Pluggable Authentication Modules for Linux

This article describes an implementation of a user-authentication API: the Pluggable-Authentication-Modules API (PAM for short).
Some details

In the remainder of this article, I will give a brief overview of how to write and use a PAM-based application. Potential authors and interested administrators should see the Linux-PAM URL (at the end) for more complete details. The intention of this article is to provide only a taste of what you can do with Linux-PAM. In particular, I will not address the issue of how to write an authentication module. For those details you should consult the full documentation available from the Linux-PAM URL.

Let us consider a generic login-type application. We will see which responsibilities are delegated to the PAM-API and which responsibilities are retained by the application. Finally, we will cover how a local administrator can configure the application to suit local taste.

Figure 1 is a graphic protraying the three components to a working PAM-based application. On the left is the application which is linked to the libpam.so shared library. In the middle we have the PAM library which parses the configuration file(s) and uses the entries listed there to load the configured selection of authentication modules (PAMs). Additionally, the application supplies a conversation function which provides a means for the modules to talk directly with the user.

In Listing 1 the skeleton of a login-type application is shown. It can be compared with the sample application given in the OSF-RFC defining PAM. The differences reflect enhancements to PAM since the RFC was written. Note, the listing is not very secure; it pays little attention to possible errors returned by the framework and is intended only to orient the reader.

The application initializes the library with a call to pam_start(), which silently parses the configuration file and loads those authentication modules that are appropriate to this application. It then enters a loop that attempts to authenticate applicant users. This process is repeated until a user is correctly authenticated, or the loaded authentication modules indicate that their patience has been exhausted.

Once a user has been authenticated, the pam_acct_mgmt() function is used to establish if the user is permitted to log in at this time. Modules of account-management type can be used to restrict users from logging in at certain times of the day/week or for enforcing password expiration. This latter case is intercepted, and the user is prevented from gaining access to the system until they have successfully updated their password with the pam_chauthtok() function.

The user's login session is surrounded with two sets of function calls. The outer function calls, pam_open_session() and pam_close_session(), mark the beginning and end of the PAM-authenticated session. Session initialization and termination typically include tasks such as making a system resource available (mounting the user's home directory) and establishing an audit trail. The inner function calls, pam_setcred(), first establish and finally release the PAM-configurable identity of the user. These can include credentials like access-tickets and supplementary group memberships.

Following logout, the user's PAM-configurable credentials are deleted, and the session is closed with a call to the pam_close_session() function.

Finally, with a call to pam_end(), the login application breaks its connection to the PAM library. The PAMs are unloaded, and the dynamically allocated memory is scrubbed and returned to the system.

This simple application demonstrates most of the functionality provided by the PAM paradigm. The conversation mechanism flexibly leaves the mode of direct interaction with the user entirely at the discretion of the application. In this way, it is possible for modules to be used simultaneously with graphically-based programs (xdm etc.) and their text based equivalents (login etc.).

Configuring an Application

Having obtained a PAM-based application, it is necessary to attach authentication modules to it. At the time of this writing there is an old way and a new way of doing this. The old way corresponds to the method advocated in the RFC and is based on a the contents of a single PAM configuration file: /etc/pam.conf. The new method is to break up the entries for the separate services into independent configuration files that are each located in the /etc/pam.d/ directory. The name of the file containing the configuration for a given application is the service name (in lower-case letters).

The function of the configuration file(s) is to provide a mapping from the application's service name to a selection of modules that provide authentication services to the raw application. In the case of the source program of Listing 1, the service name is simply “login”. (This is the first argument of the pam_start() function call.)

Along with similar entries for each of the PAM-aware services present on your system, the old configuration file (/etc/pam.conf) might contain entries of the following form:

...
# Here is the module configuration for login as it
# might appear in "/etc/pam.conf"
#
login   auth    requisite       pam_securetty.so
login   auth    required        pam_unix_auth.so
login   account required        pam_unix_acct.so
login   session optional        pam_cfs.so      \
        keys=/etc/security/cfs.keys
login   session required        pam_unix_sess.so
login   password        sufficient      pam_unix_passwd.so
login   password        required        pam_warn.so
#
...

The first four fields are: service-name, module-type, control-flag and module-filename. The fifth and greater fields are for optional arguments that are specific to the individual authentication modules.

The second field in the configuration file is the module-type, it indicates which of the four PAM management services the corresponding module will provide to the application. Our sample configuration file refers to all four groups:

  • auth: identifies the PAMs that are invoked when the application calls pam_authenticate() and pam_setcred().

  • account: maps to the pam_acct_mgmt() function.

  • session: indicates the mapping for the pam_open_session() and pam_close_session() calls.

  • password: group refers to the pam_chauthtok() function.

Generally, you only need to supply mappings for the functions that are needed by a specific application. For example, the standard password changing application, passwd, only requires a password group entry; any other entries are ignored.

The third field indicates what action is to be taken based on the success or failure of the corresponding module. Choices for tokens to fill this field are:

  • requisite: Failure instantly returns control to the application indicating the nature of the first module failure.

  • required: All these modules are required to succeed for libpam to return success to the application.

  • sufficient: Given that all preceding modules have succeeded, the success of this module leads to an immediate and successful return to the application (failure of this module is ignored).

  • optional: The success or failure of this module is generally not recorded.

The fourth field contains the name of the loadable module, pam_*.so. For the sake of readability, the full pathname of each module is not given. Before Linux-PAM-0.56 was released, there was no support for a default authentication-module directory. If you have an earlier version of Linux-PAM installed, you will have to specify the full path for each of the modules. Your distribution most likely placed these modules exclusively in one of the following directories: /lib/security/ or /usr/lib/security/.

