README

   1 systemd System and Service Manager
   2
   3 WEB SITE:
   4         https://systemd.io
   5
   6 GIT:
   7         git@github.com:systemd/systemd.git
   8         https://github.com/systemd/systemd
   9
  10 MAILING LIST:
  11         https://lists.freedesktop.org/mailman/listinfo/systemd-devel
  12
  13 IRC:
  14         #systemd on irc.libera.chat
  15
  16 BUG REPORTS:
  17         https://github.com/systemd/systemd/issues
  18
  19 OLDER DOCUMENTATION:
  20         https://0pointer.de/blog/projects/systemd.html
  21         https://www.freedesktop.org/wiki/Software/systemd
  22
  23 AUTHOR:
  24         Lennart Poettering
  25         Kay Sievers
  26         ...and many others
  27
  28 LICENSE:
  29         LGPL-2.1-or-later for all code, exceptions noted in LICENSES/README.md
  30
  31 REQUIREMENTS:
  32         Linux kernel ≥ 3.15
  33                      ≥ 4.3 for ambient capabilities
  34                      ≥ 4.5 for pids controller in cgroup v2
  35                      ≥ 4.6 for cgroup namespaces
  36                      ≥ 4.9 for RENAME_NOREPLACE support in vfat
  37                      ≥ 4.10 for cgroup-bpf egress and ingress hooks
  38                      ≥ 4.15 for cgroup-bpf device hook and cpu controller in cgroup v2
  39                      ≥ 4.17 for cgroup-bpf socket address hooks
  40                      ≥ 4.20 for PSI (used by systemd-oomd)
  41                      ≥ 5.3 for bounded loops in BPF program
  42                      ≥ 5.4 for signed Verity images
  43                      ≥ 5.7 for BPF links and the BPF LSM hook
  44
  45         ⛔ Kernel versions below 3.15 ("minimum baseline") are not supported at
  46         all, and are missing required functionality (e.g. CLOCK_BOOTTIME
  47         support for timerfd_create()).
  48
  49         ⚠️ Kernel versions below 4.15 ("recommended baseline") have significant
  50         gaps in functionality and are not recommended for use with this version
  51         of systemd (e.g. lack sufficiently comprehensive and working cgroupv2
  52         support). Taint flag 'old-kernel' will be set. systemd will most likely
  53         still function, but upstream support and testing are limited.
  54
  55         Kernel Config Options:
  56           CONFIG_DEVTMPFS
  57           CONFIG_CGROUPS (it is OK to disable all controllers)
  58           CONFIG_INOTIFY_USER
  59           CONFIG_SIGNALFD
  60           CONFIG_TIMERFD
  61           CONFIG_EPOLL
  62           CONFIG_UNIX (it requires CONFIG_NET, but every other flag in it is not necessary)
  63           CONFIG_SYSFS
  64           CONFIG_PROC_FS
  65           CONFIG_FHANDLE (libudev, mount and bind mount handling)
  66
  67         udev will fail to work with the legacy sysfs layout:
  68           CONFIG_SYSFS_DEPRECATED=n
  69
  70         Legacy hotplug slows down the system and confuses udev:
  71           CONFIG_UEVENT_HELPER_PATH=""
  72
  73         Userspace firmware loading is not supported and should be disabled in
  74         the kernel:
  75           CONFIG_FW_LOADER_USER_HELPER=n
  76
  77         Some udev rules and virtualization detection relies on it:
  78           CONFIG_DMIID
  79
  80         Support for some SCSI devices serial number retrieval, to create
  81         additional symlinks in /dev/disk/ and /dev/tape:
