User Authentication HOWTO

User Authentication HOWTO

Peter Hernberg

2000/05/02

Explains how user and group information is stored and how users are
authenticated on a Linux system (PAM), and how to secure you system's
user authentication.
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Table of Contents
[1]Introduction

[2]How this document came to be
[3]New versions
[4]Feedback
[5]Version History
[6]Copyrights and Trademarks
[7]Acknowledgements and Thanks
[8]Assumptions about the reader

[9]How User Information is Stored on Your System

[10]/etc/passwd
[11]Shadow passwords
[12]/etc/group and /etc/gshadow
[13]MD5 encrypted passwords
[14]Sifting through the mess

[15]PAM (Pluggable Authentication Modules)

[16]Why
[17]What
[18]How
[19]Getting more information

[20]Securing User Authentication

[21]A strong /etc/pam.d/other
[22]Disabling logins for user with null passwords
[23]Disable unused services
[24]Password-cracking tools
[25]Shadow and MD5 passwords

[26]Tying it all together

[27]Apache + mod_auth_pam

[28]Resources

[29]PAM
[30]General Security
[31]Offline Documentation

[32]Conclusion

Introduction

How this document came to be

When trying to add a number of (mostly unnecessary :) network services
to my existing home network, I kept running into the problem of
authentication, so I decided to figure out how authentication works on
linux systems, write a HOWTO, and call it my senior project. I hope
this document helps you understand this often-forgotten, but very
important, aspect of system administration.
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New versions

When I get my domain up running properly, you'll be able to find the
newest version of this document there. Until then,
http://www.linuxdoc.org/ will have to suffice.
_________________________________________________________________

Feedback

Comments, corrections, suggestions, flames, and flying saucer
sightings can be sent to [email protected].
_________________________________________________________________

Version History

v0.1 (May 13, 2000) first version (not released).

v0.3 (May 14, 2000) revised (not released).

v0.5 (May 15, 2000) added section on securing pam, added resources
section (not released).

v0.7 (May 15, 2000) revised; ready for release.
_________________________________________________________________

Copyrights and Trademarks

(c) 2000 Peter Hernberg

This manual may be reproduced in whole or in part, without fee,
subject to the following restrictions:

* The copyright notice above and this permission notice must be
preserved complete on all complete or partial copies
* Any translation or derived work must be approved by the author in
writing before distribution.
* If you distribute this work in part, instructions for obtaining
the complete version of this manual must be included, and a means
for obtaining a complete version provided.
* Small portions may be reproduced as illustrations for reviews or
quotes in other works without this permission notice if proper
citation is given. Exceptions to these rules may be granted for
academic purposes: Write to the author and ask. These restrictions
are here to protect us as authors, not to restrict you as learners
and educators. Any source code (aside from the SGML this document
was written in) in this document is placed under the GNU General
Public License, available via anonymous FTP from the GNU archive.
_________________________________________________________________

Acknowledgements and Thanks

Thanks to my family for putting up with me for 18 years. Thanks to the
Debian folks for making such a sweet distro for me to play with.
Thanks to [33]CGR for paying me to be a geek. Thanks to Sandy Harris
for his helpful suggestions. Finally, I'd like thank the makers of
ramen noodles, because I don't know how I'd live without them.
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Assumptions about the reader

For the purpose of this document, it is assumed that the reader is
comfortably with executing commands at the command line and editing
text configuration files.
_________________________________________________________________

How User Information is Stored on Your System

/etc/passwd

On almost all linux distributions (and commercial *nixes as well),
user information is stored in /etc/passwd, a text file which contains
the user's login, their encrypted password, a unique numerical user id
(called the uid), a numerical group id (called the gid), an optional
comment field (usually containing such items as their real name, phone
number, etc.), their home directory, and their preferred shell. A
typical entry in /etc/passwd looks something like this:
pete:K3xcO1Qnx8LFN:1000:1000:Peter Hernberg,,,1-800-FOOBAR:/home/pete:/bin/ba
sh

As you can see, it's pretty straight-forward. Each entry contains the
six fields I described above, with each field separated by a colon. If
this were as complex as user authentication got, there would be no
need for this HOWTO.
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Shadow passwords

Looking at your /etc/passwd, it's likely that you actually saw
something like this:
pete:x:1000:1000:Peter Hernberg,,,1-800-FOOBAR:/home/pete:/bin/bash

Where did the encrypted password go? Before I tell you where it went,
a bit explanation is required.

