Ubuntu Manpage: pam_systemd - Register user sessions in the systemd login manager

Provided by: libpam-systemd_245.4-4ubuntu3.24_amd64

NAME

       pam_systemd - Register user sessions in the systemd login manager

SYNOPSIS

       pam_systemd.so

DESCRIPTION

pam_systemd registers user sessions with the systemd login manager systemd-logind.service(8), and hence
the systemd control group hierarchy.

The module also applies various resource management and runtime parameters to the new session, as
configured in the JSON User Record[1] of the user, when one is defined.

On login, this module — in conjunction with systemd-logind.service — ensures the following:

1. If it does not exist yet, the user runtime directory /run/user/$UID is either created or mounted as
new "tmpfs" file system with quota applied, and its ownership changed to the user that is logging in.

2. The $XDG_SESSION_ID environment variable is initialized. If auditing is available and pam_loginuid.so
was run before this module (which is highly recommended), the variable is initialized from the
auditing session id (/proc/self/sessionid). Otherwise, an independent session counter is used.

3. A new systemd scope unit is created for the session. If this is the first concurrent session of the
user, an implicit per-user slice unit below user.slice is automatically created and the scope placed
into it. An instance of the system service user@.service, which runs the systemd user manager
instance, is started.

4. The "$TZ", "$EMAIL" and "$LANG" environment variables are configured for the user, based on the
respective data from the user's JSON record (if it is defined). Moreover, any environment variables
explicitly configured in the user record are imported, and the umask, nice level, and resource limits
initialized.

On logout, this module ensures the following:

1. If enabled in logind.conf(5) (KillUserProcesses=), all processes of the session are terminated. If
the last concurrent session of a user ends, the user's systemd instance will be terminated too, and
so will the user's slice unit.

2. If the last concurrent session of a user ends, the user runtime directory /run/user/$UID and all its
contents are removed, too.

If the system was not booted up with systemd as init system, this module does nothing and immediately
returns PAM_SUCCESS.

OPTIONS

       The following options are understood:

       class=
           Takes a string argument which sets the session class. The XDG_SESSION_CLASS environment variable (see
           below) takes precedence. One of "user", "greeter", "lock-screen" or "background". See
           sd_session_get_class(3) for details about the session class.

       type=
           Takes a string argument which sets the session type. The XDG_SESSION_TYPE environment variable (see
           below) takes precedence. One of "unspecified", "tty", "x11", "wayland" or "mir". See
           sd_session_get_type(3) for details about the session type.

       desktop=
           Takes a single, short identifier string for the desktop environment. The XDG_SESSION_DESKTOP
           environment variable (see below) takes precedence. This may be used to indicate the session desktop
           used, where this applies and if this information is available. For example: "GNOME", or "KDE". It is
           recommended to use the same identifiers and capitalization as for $XDG_CURRENT_DESKTOP, as defined by
           the Desktop Entry Specification[2]. (However, note that the option only takes a single item, and not
           a colon-separated list like $XDG_CURRENT_DESKTOP.) See sd_session_get_desktop(3) for further details.

       debug[=]
           Takes an optional boolean argument. If yes or without the argument, the module will log debugging
           information as it operates.

MODULE TYPES PROVIDED

       Only session is provided.

ENVIRONMENT

The following environment variables are initialized by the module and available to the processes of the
user's session:

$XDG_SESSION_ID
A short session identifier, suitable to be used in filenames. The string itself should be considered
opaque, although often it is just the audit session ID as reported by /proc/self/sessionid. Each ID
will be assigned only once during machine uptime. It may hence be used to uniquely label files or
other resources of this session. Combine this ID with the boot identifier, as returned by
sd_id128_get_boot(3), for a globally unique identifier for the current session.

