  The Linux-PAM System Administrators' Guide
  Andrew G. Morgan, morgan@linux.kernel.org
  DRAFT v0.68 1999/7/4

  This manual documents what a system-administrator needs to know about
  the Linux-PAM library. It covers the correct syntax of the PAM config-
  uration file and discusses strategies for maintaining a secure system.
  ______________________________________________________________________

  Table of Contents
























































  1. Introduction

  2. Some comments on the text

  3. Overview

     3.1 Getting started

  4. The Linux-PAM configuration file

     4.1 Configuration file syntax
     4.2 Directory based configuration
     4.3 Generic optional arguments
     4.4 Example configuration file entries
        4.4.1 Default policy

  5. Security issues of Linux-PAM

     5.1 If something goes wrong
     5.2 Avoid having a weak `other' configuration

  6. A reference guide for available modules

     6.1 The access module
        6.1.1 Synopsis
        6.1.2 Overview of module
        6.1.3 Account component
     6.2 Chroot
        6.2.1 Synopsis
        6.2.2 Overview of module
        6.2.3 Account component:
        6.2.4 Authentication component:
        6.2.5 Session component:
     6.3 Cracklib pluggable password strength-checker
        6.3.1 Synopsis
        6.3.2 Overview of module
        6.3.3 Password component
     6.4 The locking-out module
        6.4.1 Synopsis
        6.4.2 Overview of module
        6.4.3 Account component
        6.4.4 Authentication component
        6.4.5 Password component
        6.4.6 Session component
     6.5 Set/unset environment variables
        6.5.1 Synopsis
        6.5.2 Overview of module
        6.5.3 Authentication component
     6.6 The filter module
        6.6.1 Synopsis
        6.6.2 Overview of module
        6.6.3 Account+Authentication+Password+Session components
     6.7 Anonymous access module
        6.7.1 Synopsis
        6.7.2 Overview of module
        6.7.3 Authentication component
     6.8 The group access module
        6.8.1 Synopsis
        6.8.2 Overview of module
        6.8.3 Authentication component
     6.9 The Kerberos 4 module.
        6.9.1 Synopsis
        6.9.2 Overview of module
        6.9.3 Session component
        6.9.4 Password component
        6.9.5 Authentication component
     6.10 The last login module
        6.10.1 Synopsis
        6.10.2 Overview of module
        6.10.3 Authentication component
     6.11 The resource limits module
        6.11.1 Synopsis
        6.11.2 Overview of module
        6.11.3 Session component
     6.12 The list-file module
        6.12.1 Synopsis
        6.12.2 Overview of module
        6.12.3 Authentication component
     6.13 The mail module
        6.13.1 Synopsis
        6.13.2 Overview of module
        6.13.3 Authentication component
     6.14 The no-login module
        6.14.1 Synopsis
        6.14.2 Overview of module
        6.14.3 Authentication component
     6.15 The promiscuous module
        6.15.1 Synopsis
        6.15.2 Overview of module
        6.15.3 Account+Authentication+Password+Session components
     6.16 The Password-Database module
        6.16.1 Synopsis
        6.16.2 Overview of module
        6.16.3 Account component
        6.16.4 Authentication component
        6.16.5 Password component
        6.16.6 Session component
     6.17 The RADIUS session module
        6.17.1 Synopsis
        6.17.2 Overview of module
        6.17.3 Session component
     6.18 The rhosts module
        6.18.1 Synopsis
        6.18.2 Overview of module
        6.18.3 Authentication component
     6.19 The root access module
        6.19.1 Synopsis
        6.19.2 Overview of module
        6.19.3 Authentication component
     6.20 The securetty module
        6.20.1 Synopsis
        6.20.2 Overview of module
        6.20.3 Authentication component
     6.21 Time control
        6.21.1 Synopsis
        6.21.2 Overview of module
        6.21.3 Account component
     6.22 The userdb module
        6.22.1 Synopsis
        6.22.2 Overview of module
        6.22.3 Authentication component
     6.23 Warning logger module
        6.23.1 Synopsis
        6.23.2 Overview of module
        6.23.3 Authentication+Password component
     6.24 The wheel module
        6.24.1 Synopsis
        6.24.2 Overview of module
        6.24.3 Authentication component

  7. Files

  8. See also

  9. Notes

  10. Author/acknowledgments

  11. Bugs/omissions

  12. Copyright information for this document



  ______________________________________________________________________

  1.  Introduction

  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 /usr/lib/security and take the form of
  dynamically loadable object files (see dlopen(3)).


  2.  Some comments on the text

  Before proceeding to read the rest of this document, it should be
  noted that the text assumes that certain files are placed in certain
  directories.  Where they have been specified, the conventions we adopt
  here for locating these files are those of the relevant RFC (RFC-86.0,
  see ``bibliography'').  If you are using a distribution of Linux (or
  some other operating system) that supports PAM but chooses to
  distribute these files in a diferent way (Red Hat is one such
  distribution), you should be careful when copying examples directly
  from the text.


  As an example of the above, where it is explicit, the text assumes
  that PAM loadable object files (the modules) are to be located in the
  following directory: /usr/lib/security/. However, Red Hat Linux, in
  agreement with the Linux File System Standard (the FSSTND), places
  these files in /lib/security. Please be careful to perform the
  necessary transcription when using the examples from the text.


  3.  Overview

  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.


  Traditinally, 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 the so 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 syntax for this file is discussed in the section ``below''.

  Here is a figure that describes the overall organization of Linux-PAM.


                +----------------+
                | application: X |
                +----------------+       /  +----------+     +================+
                | authentication-[---->--\--] Linux-   |--<--| PAM config file|
                |       +        [----<--/--]   PAM    |     |================|
                |[conversation()][--+    \  |          |     | X auth .. a.so |
                +----------------+  |    /  +-n--n-----+     | X auth .. b.so |
                |                |  |       __|  |           |           _____/
                |  service user  |  A      |     |           |____,-----'
                |                |  |      V     A
                +----------------+  +------|-----|---------+ -----+------+
                                       +---u-----u----+    |      |      |
                                       |   auth....   |--[ a ]--[ b ]--[ c ]
                                       +--------------+
                                       |   acct....   |--[ b ]--[ d ]
                                       +--------------+
                                       |   password   |--[ b ]--[ c ]
                                       +--------------+
                                       |   session    |--[ e ]--[ c ]
                                       +--------------+




  By way of explanation, the left of the figure represents the applica-
  tion; application X.  Such an application interfaces with the Linux-
  PAM library and knows none of the specifics of its configured authen-
  tication method.  The Linux-PAM library (in the center) consults the
  contents of the PAM configuration file and loads the modules that are
  appropriate for application-X. These modules fall into one of four
  management groups (lower-center) and are stacked in the order they
  appear in the configuaration file. These modules, when called by
  Linux-PAM, perform the various authentication tasks for the applica-
  tion. Textual information, required from/or offered to the user, can
  be exchanged through the use of the application-supplied conversation
  function.


  3.1.  Getting started

  The following text was contributed by Seth Chaiklin:


       To this point, we have described how PAM should work in an
       ideal world, in which all applications are coded properly.
       However, at the present time (October 1998), this is far
       from the case.  Therefore, here are some practical considerations
       in trying to use PAM in your system.

       Why bother, is it really worth all the trouble?

       If you running Linux as a single user system, or in an
       environment where all the users are trusted, then there
       is no real advantage for using PAM.





  Ed: there is actually an advantage since you can dummy down the
  authentication to the point where you don't have any... Almost like
  Win95.

  In a networked environment, it is clear that you need to think a
  little more about how users etc., are authenticated:]
































































  If you are running Linux as a server, where several different
  services are being provided (e.g., WWW with areas restricted by
  password control, PPP), then there can be some real and interesting
  value for PAM.  In particular, through the use of modules, PAM can
  enable a program to search through several different password
  databases, even if that program is not explicitly coded for
  that particular database.  Here are some examples of the possibilities
  that this enables.

     o  Apache has a module that provides PAM services.  Now
     authentication
        to use particular directories can be conducted by PAM, which
        means that the range of modules that are available to PAM can
        be used, including RADIUS, NIS, NCP (which means that Novell
        password databases can be used).

     o  pppd has a PAMified version (available from RedHat)  Now it is
        possible to use a series of databases to authenticate ppp users.
        In addition to the normal Linux-based password databases (such
        as /etc/passwd and /etc/shadow), you can use PAM modules to
        authenticate against Novell password databases or NT-based
        password databases.

     o  The preceding two examples can be combined.  Imagaine that the
        persons in your office/department are already registered with a
        username and password in a Novell or NT LAN.  If you wanted to
        use this database on your Linux server (for PPP access, for
        web access, or even for normal shell access), you can use PAM
        to authenticate against this existing database, rather than
        maintain a separate database on both Linux and the LAN server.


  Can I use PAM for any program that requires authentication?

  Yes and no.   Yes, if you have access to the source code, and can
  add the appropriate PAM functions.  No, if you do not have access
  to the source code, and the binary does not have the PAM functions
  included.

  In other words, if a program is going to use PAM, then it has to
  have PAM functions explicitly coded into the program.  If they
  are not, then it is not possible to use PAM.

  How can I tell whether a program has PAM coded into it or not?

  A quick-and-dirty (but not always reliable) method is to ldd
  <programname>
  If libpam and libpam_misc are not among the libraries that the program
  uses, then it is not going to work with PAM.  However, it is possible
  that the libraries are included, but there are still problems, because
  the PAM coding in the program does not work as it should.  So a
  more reliable method is to make the follow tests.

  In the /etc/pam.d directory, one needs to make a configuration file
  for the program that one wants to run.  The exact name of the
  configuration
  file is hard-coded into the program.  Usually, it is the same name as
  the
  program, but not always.  For sake of illustration, let's assume that
  the program is named "pamprog" and the name of the configuration file
  is /etc/pam.d/pamprog.

  In the /etc/pam.d/pamprog but the following two lines:

  auth    required  pam_permit.so
  auth    required  pam_warn.so
  Now try to use pamprog.  The first line in the configuration file
  says that all users are permitted.  The second line will write a
  warning to your syslog file (or whether you syslog is writing

  messages).  If this test succeeds, then you know that you have
  a program that can understand pam, and you can start the more
  interesting work of deciding how to stack modules in your
  /etc/pam.d/pamprog  file.





  4.  The Linux-PAM configuration file

  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.  In this section we discuss the correct syntax of and
  generic options respected by entries to these files.


