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
 172         If systemd is compiled with libseccomp support on architectures which do
 173         not use socketcall() and where seccomp is supported (this effectively
 174         means x86-64 and ARM, but excludes 32-bit x86!), then nspawn will now
 175         install a work-around seccomp filter that makes containers boot even
 176         with audit being enabled. This works correctly only on kernels 3.14 and
 177         newer though. TL;DR: turn audit off, still.
 178
 179         glibc >= 2.16
 180         libcap
 181         libmount >= 2.30 (from util-linux)
 182                 (util-linux *must* be built without --enable-libmount-support-mtab)
 183         libseccomp >= 2.3.1 (optional)
 184         libblkid >= 2.24 (from util-linux) (optional)
 185         libkmod >= 15 (optional)
 186         PAM >= 1.1.2 (optional)
 187         libcryptsetup (optional), >= 2.3.0 required for signed Verity images support
 188         libaudit (optional)
 189         libacl (optional)
 190         libbpf >= 0.1.0 (optional)
 191         libfdisk >= 2.32 (from util-linux) (optional)
 192         libselinux (optional)
 193         liblzma (optional)
 194         liblz4 >= 1.3.0 / 130 (optional)
 195         libzstd >= 1.4.0 (optional)
 196         libgcrypt (optional)
 197         libqrencode (optional)
 198         libmicrohttpd (optional)
 199         libidn2 or libidn (optional)
 200         gnutls >= 3.1.4 (optional, >= 3.6.0 is required to support DNS-over-TLS with gnutls)
 201         openssl >= 1.1.0 (optional, required to support DNS-over-TLS with openssl)
 202         elfutils >= 158 (optional)
 203         polkit (optional)
 204         tzdata >= 2014f (optional)
 205         pkg-config
 206         gperf
 207         docbook-xsl (optional, required for documentation)
 208         xsltproc    (optional, required for documentation)
 209         python >= 3.7 (required by meson too, >= 3.9 is required for ukify)
 210         python-jinja2
 211         python-pefile (optional, required for ukify)
 212         python-lxml (optional, required to build the indices)
 213         pyelftools (optional, required for systemd-boot)
 214         meson >= 0.60.0
 215         ninja
 216         gcc >= 4.7
 217         awk, sed, grep, and similar tools
 218         clang >= 10.0, llvm >= 10.0 (optional, required to build BPF programs
 219                 from source code in C)
 220
 221         During runtime, you need the following additional
 222         dependencies:
 223
 224         util-linux >= v2.27.1 required (including but not limited to: mount,
 225                                         umount, swapon, swapoff, sulogin,
 226                                         agetty, fsck)
 227         dbus >= 1.4.0 (strictly speaking optional, but recommended)
 228                 NOTE: If using dbus < 1.9.18, you should override the default
 229                 policy directory (--with-dbuspolicydir=/etc/dbus-1/system.d).
 230         dracut (optional)
 231         polkit (optional)
 232
 233         To build in directory build/:
 234           meson setup build/ && ninja -C build/
 235
 236         Any configuration options can be specified as -Darg=value... arguments
 237         to meson. After the build directory is initially configured, meson will
 238         refuse to run again, and options must be changed with:
 239           meson configure -Darg=value build/
 240         meson configure without any arguments will print out available options and
 241         their current values.
 242
 243         Useful commands:
 244           ninja -C build -v some/target
 245           meson test -C build/
 246           sudo meson install -C build/ --no-rebuild
 247           DESTDIR=... meson install -C build/
 248
 249         A tarball can be created with:
 250           v=250 && git archive --prefix=systemd-$v/ v$v | zstd >systemd-$v.tar.zstd
 251
 252         When systemd-hostnamed is used, it is strongly recommended to install
 253         nss-myhostname to ensure that, in a world of dynamically changing
 254         hostnames, the hostname stays resolvable under all circumstances. In
 255         fact, systemd-hostnamed will warn if nss-myhostname is not installed.
 256
 257         nss-systemd must be enabled on systemd systems, as that's required for
 258         DynamicUser= to work. Note that we ship services out-of-the-box that
 259         make use of DynamicUser= now, hence enabling nss-systemd is not
 260         optional.