The /etc/passwd file, which contains information about all users,
including their encrypted password, is readable by all users, making
it possible for any user to get the encrypted password of everyone on
the system. Though the passwords are encrypted, password-cracking
programs are widely available. To combat this growing security threat,
shadow passwords were developed.

When a system has shadow passwords enabled, the password field in
/etc/passwd is replaced by an "x" and the user's real encrypted
password is stored in /etc/shadow. Because /etc/shadow is only
readable by the root user, malicious users cannot crack their fellow
users' passwords. Each entry in /etc/shadow contains the user's login,
their encrypted password, and a number of fields relating to password
expiration. A typical entry looks like this:
pete:/3GJllg1o4152:11009:0:99999:7:::
_________________________________________________________________

/etc/group and /etc/gshadow

Group information is stored in /etc/group. The format is similar to
that of /etc/passwd, with the entries containing fields for the group
name, password, numerical id (gid), and a comma-separated list of
group members. An entry in /etc/group looks like this:
pasta:x:103:spagetti,fettucini,linguine,vermicelli

As you can see from the "x" in the password field, group passwords can
be shadowed as well. Although groups almost never have their own
passwords, it is worth noting that shadowed group password information
is stored in /etc/gshadow.
_________________________________________________________________

MD5 encrypted passwords

Traditionally, unix passwords were encrypted with the standard crypt()
function. (For more information on the crypt() function, see the
crypt(3) manpage.) As computers grew faster, passwords encrypted with
this function became easier to crack. As the internet emerged, tools
for distributed the task of password-cracking across multiple hosts
became available. Many newer distributions ship with the option of
encrypting passwords with the stronger MD5 hash algorithm. (For more
information on the MD5 hash algorithm, consult RFC 1321.) While MD5
passwords will not eliminate the threat of password cracking, they
will make cracking your passwords much more difficult.
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Sifting through the mess

As you can see, there are a number of different ways user
authentication information can be stored on your system (shadow
passwords without MD5 encryption, /etc/passwd passwords with MD5
encryption, etc.). How do programs like login and su know how to
verify your password? Worse yet, what if you wanted to change the way
passwords are stored on your system? How will programs that need your
password know that passwords are stored differently? PAM is the
answer.
_________________________________________________________________

PAM (Pluggable Authentication Modules)

Pluggable authentication modules are at the core of user
authentication in any modern linux distribution.
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Why

Back in the good old days of linux, if a program, such as su, passwd,
login, or xlock, needed to authenticate a user, it would simply read
the necessary information from /etc/passwd. If it needed to change the
users' password, it would simply edit /etc/passwd. This simple but
clumsy method presented numerous problems for system administrators
and application developers. As MD5 and shadow passwords became
increasingly popular, each program requiring user authentication had
to know how to get the proper information when dealing with a number
of different schemes. If you wanted to change your user authentication
scheme, all these programs had to be recompiled. PAM eliminates this
mess by enabling programs to transparently authenticate users,
regardless of how user information is stored.
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What

Quoting from the [34]Linux-PAM System Administrator's Guide: "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." With PAM, it doesn't matter whether your
password is stored in /etc/passwd or on a server in Hong Kong. When a
program needs to authenticate a user, PAM provides a library
containing the functions for the proper authentication scheme. Because
this library is loaded dynamically, changing authentication schemes
can be done by simply editing a configuration file.

Flexibility is one of PAM's greatest strengths. PAM can be configured
to deny certain programs the right to authenticate users, to only
allow certain users to be authenticated, to warn when certain programs
attempt to authenticate, or even to deprive all users of login
privileges. PAM's modular design gives you complete control over how
users are authenticated.
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Distributions that support pam.

