Linux pluggable authentication modules
The Linux Pluggable Authentication Modules (PAM) provides 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 a dependency of the base meta package 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 —query —list 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 and 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:
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:
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 default policy to deny and log a warning is applied.
$ 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 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 behavior of the modules.
Additionally, 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 user can log in, or all users being able to log in.
Tip: Changes are not effective for already authenticated users; a way to work with PAM is to login preferably locally on the testing machine and develop, keeping the session constantly running, while checking the results from another user on another console.
Examples
The factual accuracy of this article or section is disputed.
Reason: (1) the use of nullok (2) the way pam handles optional modules (Discuss in Talk:PAM#Accuracy of PAM#Examples)
Two short examples to illustrate the above warning.
First, we take the following two lines:
auth required pam_unix.so try_first_pass nullok auth optional pam_permit.so
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 of pam_nologin.so at /etc/pam.d/login, creating the following file:
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. Possibly, 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:
PAM stack and module configuration
The following articles detail how to change the #PAM base-stack for special use-cases.
pam_mount detail examples for using pam_mount.so to automount encrypted directory paths on user login. ECryptfs#Auto-mounting uses pam_ecryptfs.so to automount an encrypted directory. Dm-crypt/Mounting at login shows how to use pam_exec.so to execute a custom script on a user login. Active Directory integration#Configuring PAM uses pam_winbind.so and pam_krb5.so to let users authenticate via Active Directory (LDAP, Kerberos) services. LDAP authentication#NSS and PAM is an article about integrating LDAP client or server-side authentication with pam_ldap.so . YubiKey#Linux user authentication with PAM describes how to use U2F ( pam_u2f.so ) and the proprietary Yubico OTP implementation ( pam_yubico.so ) provided by the YubiKey with PAM pam_oath shows an example to implement software based two-factor authentication with pam_oath.so . fprint employs pam_fprintd.so to setup fingerprint authentication. pam_autologin saves username and password to log in automatically. pam_usb shows how to configure pam_usb.so to use an usb-device for, optionally two-factor, authentication. SSH keys#pam_ssh uses pam_ssh.so to authenticate as a remote user. pam_abl explains how pam_abl.so can be used to limit brute-forcing attacks via ssh. EncFS may get automounted via pam_encfs.so . Google Authenticator shows how to set up two-factor authentication with pam_google_authenticator.so . Very Secure FTP Daemon#PAM with virtual users 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.
A general purpose utility relating to PAM is:
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.