  4.1.  Configuration 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 `#' 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   arguments




  Below, we explain the meaning of each of these tokens. The second (and
  more recently adopted) way of configuring Linux-PAM is via the con-
  tents of the /etc/pam.d/ directory. Once we have explained the meaning
  of the above tokens, we will 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', 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.


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

     o  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.

     o  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.

     o  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. .

     o  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.


     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:


     o  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.

     o  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.

     o  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.

     o  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
        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 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.



     o  ignore - when used with a stack of modules, the module's return
        status will not contribute to the return code the application
        obtains.
     o  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.

     o  die - equivalent to bad with the side effect of terminating the
        module stack and PAM immediately returning to the application.

     o  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.

     o  done - equivalent to ok with the side effect of terminating the
        module stack and PAM immediately returning to the application.

     o  reset - clear all memory of the state of the module stack and
        start again with the next stacked module.


        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.  Flexible eh?


      module-path
        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: /usr/lib/security (but see the notes ``above'').


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



  Any line in (one of) the confiuration 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).


  4.2.  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.  The
  other mode (and the one currently supported by Red Hat 4.2 and higher)
  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:



  o  A lower chance of misconfiguring an application. There is one less
     field to mis-type when editing the configuration files by hand.

  o  Easier to maintain. One application may be reconfigured without
     risk of interfering with other applications on the system.

  o  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.)

  o  A potential for quicker configuration file parsing. Only the
     relevant entries are parsed when a service gets bound to its
     modules.

  o  It is possible to limit read access to individual Linux-PAM
     configuration files using the file protections of the filesystem.

  o  Package management becomes simpler.  Every time a new application
     is installed, it can be accompanied by an /etc/pam.d/xxxxxx file.


  4.3.  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(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). For compatibility
        reasons we describe its use as suggested in the DCE-RFC 86.0,
        see 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.



  4.4.  Example configuration file entries

  In this section, we give some examples of entries that can be present
  in the Linux-PAM configuration file. As a first attempt at configuring
  your system you could do worse than to implement these.


  4.4.1.  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       /usr/lib/security/pam_deny.so
  OTHER   account  required       /usr/lib/security/pam_deny.so
  OTHER   password required       /usr/lib/security/pam_deny.so
  OTHER   session  required       /usr/lib/security/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 (documented in a later section) 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       /usr/lib/security/pam_warn.so
       OTHER   password required       /usr/lib/security/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       /usr/lib/security/pam_warn.so
       auth     required       /usr/lib/security/pam_deny.so
       account  required       /usr/lib/security/pam_deny.so
       password required       /usr/lib/security/pam_warn.so
       password required       /usr/lib/security/pam_deny.so
       session  required       /usr/lib/security/pam_deny.so




  This is the only explicit example we give for an /etc/pam.d/ file. In
  general, it should be clear how to transpose the remaining examples to
  this configuration scheme.


  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 UNIX access
  #
  OTHER   auth     required       /usr/lib/security/pam_unix_auth.so
  OTHER   account  required       /usr/lib/security/pam_unix_acct.so
  OTHER   password required       /usr/lib/security/pam_unix_passwd.so
  OTHER   session  required       /usr/lib/security/pam_unix_session.so




  In general this will provide a starting place for most applications.
  Unfortunately, most is not all. One application that might require
  additional lines is ftpd if you wish to enable anonymous-ftp.


  To enable anonymous-ftp, the following lines might be used to replace
  the default (OTHER) ones. (*WARNING* as of 1996/12/28 this does not
  work correctly with any ftpd. Consequently, this description may be
  subject to change or the application will be fixed.)


       #
       # ftpd; add ftp-specifics. These lines enable anonymous ftp over
       #       standard UNIX access (the listfile entry blocks access to
       #       users listed in /etc/ftpusers)
       #
       ftpd    auth    sufficient  /usr/lib/security/pam_ftp.so
       ftpd    auth    required    /usr/lib/security/pam_unix_auth.so use_first_pass
       ftpd    auth    required    /usr/lib/security/pam_listfile.so \
                               onerr=succeed item=user sense=deny file=/etc/ftpusers




  Note, the second line is necessary since the default entries are
  ignored by a service application (here ftpd) if there are any entries
  in /etc/pam.conf for that specified service.  Again, this is an exam-
  ple of authentication module stacking.  Note the use of the sufficient
  control-flag. It says that ``if this module authenticates the user,
  ignore the subsequent auth modules''. Also note the use of the
  ``use_first_pass'' module-argument, this instructs the UNIX authenti-
  cation module that it is not to prompt for a password but rely one
  already having been obtained by the ftp module.


  The standard UNIX modules, used above, are strongly tied to using the
  default `libc' user database functions (see for example, man
  getpwent).  It is the opinion of the author that these functions are
  not sufficently flexible to make full use of the power of Linux-PAM.
  For this reason, and as a small plug, I mention in passing that there
  is a pluggable replacement for the pam_unix_..  modules; pam_pwdb. See
  the section below for a more complete description.



  5.  Security issues of Linux-PAM

  This section will discuss good practices for using Linux-PAM in a
  secure manner.  It is currently sadly lacking...suggestions are
  welcome!





  5.1.  If something goes wrong

  Linux-PAM has the potential to seriously change the security of your
  system.  You can choose to have no security or absolute security (no
  access permitted).  In general, Linux-PAM errs towards the latter.
  Any number of configuration errors can dissable access to your system
  partially, or completely.


  The most dramatic problem that is likely to be encountered when
  configuring Linux-PAM is that of deleting the configuration file(s):
  /etc/pam.d/* and/or /etc/pam.conf.  This will lock you out of your own
  system!


  To recover, your best bet is to reboot the system in single user mode
  and set about correcting things from there.  The following has been
  adapted from a life-saving email on the subject from David Wood:
















































  > What the hell do I do now?

  OK, don't panic. The first thing you have to realize is that
  this happens to 50% of users who ever do anything with PAM.
  It happened here, not once, not twice, but three times, all
  different, and in the end, the solution was the same every
  time.

  First, I hope you installed LILO with a delay. If you can,
  reboot, hit shift or tab or something and type:

      LILO boot: linux single

  (Replace 'linux' with 'name-of-your-normal-linux-image').
  This will let you in without logging in.  Ever wondered how
  easy it is to break into a linux machine from the console?
  Now you know.

  If you can't do that, then get yourself a bootkernel floppy
  and a root disk a-la slackware's rescue.gz.  (Red Hat's
  installation disks can be used in this mode too.)

  In either case, the point is to get back your root prompt.

  Second, I'm going to assume that you haven't completely
  nuked your pam installation - just your configuration files.
  Here's how you make your configs nice again:

      cd /etc
      mv pam.conf pam.conf.orig
      mv pam.d pam.d.orig
      mkdir pam.d
      cd pam.d

  and then use vi to create a file called "other" in this
  directory.  It should contain the following four lines:

      auth     required       pam_unix_auth.so
      account  required       pam_unix_acct.so
      password required       pam_unix_passwd.so
      session  required       pam_unix_session.so

  Now you have the simplest possible PAM configuration that
  will work the way you're used to.  Everything should
  magically start to work again.  Try it out by hitting ALT-F2
  and logging in on another virtual console.  If it doesn't
  work, you have bigger problems, or you've mistyped
  something.  One of the wonders of this system (seriously,
  perhaps) is that if you mistype anything in the conf files,
  you usually get no error reporting of any kind on the
  console - just some entries in the log file.  So look there!
  (Try 'tail /var/log/messages'.)

  From here you can go back and get a real configuration
  going, hopefully after you've tested it first on a machine
  you don't care about screwing up.  :/

  Some pointers (to make everything "right" with Red Hat...):

      Install the newest pam, pamconfig, and pwdb from the
      redhat current directory, and do it all on the same
      command line with rpm...

          rpm -Uvh [maybe --force too] pam-* pamconfig-* pwdb-*

      Then make sure you install (or reinstall) the newest
      version of libc, util-linux, wuftp, and NetKit. For
      kicks you might try installing the newest versions of
      the affected x apps, like xlock, but I haven't gotten
      those to work at all yet.




  5.2.  Avoid having a weak `other' configuration

  It is not a good thing to have a weak default (OTHER) entry.  This
  service is the default configuration for all PAM aware applications
  and if it is weak, your system is likely to be vulnerable to attack.


  Here is a sample "other" configuration file. The pam_deny module will
  deny access and the pam_warn module will send a syslog message to
  auth.notice:




       #
       # The PAM configuration file for the `other' service
       #
       auth      required   pam_deny.so
       auth      required   pam_warn.so
       account   required   pam_deny.so
       account   required   pam_warn.so
       password  required   pam_deny.so
       password  required   pam_warn.so
       session   required   pam_deny.so
       session   required   pam_warn.so





  6.  A reference guide for available modules

  Here, we collect together some descriptions of the various modules
  available for Linux-PAM.  In general these modules should be freely
  available.  Where this is not the case, it will be indicated.


  Also please note the comments contained in the section ``on text
  conventions above'' when copying the examples listed below.



  6.1.  The access module

  6.1.1.  Synopsis



     Module Name:
        pam_access



     Author[s]:
        Alexei Nogin <alexei@nogin.dnttm.ru>



     Maintainer:
        Author


     Management groups provided:
        account


     Cryptographically sensitive:

     Security rating:

     Clean code base:

     System dependencies:
        Requires a configuration file /etc/security/access.conf

     Network aware:
        Through PAM_TTY if set, otherwise attempts getting tty name of
        the stdin file descriptor with ttyname().  Standard
        gethostname(), yp_get_default_domain(), gethostbyname() calls.
        NIS is used for netgroup support.



  6.1.2.  Overview of module

  Provides logdaemon style login access control.


  6.1.3.  Account component



     Recognized arguments:

     Description:
        This module provides logdaemon style login access control based
        on login names and on host (or domain) names, internet addresses
        (or network numbers), or on terminal line names in case of non-
        networked logins. Diagnostics are reported through syslog(3).
        Wietse Venema's login_access.c from logdaemon-5.6 is used with
        several changes by A. Nogin.


     Examples/suggested usage:
        Use of module is recommended, for example, on administrative
        machines such as NIS servers and mail servers where you need
        several accounts active but don't want them all to have login
        capability.

        For /etc/pam.d style configurations where your modules live in
        /lib/security, start by adding the following line to
        /etc/pam.d/login, /etc/pam.d/rlogin, /etc/pam.d/rsh and
        /etc/pam.d/ftp:



          account  required       /lib/security/pam_access.so




     Note that use of this module is not effective unless your system
     ignores .rhosts files.  See the the pam_rhosts_auth documentation.