  82           CONFIG_BLK_DEV_BSG
  83
  84         Required for PrivateNetwork= in service units:
  85           CONFIG_NET_NS
  86         Note that systemd-localed.service and other systemd units use
  87         PrivateNetwork so this is effectively required.
  88
  89         Required for PrivateUsers= in service units:
  90           CONFIG_USER_NS
  91
  92         Optional but strongly recommended:
  93           CONFIG_IPV6
  94           CONFIG_AUTOFS_FS
  95           CONFIG_TMPFS_XATTR
  96           CONFIG_{TMPFS,EXT4_FS,XFS,BTRFS_FS,...}_POSIX_ACL
  97           CONFIG_SECCOMP
  98           CONFIG_SECCOMP_FILTER (required for seccomp support)
  99           CONFIG_KCMP (for the kcmp() syscall, used to be under
 100                        CONFIG_CHECKPOINT_RESTORE before ~5.12)
 101
 102         Required for CPUShares= in resource control unit settings:
 103           CONFIG_CGROUP_SCHED
 104           CONFIG_FAIR_GROUP_SCHED
 105
 106         Required for CPUQuota= in resource control unit settings:
 107           CONFIG_CFS_BANDWIDTH
 108
 109         Required for IPAddressDeny=, IPAddressAllow=, IPIngressFilterPath=,
 110         IPEgressFilterPath= in resource control unit settings unit settings:
 111           CONFIG_BPF
 112           CONFIG_BPF_SYSCALL
 113           CONFIG_BPF_JIT
 114           CONFIG_HAVE_EBPF_JIT
 115           CONFIG_CGROUP_BPF
 116
 117         Required for SocketBind{Allow|Deny}=, RestrictNetworkInterfaces= in
 118         resource control unit settings:
 119           CONFIG_BPF
 120           CONFIG_BPF_SYSCALL
 121           CONFIG_BPF_JIT
 122           CONFIG_HAVE_EBPF_JIT
 123           CONFIG_CGROUP_BPF
 124
 125         For UEFI systems:
 126           CONFIG_EFIVAR_FS
 127           CONFIG_EFI_PARTITION
 128
 129         Required for signed Verity images support:
 130           CONFIG_DM_VERITY_VERIFY_ROOTHASH_SIG
 131         Required to verify signed Verity images using keys enrolled in the MoK
 132         (Machine-Owner Key) keyring:
 133           CONFIG_DM_VERITY_VERIFY_ROOTHASH_SIG_SECONDARY_KEYRING
 134           CONFIG_IMA_ARCH_POLICY
 135           CONFIG_INTEGRITY_MACHINE_KEYRING
 136
 137         Required for RestrictFileSystems= in service units:
 138           CONFIG_BPF
 139           CONFIG_BPF_SYSCALL
 140           CONFIG_BPF_LSM
 141           CONFIG_DEBUG_INFO_BTF
 142           CONFIG_LSM="...,bpf" or kernel booted with lsm="...,bpf".
 143
 144         We recommend to turn off Real-Time group scheduling in the kernel when
 145         using systemd. RT group scheduling effectively makes RT scheduling
 146         unavailable for most userspace, since it requires explicit assignment of
 147         RT budgets to each unit whose processes making use of RT. As there's no
 148         sensible way to assign these budgets automatically this cannot really be
 149         fixed, and it's best to disable group scheduling hence:
 150            CONFIG_RT_GROUP_SCHED=n
 151
 152         It's a good idea to disable the implicit creation of networking bonding
 153         devices by the kernel networking bonding module, so that the
 154         automatically created "bond0" interface doesn't conflict with any such
 155         device created by systemd-networkd (or other tools). Ideally there would
 156         be a kernel compile-time option for this, but there currently isn't. The
 157         next best thing is to make this change through a modprobe.d drop-in.
 158         This is shipped by default, see modprobe.d/systemd.conf.
 159
 160         Required for systemd-nspawn:
 161           CONFIG_DEVPTS_MULTIPLE_INSTANCES or Linux kernel >= 4.7
 162
 163         Required for systemd-oomd:
 164           CONFIG_PSI
 165
 166         Note that kernel auditing is broken when used with systemd's container
 167         code. When using systemd in conjunction with containers, please make
 168         sure to either turn off auditing at runtime using the kernel command
 169         line option "audit=0", or turn it off at kernel compile time using:
 170           CONFIG_AUDIT=n
 171         If systemd is compiled with libseccomp support on architectures which do
 172         not use socketcall() and where seccomp is supported (this effectively
 173         means x86-64 and ARM, but excludes 32-bit x86!), then nspawn will now
 174         install a work-around seccomp filter that makes containers boot even
 175         with audit being enabled. This works correctly only on kernels 3.14 and
 176         newer though. TL;DR: turn audit off, still.