Nearly all popular distributions have supported PAM for some time.
Here's an incomplete list of distributions that support PAM:

* Redhat since version 5.0
* Mandrake since 5.2
* Debian since version 2.1 (partial support in 2.1 -- complete
support in 2.2)
* Caldera since version 1.3
* Turbolinux since version 3.6
* SuSE since version 6.2

This list is certainly incomplete and possibly inaccurate. I'd
appreciate it if you sent any corrections or additions to this list to
<[35][email protected]>.
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Installing PAM

Installing PAM from scratch is long process, beyond the scope of this
HOWTO. If PAM isn't installed on your system, you're probably running
such an old version of your distribution that there are many other
reasons to upgrade. If you really want to do it yourself, then you're
certainly not the sort of person who needs any help from me. For all
these reasons, I'm going to assume that you already have PAM
installed.
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How

Enough talk, let's dig in.
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PAM configuration files

PAM configuration files are stored in the /etc/pam.d/ directory. (If
you don't have /etc/pam.d/ directory, don't worry, I'll cover that in
the next section) Let's go over there and take look.
~$ cd /etc/pam.d
/etc/pam.d/$ ls
chfn chsh login other passwd su xlock
/etc/pam.d/$

Your system may have a few more or a few less files in this directory,
depending on what's installed on your system. Whatever the details,
you probably saw a file for each of the programs on your system that
authenticate users. As you probably already guessed, each file
contains the PAM authentication configuration for the program it's
named after (except for the other file, which we'll talk about in a
little bit). Let's take a look the PAM configuration file for passwd
(I've condensed the file for the sake of simplicity):
/etc/pam.d/$ cat login
# PAM configuration for login
auth requisite pam_securetty.so
auth required pam_nologin.so
auth required pam_env.so
auth required pam_unix.so nulok
account required pam_unix.so
session required pam_unix.so
session optional pam_lastlog.so
password required pam_unix.so nullok obscure min=4 max=8

Before dig into this file, I must mention a little something.
_________________________________________________________________

A little something

A small percentage are probably thinking, "Oh no! I don't have a
/etc/pam.d directory! Your list of distributions says that my
distribution includes PAM, but I can't find that directory. Without
PAM, my life is empty and meaningless! What can I do?" Don't worry,
all is not lost. If you know that your distribution includes PAM, but
you have no /etc/pam.d/ directory, then your PAM configuration is
stored in /etc/pam.conf. Rather than being spread across several
files, all your PAM configuration is stored in a single file. This
adds a little twist to PAM configuration, but the proper adjustments
are pointed out in section 3.3.4.
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Configuration syntax

PAM configuration files have the following syntax:
type control module-path module-arguments

Using the login configuration file (see above) as an example let's
take a look a the syntax for PAM configuration files:

PAM configuration tokens

type
The type token tells PAM what type of authentication is to be
used for this module. Modules of the same type can be
"stacked", requiring a user to meet multiple requirements to be
authenticated. PAM recognizes four types:

account

Determines whether the user is allowed to access the service,
whether their passwords has expired, etc.

auth
Determines whether the user is who they claim to be,
usually by a password, but perhaps by a more sophistcated
means, such as biometrics.

password
Provides a mechanism for the user to change their
authentication. Again, this usually their password.

session
Things that should be done before and/or after the user
is authenticed. This might included things such as
mounting/unmounting the user home directory, logging
their login/logout, and restricting/unrestricting the
services available to the user.

In the login config file, we see at least one entry for each
type. Since this the program that allows user to login (hence
the name :), it's understandable that it needs to access all of
the different types of authentication.

control
The control token tells PAM what should be done in if
authentication by this module fails. PAM recognizes four
control types:

requisite

Failure to authenticate via this module results in immediate
denial of authentication.

required

Failure also results in denial of authentication, although PAM
will still call all the other modules listed for this
service before denying authentication.

sufficient
If authentication by this module is successful, PAM will
grant authentication, even if a previous required module
failed.

optional
Whether this module succeeds or fails is only significant
if it is the only module of its type for this service.