     A sample access.conf configuration file is included with the
     distribution.




  6.2.  Chroot

  6.2.1.  Synopsis



     Module Name:
        pam_chroot


     Author:
        Bruce Campbell <brucec@humbug.org.au>


     Maintainer:
        Author; proposed on 20/11/96 - email for status


     Management groups provided:
        account; session; authentication


     Cryptographically sensitive:

     Security rating:

     Clean code base:
        Unwritten.


     System dependencies:

     Network aware:
        Expects localhost.



  6.2.2.  Overview of module

  This module is intended to provide a transparent wrapper around the
  average user, one that puts them in a fake file-system (eg, their


  Useful if you have several classes of users, and are slightly paranoid
  about security.  Can be used to limit who else users can see on the
  system, and to limit the selection of programs they can run.


  6.2.3.  Account component:

  Need more info here.


  6.2.4.  Authentication component:

  Need more info here.




  6.2.5.  Session component:

  Need more info here.




     Recognized arguments:
        Arguments and logging levels for the PAM version are being
        worked on.


     Description:

     Examples/suggested usage:
        Do provide a reasonable list of programs - just tossing 'cat',
        'ls', 'rm',

        Don't take it to extremes (eg, you can set up a separate
        environment for each user, but its a big waste of your disk
        space.)




  6.3.  Cracklib pluggable password strength-checker

  6.3.1.  Synopsis



     Module Name:
        pam_cracklib


     Author:
        Cristian Gafton <gafton@redhat.com>


     Maintainer:
        Author.


     Management groups provided:
        password


     Cryptographically sensitive:

     Security rating:

     Clean code base:

     System dependencies:
        Requires the system library libcrack and a system dictionary:
        /usr/lib/cracklib_dict.


     Network aware:


  6.3.2.  Overview of module

  This module can be plugged into the password stack of a given
  application to provide some plug-in strength-checking for passwords.
  (XXX - note this does not necessarily work with the pam_unix module,
  although it is known to work with the pam_pwdb replacement for the
  unix module -- see example and pam_pwdb write up for more
  information).


  This module works in the following manner: it first calls the Cracklib
  routine to check the strength of the password; if crack likes the
  password, the module does an additional set of strength checks.  These
  checks are:

  o  Palindrome -

     Is the new password a palindrome of the old one?

  o  Case Change Only -

     Is the new password the the old one with only a change of case?

  o  Similar -

     Is the new password too much like the old one?  This is controlled
     by one argument, difok which is a number of characters that if
     different between the old and new are enough to accept the new
     password, this defaults to 10 or 1/2 the size of the new password
     whichever is smaller.

  o  Simple -

     Is the new password too small?  This is controlled by 5 arguments
     minlen, dcredit, ucredit, lcredit, and ocredit. See the section on
     the arguments for the details of how these work and there defaults.

  o  Rotated -

     Is the new password a rotated version of the old password?

  o  Already used -

     Was the password used in the past?  Previously used passwords are
     to be found in /etc/security/opasswd.


  This module with no arguments will work well for standard unix
  password encryption.  With md5 encryption, passwords can be longer
  than 8 characters and the default settings for this module can make it
  hard for the user to choose a satisfactory new password.  Notably, the
  requirement that the new password contain no more than 1/2 of the
  characters in the old password becomes a non-trivial constraint.  For
  example, an old password of the form "the quick brown fox jumped over
  the lazy dogs" would be difficult to change...  In addition, the
  default action is to allow passwords as small as 5 characters in
  length.  For a md5 systems it can be a good idea to increase the
  required minimum size of a password.  One can then allow more credit
  for different kinds of characters but accept that the new password may
  share most of these characters with the old password.


  6.3.3.  Password component



     Recognized arguments:
        debug; type=XXX; retry=N; difok=N; minlen=N; dcredit=N;
        ucredit=N; lcredit=N; ocredit=N;


     Description:
        The action of this module is to prompt the user for a password
        and check its strength against a system dictionary and a set of
        rules for identifying poor choices.


        The default action is to prompt for a single password, check its
        strength and then, if it is considered strong, prompt for the
        password a second time (to verify that it was typed correctly on
        the first occasion). All being well, the password is passed on
        to subsequent modules to be installed as the new authentication
        token.


        The default action may be modified in a number of ways using the
        arguments recognized by the module:

     o  debug -

        this option makes the module write information to syslog(3)
        indicating the behavior of the module (this option does not
        write password information to the log file).

     o  type=XXX -

        the default action is for the module to use the following
        prompts when requesting passwords: ``New UNIX password: '' and
        ``Retype UNIX password: ''. Using this option you can replace
        the word UNIX with XXX.

     o  retry=N -

        the default number of times this module will request a new
        password (for strength-checking) from the user is 1. Using this
        argument this can be increased to N.

     o  difok=N -

        This argument will change the default of 10 for the number of
        characters in the new password that must not be present in the
        old password.  In addition, if 1/2 of the characters in the new
        password are different then the new password will be accepted
        anyway.

     o  minlen=N -

        The minimum acceptable size for the new password plus one.  In
        addition to the number of characters in the new password, credit
        (of +1 in length) is given for each different kind of character
        (other, upper, lower and digit).  The default for this parameter
        is 9 which is good for a old style UNIX password all of the same
        type of character but may be too low to exploit the added
        security of a md5 system.  Note that there is a pair of length
        limits in Cracklib itself, a "way too short" limit of 4 which is
        hard coded in and a defined limit (6) that will be checked
        without reference to minlen.  If you want to allow passwords as
        short as 5 characters you should either not use this module or
        recompile the crack library and then recompile this module.

     o  dcredit=N -

        This is the maximum credit for having digits in the new
        password. If you have less than or N digits, each digit will
        count +1 towards meeting the current minlen value.  The default
        for dcredit is 1 which is the recommended value for minlen less
        than 10.
     o  ucredit=N -

        This is the maximum credit for having upper case letters in the
        new password.  If you have less than or N upper case letters
        each letter will count +1 towards meeting the current minlen
        value.  The default for ucredit is 1 which is the recommended
        value for minlen less than 10.

     o  lcredit=N -

        This is the maximum credit for having lower case letters in the
        new password.  If you have less than or N lower case letters,
        each letter will count +1 towards meeting the current minlen
        value.  The default for lcredit is 1 which is the recommended
        value for minlen less than 10.

     o  ocredit=N -

        This is the maximum credit for having other characters in the
        new password.  If you have less than or N other characters, each
        character will count +1 towards meeting the current minlen
        value.  The default for ocredit is 1 which is the recommended
        value for minlen less than 10.


     Examples/suggested usage:
        (At the time of writing, this module can only be stacked before
        the pam_pwdb module. Cracklib strength checking may be compiled
        by default into the pam_unix module.)


        For an example of the use of this module, we show how it may be
        stacked with the password component of pam_pwdb:


          #
          # These lines stack two password type modules. In this example the
          # user is given 3 opportunities to enter a strong password. The
          # "use_authtok" argument ensures that the pam_pwdb module does not
          # prompt for a password, but instead uses the one provided by
          # pam_cracklib.
          #
          passwd  password required       pam_cracklib.so retry=3
          passwd  password required       pam_pwdb.so use_authtok





     Another example (in the /etc/pam.d/passwd format) is for the case
     that you want to use md5 password encryption:


          #%PAM-1.0
          #
          # These lines allow a md5 systems to support passwords of at least 14
          # bytes with extra credit of 2 for digits and 2 for others the new
          # password must have at least three bytes that are not present in the
          # old password
          #
          password  required pam_cracklib.so \
                         difok=3 minlen=15 dcredit= 2 ocredit=2
          password  required pam_pwdb.so use_authtok nullok md5



  6.4.  The locking-out module

  6.4.1.  Synopsis



     Module Name:
        pam_deny


     Author:
        Andrew G. Morgan <morgan@parc.power.net>


     Maintainer:
        current Linux-PAM maintainer


     Management groups provided:
        account; authentication; password; session


     Cryptographically sensitive:

     Security rating:

     Clean code base:
        clean.


     System dependencies:

     Network aware:


  6.4.2.  Overview of module

  This module can be used to deny access. It always indicates a failure
  to the application through the PAM framework. As is commented in the
  overview section ``above'', this module might be suitable for using
  for default (the OTHER) entries.


  6.4.3.  Account component



     Recognized arguments:

     Description:
        This component does nothing other than return a failure. The
        failure type is PAM_ACCT_EXPIRED.


     Examples/suggested usage:
        Stacking this module with type account will prevent the user
        from gaining access to the system via applications that refer to
        Linux-PAM's account management function pam_acct_mgmt().


        The following example would make it impossible to login:





     #
     # add this line to your other login entries to disable all accounts
     #
     login   account  required       pam_deny.so






  6.4.4.  Authentication component



     Recognized arguments:

     Description:
        This component does nothing other than return a failure. The
        failure type is PAM_AUTH_ERR in the case that pam_authenticate()
        is called (when the application tries to authenticate the user),
        and is PAM_CRED_UNAVAIL when the application calls pam_setcred()
        (to establish and set the credentials of the user -- it is
        unlikely that this function will ever be called in practice).


     Examples/suggested usage:
        To deny access to default applications with this component of
        the pam_deny module, you might include the following line in
        your Linux-PAM configuration file:


          #
          # add this line to your existing OTHER entries to prevent
          # authentication succeeding with default applications.
          #
          OTHER   auth     required       pam_deny.so






  6.4.5.  Password component



     Recognized arguments:

     Description:
        This component of the module denies the user the opportunity to
        change their password. It always responds with PAM_AUTHTOK_ERR
        when invoked.


     Examples/suggested usage:
        This module should be used to prevent an application from
        updating the applicant user's password. For example, to prevent
        login from automatically prompting for a new password when the
        old one has expired you should include the following line in
        your configuration file:






     #
     # add this line to your other login entries to prevent the login
     # application from being able to change the user's password.
     #
     login   password required       pam_deny.so






  6.4.6.  Session component



     Recognized arguments:

     Description:
        This aspect of the module prevents an application from starting
        a session on the host computer.


