systemd System and Service Manager

WEB SITE:

https://systemd.io

GIT:

git@github.com:systemd/systemd.git

https://github.com/systemd/systemd

MAILING LIST:

https://lists.freedesktop.org/mailman/listinfo/systemd-devel

IRC:

#systemd on irc.libera.chat

BUG REPORTS:

https://github.com/systemd/systemd/issues

OLDER DOCUMENTATION:

https://0pointer.de/blog/projects/systemd.html

https://www.freedesktop.org/wiki/Software/systemd

AUTHOR:

Lennart Poettering

Kay Sievers

...and many others

LICENSE:

LGPL-2.1-or-later for all code, exceptions noted in LICENSES/README.md

REQUIREMENTS:

Linux kernel ≥ 3.15

≥ 4.3 for ambient capabilities

≥ 4.5 for pids controller in cgroup v2

≥ 4.6 for cgroup namespaces

≥ 4.9 for RENAME_NOREPLACE support in vfat

≥ 4.10 for cgroup-bpf egress and ingress hooks

≥ 4.15 for cgroup-bpf device hook and cpu controller in cgroup v2

≥ 4.17 for cgroup-bpf socket address hooks

≥ 4.20 for PSI (used by systemd-oomd)

≥ 5.3 for bounded loops in BPF program

≥ 5.4 for signed Verity images

≥ 5.7 for BPF links and the BPF LSM hook

⛔ Kernel versions below 3.15 ("minimum baseline") are not supported at

all, and are missing required functionality (e.g. CLOCK_BOOTTIME

support for timerfd_create()).

⚠️ Kernel versions below 4.15 ("recommended baseline") have significant

gaps in functionality and are not recommended for use with this version

of systemd (e.g. lack sufficiently comprehensive and working cgroupv2

support). Taint flag 'old-kernel' will be set. systemd will most likely

still function, but upstream support and testing are limited.

Kernel Config Options:

CONFIG_DEVTMPFS

CONFIG_CGROUPS (it is OK to disable all controllers)

CONFIG_INOTIFY_USER

CONFIG_SIGNALFD

CONFIG_TIMERFD

CONFIG_EPOLL

CONFIG_UNIX (it requires CONFIG_NET, but every other flag in it is not necessary)

CONFIG_SYSFS

CONFIG_PROC_FS

CONFIG_FHANDLE (libudev, mount and bind mount handling)

udev will fail to work with the legacy sysfs layout:

CONFIG_SYSFS_DEPRECATED=n

Legacy hotplug slows down the system and confuses udev:

CONFIG_UEVENT_HELPER_PATH=""

Userspace firmware loading is not supported and should be disabled in

the kernel:

CONFIG_FW_LOADER_USER_HELPER=n

Some udev rules and virtualization detection relies on it:

CONFIG_DMIID

Support for some SCSI devices serial number retrieval, to create

additional symlinks in /dev/disk/ and /dev/tape:

CONFIG_BLK_DEV_BSG

Required for PrivateNetwork= in service units:

CONFIG_NET_NS

Note that systemd-localed.service and other systemd units use

PrivateNetwork so this is effectively required.

Required for PrivateUsers= in service units:

CONFIG_USER_NS

Optional but strongly recommended:

CONFIG_IPV6

CONFIG_AUTOFS_FS

CONFIG_TMPFS_XATTR

CONFIG_{TMPFS,EXT4_FS,XFS,BTRFS_FS,...}_POSIX_ACL

CONFIG_SECCOMP

CONFIG_SECCOMP_FILTER (required for seccomp support)

CONFIG_KCMP (for the kcmp() syscall, used to be under

CONFIG_CHECKPOINT_RESTORE before ~5.12)

CONFIG_NET_SCHED

CONFIG_NET_SCH_FQ_CODEL

Required for CPUShares= in resource control unit settings:

CONFIG_CGROUP_SCHED

CONFIG_FAIR_GROUP_SCHED

Required for CPUQuota= in resource control unit settings:

CONFIG_CFS_BANDWIDTH

Required for IPAddressDeny=, IPAddressAllow=, IPIngressFilterPath=,

IPEgressFilterPath= in resource control unit settings unit settings:

CONFIG_BPF

CONFIG_BPF_SYSCALL

CONFIG_BPF_JIT

CONFIG_HAVE_EBPF_JIT

CONFIG_CGROUP_BPF

Required for SocketBind{Allow|Deny}=, RestrictNetworkInterfaces= in

resource control unit settings:

CONFIG_BPF

CONFIG_BPF_SYSCALL

CONFIG_BPF_JIT

CONFIG_HAVE_EBPF_JIT

CONFIG_CGROUP_BPF

For UEFI systems:

CONFIG_EFIVAR_FS

CONFIG_EFI_PARTITION

Required for signed Verity images support:

CONFIG_DM_VERITY_VERIFY_ROOTHASH_SIG

Required to verify signed Verity images using keys enrolled in the MoK

(Machine-Owner Key) keyring:

CONFIG_DM_VERITY_VERIFY_ROOTHASH_SIG_SECONDARY_KEYRING

CONFIG_IMA_ARCH_POLICY

CONFIG_INTEGRITY_MACHINE_KEYRING

Required for reading credentials from SMBIOS:

CONFIG_DMI

CONFIG_DMI_SYSFS

Required for RestrictFileSystems= in service units:

CONFIG_BPF

CONFIG_BPF_SYSCALL

CONFIG_BPF_LSM

CONFIG_DEBUG_INFO_BTF

CONFIG_LSM="...,bpf" or kernel booted with lsm="...,bpf".

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:

CONFIG_RT_GROUP_SCHED=n

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:

CONFIG_DEVPTS_MULTIPLE_INSTANCES or Linux kernel >= 4.7

Required for systemd-oomd:

CONFIG_PSI

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:

CONFIG_AUDIT=n

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

libcap

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)

libbpf >= 0.1.0 (optional)

libfdisk >= 2.32 (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)

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)

pkg-config

gperf

docbook-xsl (optional, required for documentation)

xsltproc (optional, required for documentation)

python >= 3.7 (required by meson too, >= 3.9 is required for ukify)

python-jinja2

python-pefile (optional, required for ukify)

python-lxml (optional, required to build the indices)

pyelftools (optional, required for systemd-boot)

meson >= 0.60.0

ninja

gcc >= 8.4

awk, sed, grep, and similar tools

clang >= 10.0, llvm >= 10.0 (optional, required to build BPF programs

from source code in C)

During runtime, you need the following additional

dependencies:

util-linux >= v2.27.1 required (including but not limited to: mount,

umount, swapon, swapoff, sulogin,

agetty, fsck)

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).

polkit (optional)

To build in directory build/:

meson setup build/ && ninja -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:

meson configure -Darg=value build/

meson configure without any arguments will print out available options and

their current values.

Useful commands:

ninja -C build -v some/target

meson test -C build/

sudo meson install -C build/ --no-rebuild

DESTDIR=... meson install -C build/

A tarball can be created with:

v=250 && git archive --prefix=systemd-$v/ v$v | zstd >systemd-$v.tar.zstd

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

optional.

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.) Split-usr and unmerged-usr systems are no

longer supported, and moving everything under /usr/ is required. Systems

with a separate /usr/ partition must mount it before transitioning into it

(i.e.: from the initrd). For more information see:

https://www.freedesktop.org/wiki/Software/systemd/separate-usr-is-broken

https://www.freedesktop.org/wiki/Software/systemd/TheCaseForTheUsrMerge

Additional packages are necessary to run some tests:

- nc (used by test/TEST-12-ISSUE-3171)

- python (test-udev which is installed is in python)

- python-pyparsing

- python-evdev (used by hwdb parsing tests)

- strace (used by test/test-functions)

- capsh (optional, used by test-execute)

POLICY FOR SUPPORT OF DISTRIBUTIONS AND ARCHITECTURES:

systemd main branch and latest major or stable releases are generally

expected to compile on current versions of popular distributions (at

least all non-EOL versions of Fedora, Debian unstable/testing/stable,

latest Ubuntu LTS and non-LTS releases, openSUSE Tumbleweed/Leap,

CentOS Stream 8 and 9, up-to-date Arch, etc.) We will generally

attempt to support also other non-EOL versions of various distros.

Features which would break compilation on slightly older distributions

will only be introduced if there are significant reasons for this

(i.e. supporting them interferes with development or requires too many

resources to support). In some cases backports of specific libraries or

tools might be required.

