Author: Ben Collins Modified by: Sam Hartman 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 this, 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-session, common-account and common-password. Under some circumstances (such as ftp auth, or auth based on tty) other modules will be required. 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 this parameter. 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. You should not use the pam_stack module in the pam config file. It's not currently in Debian so it won't work. While I cannot stop someone from packaging pam_stack for Debian, I will try to convince them that it is not the direction we want. Pam_stack (among other faults) uses different pam handles for each step in the process--the handle used for session management is not the same as the handle used for authentication. This breaks several modules. We have an alternate solution for shared PAM configuration across modules, in the form of the @include directive. Currently libpam-modules is in the base setup, so it's dependency is not needed (since the library depends on the correct version). However, if any modules other than the base set in libpam-modules are used, that package must be depended on. Applications need to depend on libpam-runtime (>= 0.76-14) to guarantee that /etc/pam.d/common-* exist. The pam_unix.so module allows programs to verify the authentication of the uid of the calling process without any set bits (uid or gid). NOTE: this means the user executing the program, you cannot authenticate against other users without suid root (root makes sure the NIS and NIS+ works too) or at least sgid shadow (wont work in the above cases). Most notably this affects programs like apache from being able to use PAM with much success since it runs as www-data which has no priviledges and cannot use pam_unix.so to auth against other users. On the other hand is does allow program like vlock to auth (but not auth the root password). 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 enfluence 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 decended 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_finish 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-' (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_.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 .conf.