     Examples/suggested usage:
        Together with another session module, that displays a message of
        the day perhaps (XXX - such a module needs to be written), this
        module can be used to block a user from starting a shell. Given
        the presence of a pam_motd module, we might use the following
        entries in the configuration file to inform the user it is
        system time:


          #
          # An example to see how to configure login to refuse the user a
          # session (politely)
          #
          login   session  required       pam_motd.so \
                                  file=/etc/system_time
          login   session  required       pam_deny.so







  6.5.  Set/unset environment variables

  6.5.1.  Synopsis



     Module Name:
        pam_env


     Author:
        Dave Kinchlea <kinch@kinch.ark.com>


     Maintainer:
        Author


     Management groups provided:
        Authentication (setcred)

     Cryptographically sensitive:

     Security rating:

     Clean code base:

     System dependencies:
        /etc/security/pam_env.conf


     Network aware:


  6.5.2.  Overview of module

  This module allows the (un)setting of environment variables. Supported
  is the use of previously set environment variables as well as
  PAM_ITEMs such as PAM_RHOST.


  6.5.3.  Authentication component



     Recognized arguments:
        debug; conffile=configuration-file-name


     Description:
        This module allows you to (un)set arbitrary environment
        variables using fixed strings, the value of previously set
        environment variables and/or PAM_ITEMs.


        All is controlled via a configuration file (by default,
        /etc/security/pam_env.conf but can be overriden with connfile
        argument).  Each line starts with the variable name, there are
        then two possible options for each variable DEFAULT and
        OVERRIDE.  DEFAULT allows and administrator to set the value of
        the variable to some default value, if none is supplied then the
        empty string is assumed.  The OVERRIDE option tells pam_env that
        it should enter in its value (overriding the default value) if
        there is one to use.  OVERRIDE is not used, "" is assumed and no
        override will be done.




          VARIABLE   [DEFAULT=[value]]  [OVERRIDE=[value]]





     (Possibly non-existent) environment variables may be used in values
     using the ${string} syntax and (possibly non-existent) PAM_ITEMs
     may be used in values using the @{string} syntax. Both the $ and @
     characters can be backslash-escaped to be used as literal values
     (as in \$.  Double quotes may be used in values (but not
     environment variable names) when white space is needed the full
     value must be delimited by the quotes and embedded or escaped
     quotes are not supported.


     The behavior of this module can be modified with one of the
     following flags:
     o  debug - write more information to syslog(3).

     o  conffile=filename - by default the file
        /etc/security/pam_env.conf is used as the configuration file.
        This option overrides the default. You must supply a complete
        path + file name.


     Examples/suggested usage:
        See sample pam_env.conf for more information and examples.














  6.6.  The filter module

  6.6.1.  Synopsis



     Module Name:
        pam_filter


     Author:
        Andrew G. Morgan <morgan@parc.power.net>


     Maintainer:
        Author.


     Management groups provided:
        account; authentication; password; session


     Cryptographically sensitive:
        Not yet.


     Security rating:

     Clean code base:
        This module compiles cleanly on Linux based systems.


     System dependencies:
        To function it requires filters to be installed on the system.


     Network aware:





  6.6.2.  Overview of module

  This module was written to offer a plug-in alternative to programs
  like ttysnoop (XXX - need a reference). Since writing a filter that
  performs this function has not occurred, it is currently only a toy.
  The single filter provided with the module simply transposes upper and
  lower case letters in the input and output streams. (This can be very
  annoying and is not kind to termcap based editors).


  6.6.3.  Account+Authentication+Password+Session components



     Recognized arguments:
        debug; new_term; non_term; runX


     Description:
        Each component of the module has the potential to invoke the
        desired filter.  The filter is always execv(2)d with the
        privilege of the calling application and not that of the user.
        For this reason it cannot usually be killed by the user without
        closing their session.


        The behavior of the module can be significantly altered by the
        arguments passed to it in the Linux-PAM configuration file:

     o  debug -

        this option increases the amount of information logged to
        syslog(3) as the module is executed.

     o  new_term -

        the default action of the filter is to set the PAM_TTY item to
        indicate the terminal that the user is using to connect to the
        application. This argument indicates that the filter should set
        PAM_TTY to the filtered pseudo-terminal.

     o  non_term - don't try to set the PAM_TTY item.

     o  runX -

        in order that the module can invoke a filter it should know when
        to invoke it. This argument is required to tell the filter when
        to do this. The arguments that follow this one are respectively
        the full pathname of the filter to be run and any command line
        arguments that the filter might expect.


        Permitted values for X are 1 and 2. These indicate the precise
        time the that filter is to be run. To explain this concept it
        will be useful to have read the Linux-PAM Module developer's
        guide. Basically, for each management group there are up to two
        ways of calling the module's functions.

        In the case of the authentication and session components there
        are actually two separate functions.  For the case of
        authentication, these functions are _authenticate and _setcred
        -- here run1 means run the filter from the _authenticate
        function and run2 means run the filter from _setcred. In the
        case of the session modules, run1 implies that the filter is
        invoked at the _open_session stage, and run2 for _close_session.

        For the case of the account component. Either run1 or run2 may
        be used.


        For the case of the password component, run1 is used to indicate
        that the filter is run on the first occasion _chauthtok is run
        (the PAM_PRELIM_CHECK phase) and run2 is used to indicate that
        the filter is run on the second occasion (the PAM_UPDATE_AUTHTOK
        phase).



     Examples/suggested usage:
        At the time of writing there is little real use to be made of
        this module. For fun you might try adding the following line to
        your login's configuration entries


          #
          # An example to see how to configure login to transpose upper and
          # lower case letters once the user has logged in(!)
          #
          login   session  required       pam_filter.so \
                                  run1 /usr/sbin/pam_filter/upperLOWER







  6.7.  Anonymous access module

  6.7.1.  Synopsis



     Module Name:
        pam_ftp.so


     Author:
        Andrew G. Morgan <morgan@parc.power.net>


     Maintainer:
        Author.


     Management groups provided:
        authentication


     Cryptographically sensitive:

     Security rating:

     Clean code base:

     System dependencies:

     Network aware:
        prompts for email address of user; easily spoofed (XXX - needs
        work)


  6.7.2.  Overview of module

  The purpose of this module is to provide a pluggable anonymous ftp
  mode of access.


  6.7.3.  Authentication component



     Recognized arguments:
        debug; users=XXX,YYY,...; ignore


     Description:
        This module intercepts the user's name and password. If the name
        is ``ftp'' or ``anonymous'', the user's password is broken up at
        the `@' delimiter into a PAM_RUSER and a PAM_RHOST part; these
        pam-items being set accordingly. The username is set to ``ftp''.
        In this case the module succeeds.  Alternatively, the module
        sets the PAM_AUTHTOK item with the entered password and fails.


        The behavior of the module can be modified with the following
        flags:

     o  debug - log more information to with syslog(3).

     o  users=XXX,YYY,... - instead of ``ftp'' or ``anonymous'', provide
        anonymous login to the comma separated list of users;
        ``XXX,YYY,...''. Should the applicant enter one of these
        usernames the returned username is set to the first in the list;
        ``XXX''.

     o  ignore - pay no attention to the email address of the user (if
        supplied).


     Examples/suggested usage:
        An example of the use of this module is provided in the
        configuration file section ``above''. With care, this module
        could be used to provide new/temporary account anonymous login.




  6.8.  The group access module

  6.8.1.  Synopsis



     Module Name:
        pam_group


     Author:
        Andrew G. Morgan <morgan@parc.power.net>


     Maintainer:
        Author.


     Management groups provided:
        authentication
     Cryptographically sensitive:

     Security rating:
        Sensitive to setgid status of file-systems accessible to users.


     Clean code base:

     System dependencies:
        Requires an /etc/security/group.conf file. Can be compiled with
        or without libpwdb.


     Network aware:
        Only through correctly set PAM_TTY item.



  6.8.2.  Overview of module

  This module provides group-settings based on the user's name and the
  terminal they are requesting a given service from. It takes note of
  the time of day.


  6.8.3.  Authentication component



     Recognized arguments:

     Description:
        This module does not authenticate the user, but instead it
        grants group memberships (in the credential setting phase of the
        authentication module) to the user.  Such memberships are based
        on the service they are applying for. The group memberships are
        listed in text form in the /etc/security/group.conf file.


     Examples/suggested usage:
        For this module to function correctly there must be a correctly
        formatted /etc/security/groups.conf file present. The format of
        this file is as follows. Group memberships are given based on
        the service application satisfying any combination of lines in
        the configuration file. Each line (barring comments which are
        preceded by `#' marks) has the following syntax:


          services   ;   ttys   ;   users   ;   times   ;   groups




     Here the first four fields share the syntax of the pam_time config-
     uration file; /etc/security/pam_time.conf, and the last field, the
     groups field, is a comma (or space) separated list of the text-
     names of a selection of groups. If the users application for ser-
     vice satisfies the first four fields, the user is granted member-
     ship of the listed groups.


     As stated in above this module's usefulness relies on the file-
     systems accessible to the user.  The point being that once granted
     the membership of a group, the user may attempt to create a setgid
     binary with a restricted group ownership.  Later, when the user is
     not given membership to this group, they can recover group
     membership with the precompiled binary.  The reason that the file-
     systems that the user has access to are so significant, is the fact
     that when a system is mounted nosuid the user is unable to create
     or execute such a binary file.  For this module to provide any
     level of security, all file-systems that the user has write access
     to should be mounted nosuid.


     The pam_group module fuctions in parallel with the /etc/group file.
     If the user is granted any groups based on the behavior of this
     module, they are granted in addition to those entries /etc/group
     (or equivalent).




  6.9.  The Kerberos 4 module.

  6.9.1.  Synopsis



     Module Name:
        pam_krb4


     Author:
        Derrick J. Brashear <shadow@dementia.org>


     Maintainer:
        Author.


     Management groups provided:
        authentication; password; session


     Cryptographically sensitive:
        uses API


     Security rating:

     Clean code base:

     System dependencies:
        libraries - libkrb, libdes, libcom_err, libkadm; and a set of
        Kerberos include files.


     Network aware:
        Gets Kerberos ticket granting ticket via a Kerberos key
        distribution center reached via the network.



  6.9.2.  Overview of module

  This module provides an interface for doing Kerberos verification of a
  user's password, getting the user a Kerberos ticket granting ticket
  for use with the Kerberos ticket granting service, destroying the
  user's tickets at logout time, and changing a Kerberos password.



  6.9.3.  Session component



     Recognized arguments:

     Description:
        This component of the module currently sets the user's KRBTKFILE
        environment variable (although there is currently no way to
        export this), as well as deleting the user's ticket file upon
        logout (until PAM_CRED_DELETE is supported by login).


     Examples/suggested usage:
        This part of the module won't be terribly useful until we can
        change the environment from within a Linux-PAM module.