 177
 178         glibc >= 2.16
 179         libcap
 180         libmount >= 2.30 (from util-linux)
 181                 (util-linux *must* be built without --enable-libmount-support-mtab)
 182         libseccomp >= 2.3.1 (optional)
 183         libblkid >= 2.24 (from util-linux) (optional)
 184         libkmod >= 15 (optional)
 185         PAM >= 1.1.2 (optional)
 186         libcryptsetup (optional), >= 2.3.0 required for signed Verity images support
 187         libaudit (optional)
 188         libacl (optional)
 189         libbpf >= 0.1.0 (optional)
 190         libfdisk >= 2.32 (from util-linux) (optional)
 191         libselinux (optional)
 192         liblzma (optional)
 193         liblz4 >= 1.3.0 / 130 (optional)
 194         libzstd >= 1.4.0 (optional)
 195         libgcrypt (optional)
 196         libqrencode (optional)
 197         libmicrohttpd (optional)
 198         libidn2 or libidn (optional)
 199         gnutls >= 3.1.4 (optional, >= 3.6.0 is required to support DNS-over-TLS with gnutls)
 200         openssl >= 1.1.0 (optional, required to support DNS-over-TLS with openssl)
 201         elfutils >= 158 (optional)
 202         polkit (optional)
 203         tzdata >= 2014f (optional)
 204         pkg-config
 205         gperf
 206         docbook-xsl (optional, required for documentation)
 207         xsltproc    (optional, required for documentation)
 208         python >= 3.7 (required by meson too, >= 3.9 is required for ukify)
 209         python-jinja2
 210         python-pefile (optional, required for ukify)
 211         python-lxml (optional, required to build the indices)
 212         pyelftools (optional, required for systemd-boot)
 213         meson >= 0.53.2
 214         ninja
 215         gcc >= 4.7
 216         awk, sed, grep, and similar tools
 217         clang >= 10.0, llvm >= 10.0 (optional, required to build BPF programs
 218                 from source code in C)
 219
 220         During runtime, you need the following additional
 221         dependencies:
 222
 223         util-linux >= v2.27.1 required (including but not limited to: mount,
 224                                         umount, swapon, swapoff, sulogin,
 225                                         agetty, fsck)
 226         dbus >= 1.4.0 (strictly speaking optional, but recommended)
 227                 NOTE: If using dbus < 1.9.18, you should override the default
 228                 policy directory (--with-dbuspolicydir=/etc/dbus-1/system.d).
 229         dracut (optional)
 230         polkit (optional)
 231
 232         To build in directory build/:
 233           meson setup build/ && ninja -C build/
 234
 235         Any configuration options can be specified as -Darg=value... arguments
 236         to meson. After the build directory is initially configured, meson will
 237         refuse to run again, and options must be changed with:
 238           meson configure -Darg=value build/
 239         meson configure without any arguments will print out available options and
 240         their current values.
 241
 242         Useful commands:
 243           ninja -C build -v some/target
 244           meson test -C build/
 245           sudo meson install -C build/ --no-rebuild
 246           DESTDIR=... meson install -C build/
 247
 248         A tarball can be created with:
 249           v=250 && git archive --prefix=systemd-$v/ v$v | zstd >systemd-$v.tar.zstd
 250
 251         When systemd-hostnamed is used, it is strongly recommended to install
 252         nss-myhostname to ensure that, in a world of dynamically changing
 253         hostnames, the hostname stays resolvable under all circumstances. In
 254         fact, systemd-hostnamed will warn if nss-myhostname is not installed.
 255
 256         nss-systemd must be enabled on systemd systems, as that's required for
 257         DynamicUser= to work. Note that we ship services out-of-the-box that
 258         make use of DynamicUser= now, hence enabling nss-systemd is not
 259         optional.
 260
 261         Note that the build prefix for systemd must be /usr. (Moreover, packages
 262         systemd relies on — such as D-Bus — really should use the same prefix,
 263         otherwise you are on your own.) -Dsplit-usr=false (which is the default
 264         and does not need to be specified) is the recommended setting.
 265         -Dsplit-usr=true can be used to give a semblance of support for systems
 266         with programs installed split between / and /usr. Moving everything
 267         under /usr is strongly encouraged.
 268
 269         Additional packages are necessary to run some tests:
 270         - nc                 (used by test/TEST-12-ISSUE-3171)
 271         - python             (test-udev which is installed is in python)
 272         - python-pyparsing
 273         - python-evdev       (used by hwdb parsing tests)
 274         - strace             (used by test/test-functions)
 275         - capsh              (optional, used by test-execute)
 276
 277 POLICY FOR SUPPORT OF DISTRIBUTIONS AND ARCHITECTURES:
 278         systemd main branch and latest major or stable releases are generally
 279         expected to compile on current versions of popular distributions (at
 280         least all non-EOL versions of Fedora, Debian unstable/testing/stable,
 281         latest Ubuntu LTS and non-LTS releases, openSUSE Tumbleweed/Leap,
 282         CentOS Stream 8 and 9, up-to-date Arch, etc.)  We will generally
 283         attempt to support also other non-EOL versions of various distros.
