man
5 SYSTEMD.PRESET
SYSTEMD.PRESET(5) systemd.preset SYSTEMD.PRESET(5)
NAME
systemd.preset - Service enablement presets
SYNOPSIS
/etc/systemd/system-preset/*.preset
/run/systemd/system-preset/*.preset
/usr/lib/systemd/system-preset/*.preset
/etc/systemd/user-preset/*.preset
/run/systemd/user-preset/*.preset
/usr/lib/systemd/user-preset/*.preset
DESCRIPTION
Preset files may be used to encode policy which units shall be enabled
by default and which ones shall be disabled. They are read by systemctl
preset which uses this information to enable or disable a unit.
Depending on that policy, systemctl preset is identical to systemctl
enable or systemctl disable. systemctl preset is used by the post
install scriptlets of rpm packages (or other OS package formats), to
enable/disable specific units by default on package installation,
enforcing distribution, spin or administrator preset policy. This
allows choosing a certain set of units to be enabled/disabled even
before installing the actual package. For more information, see
systemctl(1).
It is not recommended to ship preset files within the respective
software packages implementing the units, but rather centralize them in
a distribution or spin default policy, which can be amended by
administrator policy, see below.
If no preset files exist, preset operations will enable all units that
are installed by default. If this is not desired and all units shall
rather be disabled, it is necessary to ship a preset file with a
single, catchall "disable *" line. (See example 1, below.)
When the machine is booted for the first time, systemd(1) will
enable/disable all units according to preset policy, similarly to
systemctl preset-all. Also see "First Boot Semantics" in machine-id(5).
PRESET FILE FORMAT
The preset files contain a list of directives consisting of either the
word "enable" or "disable" followed by a space and a unit name
(possibly with shell style wildcards), separated by newlines. Empty
lines and lines whose first non-whitespace character is "#" or ";" are
ignored. Multiple instance names for unit templates may be specified as
a space separated list at the end of the line instead of the customary
position between "@" and the unit suffix.
Presets must refer to the "real" unit file, and not to any aliases. See
systemd.unit(5) for a description of unit aliasing.
Two different directives are understood: "enable" may be used to enable
units by default, "disable" to disable units by default.
If multiple lines apply to a unit name, the first matching one takes
precedence over all others.
Each preset file shall be named in the style of
<priority>-<policy-name>.preset. Files in /etc/ override files with the
same name in /usr/lib/ and /run/. Files in /run/ override files with
the same name in /usr/lib/. Packages should install their preset files
in /usr/lib/. Files in /etc/ are reserved for the local administrator,
who may use this logic to override the preset files installed by vendor
packages. All preset files are sorted by their filename in
lexicographic order, regardless of which of the directories they reside
in. If multiple files specify the same unit name, the entry in the file
with the lexicographically earliest name will be applied. It is
recommended to prefix all filenames with a two-digit number and a dash,
to simplify the ordering of the files.
If the administrator wants to disable a preset file supplied by the
vendor, the recommended way is to place a symlink to /dev/null in
/etc/systemd/system-preset/ bearing the same filename.
EXAMPLES
Example 1. Default to off
# /usr/lib/systemd/system-preset/99-default.preset
disable *
This disables all units. Due to the filename prefix "99-", it will be
read last and hence can easily be overridden by spin or administrator
preset policy.
Example 2. Enable multiple template instances
# /usr/lib/systemd/system-preset/80-dirsrv.preset
enable dirsrv@.service foo bar baz
This enables all three of dirsrv@foo.service, dirsrv@bar.service and
dirsrv@baz.service.
Example 3. A GNOME spin
# /usr/lib/systemd/system-preset/50-gnome.preset
enable gdm.service
enable colord.service
enable accounts-daemon.service
enable avahi-daemon.*
This enables the three mentioned units, plus all avahi-daemon
regardless of which unit type. A file like this could be useful for
inclusion in a GNOME spin of a distribution. It will ensure that the
units necessary for GNOME are properly enabled as they are installed.
It leaves all other units untouched, and subject to other (later)
preset files, for example like the one from the first example above.
Example 4. Administrator policy
# /etc/systemd/system-preset/00-lennart.preset
enable httpd.service
enable sshd.service
enable postfix.service
disable *
This enables three specific services and disables all others. This is
useful for administrators to specifically select the units to enable,
and disable all others. Due to the filename prefix "00-" it will be
read early and override all other preset policy files.
MOTIVATION FOR THE PRESET LOGIC
Different distributions have different policies on which services shall
be enabled by default when the package they are shipped in is
installed. On Fedora all services stay off by default, so that
installing a package will not cause a service to be enabled (with some
exceptions). On Debian all services are immediately enabled by default,
so that installing a package will cause its services to be enabled
right-away.
Even within a single distribution, different spins (flavours, remixes,
whatever you might want to call them) of a distribution also have
different policies on what services to enable, and what services to
leave off. For example, Fedora Workstation will enable gdm as display
manager by default, while the Fedora KDE spin will enable sddm instead.
Different sites might also have different policies what to turn on by
default and what to turn off. For example, one administrator would
prefer to enforce the policy of "sshd should be always on, but
everything else off", while another one might say "snmpd always on, and
for everything else use the distribution policy defaults".
Traditionally, policy about which services shall be enabled were
implemented in each package individually. This made it cumbersome to
implement different policies per spin or per site, or to create
software packages that do the right thing on more than one
distribution. The enablement mechanism was also encoding the enablement
policy.
The preset mechanism allows clean separation of the enablement
mechanism (inside the package scriptlets, by invoking systemctl preset)
and enablement policy (centralized in the preset files), and lifts the
configuration out of individual packages. Preset files may be written
for specific distributions, for specific spins or for specific sites,
in order to enforce different policies as needed. It is recommended to
apply the policy encoded in preset files in package installation
scriptlets.
SEE ALSO
systemd(1), systemctl(1), systemd-delta(1)
daemon(7) has a discussion of packaging scriptlets.
Fedora page introducing the use of presets: Features/PackagePresets[1].
NOTES
1. Features/PackagePresets
https://fedoraproject.org/wiki/Features/PackagePresets
systemd 252 SYSTEMD.PRESET(5)