Introduction to Linux PAM

The Linux PAM package contains Pluggable Authentication Modules used to enable the local system administrator to choose how applications authenticate users.

This package is known to build and work properly using an LFS-7.8 platform.

Package Information

Additional Downloads

Optional Documentation

Linux PAM Dependencies


Berkeley DB-6.1.26, CrackLib-2.9.6, libtirpc-0.3.2 and Prelude

Optional (To Rebuild the Documentation)

docbook-xml-4.5, docbook-xsl-1.78.1, fop-2.0, libxslt-1.1.28 and w3m-0.5.3

User Notes:

Installation of Linux PAM

If you downloaded the documentation, unpack the tarball by issuing the following command.

tar -xf ../Linux-PAM-1.2.0-docs.tar.bz2 --strip-components=1

Install Linux PAM by running the following commands:

./configure --prefix=/usr \
            --sysconfdir=/etc \
            --libdir=/usr/lib \
            --enable-securedir=/lib/security \
            --docdir=/usr/share/doc/Linux-PAM-1.2.1 &&

To test the results, a suitable /etc/pam.d/other configuration file must exist.


Reinstallation or upgrade of Linux PAM

If you have a system with Linux PAM installed and working, be careful when modifying the files in /etc/pam.d, since your system may become totally unusable. If you want to run the tests, you do not need to create another /etc/pam.d/other file. The installed one can be used for that purpose.

You should also be aware that make install overwrites the configuration files in /etc/security as well as /etc/environment. In case you have modified those files, be sure to backup them.

For a first installation, create the configuration file by issuing the following commands as the root user:

install -v -m755 -d /etc/pam.d &&

cat > /etc/pam.d/other << "EOF"
auth     required
account  required
password required
session  required

Now run the tests by issuing make check. Ensure there are no errors produced by the tests before continuing the installation. Note that the checks are quite long. It may be useful to redirect the output to a log file in order to inspect it thoroughly.

Only in case of a first installation, remove the configuration file created earlier by issuing the following command as the root user:

rm -fv /etc/pam.d/*

Now, as the root user:

make install &&
chmod -v 4755 /sbin/unix_chkpwd &&

for file in pam pam_misc pamc
  mv -v /usr/lib/lib${file}.so.* /lib &&
  ln -sfv ../../lib/$(readlink /usr/lib/lib${file}.so) /usr/lib/lib${file}.so

Command Explanations

--enable-securedir=/lib/security: This switch sets install location for the PAM modules.

chmod -v 4755 /sbin/unix_chkpwd: The unix_chkpwd helper program must be setuid so that non-root processes can access the shadow file.

Configuring Linux-PAM

Config Files

/etc/security/* and /etc/pam.d/*

Configuration Information

Configuration information is placed in /etc/pam.d/. Below is an example file:

# Begin /etc/pam.d/other

auth            required     nullok
account         required
session         required
password        required     nullok

# End /etc/pam.d/other

Now set up some generic files. As root:

cat > /etc/pam.d/system-account << "EOF"
# Begin /etc/pam.d/system-account

account   required

# End /etc/pam.d/system-account

cat > /etc/pam.d/system-auth << "EOF"
# Begin /etc/pam.d/system-auth

auth      required

# End /etc/pam.d/system-auth

cat > /etc/pam.d/system-session << "EOF"
# Begin /etc/pam.d/system-session

session   required

# End /etc/pam.d/system-session

The remaining generic file depends on wheather CrackLib-2.9.6 is installed. If it is installed, use:

cat > /etc/pam.d/system-password << "EOF"
# Begin /etc/pam.d/system-password

# check new passwords for strength (man pam_cracklib)
password  required   type=Linux retry=3 difok=5 \
                                        difignore=23 minlen=9 dcredit=1 \
                                        ucredit=1 lcredit=1 ocredit=1 \
# use sha512 hash for encryption, use shadow, and use the
# authentication token (chosen password) set by pam_cracklib
# above (or any previous modules)
password  required       sha512 shadow use_authtok

# End /etc/pam.d/system-password


In its default configuration, pam_cracklib will allow multiple case passwords as short as 6 characters, even with the minlen value set to 11. You should review the pam_cracklib(8) man page and determine if these default values are acceptable for the security of your system.

If CrackLib-2.9.6 is NOT installed, use:

cat > /etc/pam.d/system-password << "EOF"
# Begin /etc/pam.d/system-password

# use sha512 hash for encryption, use shadow, and try to use any previously
# defined authentication token (chosen password) set by any prior module
password  required       sha512 shadow try_first_pass

# End /etc/pam.d/system-password

Now add a restrictive /etc/pam.d/other configuration file. With this file, programs that are PAM aware will not run unless a configuration file specifically for that application is created.

cat > /etc/pam.d/other << "EOF"
# Begin /etc/pam.d/other

auth        required
auth        required
account     required
account     required
password    required
password    required
session     required
session     required

# End /etc/pam.d/other

The PAM man page (man pam) provides a good starting point for descriptions of fields and allowable entries. The Linux-PAM System Administrators' Guide is recommended for additional information.

Refer to for a list of various third-party modules available.



You should now reinstall the Shadow-4.2.1 package.


Installed Program: mkhomedir_helper, pam_tally, pam_tally2, pam_timestamp_check, unix_chkpwd and unix_update
Installed Libraries:, and
Installed Directories: /etc/security, /lib/security, /usr/include/security and /usr/share/doc/Linux-PAM-1.2.1

Short Descriptions


is a helper binary that creates home directories.


is used to interrogate and manipulate the login counter file.


is used to interrogate and manipulate the login counter file, but does not have some limitations that pam_tally does.


is used to check if the default timestamp is valid


is a helper binary that verifies the password of the current user.


is a helper binary that updates the password of a given user.

provides the interfaces between applications and the PAM modules.

Last updated on 2015-09-18 11:02:25 -0500