The pam_deny(8) module is one of the simplest modules available; it responds to any request with PAM_AUTH_ERR. It is useful for quickly disabling a service (add it to the top of every chain), or for terminating chains of sufficient modules.

The pam_echo(8) module simply passes its arguments to the conversation function as a PAM_TEXT_INFO message. It is mostly useful for debugging, but can also serve to display messages such as “Unauthorized access will be prosecuted” before starting the authentication procedure.

The pam_exec(8) module takes its first argument to be the name of a program to execute, and the remaining arguments are passed to that program as command-line arguments. One possible application is to use it to run a program at login time which mounts the user's home directory.

The pam_ftpusers(8) module

The pam_group(8) module accepts or rejects applicants on the basis of their membership in a particular file group (normally wheel for su(1)). It is primarily intended for maintaining the traditional behaviour of BSD su(1), but has many other uses, such as excluding certain groups of users from a particular service.

The pam_guest(8) module allows guest logins using fixed login names. Various requirements can be placed on the password, but the default behaviour is to allow any password as long as the login name is that of a guest account. The pam_guest(8) module can easily be used to implement anonymous FTP logins.

The pam_krb5(8) module

The pam_ksu(8) module

The pam_lastlog(8) module

The pam_login_access(8) module provides an implementation of the account management primitive which enforces the login restrictions specified in the login.access(5) table.

The pam_nologin(8) module refuses non-root logins when /var/run/nologin exists. This file is normally created by shutdown(8) when less than five minutes remain until the scheduled shutdown time.

The pam_opie(8) module implements the opie(4) authentication method. The opie(4) system is a challenge-response mechanism where the response to each challenge is a direct function of the challenge and a passphrase, so the response can be easily computed “just in time” by anyone possessing the passphrase, eliminating the need for password lists. Moreover, since opie(4) never reuses a challenge that has been correctly answered, it is not vulnerable to replay attacks.

The pam_opieaccess(8) module is a companion module to pam_opie(8). Its purpose is to enforce the restrictions codified in opieaccess(5), which regulate the conditions under which a user who would normally authenticate herself using opie(4) is allowed to use alternate methods. This is most often used to prohibit the use of password authentication from untrusted hosts.

In order to be effective, the pam_opieaccess(8) module must be listed as requisite immediately after a sufficient entry for pam_opie(8), and before any other modules, in the auth chain.

The pam_passwdqc(8) module

The pam_permit(8) module is one of the simplest modules available; it responds to any request with PAM_SUCCESS. It is useful as a placeholder for services where one or more chains would otherwise be empty.

The pam_radius(8) module

The pam_rhosts(8) module

The pam_rootok(8) module reports success if and only if the real user id of the process calling it (which is assumed to be run by the applicant) is 0. This is useful for non-networked services such as su(1) or passwd(1), to which the root should have automatic access.

The pam_securetty(8) module

The pam_self(8) module reports success if and only if the names of the applicant matches that of the target account. It is most useful for non-networked services such as su(1), where the identity of the applicant can be easily verified.

The pam_ssh(8) module provides both authentication and session services. The authentication service allows users who have passphrase-protected SSH secret keys in their ~/.ssh directory to authenticate themselves by typing their passphrase. The session service starts ssh-agent(1) and preloads it with the keys that were decrypted in the authentication phase. This feature is particularly useful for local logins, whether in X (using xdm(1) or another PAM-aware X login manager) or at the console.

The pam_tacplus(8) module

The pam_unix(8) module implements traditional UNIX® password authentication, using getpwnam(3) to obtain the target account's password and compare it with the one provided by the applicant. It also provides account management services (enforcing account and password expiration times) and password-changing services. This is probably the single most useful module, as the great majority of admins will want to maintain historical behaviour for at least some services.