 284         Features which would break compilation on slightly older distributions
 285         will only be introduced if there are significant reasons for this
 286         (i.e. supporting them interferes with development or requires too many
 287         resources to support). In some cases backports of specific libraries or
 288         tools might be required.
 289
 290         The policy is similar for architecture support. systemd is regularly
 291         tested on popular architectures (currently amd64, i386, arm64, ppc64el,
 292         and s390x), but should compile and work also on other architectures, for
 293         which support has been added. systemd will emit warnings when
 294         architecture-specific constants are not defined.
 295
 296 STATIC COMPILATION AND "STANDALONE" BINARIES:
 297
 298         systemd provides a public shared libraries libsystemd.so and
 299         libudev.so. The latter is deprecated, and the sd-device APIs in
 300         libsystemd should be used instead for new code. In addition, systemd is
 301         built with a private shared library, libsystemd-shared-<suffix>.so,
 302         that also includes the libsystemd code, and by default most systemd
 303         binaries are linked to it. Using shared libraries saves disk space and
 304         memory at runtime, because only one copy of the code is needed.
 305
 306         It is possible to build static versions of systemd public shared
 307         libraries (via the configuration options '-Dstatic-libsystemd' and
 308         '-Dstatic-libudev'). This allows the libsystemd and libudev code to be
 309         linked statically into programs. In addition, it is possible to disable
 310         the use of libsystemd-shared-<suffix>.so for various components (via
 311         the configuration options '-Dlink-*-shared'). In this mode, the
 312         libsystemd and libsystemd-shared code is linked statically into
 313         selected binaries. This option is intended for systems where some of
 314         the components are intended to be delivered independently of the main
 315         systemd package. Finally, some binaries can be compiled in a second
 316         version (via the configuration option '-Dstandalone-binaries'). The
 317         version suffixed with ".standalone" has the libsystemd and
 318         libsystemd-shared code linked statically. Those binaries are intended
 319         as replacements to be used in limited installations where the full
 320         systemd is not installed. Yet another option is to rebuild systemd with
 321         a different '-Dshared-lib-tag' setting, allowing different systemd
 322         binaries to be linked to instances of the private shared library that
 323         can be installed in parallel.
 324
 325         Using the default shared linking is recommended. Mixing versions of
 326         systemd components that would normally be built and used together (in
 327         particular various daemons and the manager) is not recommended: we do
 328         not test such combinations upstream and cannot provide support.
 329         Distributors making use of those options are responsible if things do
 330         not work as expected.
 331
 332 USERS AND GROUPS:
 333         Default udev rules use the following standard system group names, which
 334         need to be resolvable by getgrnam() at any time, even in the very early
 335         boot stages, where no other databases and network are available:
 336
 337         audio, cdrom, dialout, disk, input, kmem, kvm, lp, render, tape, tty, video
 338
 339         During runtime, the journal daemon requires the "systemd-journal" system
 340         group to exist. New journal files will be readable by this group (but
 341         not writable), which may be used to grant specific users read access. In
 342         addition, system groups "wheel" and "adm" will be given read-only access
 343         to journal files using systemd-tmpfiles-setup.service.
 344
 345         The journal remote daemon requires the "systemd-journal-remote" system
 346         user and group to exist. During execution this network facing service
 347         will drop privileges and assume this uid/gid for security reasons.
 348
 349         Similarly, the network management daemon requires the "systemd-network"
 350         system user and group to exist.
 351
 352         Similarly, the name resolution daemon requires the "systemd-resolve"
 353         system user and group to exist.
 354
 355         Similarly, the coredump support requires the "systemd-coredump" system
 356         user and group to exist.
 357
 358 GLIBC NSS:
 359         systemd ships with four glibc NSS modules:
 360
 361         nss-myhostname resolves the local hostname to locally configured IP
 362         addresses, as well as "localhost" to 127.0.0.1/::1.
 363
 364         nss-resolve enables DNS resolution via the systemd-resolved DNS/LLMNR
 365         caching stub resolver "systemd-resolved".
 366
 367         nss-mymachines enables resolution of all local containers registered
 368         with machined to their respective IP addresses.