 261
 262         Note that the build prefix for systemd must be /usr/. (Moreover, packages
 263         systemd relies on — such as D-Bus — really should use the same prefix,
 264         otherwise you are on your own.) Split-usr and unmerged-usr systems are no
 265         longer supported, and moving everything under /usr/ is required. Systems
 266         with a separate /usr/ partition must mount it before transitioning into it
 267         (i.e.: from the initrd). For more information see:
 268         https://www.freedesktop.org/wiki/Software/systemd/separate-usr-is-broken
 269         https://www.freedesktop.org/wiki/Software/systemd/TheCaseForTheUsrMerge
 270
 271         Additional packages are necessary to run some tests:
 272         - nc                 (used by test/TEST-12-ISSUE-3171)
 273         - python             (test-udev which is installed is in python)
 274         - python-pyparsing
 275         - python-evdev       (used by hwdb parsing tests)
 276         - strace             (used by test/test-functions)
 277         - capsh              (optional, used by test-execute)
 278
 279 POLICY FOR SUPPORT OF DISTRIBUTIONS AND ARCHITECTURES:
 280         systemd main branch and latest major or stable releases are generally
 281         expected to compile on current versions of popular distributions (at
 282         least all non-EOL versions of Fedora, Debian unstable/testing/stable,
 283         latest Ubuntu LTS and non-LTS releases, openSUSE Tumbleweed/Leap,
 284         CentOS Stream 8 and 9, up-to-date Arch, etc.)  We will generally
 285         attempt to support also other non-EOL versions of various distros.
 286         Features which would break compilation on slightly older distributions
 287         will only be introduced if there are significant reasons for this
 288         (i.e. supporting them interferes with development or requires too many
 289         resources to support). In some cases backports of specific libraries or
 290         tools might be required.
 291
 292         The policy is similar for architecture support. systemd is regularly
 293         tested on popular architectures (currently amd64, i386, arm64, ppc64el,
 294         and s390x), but should compile and work also on other architectures, for
 295         which support has been added. systemd will emit warnings when
 296         architecture-specific constants are not defined.
 297
 298 STATIC COMPILATION AND "STANDALONE" BINARIES:
 299         systemd provides a public shared libraries libsystemd.so and
 300         libudev.so. The latter is deprecated, and the sd-device APIs in
 301         libsystemd should be used instead for new code. In addition, systemd is
 302         built with a private shared library, libsystemd-shared-<suffix>.so,
 303         that also includes the libsystemd code, and by default most systemd
 304         binaries are linked to it. Using shared libraries saves disk space and
 305         memory at runtime, because only one copy of the code is needed.
 306
 307         It is possible to build static versions of systemd public shared
 308         libraries (via the configuration options '-Dstatic-libsystemd' and
 309         '-Dstatic-libudev'). This allows the libsystemd and libudev code to be
 310         linked statically into programs. Note that mixing & matching different
 311         versions of libsystemd and systemd is generally not recommended, since
 312         various of its APIs wrap internal state and protocols of systemd
 313         (e.g. logind and udev databases), which are not considered
 314         stable. Hence, using static libraries is not recommended since it
 315         generally means that version of the static libsystemd linked into
 316         applications and the host systemd are not in sync, and will thus create
 317         compatibility problems.
 318
 319         In addition, it is possible to disable the use of
 320         libsystemd-shared-<suffix>.so for various components (via the
 321         configuration options '-Dlink-*-shared'). In this mode, the libsystemd
 322         and libsystemd-shared code is linked statically into selected
 323         binaries. This option is intended for systems where some of the
 324         components are intended to be delivered independently of the main
 325         systemd package. Finally, some binaries can be compiled in a second
 326         version (via the configuration option '-Dstandalone-binaries'). The
 327         version suffixed with ".standalone" has the libsystemd and
 328         libsystemd-shared code linked statically. Those binaries are intended
 329         as replacements to be used in limited installations where the full
 330         systemd suite is not installed. Yet another option is to rebuild
 331         systemd with a different '-Dshared-lib-tag' setting, allowing different
 332         systemd binaries to be linked to instances of the private shared
 333         library that can be installed in parallel.