  6.9.4.  Password component



     Recognized arguments:
        use_first_pass; try_first_pass


     Description:
        This component of the module changes a user's Kerberos password
        by first getting and using the user's old password to get a
        session key for the password changing service, then sending a
        new password to that service.


     Examples/suggested usage:
        This should only be used with a real Kerberos v4 kadmind. It
        cannot be used with an AFS kaserver unless special provisions
        are made. Contact the module author for more information.



  6.9.5.  Authentication component



     Recognized arguments:
        use_first_pass; try_first_pass


     Description:
        This component of the module verifies a user's Kerberos password
        by requesting a ticket granting ticket from the Kerberos server
        and optionally using it to attempt to retrieve the local
        computer's host key and verifying using the key file on the
        local machine if one exists.

        It also writes out a ticket file for the user to use later, and
        deletes the ticket file upon logout (not until PAM_CRED_DELETE
        is called from login).


     Examples/suggested usage:
        This module can be used with a real Kerberos server using MIT v4
        Kerberos keys. The module or the system Kerberos libraries may
        be modified to support AFS style Kerberos keys. Currently this
        is not supported to avoid cryptography constraints.
  6.10.  The last login module

  6.10.1.  Synopsis



     Module Name:
        pam_lastlog


     Author:
        Andrew G. Morgan <morgan@parc.power.net>


     Maintainer:
        Author


     Management groups provided:
        auth


     Cryptographically sensitive:

     Security rating:

     Clean code base:

     System dependencies:
        uses information contained in the /var/log/wtmp file.


     Network aware:


  6.10.2.  Overview of module

  This session module maintains the /var/log/wtmp file.  Adding an open
  entry when called via the pam_open_seesion() function and completing
  it when pam_close_session() is called.  This module can also display a
  line of information about the last login of the user.  If an
  application already performs these tasks, it is not necessary to use
  this module.


  6.10.3.  Authentication component



     Recognized arguments:
        debug; nodate; noterm; nohost; silent; never


     Description:

        This module can be used to provide a ``Last login on ...''
        message. when the user logs into the system from what ever
        application uses the PAM libraries.  In addition, the module
        maintains the /var/log/wtmp file.


        The behavior of this module can be modified with one of the
        following flags:



     o  debug - write more information to syslog(3).

     o  nodate - neglect to give the date of the last login when
        displaying information about the last login on the system.

     o  noterm - neglect to diplay the terminal name on which the last
        login was attempt.

     o  nohost - neglect to indicate from which host the last login was
        attempted.

     o  silent - neglect to inform the user about any previous login:
        just update the /var/log/wtmp file.

     o  never - if the /var/log/wtmp file does not contain any old
        entries for the user, indicate that the user has never
        previously logged in with a ``welcome..." message.


     Examples/suggested usage:
        This module can be used to indicate that the user has new mail
        when they login to the system. Here is a sample entry for your
        /etc/pam.conf file:


          #
          # do we have any mail?
          #
          login   session  optional       pam_lastlog.so





     Note, some applications may perform this function themselves. In
     such cases, this module is not necessary.




  6.11.  The resource limits module

  6.11.1.  Synopsis



     Module Name:
        pam_limits


     Authors:
        Cristian Gafton <gafton@redhat.com>
        Thanks are also due to Elliot Lee <sopwith@redhat.com> for his
        comments on improving this module.


     Maintainer:
        Cristian Gafton - 1996/11/20


     Management groups provided:
        session


     Cryptographically sensitive:

     Security rating:

     Clean code base:

     System dependencies:
        requires an /etc/security/limits.conf file and kernel support
        for resource limits. Also uses the library, libpwdb.


     Network aware:


  6.11.2.  Overview of module

  This module, through the Linux-PAM open-session hook, sets limits on
  the system resources that can be obtained in a user-session. Its
  actions are dictated more explicitly through the configuration file
  discussed below.


  6.11.3.  Session component



     Recognized arguments:
        debug; conf=/path/to/file.conf


     Description:
        Through the contents of the configuration file,
        /etc/security/limits.conf, resource limits are placed on users'
        sessions. Users of uid=0 are not affected by this restriction.


        The behavior of this module can be modified with the following
        arguments:

     o  debug - verbose logging to syslog(3).

     o  conf=/path/to/file.conf - indicate an alternative limits
        configuration file to the default.


     Examples/suggested usage:
        In order to use this module the system administrator must first
        create a root-only-readable file (default is
        /etc/security/limits.conf).  This file describes the resource
        limits the superuser wishes to impose on users and groups. No
        limits are imposed on uid=0 accounts.


        Each line of the configuration file describes a limit for a user
        in the form:


          <domain>        <type>  <item>          <value>





     The fields listed above should be filled as follows...
     <domain> can be:

     o  a username

     o  a groupname, with @group syntax

     o  the wild-card *, for default entry


        <type> can have the two values:

     o  hard for enforcing hard resource limits. These limits are set by
        the superuser and enforced by the Linux Kernel. The user cannot
        raise his requirement of system resources above such values.

     o  soft for enforcing soft resource limits. These limits are ones
        that the user can move up or down within the permitted range by
        any pre-exisiting hard limits. The values specified with this
        token can be thought of as default values, for normal system
        usage.


        <item> can be one of the following:

     o  core - limits the core file size (KB)

     o  data - max data size (KB)

     o  fsize - maximum filesize (KB)

     o  memlock - max locked-in-memory address space (KB)

     o  nofile - max number of open files

     o  rss - max resident set size (KB)

     o  stack - max stack size (KB)

     o  cpu - max CPU time (MIN)

     o  nproc - max number of processes

     o  as - address space limit

     o  maxlogins - max number of logins for this user.

     o  priority - the priority to run user process with


        To completely disable limits for a user (or a group), a single
        dash (-) will do (Example: ``bin -'', ``@admin -''). Please
        remember that individual limits have priority over group limits,
        so if you impose no limits for admin group, but one of the
        members in this group have a limits line, the user will have its
        limits set according to this line.


        Also, please note that all limit settings are set per login.
        They are not global, nor are they permanent; existing only for
        the duration of the session.


        In the limits configuration file, the ``#'' character introduces
        a comment - after which the rest of the line is ignored.


        The pam_limits module does its best to report configuration
        problems found in its configuration file via syslog(3).


        The following is an example configuration file:


          # EXAMPLE /etc/security/limits.conf file:
          # =======================================
          # <domain>      <type>  <item>          <value>
          *               soft    core            0
          *               hard    rss             10000
          @student        hard    nproc           20
          @faculty        soft    nproc           20
          @faculty        hard    nproc           50
          ftp             hard    nproc           0
          @student        -       maxlogins       4




     Note, the use of soft and hard limits for the same resource (see
     @faculty) -- this establishes the default and permitted extreme
     level of resources that the user can can obtain in a given service-
     session.


     For the services that need resources limits (login for example) put
     a the following line in /etc/pam.conf as the last line for that
     service (usually after the pam_unix session line:


          #
          # Resource limits imposed on login sessions via pam_limits
          #
          login   session    required     pam_limits.so







  6.12.  The list-file module

  6.12.1.  Synopsis



     Module Name:
        pam_listfile


     Author:
        Elliot Lee <sopwith@cuc.edu>


     Maintainer:
        Red Hat Software:
        Michael K. Johnson <johnsonm@redhat.com>  1996/11/18
        (if unavailable, contact Elliot Lee <sopwith@cuc.edu>).


     Management groups provided:
        authentication


     Cryptographically sensitive:


     Security rating:

     Clean code base:
        clean


     System dependencies:

     Network aware:


  6.12.2.  Overview of module

  The list-file module provides a way to deny or allow services based on
  an arbitrary file.


  6.12.3.  Authentication component



     Recognized arguments:
        onerr=succeed|fail; sense=allow|deny; file=filename;
        item=user|tty|rhost|ruser|group|shell apply=user|@group


     Description:
        The module gets the item of the type specified -- user specifies
        the username, PAM_USER; tty specifies the name of the terminal
        over which the request has been made, PAM_TTY; rhost specifies
        the name of the remote host (if any) from which the request was
        made, PAM_RHOST; and ruser specifies the name of the remote user
        (if available) who made the request, PAM_RUSER -- and looks for
        an instance of that item in the file filename.  filename
        contains one line per item listed.  If the item is found, then
        if sense=allow, PAM_SUCCESS is returned, causing the
        authorization request to succeed; else if sense=deny,
        PAM_AUTH_ERR is returned, causing the authorization request to
        fail.


        If an error is encountered (for instance, if filename does not
        exist, or a poorly-constructed argument is encountered), then if
        onerr=succeed, PAM_SUCCESS is returned, otherwise if onerr=fail,
        PAM_AUTH_ERR or PAM_SERVICE_ERR (as appropriate) will be
        returned.


        An additional argument, apply=, can be used to restrict the
        application of the above to a specific user (apply=username) or
        a given group (apply=@groupname).  This added restriction is
        only meaningful when used with the tty, rhost and shell items.


        Besides this last one, all arguments should be specified; do not
        count on any default behavior, as it is subject to change.


        No credentials are awarded by this module.


     Examples/suggested usage:
        Classic ``ftpusers'' authentication can be implemented with this
        entry in /etc/pam.conf:


     #
     # deny ftp-access to users listed in the /etc/ftpusers file
     #
     ftp     auth     required       pam_listfile.so \
             onerr=succeed item=user sense=deny file=/etc/ftpusers




     Note, users listed in /etc/ftpusers file are (counterintuitively)
     not allowed access to the ftp service.


     To allow login access only for certain users, you can use an
     pam.conf entry like this:


          #
          # permit login to users listed in /etc/loginusers
          #
          login   auth     required       pam_listfile.so \
                  onerr=fail item=user sense=allow file=/etc/loginusers





     For this example to work, all users who are allowed to use the
     login service should be listed in the file /etc/loginusers.  Unless
     you are explicitly trying to lock out root, make sure that when you
     do this, you leave a way for root to log in, either by listing root
     in /etc/loginusers, or by listing a user who is able to su to the
     root account.




  6.13.  The mail module

  6.13.1.  Synopsis



     Module Name:
        pam_mail


     Author:
        Andrew G. Morgan <morgan@linux.kernel.org>


     Maintainer:
        Author


     Management groups provided:
        auth


     Cryptographically sensitive:

     Security rating:

     Clean code base:


     System dependencies:
        Default mail directory /var/spool/mail/


     Network aware:


  6.13.2.  Overview of module

  This module looks at the user's mail directory and indicates whether
  the user has any mail in it.