In the configuration file for login, we see nearly all of the
different control types. Most of the required modules are
pam_unix.so (the main authentication module), the single
requisite module is pam_securetty.so (checks make sure the user
is logging in on a secure console), and the only optional
module is pam_lastlogin.so (the module that retrieves
information on the user's most recent login).

module-path
The module-path tells PAM which module to use and (optionally)
where to find it. Most configurations only contain the module's
name, as is the case in our login configuration file. When this
is the case, PAM looks for the modules in the default PAM
module directory, normally /usr/lib/security. However, if your
linux distribution conforms to the Linux Filesystem standard,
PAM modules can be found in /lib/security.

module-arguments
The module-arguments are arguments to be passed to the module.
Each module has its own arguments. For example, in our login
configuration, the "nulok" ("null ok", argument being passed to
pam_unix.so module, indicating the a blank ("null") password is
acceptable ("ok").
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pam.conf configuration

If your PAM configuration is stored in /etc/pam.conf rather than
/etc/pam.d/, PAM configuration lines are a bit different. Rather than
each service having its own configuration file, all configurations are
stored in /etc/pam.conf with the service name as the first token in a
configuration line. For example, the following line in
/etc/pam.d/login:
auth required pam_unix.so nulok

would become the following line in /etc/pam.conf:
login auth required pam_unix.so nulok

Except for this minor difference, all the rest of the PAM syntax
applies.
_________________________________________________________________

Getting more information

For more information on configuring PAM and complete PAM module
reference, consult the [36]Linux-PAM System Administrator's Guide.
This guide serves as a thorough and up-to-date reference on PAM
configuration.
_________________________________________________________________

Securing User Authentication

Many linux distributions ship with user authentication that is not
adequately secure. This section discusses some of the ways you make
user authentication secure on your system. While doing these things
will make your system more secure, do not be so naive as to think they
make you invulnerable.
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A strong /etc/pam.d/other

All of the files in /etc/pam.d/ contain the configuration for a
particular service. The notable exception to this rule is the
/etc/pam.d/other file. This file contains the configuration for any
services which do not have their own configuration file. For example,
is the (imaginary) xyz service attempted authentication PAM would look
for a /etc/pam.d/xyz file. Not finding one, authentication for xyz
would be determined by the /etc/pam.d/other file. Since
/etc/pam.d/other is the configuration to which PAM services fallback,
it is important that it is secure. We will discuss two secure
configurations of /etc/pam.d/other, one which is quite nearly paranoid
and which is gentler.
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A paranoid configuration

A paranoid configuration of /etc/pam.d/other is as follows:
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

With this configuration, whenever an unknown service attempts to
access any of the four configuration types, PAM denies authentication
(via the pam_deny.so module) and then logs a syslog warning (via the
pam_warn.so module). Short of a bug in PAM, this configuration is
brutally secure. The only problem with that brutality is it may cause
problems if your accidentally delete the configuration of another
service. If your /etc/pam.d/login was mistakenly deleted, no one would
be able to login!
_________________________________________________________________

A kinder configuration

Here's configuration that isn't quite so mean:
auth required pam_unix.so
auth required pam_warn.so
account required pam_unix.so
account required pam_warn.so
password required pam_deny.so
password required pam_warn.so
session required pam_unix.so
session required pam_warn.so

This configuration will allow an unknown service to authenticate (via
the pam_unix.so module), although it will not allow it to change the
user's password. Although it allows authentication by unknown
services, it logs a syslog warning whenever such a service attempts
authentication.
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Choosing a /etc/pam.d/other

I would strongly reccomend that you implement the first
/etc/pam.d/other configuration unless you have a very good reason not
to. It always a good idea to be 'secure by default'. If you ever do
need to grant a new service authentication privileges, you can simply
create a PAM configuration file for that service.
_________________________________________________________________

