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14 KiB

systemd System and Service Manager





#systemd on


Lennart Poettering
Kay Sievers
...and many others

LGPLv2.1+ for all code
- except src/basic/MurmurHash2.c which is Public Domain
- except src/basic/siphash24.c which is CC0 Public Domain
- except src/journal/lookup3.c which is Public Domain
- except src/udev/* which is (currently still) GPLv2, GPLv2+
- except tools/chromiumos/* which is BSD-style

Linux kernel >= 3.13
Linux kernel >= 4.2 for unified cgroup hierarchy support
Linux kernel >= 5.4 for signed Verity images support

Kernel Config Options:
CONFIG_CGROUPS (it is OK to disable all controllers)
CONFIG_FHANDLE (libudev, mount and bind mount handling)

Kernel crypto/hash API

udev will fail to work with the legacy sysfs layout:

Legacy hotplug slows down the system and confuses udev:

Userspace firmware loading is not supported and should
be disabled in the kernel:

Some udev rules and virtualization detection relies on it:

Support for some SCSI devices serial number retrieval, to
create additional symlinks in /dev/disk/ and /dev/tape:

Required for PrivateNetwork= in service units:
Note that systemd-localed.service and other systemd units use
PrivateNetwork so this is effectively required.

Required for PrivateUsers= in service units:

Optional but strongly recommended:
CONFIG_SECCOMP_FILTER (required for seccomp support)
CONFIG_CHECKPOINT_RESTORE (for the kcmp() syscall)

Required for CPUShares= in resource control unit settings

Required for CPUQuota= in resource control unit settings

Required for IPAddressDeny= and IPAddressAllow= in resource control
unit settings

For UEFI systems:

Required for signed Verity images support:

We recommend to turn off Real-Time group scheduling in the
kernel when using systemd. RT group scheduling effectively
makes RT scheduling unavailable for most userspace, since it
requires explicit assignment of RT budgets to each unit whose
processes making use of RT. As there's no sensible way to
assign these budgets automatically this cannot really be
fixed, and it's best to disable group scheduling hence.

It's a good idea to disable the implicit creation of networking bonding
devices by the kernel networking bonding module, so that the
automatically created "bond0" interface doesn't conflict with any such
device created by systemd-networkd (or other tools). Ideally there
would be a kernel compile-time option for this, but there currently
isn't. The next best thing is to make this change through a modprobe.d
drop-in. This is shipped by default, see modprobe.d/systemd.conf.

Required for systemd-nspawn:

Note that kernel auditing is broken when used with systemd's
container code. When using systemd in conjunction with
containers, please make sure to either turn off auditing at
runtime using the kernel command line option "audit=0", or
turn it off at kernel compile time using:
If systemd is compiled with libseccomp support on
architectures which do not use socketcall() and where seccomp
is supported (this effectively means x86-64 and ARM, but
excludes 32-bit x86!), then nspawn will now install a
work-around seccomp filter that makes containers boot even
with audit being enabled. This works correctly only on kernels
3.14 and newer though. TL;DR: turn audit off, still.

glibc >= 2.16
libmount >= 2.30 (from util-linux)
(util-linux *must* be built without --enable-libmount-support-mtab)
libseccomp >= 2.3.1 (optional)
libblkid >= 2.24 (from util-linux) (optional)
libkmod >= 15 (optional)
PAM >= 1.1.2 (optional)
libcryptsetup (optional), >= 2.3.0 required for signed Verity images support
libaudit (optional)
libacl (optional)
libfdisk >= 2.33 (from util-linux) (optional)
libselinux (optional)
liblzma (optional)
liblz4 >= 1.3.0 / 130 (optional)
libzstd >= 1.4.0 (optional)
libgcrypt (optional)
libqrencode (optional)
libmicrohttpd (optional)
libpython (optional)
libidn2 or libidn (optional)
gnutls >= 3.1.4 (optional, >= 3.6.0 is required to support DNS-over-TLS with gnutls)
openssl >= 1.1.0 (optional, required to support DNS-over-TLS with openssl)
elfutils >= 158 (optional)
polkit (optional)
tzdata >= 2014f (optional)
docbook-xsl (optional, required for documentation)
xsltproc (optional, required for documentation)
python-lxml (optional, required to build the indices)
python >= 3.5
meson >= 0.46 (>= 0.49 is required to build position-independent executables)
gcc, awk, sed, grep, m4, and similar tools

During runtime, you need the following additional

util-linux >= v2.27.1 required
dbus >= 1.4.0 (strictly speaking optional, but recommended)
NOTE: If using dbus < 1.9.18, you should override the default
policy directory (--with-dbuspolicydir=/etc/dbus-1/system.d).
dracut (optional)
polkit (optional)

To build in directory build/:
meson setup build/ && meson compile -C build/

Any configuration options can be specified as -Darg=value... arguments
to meson. After the build directory is initially configured, meson will
refuse to run again, and options must be changed with:
mesonconf -Darg=value...
mesonconf without any arguments will print out available options and
their current values.

