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Author: Ben Collins <bcollins@debian.org>
Modified by: Sam Hartman <hartmans@debian.org>,
Steve Langasek <vorlon@debian.org>
Objective: To document a base set of policies regarding PAM (Pluggable
Authentication Modules) usage in Debian packages.
===========================================================================
In order to have a consistent and stable implementation across packages
that use PAM, these guidelines will help to avoid some common mistakes and
be usable as a cross reference for FAQ's.
This document will not go into the details of how to add PAM usage to
existing code; please read the documentation in the libpam-doc package for
info on that. However, it does specify behavior needed to make sure PAM
modules in Debian will work with your application.
==================
PAM Applications
==================
Each application that uses PAM also must contain a file in /etc/pam.d/.
This file specifies which PAM modules will be used for the common PAM
functions in that application. There are several notes concerning what
modules to use in this file. Most commonly, this file should use the
@include directive to include common-auth, common-account, and
common-password, and one of either common-session or
common-session-noninteractive.
The selection of common-session or common-session-noninteractive is based
on whether the service provides "shell-like" interactive capabilities to
the user (e.g.: login, ssh, gdm) or is a non-interactive session or a
session mediated by a structured protocol (e.g.: cron, cups, samba, ppp).
This allows a service to avoid calling some modules, such as
pam_ck_connector, that only make sense in an interactive context and should
be avoided otherwise. It is expected that the modules used for
noninteractive sessions will always be a subset of those used for
interactive sessions.
Under some circumstances (such as ftp auth, or auth based on tty) other
service-specific modules will need to be listed in the service's /etc/pam.d
file.
Here is an example of a PAM configuration file that just includes the
common module fragments:
#
# /etc/pam.d/other - specify the PAM fallback behaviour
#
# Note that this file is used for any unspecified service; for example
#if /etc/pam.d/cron specifies no session modules but cron calls
#pam_open_session, the session module out of /etc/pam.d/other is
#used. If you really want nothing to happen then use pam_permit.so or
#pam_deny.so as appropriate.
# We fall back to the system default in /etc/pam.d/common-*
#
@include common-auth
@include common-account
@include common-password
@include common-session
The name of this file is determined by the call to pam_start() in the
application source code. The first parameter will be a string containing
the "service" name (eg. "login", "httpd", etc..). Please make sure that
the filename coincides with the value of this parameter used in your
application.
The file should _not_ reference the full path of the modules. It only needs
to reference the basename (eg. "pam_unix.so"). This will ensure that the
program continues to work even if the module location changes, since
libpam itself will resolve the location.
Packages which configure their services by default to use modules other than
those provided by /etc/pam.d/common-* must depend on the package providing
those modules. E.g., /etc/pam.d/login includes the line:
session required pam_limits.so
therefore it must depend on libpam-modules, which provides
/lib/security/pam_limits.so.
Applications need to depend on libpam-runtime to guarantee that
/etc/pam.d/common-* exist.
Applications that use common-session-noninteractive must depend
on libpam-runtime for this file.
The pam_unix.so module allows programs to authenticate the uid of the
calling process without being setuid or setgid. NOTE: this means the user
executing the program; you cannot authenticate other users without suid
root (root makes sure the NIS and NIS+ works too) or at least sgid shadow
(won't work in the above cases). Most notably this affects programs like
apache being able to use PAM since it runs as www-data which has no
privileges and cannot use pam_unix.so to authenticate other users. On the
other hand it does allow programs like vlock to authenticate.
The application needs to follow the following rules to make sure PAM
modules work:
1) Use the same PAM handle for all operations. This means it is not OK to
call pam_start once for authentication and then later for session
management. Modules need to be able to store pam_data between entry
points.
2) The pam_open_session and pam_setcred calls must be made in a parent
process of the eventual session. They need to be able to influence the
environment of the session.
3) If you are started as root or have root privs for some other reason,
pam_open_session and pam_setcred should be called while still root.
4) Implied by 1, make sure that pam_close_session and pam_end are called in
the same process or a process descended from the execution context as
pam_open_session and pam_setcred. The pam_close_session call may need
state stored in the handle by the open session entry point to clean up
properly. The pam_end call may need to free data (thus influencing system
state in some cases) allocated in the earlier calls.
=============
PAM Modules
=============
Separately packaged PAM modules should adhere to a few basic setup rules:
1) Packages should use the naming scheme of `libpam-<name>' (eg.
libpam-ldap).
2) The modules should be located in the directory of the most recent
libpam-modules (currently /lib/security).
3) The module should be named as pam_<name>.so. The module should not
contain a version suffix.
4) The module should be linked to libpam (-lpam) when compiled so that
proper version dependencies will work.
5) Any config files should be located in /etc/security. The filename
will be in the form of <name>.conf.
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