Disabling logins for user with null passwords

On most linux systems, there a number of "dummy" user accounts, used
to assign privileges to certain system services like ftp, webservers,
and mail gateways. Having these accounts allows your system to be more
secure, because if these services are compromised, an attacker will
only gain the limited privileges available to the dummy account,
rather than the full privileges of a service running as root. However,
allowing these dummy account login privileges is a security risk, as
they usually have blank (null) passwords. The configuration option
that enables null passwords is the "nullok" module-argument. You'll
want remove this argument from any modules of 'auth' type for services
that allow login. This is usually the login service, may also include
services like rlogin and ssh. Hence, the following line in
/etc/pam.d/login:
auth required pam_unix.so nullok

should be changed to:
auth required pam_unix.so
_________________________________________________________________

Disable unused services

Looking at the files in /etc/pam.d/, you'll probably see configuration
files for a number of programs you don't use and maybe even a few
you've never heard of. Although allowing authentication to these
services probably won't open any huge security holes, you're better
off denying them authentication. The best way to disable PAM
authentication for these programs is to rename these files. Not
finding the file named after the service requesting authentication,
PAM will fallback to the (hopefully) very secure /etc/pam.d/other. If
you later find that you need one of these programs, you can simply
rename the file to its original name and everything will work as it
was intended.
_________________________________________________________________

Password-cracking tools

While password-cracking tools can be by attackers used to compromise a
system, they can also be used by system administrators as proactive
tool to ensure the strength of passwords on their system. The two most
commonly used password-cracking tools are "crack" and "John the
Ripper". Crack is probably included in your facorite distribution.
John the Ripper can be obtained from
[37]http://www.false.com/security/john/index.html. Run the tools
against your password database and you'll probably be surprised with
what they come up with.

Additionally, there is a PAM module which utilizes the crack library
to check the strength of a users password whenever it changed. When
this module is installed, the user can only change their password to
one which meets the minimum password strength.
_________________________________________________________________

Shadow and MD5 passwords

As was discussed in the first section of this document, Shadow and MD5
passwords can make your system more secure. During the installation
procedure, most modern distributions will ask whether you want to
install MD5 and/or Shadow passwords. Unless you have a good reason not
to, you should enable these. The process of converting from
non-shadowed/non-MD5 passwords is a complicated process, and is beyond
the scope of this document. The [38]Shadow Password HOWTO is outdated,
but it might be of some help.
_________________________________________________________________

Tying it all together

In this section, I'll give a simple example which ought to help tie
together what's in the previous section.
_________________________________________________________________

Apache + mod_auth_pam

As our example, we'll install and configure mod_auth_pam, an Apache
module that allows you to use authenticate users of your webserver
using PAM. For the purpose of this example, I'll assume you have
apache installed. If it's not installed already you should be able
find installation packages from your distributor.
_________________________________________________________________

Our example

Our goal will be to configure a restricted area of our webserver, a
family/ directory, to authenticate users via PAM. This directory
contains private family information, and should only be accessible to
members of the user group family.
_________________________________________________________________

Installing mod_auth_pam

First, you'll want to download mod_auth_pam from
[39]http://blank.pages.de/pam/mod_auth_pam/. The following commands
will compile mod_auth_pam (you must be logged in as root):
~# tar xzf mod_auth_pam.tar.gz
~# cd mod_auth_pam-1.0a
~/mod_auth_pam-1.0a# make
~/mod_auth_pam-1.0a# make install

If you have any trouble installing the mod_auth_pam module, make sure
you've installed your distributions apache-dev package. After you've
installed mod_auth_pam, you'll need to restart apache. Apache can
usually by restarted by typing the following command (again, you must
be root):
~# /etc/init.d/apache restart
_________________________________________________________________

Configuring PAM

PAM configuration for Apache is stored in /etc/pam.d/httpd. The
default configuration (which was installed when you installed
mod_auth_pam) is secure, but it uses a module (pam_pwdb.so) which may
not be available on many systems. (Besides, configuring it from
scratch will be fun!) So delete the /etc/pam.d/httpd file, and let's
start fresh.
_________________________________________________________________