The equivalent functionality for our login application can be obtained with the new configuration arrangement via an independent login configuration file:

#%PAM-1.0
#(The above "magic" header is optional)
# The modules for login as they might appear in
# "/etc/pam.d/login" this configuration is
# accepted by Linux-PAM-0.56 and higher.
#
auth    requisite       pam_securetty.so
auth    required        pam_unix_auth.so
account required        pam_unix_acct.so
session optional        pam_cfs.so      \
        keys=/etc/security/cfs.keys
session required        pam_unix_sess.so
password        sufficient      pam_unix_passwd.so
password        required        pam_warn.so
# end of file.

The newer configuration file is distinct from the old in that it is missing a service-name field. This field is not needed, as the name of the service-specific configuration file is by definition the service-name of the application.

It should be noted that the content of an /etc/pam.d/ directory takes precedence over the contents of any /etc/pam.conf file.

Note that the example contains more than a single module mapping for the auth, session and password management groups. This feature is referred to as stacking and enables a single application to make use of more than one module at a time. The order in which the modules are stacked is the same as the order in which they are invoked.

The two stacked auth modules are used to pam_authenticate() the user. The first module (pam_securetty.so) checks to see if the user is root and prevents root from logging in from an insecure terminal. The value requisite for control-flag is used to force immediate authentication failure if the securetty module fails. If this occurs, no more of the auth modules are executed. This has the benefit of preventing root from mistakenly typing a password over an insecure terminal line. Another popular module that can be used to prevent log in attempts like this is pam_listfile.so. It can be configured to perform many types of access control based on a list of tokens in a specified file.

When a non-superuser, joe for example, is successfully evaluated by the securetty module, control is passed to the next module in the stack, pam_unix_auth.so. This module performs standard Unix authentication. It prompts for a password and checks it against that stored in the local system. Providing your libc can handle it, it works on both shadowed and non-shadowed systems. An enhanced alternative to pam_unix_...so is the pam_pwdb.so module. This module makes use of the password database library, libpwdb.so, and can do things like MD5 passwords and offer RADIUS support. The important point is that the system administrator is the one deciding which authentication policy to implement by simply plugging in the corresponding module.

The auth module also supplies a binding for the pam_setcred() function. It is linked to the authentication process because the method by which a user is authenticated is strongly tied to the user's identity. Kerberos, for example, requires a network-based authentication and yields a ticket (the user's credential) with which they can obtain network services, such as remote login and print requests.

The account module line in our configuration file is used to check that the user is permitted to login. This is different from establishing whether the user is who they say they are. Account management deals with enforcing the expiration of passwords and preventing logins during system time. Our login example uses the standard pam_unix_acct.so module to enforce shadow password aging. Here, it is used (in conjunction with the password module type) to force the renewal of a user's password.

For experiments in this area, the administrator might like to try pam_time.so. This module can be configured to permit or deny access to users based on their terminal line, the time they are logging in and what they intend to do.

Next, we come to the session modules. The first in the stack, pam_cfs.so does not currently exist. (Because of ITAR export restrictions, I will not be writing it.) However, I have included it to illustrate the PAM session concept. At the start (and end) of the user's session on the system, this module would mount/unmount the user's cryptographically-secured home-directory, obtaining the user's home-directory/key-mapping information from the cfs.keys file. With PAM, someone could make a single module available, and that module could be used in any PAM aware application. The use of the value optional for control-flag ensures that the user can log in even when no such directory is available.

The second module, pam_unix_sess.so, logs a message with syslog(3) to announce the user's entry to and exit from the system.

Finally, we come to the password management group. Here, the stacked modules are invoked when users change their authentication token(s). Traditionally, this change could be for updating their password, but it has the potential to be extended to refreshing a smart card or a yearly update of an employee's retinal scan. In the case of the login example, we simply request a replacement for the user's Unix password. Because the pam_unix_passwd.so module is marked as sufficient, a warning is logged by the pam_warn.so module only in the case that the user fails to successfully enter a new password.

In addition to configuring specific service names, there is also a default mapping, given the service name other. It can be used to ease the integration of new services by providing a default selection of modules appropriate to the local security policy. Instead, it can be used to deny access to any application that does not have a specific pam.conf entry. This is the recommended usage, for example, we can make use of the pam_deny.so (always deny access) and pam_warn (syslog(3) an informative warning) modules as follows:

#%PAM-1.0
#(The above "magic" header is optional)
# The modules for defaulting services as defined
# in "/etc/pam.d/other" this configuration is
# accepted by Linux-PAM-0.56 and higher.
#
auth    required        pam_deny.so
auth    required        pam_warn.so
account required        pam_deny.so
session required        pam_deny.so
password        required        pam_warn.so
password        required        pam_deny.so
# end of file.

This configuration always denies user access to an application. As before, the pam_warn.so is used to send a warning message to syslog(3) for administrative action. This configuration can be used to make sure that only specific services are available on your system. Note, if you write an application that uses PAM and this configuration file is not sufficient to block service from it, your application is not using PAM in the correct manner.

______________________

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PAM for ITDS (LDAP) expiration question

Tony M.'s picture

Mr. Morgan,

I ran across an article you put out in Linux Journal back in Dec/2007 (little over a year ago) concerning the PAM modules for Linux. We run linux on the Mainframe and are using the delivered PAM there (under z/VM).

However, we have a problem with password expirations…..when this happens the end user gets “unknown” or “sorry” for the return message.

I think this is because MF LDAP (ITDS) returns a different set of codes what PAM doesn’t understand. So, is there a way

To capture those codes and give the user MORE details about around the “expired” password. Better yet, prompt for reset of password.

In our case, we want the linux to authenticate users against the MF LDAP (ITDS) ..AND..to detect if the password is expired.

Any info. would be greatly appreciated.

Thanks.

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