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
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
Linux PAM Dependencies
Optional
libnsl-2.0.1, libtirpc-1.3.4, rpcsvc-proto-1.4.4, Berkeley DB (deprecated), libaudit, libeconf, and Prelude
Note
Shadow-4.14.2 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.
Installation of Linux PAM
First, prevent the installation of an unneeded systemd file:
sed -e /service_DATA/d \
-i modules/pam_namespace/Makefile.am &&
autoreconf
If you downloaded the documentation, unpack the tarball by issuing the following command.
tar -xf ../Linux-PAM-1.6.0-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-1.6.0 &&
make
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 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.