LPI Linux Certification/PAM Authentication
Detailed Objective 
The candidate should be able to configure PAM to support authentication using various available methods.
- Key knowledge area(s):
- PAM configuration files, terms and utilities
- passwd and shadow passwords
- The following is a partial list of the used files, terms and utilities:
== PAM authentication == test
PAM (Pluggable Authentication Modules) is a flexible mechanism for authenticating users.
Since the beginnings of UNIX, authenticating a user has been accomplished via the user entering a password and the system checking if the entered password corresponds to the encrypted official password that is stored in /etc/passwd . The idea being that the user *is* really that user if and only if they can correctly enter their secret password.
That was in the beginning. Since then, a number of new ways of authenticating users have become popular. Including more complicated replacements for the /etc/passwd file, and hardware devices Smart cards etc.. The problem is that each time a new authentication scheme is developed, it requires all the necessary programs (login, ftpd etc...) to be rewritten to support it.
PAM provides a way to develop programs that are independent of authentication scheme. These programs need "authentication modules" to be attatched to them at run-time in order to work. Which authentication module is to be attatched is dependent upon the local system setup and is at the discretion of the local system administrator.
Linux-PAM (Pluggable Authentication Modules for Linux) is a suite of shared libraries that enable the local system administrator to choose how applications authenticate users.
In other words, without (rewriting and) recompiling a PAM-aware application, it is possible to switch between the authentication mechanism(s) it uses. Indeed, one may entirely upgrade the local authentication system without touching the applications themselves.
Historically an application that has required a given user to be authenticated, has had to be compiled to use a specific authentication mechanism. For example, in the case of traditional UN*X systems, the identity of the user is verified by the user entering a correct password. This password, after being prefixed by a two character ``salt, is encrypted (with crypt(3)). The user is then authenticated if this encrypted password is identical to the second field of the user's entry in the system password database (the /etc/passwd file). On such systems, most if not all forms of privileges are granted based on this single authentication scheme. Privilege comes in the form of a personal user-identifier (uid) and membership of various groups. Services and applications are available based on the personal and group identity of the user. Traditionally, group membership has been assigned based on entries in the /etc/group file.
Unfortunately, increases in the speed of computers and the widespread introduction of network based computing, have made once secure authentication mechanisms, such as this, vulnerable to attack. In the light of such realities, new methods of authentication are continuously being developed. It is the purpose of the Linux-PAM project to separate the development of privilege granting software from the development of secure and appropriate authentication schemes. This is accomplished by providing a library of functions that an application may use to request that a user be authenticated. This PAM library is configured locally with a system file, /etc/pam.conf (or a series of configuration files located in /etc/pam.d/) to authenticate a user request via the locally available authentication modules. The modules themselves will usually be located in the directory /lib/security and take the form of dynamically loadable object files (see dlopen(3)).
For the uninitiated, we begin by considering an example. We take an application that grants some service to users; login is one such program. Login does two things, it first establishes that the requesting user is whom they claim to be and second provides them with the requested service: in the case of login the service is a command shell (bash, tcsh, zsh, etc.) running with the identity of the user.
Traditionally, the former step is achieved by the login application prompting the user for a password and then verifying that it agrees with that located on the system; hence verifying that as far as the system is concerned the user is who they claim to be. This is the task that is delegated to Linux-PAM. From the perspective of the application programmer (in this case the person that wrote the login application), Linux-PAM takes care of this authentication task -- verifying the identity of the user.
The flexibility of Linux-PAM is that you, the system administrator, have the freedom to stipulate which authentication scheme is to be used. You have the freedom to set the scheme for any/all PAM-aware applications on your Linux system. That is, you can authenticate from anything as naive as simple trust (pam_permit) to something as paranoid as a combination of a retinal scan, a voice print and a one-time password!
To illustrate the flexibility you face, consider the following situation: a system administrator (parent) wishes to improve the mathematical ability of her users (children). She can configure their favorite ``Shoot 'em up game (PAM-aware of course) to authenticate them with a request for the product of a couple of random numbers less than 12. It is clear that if the game is any good they will soon learn their multiplication tables. As they mature, the authentication can be upgraded to include (long) division!
Linux-PAM deals with four separate types of (management) task. These are: authentication management; account management; session management; and password management. The association of the preferred management scheme with the behavior of an application is made with entries in the relevant Linux-PAM configuration file. The management functions are performed by modules specified in the configuration file.
The Linux-PAM library consults the contents of the PAM configuration file and loads the modules that are appropriate for an application. These modules fall into one of four management groups and are stacked in the order they appear in the configuration file. These modules, when called by Linux-PAM, perform the various authentication tasks for the application. Textual information, required from/or offered to the user, can be exchanged through the use of the application-supplied conversation function.
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 one of two places: either the single system file, /etc/pam.conf; or the /etc/pam.d/ directory.
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 `#' and extend to the next end-of-line; also, module specification lines may be extended with a `\' escaped newline.
A general configuration line of the /etc/pam.conf file has the following form: : service-name module-type control-flag module-path args
Below, 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 above tokens, we will describe this method.
