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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
/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, systemctl preset 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.)
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 /lib/. Packages should install their preset files in /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
# /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
# /lib/systemd/system-preset/80-dirsrv.preset
enable dirsrv@.service foo bar baz
This enables all three of dirsrv AT foo.service, dirsrv AT bar.service and dirsrv AT baz.service.
Example 3. A GNOME spin
# /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 249 SYSTEMD.PRESET(5)
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