  6.13.3.  Authentication component



     Recognized arguments:
        debug; dir=direcory-name; nopen; close; noenv; empty;
        hash=hashcount


     Description:
        This module provides the ``you have new mail'' service to the
        user. It can be plugged into any application that has credential
        hooks. It gives a single message indicating the newness of any
        mail it finds in the user's mail folder. This module also sets
        the Linux-PAM environment variable, MAIL, to the user's mail
        directory.


        Although the module supplies functions for the authentication
        management group of functions, it cannot be used to authenticate
        a user; its authentication function instructs libpam to simply
        ignore it when authenticating the user.


        The behavior of this module can be modified with one of the
        following flags:



     o  debug - write more information to syslog(3).

     o  dir=pathname - look for the users' mail in an alternative
        directory given by pathname.  The default location for mail is
        /var/spool/mail. Note, if the supplied pathname is prefixed by a
        `~', the directory is interpreted as indicating a file in the
        user's home directory.

     o  nopen - instruct the module to not print any mail information
        when the user's credentials are acquired. This flag is useful to
        get the MAIL environment variable set, but to not display any
        information about it.

     o  close - instruct the module to indicate if the user has any mail
        at the as the user's credentials are revoked.

     o  noenv - do not set the MAIL environment variable.

     o  empty - indicate that the user's mail directory is empty if this
        is found to be the case.

     o  hash=hashcount - mail directory hash depth.  For example, a
        hashcount of 2 would make the mailfile be
        /var/spool/mail/u/s/user.
     Examples/suggested usage:
        This module can be used to indicate that the user has new mail
        when they login to the system. Here is a sample entry for your
        /etc/pam.conf file:


          #
          # do we have any mail?
          #
          login   auth     optional       pam_mail.so





     Note, some applications may perform this function themselves. In
     such cases, this module is not necessary.




  6.14.  The no-login module

  6.14.1.  Synopsis



     Module Name:
        pam_nologin


     Author:
        Written by Michael K. Johnson <johnsonm@redhat.com>
        (based on code taken from a module written by Andrew G. Morgan
        <morgan@parc.power.net>).


     Maintainer:
        Michael K. Johnson <johnsonm@redhat.com>


     Management groups provided:
        authentication


     Cryptographically sensitive:

     Security rating:

     Clean code base:
        1 warning about dropping const


     System dependencies:

     Network aware:


  6.14.2.  Overview of module

  Provides standard Unix nologin authentication.





  6.14.3.  Authentication component



     Recognized arguments:

     Description:
        Provides standard Unix nologin authentication.  If the file
        /etc/nologin exists, only root is allowed to log in; other users
        are turned away with an error message.  All users (root or
        otherwise) are shown the contents of /etc/nologin.


        If the file /etc/nologin does not exist, this module succeeds
        silently.


     Examples/suggested usage:
        In order to make this module effective, all login methods should
        be secured by it.  It should be used as a required method listed
        before any sufficient methods in order to get standard Unix
        nologin semantics.




  6.15.  The promiscuous module

  6.15.1.  Synopsis



     Module Name:
        pam_permit


     Author:
        Andrew G. Morgan, <morgan@parc.power.net>


     Maintainer:
        Linux-PAM maintainer.


     Management groups provided:
        account; authentication; password; session


     Cryptographically sensitive:

     Security rating:
        VERY LOW. Use with extreme caution.


     Clean code base:
        Clean.


     System dependencies:

     Network aware:





  6.15.2.  Overview of module

  This module is very dangerous. It should be used with extreme caution.
  Its action is always to permit access. It does nothing else.


  6.15.3.  Account+Authentication+Password+Session components



     Recognized arguments:

     Description:
        No matter what management group, the action of this module is to
        simply return PAM_SUCCESS -- operation successful.


        In the case of authentication, the user's name will be acquired.
        Many applications become confused if this name is unknown.


     Examples/suggested usage:
        It is seldom a good idea to use this module. However, it does
        have some legitimate uses.  For example, if the system-
        administrator wishes to turn off the account management on a
        workstation, and at the same time continue to allow logins, then
        she might use the following configuration file entry for login:


          #
          # add this line to your other login entries to disable account
          # management, but continue to permit users to log in...
          #
          login   account  required       pam_permit.so







  6.16.  The Password-Database module

  6.16.1.  Synopsis



     Module Name:
        pam_pwdb


     Author:
        Cristian Gafton <gafton@redhat.com>
        and Andrew G. Morgan <morgan@linux.kernel.org>


     Maintainer:
        Authors.


     Management groups provided:
        account; authentication; password; session


     Cryptographically sensitive:

     Security rating:

     Clean code base:

     System dependencies:
        Requires properly configured libpwdb


     Network aware:


  6.16.2.  Overview of module

  This module is a pluggable replacement for the pam_unix_..  modules.
  It uses the generic interface of the Password Database library
  http://linux.kernel.org/morgan/libpwdb/index.html.


  6.16.3.  Account component



     Recognized arguments:
        debug


     Description:
        The debug argument makes the accounting functions of this module
        syslog(3) more information on its actions. (Remaining arguments
        supported by the other functions of this module are silently
        ignored, but others are logged as errors through syslog(3)).

        Based on the following pwdb_elements: expire; last_change;
        max_change; defer_change; warn_change, this module performs the
        task of establishing the status of the user's account and
        password. In the case of the latter, it may offer advice to the
        user on changing their password or, through the
        PAM_AUTHTOKEN_REQD return, delay giving service to the user
        until they have established a new password. The entries listed
        above are documented in the Password Database Library Guide (see
        pointer above). Should the user's record not contain one or more
        of these entries, the corresponding shadow check is not
        performed.


     Examples/suggested usage:
        In its accounting mode, this module can be inserted as follows:


          #
          # Ensure users account and password are still active
          #
          login   account  required       pam_pwdb.so






  6.16.4.  Authentication component



     Recognized arguments:
        debug; use_first_pass; try_first_pass; nullok; nodelay; likeauth

     Description:
        The debug argument makes the authentication functions of this
        module syslog(3) more information on its actions.


        The default action of this module is to not permit the user
        access to a service if their official password is blank. The
        nullok argument overrides this default.


        When given the argument try_first_pass, before prompting the
        user for their password, the module first tries the previous
        stacked auth-module's password in case that satisfies this
        module as well. The argument use_first_pass forces the module to
        use such a recalled password and will never prompt the user - if
        no password is available or the password is not appropriate, the
        user will be denied access.


        The argument, nodelay, can be used to discourage the
        authentication component from requesting a delay should the
        authentication as a whole fail.  The default action is for the
        module to request a delay-on-failure of the order of one second.


        Remaining arguments, supported by the other functions of this
        module, are silently ignored. Other arguments are logged as
        errors through syslog(3).


        A helper binary, pwdb_chkpwd, is provided to check the user's
        password when it is stored in a read protected database.  This
        binary is very simple and will only check the password of the
        user invoking it.  It is called transparently on behalf of the
        user by the authenticating component of this module.  In this
        way it is possible for applications like xlock to work without
        being setuid-root.


        The likeauth argument makes the module return the same value
        when called as a credential setting module and an authentication
        module.  This will help libpam take a sane path through the auth
        component of your configuration file.


     Examples/suggested usage:
        The correct functionality of this module is dictated by having
        an appropriate /etc/pwdb.conf file, the user databases specified
        there dictate the source of the authenticated user's record.



  6.16.5.  Password component



     Recognized arguments:
        debug; nullok; not_set_pass; use_authtok; try_first_pass;
        use_first_pass; md5; bigcrypt; shadow; radius; unix


     Description:
        This part of the pam_pwdb module performs the task of updating
        the user's password.  Thanks to the flexibility of libpwdb this
        module is able to move the user's password from one database to
        another, perhaps securing the user's database entry in a dynamic
        manner (this is very ALPHA code at the moment!) - this is the
        purpose of the shadow, radius and unix arguments.


        In the case of conventional unix databases (which store the
        password encrypted) the md5 argument is used to do the
        encryption with the MD5 function as opposed to the conventional
        crypt(3) call.  As an alternative to this, the bigcrypt argument
        can be used to encrypt more than the first 8 characters of a
        password with DEC's (Digital Equipment Cooperation) `C2'
        extension to the standard UNIX crypt() algorithm.


        The nullok module is used to permit the changing of a password
        from an empty one. Without this argument, empty passwords are
        treated as account-locking ones.


        The argument use_first_pass is used to lock the choice of old
        and new passwords to that dictated by the previously stacked
        password module.  The try_first_pass argument is used to avoid
        the user having to re-enter an old password when pam_pwdb
        follows a module that possibly shared the user's old password -
        if this old password is not correct the user will be prompted
        for the correct one.  The argument use_authtok is used to force
        this module to set the new password to the one provided by the
        previously stacked password module (this is used in an example
        of the stacking of the Cracklib module documented above).


        The not_set_pass argument is used to inform the module that it
        is not to pay attention to/make available the old or new
        passwords from/to other (stacked) password modules.


        The debug argument makes the password functions of this module
        syslog(3) more information on its actions. Other arguments may
        be logged as erroneous to syslog(3).


     Examples/suggested usage:
        An example of the stacking of this module with respect to the
        pluggable password checking module, pam_cracklib, is given in
        that modules section above.


  6.16.6.  Session component



     Recognized arguments:

     Description:
        No arguments are recognized by this module component. Its action
        is simply to log the username and the service-type to syslog(3).
        Messages are logged at the beginning and end of the user's
        session.


     Examples/suggested usage:
        The use of the session modules is straightforward:





     #
     # pwdb - unix like session opening and closing
     #
     login   session  required       pam_pwdb.so







  6.17.  The RADIUS session module

  6.17.1.  Synopsis



     Module Name:
        pam_radius


     Author:
        Cristian Gafton <gafton@redhat.com>


     Maintainer:
        Author.


     Management groups provided:
        session


     Cryptographically sensitive:
        This module does not deal with passwords


     Security rating:

     Clean code base:
        gcc reports 1 warning when compiling /usr/include/rpc/clnt.h.
        Hey, is not my fault !


     System dependencies:

     Network aware:
        yes; this is a network module (independent of application).



  6.17.2.  Overview of module

  This module is intended to provide the session service for users
  autheticated with a RADIUS server.  At the present stage, the only
  option supported is the use of the RADIUS server as an accounting
  server.


  6.17.3.  Session component



     Recognized arguments:
        debug - verbose logging to syslog(3).