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. Note, when there is a module specified for a named service, the `OTHER' entries are ignored.
PAM authentication 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. Firstly, it establishes that the user is who they claim to be, by instructing the application to prompt the user for a password or other means of identification. Secondly, the module can grant group membership (independently of the /etc/groups file discussed above) 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 applicant user---`root' login only on the console.
session; primarily, this module is associated with doing things that need to be done for the user before/after they can be given service. Such things include the logging of information concerning the opening/closing of some data exchange with a user, mounting directories, etc. .
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' based authentication (auth) module-type. PAM authentication
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, however, in the case that 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. Note, 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. (Note, 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. PAM authentication
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 delimeted with square brackets and consists of a series of value=action tokens:
[value1=action1 value2=action2 ...]
Here, valueI 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 actionI 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 modules 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 - clear all memory of the state of the module stack and start again with the next stacked module. PAM authentication 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 is something that 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 fall over into an alternative authentication mode for older, legacy, applications.
The path-name 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
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(3). For a list of generic options see the next section.
Any line in (one of) the configuration file(s), 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).
Directory based configuration
More flexible than the single configuration file, as of version 0.56, it is possible to configure libpam via the contents of the /etc/pam.d/ directory. In this case the directory is filled with files each of which has a filename equal to a service-name (in lower-case): it is the personal configuration file for the named service.
Linux-PAM can be compiled in one of two modes. The preferred mode uses either /etc/pam.d/ or /etc/pam.conf configuration but not both. That is to say, if there is a /etc/pam.d/ directory then libpam only uses the files contained in this directory. However, in the absence of the /etc/pam.d/ directory the /etc/pam.conf file is used (this is likely to be the mode your preferred distribution uses). The other mode is to use both /etc/pam.d/ and /etc/pam.conf in sequence. In this mode, entries in /etc/pam.d/ override those of /etc/pam.conf. The syntax of each file in /etc/pam.d/ is similar to that of the /etc/pam.conf file and is made up of lines of the following form:
module-type control-flag module-path arguments The only difference being that the service-name is not present. The service-name is of course the name of the given configuration file. For example, /etc/pam.d/login contains the configuration for the login service.
This method of configuration has a number of advantages over the single file approach. We list them here to assist the reader in deciding which scheme to adopt:
A lower chance of misconfiguring an application. There is one less field to mis-type when editing the configuration files by hand.
Easier to maintain. One application may be reconfigured without risk of interfering with other applications on the system.
It is possible to symbolically link different services configuration files to a single file. This makes it easier to keep the system policy for access consistent across different applications. (It should be noted, to conserve space, it is equally possible to hard link a number of configuration files. However, care should be taken when administering this arrangement as editing a hard linked file is likely to break the link.)
A potential for quicker configuration file parsing. Only the relevant entries are parsed when a service gets bound to its modules. It is possible to limit read access to individual Linux-PAM configuration files using the file protections of the filesystem.
Package management becomes simpler. Every time a new application is installed, it can be accompanied by an /etc/pam.d/xxxxxx file.
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(3) 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).
expose_account : In general the leakage of some information about user accounts is not a secure policy for modules to adopt. Sometimes information such as users names or home directories, or preferred shell, can be used to attack a user's account. In some circumstances, however, this sort of information is not deemed a threat: displaying a user's full name when asking them for a password in a secured environment could also be called being 'friendly'. The expose_account argument is a standard module argument to encourage a module to be less discrete about account information as it is deemed appropriate by the local administrator.
Example configuration file entries
Default policy : If a system is to be considered secure, it had better have a reasonably secure `OTHER' entry. The following is a paranoid setting (which is not a bad place to start!):
- default; deny access
OTHER auth required pam_deny.so OTHER account required pam_deny.so OTHER password required pam_deny.so OTHER session required pam_deny.so
Whilst fundamentally a secure default, this is not very sympathetic to a misconfigured system. For example, such a system is vulnerable to locking everyone out should the rest of the file become badly written. The module pam_deny is not very sophisticated. For example, it logs no information when it is invoked so unless the users of a system contact the administrator when failing to execute a service application, the administrator may go for a long while in ignorance of the fact that his system is misconfigured.
The addition of the following line before those in the above example would provide a suitable warning to the administrator.
- default; wake up! This application is not configured
OTHER auth required pam_warn.so OTHER password required pam_warn.so
Having two ``OTHER auth lines is an example of stacking. On a system that uses the /etc/pam.d/ configuration, the corresponding default setup would be achieved with the following file:
- default configuration: /etc/pam.d/other
auth required pam_warn.so auth required pam_deny.so account required pam_deny.so password required pam_warn.so password required pam_deny.so session required pam_deny.so
On a less sensitive computer, one on which the system administrator wishes to remain ignorant of much of the power of Linux-PAM, the following selection of lines (in /etc/pam.conf) is likely to mimic the historically familiar Linux setup.
- default; standard UN*X access
OTHER auth required pam_unix.so OTHER account required pam_unix.so OTHER password required pam_unix.so OTHER session required pam_unix.so
PAM authentication Key terms, files and utilities : /etc/pam.d /etc/pam.conf /lib/libpam.so.*