Beyond Linux® From Scratch (systemd Edition) - Version r12.2-440

Chapter 4. Security

Linux-PAM-1.6.1

Introduction to Linux PAM

The Linux PAM package contains Pluggable Authentication Modules used by the local system administrator to control how application programs authenticate users.

[Note]

Note

Development versions of BLFS may not build or run some packages properly if LFS or dependencies have been updated since the most recent stable versions of the books.

Package Information

Additional Downloads

Optional Documentation

Linux PAM Dependencies

Optional

libnsl-2.0.1, libtirpc-1.3.6, rpcsvc-proto-1.4.4, Berkeley DB (deprecated), libaudit, libeconf, and Prelude

[Note]

Note

Shadow-4.16.0 and Systemd-256.5 must be reinstalled and reconfigured after installing and configuring Linux PAM.

With Linux-PAM-1.4.0 and higher, the pam_cracklib module is not installed by default. Use libpwquality-1.4.5 to enforce strong passwords.

Kernel Configuration

For the PAM module pam_loginuid.so (referred by the PAM configuration file system-session if Systemd-256.5 is rebuilt with PAM support later) to work, a kernel configuration parameter need to be set or the module will just do nothing: 

General setup --->
  [*] Auditing support                                                   [AUDIT]

Installation of Linux PAM

The shipped libtool.m4 file has a configuration inconsistent with LFS /usr hierarchy. This issue would cause libpam_misc.so linked with an rpath flag which may sometimes cause troubles or even security issues. Regenerate the building system to fix the inconsistency: 

autoreconf -fi

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

tar -xf ../Linux-PAM-1.6.1-docs.tar.xz --strip-components=1

Compile and link Linux PAM by running the following commands: 

./configure --prefix=/usr                        \
            --sbindir=/usr/sbin                  \
            --sysconfdir=/etc                    \
            --libdir=/usr/lib                    \
            --enable-securedir=/usr/lib/security \
            --docdir=/usr/share/doc/Linux-PAM-1.6.1 &&
make

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

[Caution]

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 existing file can be used for the tests.

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

For a first-time installation, create a 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       pam_deny.so
account  required       pam_deny.so
password required       pam_deny.so
session  required       pam_deny.so
EOF

Now run the tests by issuing make check. Be sure the tests produced no errors before continuing the installation. Note that the tests are very long. Redirect the output to a log file, so you can inspect it thoroughly.

For a first-time installation, remove the configuration file created earlier by issuing the following command as the root user: 

rm -fv /etc/pam.d/other

Now, as the root user: 

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

Command Explanations

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

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

Configuring Linux-PAM

Configuration Files

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

Configuration Information

Configuration information is placed in /etc/pam.d/. Here is a sample file: 

# Begin /etc/pam.d/other

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

# End /etc/pam.d/other

Now create some generic configuration files. As the root user: 

install -vdm755 /etc/pam.d &&
cat > /etc/pam.d/system-account << "EOF" &&
# Begin /etc/pam.d/system-account

account   required    pam_unix.so

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

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

auth      required    pam_unix.so

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

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

session   required    pam_unix.so

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

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

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

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

If you wish to enable strong password support, install libpwquality-1.4.5, and follow the instructions on that page to configure the pam_pwquality PAM module with strong password support.

Next, 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 exists. 

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

auth        required        pam_warn.so
auth        required        pam_deny.so
account     required        pam_warn.so
account     required        pam_deny.so
password    required        pam_warn.so
password    required        pam_deny.so
session     required        pam_warn.so
session     required        pam_deny.so

# End /etc/pam.d/other
EOF

The PAM man page (man pam) provides a good starting point to learn about the several fields, and allowable entries. The Linux-PAM System Administrators' Guide is recommended for additional information.

[Important]

Important

You should now reinstall the Shadow-4.16.0 and Systemd-256.5 packages.

Contents

Installed Program: faillock, mkhomedir_helper, pam_namespace_helper, pam_timestamp_check, pwhistory_helper, unix_chkpwd and unix_update
Installed Libraries: libpam.so, libpamc.so and libpam_misc.so
Installed Directories: /etc/security, /usr/lib/security, /usr/include/security and /usr/share/doc/Linux-PAM-1.6.1

Short Descriptions

faillock

displays and modifies the authentication failure record files

mkhomedir_helper

is a helper binary that creates home directories

pam_namespace_helper

is a helper program used to configure a private namespace for a user session

pwhistory_helper

is a helper program that transfers password hashes from passwd or shadow to opasswd

pam_timestamp_check

is used to check if the default timestamp is valid

unix_chkpwd

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

unix_update

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

libpam.so

provides the interfaces between applications and the PAM modules