     Description:
        This module is intended to provide the session service for users
        autheticated with a RADIUS server. At the present stage, the
        only option supported is the use of the RADIUS server as an
        accounting server.


        (There are few things which needs to be cleared out first in the
        PAM project until one will be able to use this module and expect
        it to magically start pppd in response to a RADIUS server
        command to use PPP for this user, or to initiate a telnet
        connection to another host, or to hang and call back the user
        using parameters provided in the RADIUS server response. Most of
        these things are better suited for the radius login application.
        I hope to make available Real Soon (tm) patches for the login
        apps to make it work this way.)


        When opening a session, this module sends an ``Accounting-
        Start'' message to the RADIUS server, which will
        log/update/whatever a database for this user. On close, an
        ``Accounting-Stop'' message is sent to the RADIUS server.


        This module has no other prerequisites for making it work.  One
        can install a RADIUS server just for fun and use it as a
        centralized accounting server and forget about wtmp/last/sac
        etc. .


     Examples/suggested usage:
        For the services that need this module (login for example) put
        the following line in /etc/pam.conf as the last line for that
        service (usually after the pam_unix session line):


          login   session  required       pam_radius.so




     Replace login for each service you are using this module.


     This module make extensive use of the API provided in libpwdb
     0.54preB or later. By default, it will read the radius server
     configuration (hostname and secret) from /etc/raddb/server.  This
     is a default compiled into libpwdb, and curently there is no way to
     modify this default without recompiling libpwdb. I am working on
     extending the radius support from libpwdb to provide a possibility
     to make this runtime-configurable.

     Also please note that libpwdb will require also the RADIUS
     dictionary to be present (/etc/raddb/dictionary).





  6.18.  The rhosts module

  6.18.1.  Synopsis




     Module Name:
        pam_rhosts_auth


     Author:
        Al Longyear <longyear@netcom.com>


     Maintainer:

     Management groups provided:
        authentication


     Cryptographically sensitive:

     Security rating:

     Clean code base:
        Clean.


     System dependencies:

     Network aware:
        Standard inet_addr(), gethostbyname() function calls.



  6.18.2.  Overview of module

  This module performs the standard network authentication for services,
  as used by traditional implementations of rlogin and rsh etc.


  6.18.3.  Authentication component



     Recognized arguments:
        no_hosts_equiv; no_rhosts; debug; no_warn; privategroup;
        promiscuous; suppress


     Description:
        The authentication mechanism of this module is based on the
        contents of two files; /etc/hosts.equiv (or _PATH_HEQUIV in
        #include <netdb.h>) and ~/.rhosts.  Firstly, hosts listed in the
        former file are treated as equivalent to the localhost.
        Secondly, entries in the user's own copy of the latter file is
        used to map "remote-host remote-user" pairs to that user's
        account on the current host. Access is granted to the user if
        their host is present in /etc/hosts.equiv and their remote
        account is identical to their local one, or if their remote
        account has an entry in their personal configuration file.


        Some restrictions are applied to the attributes of the user's
        personal configuration file: it must be a regular file (as
        defined by S_ISREG(x) of POSIX.1); it must be owned by the
        superuser or the user; it must not be writable by any user
        besides its owner.


        The module authenticates a remote user (internally specified by
        the item PAM_RUSER) connecting from the remote host (internally
        specified by the item PAM_RHOST).  Accordingly, for applications
        to be compatible this authentication module they must set these
        items prior to calling pam_authenticate().  The module is not
        capable of independently probing the network connection for such
        information.


        In the case of root-access, the /etc/host.equiv file is ignored.
        Instead, the superuser must have a correctly configured personal
        configuration file.


        The behavior of the module is modified by flags:

     o  debug - log more information to syslog(3). (XXX - actually, this
        module does not do any logging currently, please volunteer to
        fix this!)

     o  no_warn - do not give verbal warnings to the user about failures
        etc. (XXX - this module currently does not issue any warnings,
        please volunteer to fix this!)

     o  no_hosts_equiv - ignore the contents of the /etc/hosts.equiv
        file.

     o  no_rhosts - ignore the contents of all user's personal
        configuration file ~/.rhosts.

     o  privategroup - normally, the ~/.rhosts file must not be writable
        by anyone other than its owner.  This option overlooks group
        write access in the case that the group owner of this file has
        the same name as the user being authenticated.  To lessen the
        security problems associated with this option, the module also
        checks that the user is the only member of their private group.

     o  promiscuous - A host entry of `+' will lead to all hosts being
        granted access. Without this option, '+' entries will be
        ignored. Note, that the debug option will syslog a warning in
        this latter case.

     o  suppress - This will prevent the module from syslog(3)ing a
        warning message when this authentication fails.  This option is
        mostly for keeping logs free of meaningless errors, in
        particular when the module is used with the sufficient control
        flag.

     Examples/suggested usage:
        To allow users to login from trusted remote machines, you should
        try adding the following line to your /etc/pam.conf file before
        the line that would otherwise prompt the user for a password:


          #
          # No passwords required for users from hosts listed above.
          #
          login  auth  sufficient  pam_rhosts_auth.so no_rhosts




     Note, in this example, the system administrator has turned off all
     personal rhosts configuration files. Also note, that this module
     can be used to only allow remote login from hosts specified in the
     /etc/host.equiv file, by replacing sufficient in the above example
     with required.

  6.19.  The root access module

  6.19.1.  Synopsis



     Module Name:
        pam_rootok


     Author:
        Andrew G. Morgan <morgan@parc.power.net>


     Maintainer:
        Linux-PAM maintainer


     Management groups provided:
        authentication


     Cryptographically sensitive:

     Security rating:

     Clean code base:
        Clean.


     System dependencies:

     Network aware:


  6.19.2.  Overview of module

  This module is for use in situations where the superuser wishes to
  gain access to a service without having to enter a password.


  6.19.3.  Authentication component



     Recognized arguments:
        debug


     Description:
        This module authenticates the user if their uid is 0.
        Applications that are created setuid-root generally retain the
        uid of the user but run with the authority of an enhanced
        effective-uid. It is the real uid that is checked.


     Examples/suggested usage:
        In the case of the su application the historical usage is to
        permit the superuser to adopt the identity of a lesser user
        without the use of a password. To obtain this behavior under
        Linux-PAM the following pair of lines are needed for the
        corresponding entry in the configuration file:




     #
     # su authentication. Root is granted access by default.
     #
     su      auth     sufficient     pam_rootok.so
     su      auth     required       pam_unix_auth.so





     Note. For programs that are run by the superuser (or started when
     the system boots) this module should not be used to authenticate
     users.




  6.20.  The securetty module

  6.20.1.  Synopsis



     Module Name:
        pam_securetty


     Author[s]:
        Elliot Lee <sopwith@cuc.edu>


     Maintainer:
        Red Hat Software:
        currently Michael K. Johnson <johnsonm@redhat.com>
        (if unavailable, contact Elliot Lee <sopwith@cuc.edu>).


     Management groups provided:
        authentication


     Cryptographically sensitive:

     Security rating:

     Clean code base:

     System dependencies:
        /etc/securetty file


     Network aware:
        Requires the application to fill in the PAM_TTY item correctly
        in order to act meaningfully.



  6.20.2.  Overview of module

  Provides standard Unix securetty checking.


  6.20.3.  Authentication component



     Recognized arguments:

     Description:
        Provides standard Unix securetty checking, which causes
        authentication for root to fail unless PAM_TTY is set to a
        string listed in the /etc/securetty file.  For all other users,
        it succeeds.


     Examples/suggested usage:
        For canonical usage, should be listed as a required
        authentication method before any sufficient authentication
        methods.




  6.21.  Time control

  6.21.1.  Synopsis



     Module Name:
        pam_time


     Author:
        Andrew G. Morgan <morgan@parc.power.net>


     Maintainer:
        Author


     Management groups provided:
        account


     Cryptographically sensitive:

     Security rating:

     Clean code base:

     System dependencies:
        Requires a configuration file /etc/security/time.conf


     Network aware:
        Through the PAM_TTY item only



  6.21.2.  Overview of module

  Running a well regulated system occasionally involves restricting
  access to certain services in a selective manner.  This module offers
  some time control for access to services offered by a system.  Its
  actions are determined with a configuration file.  This module can be
  configured to deny access to (individual) users based on their name,
  the time of day, the day of week, the service they are applying for
  and their terminal from which they are making their request.



  6.21.3.  Account component



     Recognized arguments:

     Description:
        This module bases its actions on the rules listed in its
        configuration file: /etc/security/pam.conf.  Each rule has the
        following form,

          services;ttys;users;times


     In words, each rule occupies a line, terminated with a newline or
     the beginning of a comment; a `#'.  It contains four fields sepa-
     rated with semicolons, `;'. The fields are as follows:



     o  services - a logic list of service names that are affected by
        this rule.

     o  ttys - a logic list of terminal names indicating those terminals
        covered by the rule.

     o  user - a logic list of usernames to which this rule applies


        By a logic list we mean a sequence of tokens (associated with
        the appropriate PAM_ item), containing no more than one wildcard
        character; `*', and optionally prefixed with a negation
        operator; `!'. Such a sequence is concatenated with one of two
        logical operators: & (logical AND) and | (logical OR).  Two
        examples are: !morgan&!root, indicating that this rule does not
        apply to the user morgan nor to root; and tty*&!ttyp*, which
        indicates that the rule applies only to console terminals but
        not pseudoterminals.


     o  times - a logic list of times at which this rule applies. The
        format of each element is a day/time-range. The days are
        specified by a sequence of two character entries.  For example,
        MoTuSa, indicates Monday Tuesday and Saturday.  Note that
        repeated days are unset; MoTuMo indicates Tuesday, and MoWk
        means all weekdays bar Monday.  The two character combinations
        accepted are,


          Mo Tu We Th Fr Sa Su Wk Wd Al




     The last two of these being weekend days and all 7 days of the week
     respectively.


     The time range part is a pair of 24-hour times, HHMM, separated by
     a hyphen -- indicating the start and finish time for the rule.  If
     the finsish time is smaller than the start time, it is assumed to
     apply on the following day. For an example, Mo1800-0300 indicates
     that the permitted times are Monday night from 6pm to 3am the
     following morning.


     Note, that the given time restriction is only applied when the
     first three fields are satisfied by a user's application for
     service.


     For convenience and readability a rule can be extended beyond a
     single line with a `\newline'.


