgan@kernel.org>

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