Useful commands:
meson compile -v -C build/ some/target
meson test -C build/
sudo meson install -C build/
DESTDIR=... meson install -C build/

A tarball can be created with:
git archive --format=tar --prefix=systemd-222/ v222 | xz > systemd-222.tar.xz

When systemd-hostnamed is used, it is strongly recommended to
install nss-myhostname to ensure that, in a world of
dynamically changing hostnames, the hostname stays resolvable
under all circumstances. In fact, systemd-hostnamed will warn
if nss-myhostname is not installed.

nss-systemd must be enabled on systemd systems, as that's required for
DynamicUser= to work. Note that we ship services out-of-the-box that
make use of DynamicUser= now, hence enabling nss-systemd is not

Note that the build prefix for systemd must be /usr. (Moreover,
packages systemd relies on — such as D-Bus — really should use the same
prefix, otherwise you are on your own.) -Dsplit-usr=false (which is the
default and does not need to be specified) is the recommended setting,
and -Dsplit-usr=true should be used on systems which have /usr on a
separate partition.

Additional packages are necessary to run some tests:
- busybox (used by test/TEST-13-NSPAWN-SMOKE)
- nc (used by test/TEST-12-ISSUE-3171)
- python3-pyparsing
- python3-evdev (used by hwdb parsing tests)
- strace (used by test/test-functions)
- capsh (optional, used by test-execute)

Default udev rules use the following standard system group
names, which need to be resolvable by getgrnam() at any time,
even in the very early boot stages, where no other databases
and network are available:

audio, cdrom, dialout, disk, input, kmem, kvm, lp, render, tape, tty, video

During runtime, the journal daemon requires the
"systemd-journal" system group to exist. New journal files will
be readable by this group (but not writable), which may be used
to grant specific users read access. In addition, system
groups "wheel" and "adm" will be given read-only access to
journal files using systemd-tmpfiles.service.

The journal remote daemon requires the
"systemd-journal-remote" system user and group to
exist. During execution this network facing service will drop
privileges and assume this uid/gid for security reasons.

Similarly, the network management daemon requires the
"systemd-network" system user and group to exist.

Similarly, the name resolution daemon requires the
"systemd-resolve" system user and group to exist.

Similarly, the coredump support requires the
"systemd-coredump" system user and group to exist.

systemd ships with four glibc NSS modules:

nss-myhostname resolves the local hostname to locally configured IP
addresses, as well as "localhost" to

nss-resolve enables DNS resolution via the systemd-resolved DNS/LLMNR
caching stub resolver "systemd-resolved".

nss-mymachines enables resolution of all local containers registered
with machined to their respective IP addresses.

nss-systemd enables resolution of users/group registered via the
User/Group Record Lookup API (,
including all dynamically allocated service users. (See the
DynamicUser= setting in unit files.)

To make use of these NSS modules, please add them to the "hosts:",
"passwd:" and "group:" lines in /etc/nsswitch.conf. The "resolve"
module should replace the glibc "dns" module in this file (and don't
worry, it chain-loads the "dns" module if it can't talk to resolved).

The four modules should be used in the following order:

passwd: compat systemd
group: compat systemd
hosts: files mymachines resolve [!UNAVAIL=return] dns myhostname

When calling "systemctl enable/disable/is-enabled" on a unit which is a
SysV init.d script, it calls /usr/lib/systemd/systemd-sysv-install;
this needs to translate the action into the distribution specific
mechanism such as chkconfig or update-rc.d. Packagers need to provide
this script if you need this functionality (you don't if you disabled
SysV init support).

Please see src/systemctl/systemd-sysv-install.SKELETON for how this
needs to look like, and provide an implementation at the marked places.

systemd will warn during early boot if /usr is not already mounted at
this point (that means: either located on the same file system as / or
already mounted in the initrd). While in systemd itself very little
will break if /usr is on a separate, late-mounted partition, many of
its dependencies very likely will break sooner or later in one form or
another. For example, udev rules tend to refer to binaries in /usr,
binaries that link to libraries in /usr or binaries that refer to data
files in /usr. Since these breakages are not always directly visible,
systemd will warn about this, since this kind of file system setup is
not really supported anymore by the basic set of Linux OS components.

systemd requires that the /run mount point exists. systemd also
requires that /var/run is a symlink to /run.

For more information on this issue consult

To run systemd under valgrind, compile with meson option
-Dvalgrind=true and have valgrind development headers installed
(i.e. valgrind-devel or equivalent). Otherwise, false positives will be
triggered by code which violates some rules but is actually safe. Note
that valgrind generates nice output only on exit(), hence on shutdown
we don't execve() systemd-shutdown.

Stable branches with backported patches are available in the
systemd-stable repo at

Stable branches are started for certain releases of systemd and named
after them, e.g. v238-stable. Stable branches are managed by
distribution maintainers on an as needed basis. See for some
more information and examples.

Kinvolk ( offers professional engineering
and consulting services for systemd. Please contact Chris Kühl
<> for more information.