Deciding how to configure PAM

If we're going to configure how PAM deals with Apache's authentication
requests, we need to figure out exactly what we need PAM to check for.
First, we want PAM to make sure the user's password matches their
password in the standard unix password database. This sounds like the
'auth' type and the pam_unix.so module. We'll want the module's
control type to be set to 'required', so authentication will fail
without a correct password. Here's what the first line of our
/etc/pam.d/httpd looks like:
auth required pam_unix.so

Secondly, we must make sure that the users account is valid (i.e.
their password has not expired or any such nastiness). This is the
'account' type and is also provided by the pam_unix.so module. Again,
we'll set this module's control type to 'required'. After adding this
line, our /etc/pam.d/httpd configuration looks like this:
auth required pam_unix.so
account required pam_unix.so

It's not terribly sophisticated, but it does the job. It ought to be a
good start for learning how to configure PAM services.
_________________________________________________________________

Configuring Apache

Now that PAM is configured to authenticate apache's requests, we'll
configure apache to properly utilize PAM authentication to restrict
access to the family/ directory. To do so, add the following lines to
your httpd.conf (usually stored in /etc/apache/ or /etc/httpd):
<Directory /var/www/family>
AuthPAM_Enabled on
AllowOverride None
AuthName "Family Secrets"
AuthType "basic"
require group family
</Directory>

You may need to replace /var/www/ with the default location of web
documents, which is often /home/httpd/. Wherever that is, you'll need
to create the family directory.

Before we test our setup, I'll to take a moment to explain the Apache
configuration you just entered. The <Directory> directive is used to
encapsulate configuration data for this directory. Inside this
directive, we've enabled PAM authentication ("AuthPAM_enabled on"),
turned off any overriding of this configuration ("AllowOverride
none"), named this authentication zone "Family Secrets" ("AuthName
"Family Secrets""), set the http authentication (not the PAM
authentication) type to the default ("AuthType "basic""), and required
the user group family ("require group family").
_________________________________________________________________

Testing our setup

Now that we've got everything setup up properly, it's time to revel in
our success. Fire up your favorite web browser and head over to
http://your-domain/family/ (replacing your-domain with, well, your
domain). You are now an uber-authenticator!
_________________________________________________________________

Resources

There are a number of resources, both online and offline, where you
can more information about user authentication. If you know of any
resources that ought to be added to this list, drop me a line at
<[40][email protected]>
_________________________________________________________________

PAM

* [41]Linux-PAM System Administrator's Guide
* [42]Linux-PAM Module Writer's Manual
* [43]Linux-PAM Application Developer's Manual
_________________________________________________________________

General Security

* [44]linuxsecurity.com
* [45]securitywatch.com
* [46]Security HOWTO
* [47]Packetstorm
_________________________________________________________________

Offline Documentation

A lot of information can be gathered from your system's manual pages.
The following are some manpages relating to user authentication. The
number in parentheses refers to the manpage section. To view the
passwd(5) manpage, you would enter man 5 passwd.

* passwd(5)
* crypt(3)
* pam.d(5)
* group(5)
* shadow(5)
_________________________________________________________________

Conclusion

I hope you found this HOWTO helpful. If you have any questions,
comments, or suggestions, I'd love to hear from you. You can email me
at <[48][email protected]>.