The policy is similar for architecture support. systemd is regularly

tested on popular architectures (currently amd64, i386, arm64, ppc64el,

and s390x), but should compile and work also on other architectures, for

which support has been added. systemd will emit warnings when

architecture-specific constants are not defined.

STATIC COMPILATION AND "STANDALONE" BINARIES:

systemd provides a public shared libraries libsystemd.so and

libudev.so. The latter is deprecated, and the sd-device APIs in

libsystemd should be used instead for new code. In addition, systemd is

built with a private shared library, libsystemd-shared-<suffix>.so,

that also includes the libsystemd code, and by default most systemd

binaries are linked to it. Using shared libraries saves disk space and

memory at runtime, because only one copy of the code is needed.

It is possible to build static versions of systemd public shared

libraries (via the configuration options '-Dstatic-libsystemd' and

'-Dstatic-libudev'). This allows the libsystemd and libudev code to be

linked statically into programs. Note that mixing & matching different

versions of libsystemd and systemd is generally not recommended, since

various of its APIs wrap internal state and protocols of systemd

(e.g. logind and udev databases), which are not considered

stable. Hence, using static libraries is not recommended since it

generally means that version of the static libsystemd linked into

applications and the host systemd are not in sync, and will thus create

compatibility problems.

In addition, it is possible to disable the use of

libsystemd-shared-<suffix>.so for various components (via the

configuration options '-Dlink-*-shared'). In this mode, the libsystemd

and libsystemd-shared code is linked statically into selected

binaries. This option is intended for systems where some of the

components are intended to be delivered independently of the main

systemd package. Finally, some binaries can be compiled in a second

version (via the configuration option '-Dstandalone-binaries'). The

version suffixed with ".standalone" has the libsystemd and

libsystemd-shared code linked statically. Those binaries are intended

as replacements to be used in limited installations where the full

systemd suite is not installed. Yet another option is to rebuild

systemd with a different '-Dshared-lib-tag' setting, allowing different

systemd binaries to be linked to instances of the private shared

library that can be installed in parallel.

Again: Using the default shared linking is recommended, building static

or "standalone" versions is not. Mixing versions of systemd components

that would normally be built and used together (in particular various

daemons and the manager) is not recommended: we do not test such

combinations upstream and cannot provide support. Distributors making

use of those options are responsible if things do not work as expected.

USERS AND GROUPS:

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-setup.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.

GLIBC NSS:

systemd ships with four glibc NSS modules:

nss-myhostname resolves the local hostname to locally configured IP

addresses, as well as "localhost" to 127.0.0.1/::1.

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 (https://systemd.io/USER_GROUP_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:", "group:", "shadow:" and "gshadow:" lines in

/etc/nsswitch.conf.

The four modules should be used in the following order:

passwd: files systemd

group: files [SUCCESS=merge] systemd

shadow: files systemd

gshadow: files systemd

hosts: mymachines resolve [!UNAVAIL=return] files myhostname dns

SYSV INIT.D SCRIPTS:

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.

WARNINGS and TAINT FLAGS:

systemd requires that the /run mount point exists. systemd also

requires that /var/run is a symlink to /run. Taint flag 'var-run-bad'

will be set when this condition is detected.

Systemd will also warn when the cgroup support is unavailable in the

kernel (taint flag 'cgroups-missing'), the system is using the old

cgroup hierarchy (taint flag 'cgroupsv1'), the hardware clock is

running in non-UTC mode (taint flag 'local-hwclock'), the kernel

overflow UID or GID are not 65534 (taint flags 'overflowuid-not-65534'

and 'overflowgid-not-65534'), the UID or GID range assigned to the

running systemd instance covers less than 0…65534 (taint flags

'short-uid-range' and 'short-gid-range').

Taint conditions are logged during boot, but may also be checked at any

time with:

busctl get-property org.freedesktop.systemd1 /org/freedesktop/systemd1 org.freedesktop.systemd1.Manager Tainted

See org.freedesktop.systemd1(5) for more information.

VALGRIND:

To run systemd under valgrind, compile systemd with the valgrind

development headers available (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 AND BACKPORTS:

Stable branches with backported patches are available in the

systemd-stable repo at https://github.com/systemd/systemd-stable.

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

https://www.freedesktop.org/wiki/Software/systemd/Backports for some

more information and examples.