2. The Linux-PAM configuration file

Contents of this section

Linux-PAM is designed to provide the system administrator with a great deal of flexibility in configuring the privilege granting applications of their system. The local configuration of those aspects of system security controlled by Linux-PAM is contained in the system file, /etc/pam.conf. In this section we discuss the correct syntax of and generic options respected by entries to this file.

2.1 File syntax

The reader should note that the Linux-PAM specific tokens in this file are case insensitive. The module paths, however, are case sensitive since they indicate a file's name and reflect the case dependence of typical Linux file-systems. The case-sensitivity of the arguments to any given module is defined for each module in turn.

In addition to the lines described below, there are two special characters provided for the convenience of the system administrator: comments are preceded by a `#', the comment extending to the next end of line; also, module specification lines may be extended with a `\'.

A general configuration line the the /etc/pam.conf file has the following form:

service-name   module-type   control-flag   module-path   arguments
Here we explain the meaning of each of these tokens.

service-name

The name of the service associated with this entry. Frequently the service name is the conventional name of the given application. For example, `ftpd', `rlogind' and `su', etc. .

There is a special service-name, reserved for defining a default authentication mechanism. It has the name `OTHER' and may be specified in either lower or upper case characters.

module-type

One of (currently) four types of module. The four types are as follows:

control-flag

One of (currently) three tokens that indicate the severity of concern associated with the success or failure of a given module. Linux-PAM provides for the stacking of similar modules, providing a method of simultaneously exposing the user to more than one authentication mechanism. The application is not made aware of the individual success or failure of modules listed in the `/etc/pam.conf' file. Instead, it receives a summary success or fail response from the Linux-PAM library. The order of execution of these modules is that of the entries in the /etc/pam.conf file; earlier entries are executed before later ones.

The policy for determining this response is based on these three control-flags:

module-path

The path-name of the dynamically loadable object file; the pluggable module itself.

args

The args are a list of tokens that are passed to the module when it is invoked. Much like arguments to a typical Linux shell command. Generally, valid arguments are optional and are specific to any given module. Invalid arguments are ignored by a module, however, when encountering an invalid argument, the module is required to write an error to syslog(2). For a list of generic options see the next section.

Any line, in /etc/pam.conf, that is not formatted correctly is ignored by Linux-PAM; however, a corresponding error is written to the system log files with a call to syslog(2).

2.2 Generic optional arguments

The following are optional arguments which are likely to be understood by any module. Arguments (including these) are in general optional.

debug

Use the syslog(2) call to log debugging information to the system log files.

no_warn

Instruct module to not give warning messages to the application.

use_first_pass

The module should not prompt the user for a password. Instead, it should obtain the previously typed password (from the preceding auth module), and use that. If that doesn't work, then the user will not be authenticated. (This option is intended for auth and password modules only).

try_first_pass

The module should attempt authentication with the previously typed password (from the preceding auth module). If that doesn't work, then the user is prompted for a password. (This option is intended for auth modules only).

use_mapped_pass

This argument is not currently supported by any of the modules in the Linux-PAM distribution because of possible consequences associated with U.S. encryption exporting restrictions. Within the U.S., module developers are, of course, free to implement it (as are developers in other countries). For compatibility reasons we describe its use as suggested in the DCE-RFC 86.0, see the section bibliography for a pointer to this document.

The use_mapped_pass argument instructs the module to take the clear text authentication token entered by a previous module (that requests such a token) and use it to generate an encryption/decryption key with which to safely store/retrieve the authentication token required for this module. In this way the user can enter a single authentication token and be quietly authenticated by a number of stacked modules. Obviously a convenient feature that necessarily requires some reliably strong encryption to make it secure. This argument is intended for the auth and password module types only.


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