$XDG_RUNTIME_DIR
Path to a user-private user-writable directory that is bound to the user login time on the machine.
It is automatically created the first time a user logs in and removed on the user's final logout. If
a user logs in twice at the same time, both sessions will see the same $XDG_RUNTIME_DIR and the same
contents. If a user logs in once, then logs out again, and logs in again, the directory contents will
have been lost in between, but applications should not rely on this behavior and must be able to deal
with stale files. To store session-private data in this directory, the user should include the value
of $XDG_SESSION_ID in the filename. This directory shall be used for runtime file system objects such
as AF_UNIX sockets, FIFOs, PID files and similar. It is guaranteed that this directory is local and
offers the greatest possible file system feature set the operating system provides. For further
details, see the XDG Base Directory Specification[3]. $XDG_RUNTIME_DIR is not set if the current
user is not the original user of the session.

$TZ, $EMAIL, $LANG
If a JSON user record is known for the user logging in these variables are initialized from the
respective data in the record.

The following environment variables are read by the module and may be used by the PAM service to pass
metadata to the module. If these variables are not set when the PAM module is invoked but can be
determined otherwise they are set by the module, so that these variables are initialized for the session
and applications if known at all.

$XDG_SESSION_TYPE
The session type. This may be used instead of type= on the module parameter line, and is usually
preferred.

$XDG_SESSION_CLASS
The session class. This may be used instead of class= on the module parameter line, and is usually
preferred.

$XDG_SESSION_DESKTOP
The desktop identifier. This may be used instead of desktop= on the module parameter line, and is
usually preferred.

$XDG_SEAT
The seat name the session shall be registered for, if any.

$XDG_VTNR
The VT number the session shall be registered for, if any. (Only applies to seats with a VT
available, such as "seat0")

If not set, pam_systemd will initialize $XDG_SEAT and $XDG_VTNR based on the $DISPLAY variable (if the
latter is set).

SESSION LIMITS

       PAM modules earlier in the stack, that is those that come before pam_systemd.so, can set session scope
       limits using the PAM context objects. The data for these objects is provided as NUL-terminated C strings
       and maps directly to the respective unit resource control directives. Note that these limits apply to
       individual sessions of the user, they do not apply to all user processes as a combined whole. In
       particular, the per-user user@.service unit instance, which runs the systemd --user manager process and
       its children, and is tracked outside of any session, being shared by all the user's sessions, is not
       covered by these limits.

       See systemd.resource-control(5) for more information about the resources. Also, see pam_set_data(3) for
       additional information about how to set the context objects.

       systemd.memory_max
           Sets unit MemoryMax=.

       systemd.tasks_max
           Sets unit TasksMax=.

       systemd.cpu_weight
           Sets unit CPUWeight=.

       systemd.io_weight
           Sets unit IOWeight=.

       systemd.runtime_max_sec
           Sets unit RuntimeMaxSec=.

       Example data as can be provided from an another PAM module:

           pam_set_data(handle, "systemd.memory_max", (void *)"200M", cleanup);
           pam_set_data(handle, "systemd.tasks_max",  (void *)"50",   cleanup);
           pam_set_data(handle, "systemd.cpu_weight", (void *)"100",  cleanup);
           pam_set_data(handle, "systemd.io_weight",  (void *)"340",  cleanup);
           pam_set_data(handle, "systemd.runtime_max_sec", (void *)"3600", cleanup);

EXAMPLE

       Here's an example PAM configuration fragment that allows users sessions to be managed by
       systemd-logind.service:

           #%PAM-1.0
           auth     sufficient pam_unix.so
           auth     required   pam_deny.so

           account  required   pam_nologin.so
           account  sufficient pam_unix.so
           account  required   pam_permit.so

           password sufficient pam_unix.so sha512 shadow try_first_pass try_authtok
           password required   pam_deny.so

           -session optional   pam_loginuid.so
           -session optional   pam_systemd.so
           session  required   pam_unix.so

NOTES

        1. JSON User Record
           https://systemd.io/USER_RECORD

        2. Desktop Entry Specification
           http://standards.freedesktop.org/desktop-entry-spec/latest/

        3. XDG Base Directory Specification
           http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html