The pam.conf file contains a listing of services. Each service is paired with a corresponding service module. When a service is requested, its associated module is invoked. Each entry has the following format:

service_name module_type control_flag module_path options

The following is an example of the pam.conf configuration file with support for authentication, account management, and session management modules.

login   auth requisite     pam_authtok_get.so.1
login   auth required      pam_dhkeys.so.1
login   auth required      pam_unix_auth.so.1
login   auth required      pam_dial_auth.so.1

other   session required   pam_unix_session.so.1

service_name denotes the service (for example, login, dtlogin, or rlogin). The keyword, other, indicates the module all other applications which have not been specified should use.

The other keyword can also be used if all services of the same module_type have the same requirements.

In the example, since all of the services use the same session module, they could have been replaced by a single other line.

module_type denotes the service module type: authentication (auth), account management (account), session management (session), or password management (password).

The control_flag field determines the behavior of stacking.

The module_path field specifies the relative pathname to a shared library object which implements the service functionality. If the pathname is not absolute, it is assumed to be relative to /usr/lib/security/$ISA/.

The ISA token is replaced by an implementation defined directory name which defines the path relative to the calling program's instruction set architecture.

The options field is used by the PAM framework layer to pass module specific options to the modules. It is up to the module to parse and interpret the options.

This field can be used by the modules to turn on debugging or to pass any module specific parameters such as a TIMEOUT value. It can also be used to support unified login. The options supported by the modules are documented in their respective manual pages.

When a service_name of the same module_type is defined more than once, the service is said to be stacked. Each module referenced in the module_path for that service is then processed in the order that it occurs in the configuration file. The control_flag field specifies the continuation and failure semantics of the modules, and can be requisite, required, optional, or sufficient.

The PAM framework processes each service module in the stack. If all requisite and required modules in the stack succeed, then success is returned, and optional and sufficient error values are ignored. If one or more requisite or required modules fail, then the error value from the first requisite or required module that failed is returned.

If none of the service modules in the stack are designated as requisite or required, then the PAM framework requires that at least one optional or sufficient module succeed.

If all fail then the error value from the first service module in the stack is returned.

The requisite and sufficient flags cause two exceptions to the above semantics. If a service module that is designated as requisite fails, then the PAM framework immediately returns an error to the application, and all subsequent service modules in the stack are ignored. If a prior required service module has failed, then that error is returned. If no prior required service module failed, then the error from the failed requisite service module is returned.

If a service module that is designated as sufficient succeeds, then the PAM framework immediately returns success to the application, and all subsequent services modules in the stack, even requisite and required ones, are ignored, given that all prior requisite and required modules have also succeeded. If a prior required module has failed, then the error value from that module is returned.

If any entry in pam.conf is incorrect, or if a module does not exist or cannot be opened, then all PAM services fail and users are not be permitted access to the system. An error is logged through syslog(3C) at the LOG_CRIT level. To fix incorrect entries in pam.conf, a system administrator can boot the system in maintenance mode (single user) to edit the file.

The following is a sample configuration file that stacks the su, login, and rlogin services.

su     auth requisite      pam_inhouse.so.1
su     auth requisite      pam_authtok_get.so.1
su     auth required       pam_dhkeys.so.1
su     auth required       pam_unix_auth.so.1

login   auth requisite     pam_authtok_get.so.1
login   auth required      pam_dhkeys.so.1
login   auth required      pam_unix_auth.so.1
login   auth required      pam_dial_auth.so.1
login   auth optional      pam_inhouse.so.1

rlogin  auth sufficient    pam_rhosts_auth.so.1
rlogin  auth requisite      pam_authtok_get.so.1
rlogin  auth required      pam_dhkeys.so.1
rlogin  auth required      pam_unix_auth.so.1

In the case of su, the user is authenticated by the inhouseand authtok_get, dhkesys and unix_auth authentication modules. Because the inhouse and the other authentication modules are requisite and required, respectively, an error is returned back to the application if any module fails. In addition, if the requisite authentication (inhouse authentication) fails, the other authentication modules is never invoked, and the error is returned immediately back to the application.

In the case of login, the required keyword for control_flag requires that the user be allowed to login only if the user is authenticated by all the service modules. If authtok_get authentication fails, control continues to proceed down the stack, and the inhouse authentication module is invoked. inhouse authentication is optional by virtue of the optional keyword in the control_flag field. The user can still log in even if inhouse authentication fails, assuming the modules stacked above succeeded.

