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PAM Configuration Syntax

In this section we discuss the correct syntax of and generic options respected by entries to these files. PAM-specific tokens in the configuration 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 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 “#” and extend to the next end-of-line; also, module specification lines may be extended with a “\”-escaped newline.

If the PAM authentication module (loadable link library file) is located in the default location, then it is not necessary to specify the path. In the case of Linux, the default location is /lib/security. If the module is located outside the default, then the path must be specified as:

auth  required  /other_path/pam_strange_module.so

Anatomy of /etc/pam.d Entries

The remaining information in this subsection was taken from the documentation of the Linux-PAM project. For more information on PAM, see the Official Linux-PAM home page.

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

service-name   module-type   control-flag   module-path   args

We explain the meaning of each of these tokens. The second (and more recently adopted) way of configuring Linux-PAM is via the contents of the /etc/pam.d/ directory. Once we have explained the meaning of the tokens, we describe this method.

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 , and so on.

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 uppercase characters. Note, when there is a module specified for a named service, the OTHER entries are ignored.

module-type

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

  • auth: This module type provides two aspects of authenticating the user. It establishes that the user is who he or she claims to be by instructing the application to prompt the user for a password or other means of identification. Second, the module can grant group membership (independently of the /etc/groups file) or other privileges through its credential-granting properties.

  • account: This module performs non-authentication-based account management. It is typically used to restrict/permit access to a service based on the time of day, currently available system resources (maximum number of users), or perhaps the location of the user login. For example, the “root” login may be permitted only on the console.

  • session: Primarily, this module is associated with doing things that need to be done for the user before and after he or she can be given service. Such things include logging information concerning the opening and closing of some data exchange with a user, mounting directories, and so on.

  • password: This last module type is required for updating the authentication token associated with the user. Typically, there is one module for each “challenge/response” authentication (auth) module type.

control-flag

The control-flag is used to indicate how the PAM library will react to the success or failure of the module it is associated with. Since modules can be stacked (modules of the same type execute in series, one after another), the control-flags determine the relative importance of each module. 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. As of Linux-PAM v0.60, this control-flag can be defined with one of two syntaxes.

The simpler (and historical) syntax for the control-flag is a single keyword defined to indicate the severity of concern associated with the success or failure of a specific module. There are four such keywords: required , requisite , sufficient , and optional .

The Linux-PAM library interprets these keywords in the following manner:

  • required: This indicates that the success of the module is required for the module-type facility to succeed. Failure of this module will not be apparent to the user until all of the remaining modules (of the same module-type) have been executed.

  • requisite: Like required, except that if such a module returns a failure, control is directly returned to the application. The return value is that associated with the first required or requisite module to fail. This flag can be used to protect against the possibility of a user getting the opportunity to enter a password over an unsafe medium. It is conceivable that such behavior might inform an attacker of valid accounts on a system. This possibility should be weighed against the not insignificant concerns of exposing a sensitive password in a hostile environment.

  • sufficient: The success of this module is deemed sufficient to satisfy the Linux-PAM library that this module-type has succeeded in its purpose. In the event that no previous required module has failed, no more “stacked” modules of this type are invoked. (In this case, subsequent required modules are not invoked). A failure of this module is not deemed as fatal to satisfying the application that this module-type has succeeded.

  • optional: As its name suggests, this control-flag marks the module as not being critical to the success or failure of the user's application for service. In general, Linux-PAM ignores such a module when determining if the module stack will succeed or fail. However, in the absence of any definite successes or failures of previous or subsequent stacked modules, this module will determine the nature of the response to the application. One example of this latter case is when the other modules return something like PAM_IGNORE.

The more elaborate (newer) syntax is much more specific and gives the administrator a great deal of control over how the user is authenticated. This form of the control-flag is delimited with square brackets and consists of a series of value=action tokens:

[value1=action1 value2=action2 ...]

Here, value1 is one of the following return values:


success; open_err; symbol_err; service_err; system_err; buf_err;


perm_denied; auth_err; cred_insufficient; authinfo_unavail;


user_unknown; maxtries; new_authtok_reqd; acct_expired; session_err;


cred_unavail; cred_expired; cred_err; no_module_data; conv_err;


authtok_err; authtok_recover_err; authtok_lock_busy;


authtok_disable_aging; try_again; ignore; abort; authtok_expired;


module_unknown; bad_item;
 and 
default
.

The last of these ( default ) can be used to set the action for those return values that are not explicitly defined.

The action1 can be a positive integer or one of the following tokens: ignore ; ok ; done ; bad ; die ; and reset . A positive integer, J, when specified as the action, can be used to indicate that the next J modules of the current module-type will be skipped. In this way, the administrator can develop a moderately sophisticated stack of modules with a number of different paths of execution. Which path is taken can be determined by the reactions of individual modules.

  • ignore: When used with a stack of modules, the module's return status will not contribute to the return code the application obtains.

  • bad: This action indicates that the return code should be thought of as indicative of the module failing. If this module is the first in the stack to fail, its status value will be used for that of the whole stack.

  • die: Equivalent to bad with the side effect of terminating the module stack and PAM immediately returning to the application.

  • ok: This tells PAM that the administrator thinks this return code should contribute directly to the return code of the full stack of modules. In other words, if the former state of the stack would lead to a return of PAM_SUCCESS, the module's return code will override this value. Note, if the former state of the stack holds some value that is indicative of a module's failure, this ok value will not be used to override that value.

  • done: Equivalent to ok with the side effect of terminating the module stack and PAM immediately returning to the application.

  • reset: Clears all memory of the state of the module stack and starts again with the next stacked module.

Each of the four keywords, required ; requisite ; sufficient ; and optional , have an equivalent expression in terms of the [...] syntax. They are as follows:

  • required is equivalent to [success=ok new_authtok_reqd=ok ignore=ignore default=bad] .

  • requisite is equivalent to [success=ok new_authtok_reqd=ok ignore=ignore default=die] .

  • sufficient is equivalent to [success=done new_authtok_reqd=done default=ignore] .

  • optional is equivalent to [success=ok new_authtok_reqd=ok default=ignore] .

Just to get a feel for the power of this new syntax, here is a taste of what you can do with it. With Linux-PAM-0.63, the notion of client plug-in agents was introduced. This makes it possible for PAM to support machine-machine authentication using the transport protocol inherent to the client/server application. With the [ ... value=action ... ] control syntax, it is possible for an application to be configured to support binary prompts with compliant clients, but to gracefully fail over into an alternative authentication mode for legacy applications.

module-path

The pathname of the dynamically loadable object file; the pluggable module itself. If the first character of the module path is “/”, it is assumed to be a complete path. If this is not the case, the given module path is appended to the default module path: /lib/security (but see the previous notes).

The arguments 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(3). For a list of generic options, see the next section.

If you wish to include spaces in an argument, you should surround that argument with square brackets. For example:

squid auth required pam_mysql.so user=passwd_query passwd=mada \
db=eminence [query=select user_name from internet_service where \
user_name=“%u” and password=PASSWORD(“%p”) and service=“web_proxy”]

When using this convention, you can include “[” characters inside the string, and if you wish to have a “]” character inside the string that will survive the argument parsing, you should use “\[”. In other words,

[..[..\]..]    -->   ..[..]..

Any line in one of the configuration files that is not formatted correctly will generally tend (erring on the side of caution) to make the authentication process fail. A corresponding error is written to the system log files with a call to syslog(3).

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