 369
 370         nss-systemd enables resolution of users/group registered via the
 371         User/Group Record Lookup API (https://systemd.io/USER_GROUP_API),
 372         including all dynamically allocated service users. (See the
 373         DynamicUser= setting in unit files.)
 374
 375         To make use of these NSS modules, please add them to the "hosts:",
 376         "passwd:" and "group:" lines in /etc/nsswitch.conf. The "resolve" module
 377         should replace the glibc "dns" module in this file (and don't worry, it
 378         chain-loads the "dns" module if it can't talk to resolved).
 379
 380         The four modules should be used in the following order:
 381
 382                 passwd: compat systemd
 383                 group: compat systemd
 384                 hosts: files mymachines resolve [!UNAVAIL=return] dns myhostname
 385
 386 SYSV INIT.D SCRIPTS:
 387         When calling "systemctl enable/disable/is-enabled" on a unit which is a
 388         SysV init.d script, it calls /usr/lib/systemd/systemd-sysv-install;
 389         this needs to translate the action into the distribution specific
 390         mechanism such as chkconfig or update-rc.d. Packagers need to provide
 391         this script if you need this functionality (you don't if you disabled
 392         SysV init support).
 393
 394         Please see src/systemctl/systemd-sysv-install.SKELETON for how this
 395         needs to look like, and provide an implementation at the marked places.
 396
 397 WARNINGS and TAINT FLAGS:
 398         systemd will warn during early boot if /usr is not already mounted at
 399         this point (that means: either located on the same file system as / or
 400         already mounted in the initrd). While in systemd itself very little
 401         will break if /usr is on a separate late-mounted partition, many of its
 402         dependencies very likely will break sooner or later in one form or
 403         another. For example, udev rules tend to refer to binaries in /usr,
 404         binaries that link to libraries in /usr, or binaries that refer to data
 405         files in /usr. Since these breakages are not always directly visible,
 406         systemd will warn about this. Such setups are not really supported by
 407         the basic set of Linux OS components. Taint flag 'split-usr' will be
 408         set when this condition is detected.
 409
 410         For more information on this issue consult
 411         https://www.freedesktop.org/wiki/Software/systemd/separate-usr-is-broken
 412
 413         systemd will warn if the filesystem is not usr-merged (i.e.: /bin, /sbin
 414         and /lib* are not symlinks to their counterparts under /usr). Taint flag
 415         'unmerged-usr' will be set when this condition is detected.
 416
 417         For more information on this issue consult
 418         https://www.freedesktop.org/wiki/Software/systemd/TheCaseForTheUsrMerge
 419
 420         systemd requires that the /run mount point exists. systemd also
 421         requires that /var/run is a symlink to /run. Taint flag 'var-run-bad'
 422         will be set when this condition is detected.
 423
 424         Systemd will also warn when the cgroup support is unavailable in the
 425         kernel (taint flag 'cgroups-missing'), the system is using the old
 426         cgroup hierarchy (taint flag 'cgroupsv1'), the hardware clock is
 427         running in non-UTC mode (taint flag 'local-hwclock'), the kernel
 428         overflow UID or GID are not 65534 (taint flags 'overflowuid-not-65534'
 429         and 'overflowgid-not-65534'), the UID or GID range assigned to the
 430         running systemd instance covers less than 0…65534 (taint flags
 431         'short-uid-range' and 'short-gid-range').
 432
 433         Taint conditions are logged during boot, but may also be checked at any
 434         time with:
 435
 436           busctl get-property org.freedesktop.systemd1 /org/freedesktop/systemd1 org.freedesktop.systemd1.Manager Tainted
 437
 438         See org.freedesktop.systemd1(5) for more information.
 439
 440 VALGRIND:
 441         To run systemd under valgrind, compile systemd with the valgrind
 442         development headers available (i.e. valgrind-devel or equivalent).
 443         Otherwise, false positives will be triggered by code which violates
 444         some rules but is actually safe. Note that valgrind generates nice
 445         output only on exit(), hence on shutdown we don't execve()
 446         systemd-shutdown.
 447
 448 STABLE BRANCHES AND BACKPORTS:
 449         Stable branches with backported patches are available in the
 450         systemd-stable repo at https://github.com/systemd/systemd-stable.
 451
 452         Stable branches are started for certain releases of systemd and named
 453         after them, e.g. v238-stable. Stable branches are managed by
 454         distribution maintainers on an as needed basis. See
 455         https://www.freedesktop.org/wiki/Software/systemd/Backports for some
 456         more information and examples.