References

1. file://localhost/export/sunsite/users/gferg/howto/00_User-Authentication-HOWTO.html#AEN20
2. file://localhost/export/sunsite/users/gferg/howto/00_User-Authentication-HOWTO.html#AEN22
3. file://localhost/export/sunsite/users/gferg/howto/00_User-Authentication-HOWTO.html#AEN25
4. file://localhost/export/sunsite/users/gferg/howto/00_User-Authentication-HOWTO.html#AEN28
5. file://localhost/export/sunsite/users/gferg/howto/00_User-Authentication-HOWTO.html#AEN31
6. file://localhost/export/sunsite/users/gferg/howto/00_User-Authentication-HOWTO.html#AEN37
7. file://localhost/export/sunsite/users/gferg/howto/00_User-Authentication-HOWTO.html#AEN50
8. file://localhost/export/sunsite/users/gferg/howto/00_User-Authentication-HOWTO.html#AEN54
9. file://localhost/export/sunsite/users/gferg/howto/00_User-Authentication-HOWTO.html#AEN57
10. file://localhost/export/sunsite/users/gferg/howto/00_User-Authentication-HOWTO.html#AEN59
11. file://localhost/export/sunsite/users/gferg/howto/00_User-Authentication-HOWTO.html#AEN67
12. file://localhost/export/sunsite/users/gferg/howto/00_User-Authentication-HOWTO.html#AEN81
13. file://localhost/export/sunsite/users/gferg/howto/00_User-Authentication-HOWTO.html#AEN92
14. file://localhost/export/sunsite/users/gferg/howto/00_User-Authentication-HOWTO.html#AEN95
15. file://localhost/export/sunsite/users/gferg/howto/00_User-Authentication-HOWTO.html#AEN101
16. file://localhost/export/sunsite/users/gferg/howto/00_User-Authentication-HOWTO.html#AEN104
17. file://localhost/export/sunsite/users/gferg/howto/00_User-Authentication-HOWTO.html#AEN113
18. file://localhost/export/sunsite/users/gferg/howto/00_User-Authentication-HOWTO.html#AEN139
19. file://localhost/export/sunsite/users/gferg/howto/00_User-Authentication-HOWTO.html#AEN245
20. file://localhost/export/sunsite/users/gferg/howto/00_User-Authentication-HOWTO.html#AEN249
21. file://localhost/export/sunsite/users/gferg/howto/00_User-Authentication-HOWTO.html#AEN252
22. file://localhost/export/sunsite/users/gferg/howto/00_User-Authentication-HOWTO.html#AEN281
23. file://localhost/export/sunsite/users/gferg/howto/00_User-Authentication-HOWTO.html#AEN288
24. file://localhost/export/sunsite/users/gferg/howto/00_User-Authentication-HOWTO.html#AEN293
25. file://localhost/export/sunsite/users/gferg/howto/00_User-Authentication-HOWTO.html#AEN298
26. file://localhost/export/sunsite/users/gferg/howto/00_User-Authentication-HOWTO.html#AEN302
27. file://localhost/export/sunsite/users/gferg/howto/00_User-Authentication-HOWTO.html#AEN305
28. file://localhost/export/sunsite/users/gferg/howto/00_User-Authentication-HOWTO.html#AEN362
29. file://localhost/export/sunsite/users/gferg/howto/00_User-Authentication-HOWTO.html#AEN366
30. file://localhost/export/sunsite/users/gferg/howto/00_User-Authentication-HOWTO.html#AEN378
31. file://localhost/export/sunsite/users/gferg/howto/00_User-Authentication-HOWTO.html#AEN393
32. file://localhost/export/sunsite/users/gferg/howto/00_User-Authentication-HOWTO.html#AEN423
33. http://www.cgr.org/
34. http://www.kernel.org/pub/linux/libs/pam/Linux-PAM-html/pam.html
35. mailto:[email protected]
36. http://www.kernel.org/pub/linux/libs/pam/Linux-PAM-html/pam.html
37. http://www.false.com/security/john/index.html
38. http://www.linuxdoc.org/HOWTO/Shadow-Password-HOWTO.html
39. http://blank.pages.de/pam/mod_auth_pam/
40. mailto:[email protected]
41. http://www.kernel.org/pub/linux/libs/pam/Linux-PAM-html/pam.html
42. http://www.kernel.org/pub/linux/libs/pam/Linux-PAM-html/pam_modules.html
43. http://www.kernel.org/pub/linux/libs/pam/Linux-PAM-html/pam_modules.html
44. http://www.linuxsecurity.com/
45. http://www.securitywatch.com/
46. http://www.linuxdoc.org/HOWTO/Security-HOWTO.html
47. http://packetstorm.securify.com/
48. mailto:[email protected]