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* Avoid traling slash as most links are defined without.
* Always use https:// protocol and www. subdomain

Allows for easier tree-wide linkvalidation
for our migration to systemd.io.
63 contributors

Users who have contributed to this file

@poettering @keszybz @kaysievers @yuwata @teg @mbiebl @jengelh @floppym @karelzak @zonque @ronnychevalier @bluca
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:
http://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 4.15 have significant gaps in functionality and
are not recommended for use with this version of systemd. 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)
Kernel crypto/hash API:
CONFIG_CRYPTO_USER_API_HASH
CONFIG_CRYPTO_HMAC
CONFIG_CRYPTO_SHA256
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)
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 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 >= 0.2.0 (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)
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)
pkg-config
gperf
docbook-xsl (optional, required for documentation)
xsltproc (optional, required for documentation)
python-jinja2
python-lxml (optional, required to build the indices)
python >= 3.5
meson >= 0.53.2
ninja
gcc, awk, sed, grep, and similar tools
clang >= 10.0, llvm >= 10.0 (optional, required to build BPF programs
from source code in C)
gnu-efi >= 3.0.5 (optional, required for systemd-boot)
During runtime, you need the following additional
dependencies:
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/ && 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.) -Dsplit-usr=false (which is the default
and does not need to be specified) is the recommended setting.
-Dsplit-usr=true can be used to give a semblance of support for systems
with programs installed split between / and /usr. Moving everything
under /usr is strongly encouraged.
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)
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 wrt. 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.
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.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.
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:" 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
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 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. Such setups are not really supported by
the basic set of Linux OS components. Taint flag 'split-usr' will be
set when this condition is detected.
For more information on this issue consult
https://www.freedesktop.org/wiki/Software/systemd/separate-usr-is-broken
systemd will warn if the filesystem is not usr-merged (i.e.: /bin, /sbin
and /lib* are not symlinks to their counterparts under /usr). Taint flag
'unmerged-usr' will be set when this condition is detected.
For more information on this issue consult
https://www.freedesktop.org/wiki/Software/systemd/TheCaseForTheUsrMerge
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
VALGRIND:
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 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.