In the case of rlogin, the sufficient keyword for control_flag specifies that if the rhosts authentication check succeeds, then PAM should return success to rlogin and rlogin should not prompt the user for a password. The other authentication modules, which are in the stack, will only be invoked if the rhosts check fails. This gives the system administrator the flexibility to determine if rhosts alone is sufficient enough to authenticate a remote user.

Some modules return PAM_IGNORE in certain situations. In these cases the PAM framework ignores the entire entry in pam.conf regardless of whether or not it is requisite, required, optional or sufficient.

The following is a list of the utilities that use PAM: login, passwd, su, rlogind, rshd, telnetd, ftpd, rpc.rexd, uucpd, init, sac, cron, ppp, dtsession, ssh and ttymon.

The utility dtlogin also uses PAM. dtlogin is the login service utility for the Common Desktop Environment (CDE).

The PAM configuration file does not dictate either the name or the location of the service specific modules. The convention, however, is the following:

pam_module_name.so.x

File that implements various function of specific authentication services. As the relative pathname specified, /usr/lib/security/$ISA is prepended to it.

/etc/pam.conf

Configuration file.

/usr/lib/$ISA/libpam.so.1

File that implements the PAM framework library.

The following is a sample pam.conf configuration file.

Lines that begin with the # symbol are treated as comments, and therefore ignored.

# PAM configuration
#
# Unless explicitly defined, all services use the modules
# defined in the "other" section.
#
# Modules are defined with relative pathnames, i.e., they are
# relative to /usr/lib/security/$ISA. Absolute path names, as
# present in this file in previous releases are still acceptable.
#
# Authentication management
#
# login service (explicit because of pam_dial_auth)
#
login   auth requisite           pam_authtok_get.so.1
login   auth required           pam_dhkeys.so.1
login   auth required           pam_unix_auth.so.1
login   auth required           pam_dial_auth.so.1
#
# rlogin service (explicit because of pam_rhost_auth)
#
rlogin  auth sufficient         pam_rhosts_auth.so.1
rlogin  auth requisite           pam_authtok_get.so.1
rlogin  auth required           pam_dhkeys.so.1
rlogin  auth required           pam_unix_auth.so.1
#
# rsh service (explicit because of pam_rhost_auth)
# and pam_unix_auth for meaningful pam_setcred)
#
rsh     auth sufficient         pam_rhosts_auth.so.1
rsh     auth required           pam_authtok_get.so.1
#
# ppp service (explicit because of pam_dial_auth)
#
ppp     auth requisite          pam_authtok_get.so.1
ppp     auth required           pam_dhkeys.so.1
ppp     auth required           pam_unix_auth.so.1
ppp     auth required           pam_dial_auth.so.1
#
# Default definitions for Authentication management
# Used when service name is not explicitly mentioned for authenctication
#
other   auth requisite           pam_authtok_get.so.1
other   auth required           pam_dhkeys.so.1
other   auth required           pam_unix_auth.so.1
#
# passwd command (explicit because of a different authentication module)
#
passwd  auth required           pam_passwd_auth.so.1
#
# cron service (explicit because of non-usage of pam_roles.so.1)
#
cron    account required        pam_projects.so.1
cron    account required        pam_unix_account.so.1
#
# Default definition for Account management
# Used when service name is not explicitly mentioned for account management
#
other   account requisite       pam_roles.so.1
other   account required        pam_projects.so.1
other   account required        pam_unix_account.so.1
#
# Default definition for Session management
# Used when service name is not explicitly mentioned for session management
#
other   session required        pam_unix_session.so.1
#
# Default definition for  Password management
# Used when service name is not explicitly mentioned for password management
#
other   password required       pam_dhkeys.so.1
other   password requisite      pam_authtok_get.so.1
other   password requisite      pam_authtok_check.so.1
other   password required       pam_authtok_store.so.1
#
# Support for Kerberos V5 authentication (uncomment to use Kerberos)
#
#rlogin auth optional           pam_krb5.so.1 try_first_pass
#login  auth optional           pam_krb5.so.1 try_first_pass
#other  auth optional           pam_krb5.so.1 try_first_pass
#cron   account optional        pam_krb5.so.1
#other  account optional        pam_krb5.so.1
#other  session optional        pam_krb5.so.1
#other  password optional       pam_krb5.so.1 try_first_pass