     Examples/suggested usage:
        The use of this module is initiated with an entry in the Linux-
        PAM configuration file of the following type:


          #
          # apply pam_time accounting to login requests
          #
          login   account  required       pam_time.so




     where, here we are applying the module to the login application.


     Some examples of rules that can be placed in the
     /etc/security/time.conf configuration file are the following:


        login ; tty* & ; !ttyp* ; !root ; !Al0000-2400
           all users except for root are denied access to console-login
           at all times.


        games ; * ; !waster ; Wd0000-2400 | Wk1800-0800
           games (configured to use Linux-PAM) are only to be accessed
           out of working hours.  This rule does not apply to the user
           waster.



     Note, currently there is no daemon enforcing the end of a session.
     This needs to be remedied.


     Poorly formatted rules are logged as errors using syslog(3).




  6.22.  The userdb module

  6.22.1.  Synopsis



     Module Name:
        pam_userdb


     Author:
        Cristian Gafton <gafton@redhat.com>


     Maintainer:
        Author.
     Management groups provided:
        authentication


     Cryptographically sensitive:

     Security rating:

     Clean code base:

     System dependencies:
        Requires Berkeley DB.


     Network aware:


  6.22.2.  Overview of module

  Look up users in a .db database and verify their password against what
  is contained in that database.


  6.22.3.  Authentication component



     Recognized arguments:
        debug; icase; dump; db=XXXX; use_authtok; unknown_ok;


     Description:
        This module is used to verify a username/password pair against
        values stored in a Berkeley DB database. The database is indexed
        by the username, and the data fields corresponding to the
        username keys are the passwords, in unencrypted form, so caution
        must be exercised over the access rights to the DB database
        itself..

        The module will read the password from the user using the
        conversation mechanism. If you are using this module on top of
        another authetication module (like pam_pwdb;) then you should
        tell that module to read the entered password from the
        PAM_AUTHTOK field, which is set by this module.


        The action of the module may be modified from this default by
        one or more of the following flags in the /etc/pam.d/<service>
        file.

     o  debug - Supply more debugging information to syslog(3).

     o  icase - Perform the password comparisons case insensitive.

     o  dump - dump all the entries in the database to the log (eek,
        don't do this by default!)

     o  db=XXXX - use the database found on pathname XXXX. Note that
        Berkeley DB usually adds the needed filename extension for you,
        so you should use something like /etc/foodata instead of
        /etc/foodata.db.

     o  use_authtok - use the authentication token previously obtained
        by another that did the conversation with the application. If
        this token can not be obtained then the module will try to
        converse again. This option can be used for stacking different
        modules that need to deal with the authentication tokens.

     o  unknown_ok - do not return error when checking for a user that
        is not in the database.  This can be used to stack more than one
        pam_userdb module that will check a username/password pair in
        more than a database.


     Examples/suggested usage:
        This is a normal ftp configuration file (usually placed as
        /etc/pam.d/ftp on most systems) that will accept for login users
        whose username/password pairs are provided in the /tmp/dbtest.db
        file:



          #%PAM-1.0
          auth       required     pam_listfile.so item=user sense=deny file=/etc/ftpusers onerr=succeed
          auth       sufficient   pam_userdb.so icase db=/tmp/dbtest
          auth       required     pam_pwdb.so shadow nullok try_first_pass
          auth       required     pam_shells.so
          account    required     pam_pwdb.so
          session    required     pam_pwdb.so







  6.23.  Warning logger module

  6.23.1.  Synopsis



     Module Name:
        pam_warn


     Author:
        Andrew G. Morgan <morgan@parc.power.net>


     Maintainer:
        Author.


     Management groups provided:
        authentication; password


     Cryptographically sensitive:

     Security rating:

     Clean code base:

     System dependencies:

     Network aware:
        logs information about the remote user and host (if pam-items
        are known)



  6.23.2.  Overview of module

  This module is principally for logging information about a proposed
  authentication or application to update a password.


  6.23.3.  Authentication+Password component



     Recognized arguments:

     Description:
        Log the service, terminal, user, remote user and remote host to
        syslog(3). The items are not probed for, but instead obtained
        from the standard pam-items.


     Examples/suggested usage:
        an example is provided in the configuration file section
        ``above''.




  6.24.  The wheel module

  6.24.1.  Synopsis



     Module Name:
        pam_wheel


     Author:
        Cristian Gafton <gafton@redhat.com>


     Maintainer:
        Author.


     Management groups provided:
        authentication


     Cryptographically sensitive:

     Security rating:

     Clean code base:

     System dependencies:
        Requires libpwdb.


     Network aware:


  6.24.2.  Overview of module

  Only permit root access to members of the wheel (gid=0) group.



  6.24.3.  Authentication component



     Recognized arguments:
        debug; use_uid; trust; deny; group=XXXX


     Description:
        This module is used to enforce the so-called wheel group. By
        default, it permits root access to the system if the applicant
        user is a member of the wheel group (first, the module checks
        for the existence of a 'wheel' group. Otherwise the module
        defines the group with group-id 0 to be the wheel group).


        The action of the module may be modified from this default by
        one or more of the following flags in the /etc/pam.conf file.

     o  debug - Supply more debugging information to syslog(3).

     o  use_uid - This option modifies the behavior of the module by
        using the current uid of the process and not the getlogin(3)
        name of the user.  This option is useful for being able to jump
        from one account to another, for example with 'su'.

     o  trust - This option instructs the module to return PAM_SUCCESS
        should it find the user applying for root privilege is a member
        of the wheel group. The default action is to return PAM_IGNORE
        in this situation. By using the trust option it is possible to
        arrange for wheel-group members to become root without typing a
        password. USE WITH CARE.

     o  deny - This is used to reverse the logic of the module's
        behavior.  If the user is trying to get uid=0 access and is a
        member of the wheel group, deny access (for the wheel group,
        this is perhaps nonsense!): it is intended for use in
        conjunction with the group= argument...

     o  group=XXXX - Instead of checking the gid=0 group, use the user's
        XXXX group membership for the authentication. Here, XXXX is the
        name of the group and not its numeric identifier.


     Examples/suggested usage:
        To restrict access to superuser status to the members of the
        wheel group, use the following entries in your configuration
        file:


          #
          # root gains access by default (rootok), only wheel members can
          # become root (wheel) but Unix authenticate non-root applicants.
          #
          su      auth     sufficient     pam_rootok.so
          su      auth     required       pam_wheel.so
          su      auth     required       pam_unix_auth.so









  7.  Files



     /usr/lib/libpam.so.*
        the shared library providing applications with access to Linux-
        PAM.


     /etc/pam.conf
        the Linux-PAM configuration file.


     /usr/lib/security/pam_*.so
        the primary location for Linux-PAM dynamically loadable object
        files; the modules.



  8.  See also


  o  The Linux-PAM Application Writers' Guide.

  o  The Linux-PAM Module Writers' Guide.

  o  The V. Samar and R. Schemers (SunSoft), ``UNIFIED LOGIN WITH
     PLUGGABLE AUTHENTICATION MODULES'', Open Software Foundation
     Request For Comments 86.0, October 1995. See this url:
     http://www.pilgrim.umass.edu/pub/osf_dce/RFC/rfc86.0.txt


  9.  Notes

  I intend to put development comments here... like ``at the moment this
  isn't actually supported''. At release time what ever is in this
  section will be placed in the Bugs section below! :)


  Are we going to be able to support the use_mapped_pass module
  argument? Anyone know a cheap (free) good lawyer?!



  o  This issue may go away, as Sun have investigated adding a new
     management group for mappings. In this way, libpam would have
     mapping modules that could securely store passwords using strong
     cryptography and in such a way that they need not be distributed
     with Linux-PAM.


  10.  Author/acknowledgments

  This document was written by Andrew G. Morgan
  (morgan@linux.kernel.org) with many contributions from Chris Adams,
  Peter Allgeyer, Tim Baverstock, Tim Berger, Craig S. Bell, Derrick J.
  Brashear, Ben Buxton, Seth Chaiklin, Oliver Crow, Chris Dent, Marc
  Ewing, Cristian Gafton, Emmanuel Galanos, Brad M. Garcia, Eric Hester,
  Roger Hu, Eric Jacksch, Michael K. Johnson, David Kinchlea, Olaf
  Kirch, Marcin Korzonek, Stephen Langasek, Nicolai Langfeldt, Elliot
  Lee, Luke Kenneth Casson Leighton, Al Longyear, Ingo Luetkebohle,
  Marek Michalkiewicz, Robert Milkowski, Aleph One, Martin Pool, Sean
  Reifschneider, Jan Rekorajski, Erik Troan, Theodore Ts'o, Jeff Uphoff,
  Myles Uyema, Savochkin Andrey Vladimirovich, Ronald Wahl, David Wood,
  John Wilmes, Joseph S. D. Yao and Alex O.  Yuriev.

  Thanks are also due to Sun Microsystems, especially to Vipin Samar and
  Charlie Lai for their advice. At an early stage in the development of
  Linux-PAM, Sun graciously made the documentation for their
  implementation of PAM available. This act greatly accelerated the
  development of Linux-PAM.


  11.  Bugs/omissions

  More PAM modules are being developed all the time. It is unlikely that
  this document will ever be truely up to date!


  This manual is unfinished. Only a partial list of people is credited
  for all the good work they have done.


  12.  Copyright information for this document

  Copyright (c) Andrew G. Morgan 1996-8.  All rights reserved.
  Email: <morgan@linux.kernel.org>


  Redistribution and use in source and binary forms, with or without
  modification, are permitted provided that the following conditions are
  met:



  o  1. Redistributions of source code must retain the above copyright
     notice, and the entire permission notice in its entirety, including
     the disclaimer of warranties.

  o  2. Redistributions in binary form must reproduce the above
     copyright notice, this list of conditions and the following
     disclaimer in the documentation and/or other materials provided
     with the distribution.

  o  3. The name of the author may not be used to endorse or promote
     products derived from this software without specific prior written
     permission.


  Alternatively, this product may be distributed under the terms of the
  GNU General Public License (GPL), in which case the provisions of the
  GNU GPL are required instead of the above restrictions.  (This clause
  is necessary due to a potential bad interaction between the GNU GPL
  and the restrictions contained in a BSD-style copyright.)


  THIS SOFTWARE IS PROVIDED ``AS IS'' AND ANY EXPRESS OR IMPLIED
  WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
  MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
  IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
  INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
  BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
  OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
  ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR
  TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE
  USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH
  DAMAGE.


  $Id: pam.txt,v 1.1 2000/01/03 18:08:44 gafton Exp $




































































