systemd.preset - Service enablement presets
/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
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).
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.
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
This enables all three of
[email protected],
[email protected] and
[email protected].
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.
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.
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].
- 1.
- Features/PackagePresets