PAM Configuration File

The PAM configuration file, /etc/pam.conf, determines the authentication services to be used, and the order in which the services are used. This file can be edited to select authentication mechanisms for each system entry application.

PAM Configuration File Syntax

The PAM configuration file consists of entries with the following syntax:

service_name module_type control_flag module_path module_options

You can add comments to the pam.conf file by starting the line with a # (pound sign). Use white spaces or tabs to delimit the fields.

An entry in the PAM configuration file is ignored if one of the following conditions exists: the line has less than four fields, an invalid value is given for module_type or control_flag, or the named module does not exist.

Valid Service Names for PAM

The following table lists:

• The valid service names

• The module types that can be used with that service

• The daemon or command that is associated with the service name

Some module types are not appropriate for each service. For example, the password module type is appropriate for only the passwd command. Also, because the passwd command is not concerned with authentication, no auth module type is associated with the service.

PAM Control Flags

To determine the continuation or failure behavior from a module, you must select a control flag for each entry in the PAM configuration file, /etc/pam.conf. Each module in a stack can determine the success or failure of the request.

Continuation behavior defines if any following modules are checked. Depending on the response from a particular module, you can decide to skip any additional modules.

Failure behavior defines how error messages are logged or reported. Failures are either optional or required. A required failure causes that request to fail, even if other modules succeed. An optional failure does not always cause the request to fail.

Even though these flags apply to all module types, the following explanation assumes that these flags are being used for authentication modules. The control flags are as follows:

• binding – With this control flag, if the module is successful and no preceding modules that are flagged as required have failed, then skip the remaining modules. If a failure is returned, record a required failure and continue processing the stack.

  The binding control flag is like the required control flag, except that no additional module checking is done if the module is successful. A failure using this flag prevents the request from being successful, regardless of the response of any other modules. A success using this flag makes the request successful if no preceding required modules failed.

• required – With this control flag, if the module is successful, record a required success and continue checking any following modules. If the module fails, and if this is the first required failure, save the error message and continue checking the stack. If this is not the first failure, then continue checking the stack.

  The required control flag should be used when a particular module must succeed for the request to be successful. A failure when using this flag prevents the request from being successful, regardless of the response of any other modules. A success when using this flag does not mean that the request is successful. All of the responses from the other required modules in the stack must be successful for the request to succeed.

• requisite – With this control flag, if the module is successful, record a required success and continue checking any following modules. If the module fails, record a required failure, return the error message of the first required failure, and skip any additional checking.

  The requisite control flag is like the required control flag, except that no additional module checking is done if the module fails. A failure when using this flag prevents the request from being successful, regardless of the response of any other modules. A success when using this flag does not mean that the request is successful. All of the responses from the other required modules in the stack must be successful for the request to succeed.

• optional – With this control flag, if the module is successful, record an optional success and continue checking the stack. If the module fails, record an optional failure and continue checking the stack.

  The optional control flag should be used when successful authentication in the stack is enough for a user to be authenticated. This flag should only be used if this particular mechanism does not need to succeed. The success or failure of the request, is determined by any required failures or successes.

  If your users need to have permission associated with a specific mechanism to get their work done, then you should not label the module as optional.

• sufficient – With this control flag, if the module is successful, and no preceding modules that are flagged as required have failed, then skip the remaining modules. If the module fails, record an option failure and continue checking the stack.

  The sufficient control flag is like the optional control flag, except that no additional module checking is done if the module succeeds. A success when using this flag makes the request successful if no preceding required modules failed. A failure when using this flag causes the request to fail if no other modules succeeded.

More information about these control flags is provided in the following section, which describes the default /etc/pam.conf file.

Generic pam.conf File

The generic /etc/pam.conf file specifies the following actions:

1. When the login command is run, authentication must succeed for the pam_authtok_get, pam_dhkeys, pam_auth_unix, and the pam_dial_auth modules.

2. For the rlogin command, authentication through the pam_authtok_get, pam_dhkeys, and pam_auth_unix modules must succeed if authentication through pam_rhost_auth fails.

3. The sufficient control flag indicates that for the rlogin command, the successful authentication that is provided by the pam_rhost_auth module is sufficient. The next entry is ignored.

4. Most of the other commands that require authentication require successful authentication through the pam_authtok_get, pam_dhkeys, and pam_auth_unix modules.

5. For the rsh command, authentication through the pam_rhost_auth module is flagged as sufficient. No other authentication is required if authentication through the pam_rhost_auth module succeeds.

The OTHER service name allows a default to be set for any other commands that require authentication and are not included in the file. The OTHER option simplifies administration of the file, since many commands that are using the same module can be covered by using only one entry. Also, the OTHER service name, when used as a “catch-all,” can ensure that each access is covered by one module. By convention, the OTHER entry is included at the bottom of the section for each module type.

Normally, the entry for the module_path is “root-relative.” If the file name that you enter for module_path does not begin with a slash (/), the path /usr/lib/security/$ISA precedes the file name. A full path name must be used for modules that are located in other directories. The values for the module_options can be found in the man pages for the module. For example, the UNIX module is covered in the pam_unix(5) man page.

login   auth required           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

In this example, the login service specifies authentication through all four authentication modules. A login command fails if any one of the modules returns an error.