PAM

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The Linux Pluggable Authentication Modules (PAM) provide a framework for system-wide user authentication. To quote the project:

PAM provides a way to develop programs that are independent of authentication scheme. These programs need "authentication modules" to be attached to them at run-time in order to work. Which authentication module is to be attached is dependent upon the local system setup and is at the discretion of the local system administrator.

This article explains the Arch Linux base set-up defaults for PAM to authenticate local and remote users. Applying changes to the defaults is subject of crosslinked specialized per topic articles.

Installation

The pam package is part of the base group of packages and, thereby, normally installed on an Arch system. The PAM modules are installed into /usr/lib/security exclusively.

The repositories contain a number of optional PAM packages, the #Configuration How-Tos show examples.

Configuration

A number of /etc paths are relevant for PAM, execute pacman -Ql pam |grep /etc to see the default configuration files created. They relate to either #Security parameters for the modules, or the #PAM base-stack configuration.

Security parameters

The path /etc/security contains system-specific configuration for variables the authentication methods offer. The base install populates it with default upstream configuration files.

Note Arch Linux does not provide distribution-specific configuration for these files. For example, the /etc/security/pwquality.conf file can be used to define system-wide defaults for password quality. Yet, to enable it the pam_pwquality.so module has to be added to the #PAM base-stack of modules, which is not the case per default.

See #Security parameter configuration for some of the possibilities.

PAM base-stack

The /etc/pam.d/ path is exclusive for the PAM configuration to link the applications to the individual systems' authentication schemes. During installation of the system base it is populated by:

  • the pambase package, which contains the base-stack of Arch Linux specific PAM configuration to be used by applications, and
  • other base packages. For example, util-linux adds configuration for the central login and other programs, the shadow package adds the Arch Linux defaults to secure and modify the user database (see Users and groups).

The different configuration files of the base installation link together, are stacked during runtime. For example, on a local user logon, the login application sources the system-local-login policy, which in turn sources others:

/etc/pam.d/
login -> system-local-login -> system-login -> system-auth

For a different application, a different path may apply. For example, openssh installs its sshd PAM policy:

/etc/pam.d/
sshd -> system-remote-login -> system-login -> system-auth

Consequently, the choice of the configuration file in the stack matters. For the above example, a special authentication method could be required for sshd only, or all remote logins by changing system-remote-login; both changes would not affect local logins. Applying the change to system-login or system-auth instead would affect local and remote logins.

Like the example of sshd, any pam-aware application is required to install its policy to /etc/pam.d in order to integrate and rely on the PAM stack appropriately. If an application fails to do it, the /etc/pam.d/other policy is applied per default. A permissive policy for it is installed per default (FS#48650).

Tip: PAM is dynamically linked at runtime. For example:
$ ldd /usr/bin/login |grep pam
libpam.so.0 => /usr/lib/libpam.so.0 (0x000003d8c32d6000)
libpam_misc.so.0 => /usr/lib/libpam_misc.so.0 (0x000003d8c30d2000)
the login application is pam-aware and must, therefore, have a policy.

The PAM package manual pages pam(8) and pam.d(5) describe the standardized content of the configuration files. In particular they explain the four PAM groups: account, authentication, password, and session management, as well as the control values that may be used to configure stacking and behaviour of the modules.

Additionally, an extensive documentation is installed to /usr/share/doc/Linux-PAM/index.html which, among various guides, contains browsable man pages for each of the standard modules.

Warning: Changes to the PAM configuration fundamentally affect user authentication. Erroneous changes can result in that no or any user can log in. Since changes are not effective for already authenticated users, a good precaution is to perform changes with one user and test the result with another user in a separate console.

Examples

Two short examples to illustrate the above warning.

First, we take the following two lines:

/etc/pam.d/system-auth
auth      required  pam_unix.so     try_first_pass nullok
auth      optional  pam_permit.so

From pam_unix(8): "The authentication component pam_unix.so performs the task of checking the users credentials (password). The default action of this module is to not permit the user access to a service if their official password is blank. " - the latter being what pam_permit.so is used for. Simply swapping the control values required and optional for both lines is enough to disable password authentication, i.e. any user may logon without providing a password.

Second, as the contrary example, per default configuration creating the following file:

# touch /etc/nologin 

results in that no user other than root may login (if root logins are allowed, another default for Arch Linux). To allow logins again, remove the file from the console you created it with.

With that as background, see #PAM stack and module configuration for particular use-case configuration.

Configuration How-Tos

This section provides an overview of content detailing how to apply changes to the PAM configuration and how to integrate special new PAM modules into the PAM stack. Note the man pages for the modules can generally be reached dropping the .so extension.

Security parameter configuration

The following sections describe examples to change the default PAM parameter configuration:

shows how to enforce srong passwords with pam_cracklib.so.
shows how to limit login attempts with pam_tally.so.
limits user logons with pam_wheel.so.
detail how to configure system process limits with pam_limits.so.
shows examples to set environment variables via pam_env.so.

PAM stack and module configuration

The following articles detail how to change the #PAM base-stack for special use-cases.

PAM modules from the Official repositories:

detail examples for using pam_mount.so to automount encrypted directory paths on user login.
uses pam_ecryptfs.so to automount an encrypted directory.
shows how to use pam_exec.so to execute a custom script on a user login.
uses pam_winbind.so and pam_krb5.so to let users authenticate via Active Directory (LDAP, Kerberos) services.
is an article about integrating LDAP client or server-side authentication with pam_ldap.so.
relies on pam_yubico.so in the PAM stack to enable authentication via the proprietary Yubikey.
shows an example to implement software based two-factor authentication with pam_oath.so.
employs pam_fprintd.so to setup fingerprint authentication.

PAM modules from the Arch User Repository:

shows how to configure pam_usb.so to use an usb-device for, optionally two-factor, authentication.
uses pam_ssh.so to authenticate as a remote user.
explains how pam_abl.so can be used to limit brute-forcing attacks via ssh.
may get automounted via pam_encfs.so.
shows how to set up two-factor authentication with pam_google_authenticator.so.
explains how to configure a FTP chroot with pam_pwdfile.so to authenticate users without a local system account.

Further PAM packages

Other than those packages mentioned so far, the Arch User Repository contains a number of additional PAM modules and tools.

General purpose utilities relating to PAM are:

http://linux-pam.org/ || libx32-pamAUR
  • Pamtester — Program to test the pluggable authentication modules (PAM) facility
http://pamtester.sourceforge.net/ || pamtesterAUR

Note the AUR features a keyword tag for PAM, but not all available packages are updated to include it. Hence, searching the package description may be necessary.

See also