 334
 335         Again: Using the default shared linking is recommended, building static
 336         or "standalone" versions is not. Mixing versions of systemd components
 337         that would normally be built and used together (in particular various
 338         daemons and the manager) is not recommended: we do not test such
 339         combinations upstream and cannot provide support. Distributors making
 340         use of those options are responsible if things do not work as expected.
 341
 342 USERS AND GROUPS:
 343         Default udev rules use the following standard system group names, which
 344         need to be resolvable by getgrnam() at any time, even in the very early
 345         boot stages, where no other databases and network are available:
 346
 347         audio, cdrom, dialout, disk, input, kmem, kvm, lp, render, tape, tty, video
 348
 349         During runtime, the journal daemon requires the "systemd-journal" system
 350         group to exist. New journal files will be readable by this group (but
 351         not writable), which may be used to grant specific users read access. In
 352         addition, system groups "wheel" and "adm" will be given read-only access
 353         to journal files using systemd-tmpfiles-setup.service.
 354
 355         The journal remote daemon requires the "systemd-journal-remote" system
 356         user and group to exist. During execution this network facing service
 357         will drop privileges and assume this uid/gid for security reasons.
 358
 359         Similarly, the network management daemon requires the "systemd-network"
 360         system user and group to exist.
 361
 362         Similarly, the name resolution daemon requires the "systemd-resolve"
 363         system user and group to exist.
 364
 365         Similarly, the coredump support requires the "systemd-coredump" system
 366         user and group to exist.
 367
 368 GLIBC NSS:
 369         systemd ships with four glibc NSS modules:
 370
 371         nss-myhostname resolves the local hostname to locally configured IP
 372         addresses, as well as "localhost" to 127.0.0.1/::1.
 373
 374         nss-resolve enables DNS resolution via the systemd-resolved DNS/LLMNR
 375         caching stub resolver "systemd-resolved".
 376
 377         nss-mymachines enables resolution of all local containers registered
 378         with machined to their respective IP addresses.
 379
 380         nss-systemd enables resolution of users/group registered via the
 381         User/Group Record Lookup API (https://systemd.io/USER_GROUP_API),
 382         including all dynamically allocated service users. (See the
 383         DynamicUser= setting in unit files.)
 384
 385         To make use of these NSS modules, please add them to the "hosts:",
 386         "passwd:", "group:", "shadow:" and "gshadow:" lines in
 387         /etc/nsswitch.conf.
 388
 389         The four modules should be used in the following order:
 390
 391                 passwd: files systemd
 392                 group: files [SUCCESS=merge] systemd
 393                 shadow: files systemd
 394                 gshadow: files systemd
 395                 hosts: mymachines resolve [!UNAVAIL=return] files myhostname dns
 396
 397 SYSV INIT.D SCRIPTS:
 398         When calling "systemctl enable/disable/is-enabled" on a unit which is a
 399         SysV init.d script, it calls /usr/lib/systemd/systemd-sysv-install;
 400         this needs to translate the action into the distribution specific
 401         mechanism such as chkconfig or update-rc.d. Packagers need to provide
 402         this script if you need this functionality (you don't if you disabled
 403         SysV init support).
 404
 405         Please see src/systemctl/systemd-sysv-install.SKELETON for how this
 406         needs to look like, and provide an implementation at the marked places.
 407
 408 WARNINGS and TAINT FLAGS:
 409         systemd requires that the /run mount point exists. systemd also
 410         requires that /var/run is a symlink to /run. Taint flag 'var-run-bad'
 411         will be set when this condition is detected.
 412
 413         Systemd will also warn when the cgroup support is unavailable in the
 414         kernel (taint flag 'cgroups-missing'), the system is using the old
 415         cgroup hierarchy (taint flag 'cgroupsv1'), the hardware clock is
 416         running in non-UTC mode (taint flag 'local-hwclock'), the kernel
 417         overflow UID or GID are not 65534 (taint flags 'overflowuid-not-65534'
 418         and 'overflowgid-not-65534'), the UID or GID range assigned to the
 419         running systemd instance covers less than 0…65534 (taint flags
 420         'short-uid-range' and 'short-gid-range').
 421
 422         Taint conditions are logged during boot, but may also be checked at any
 423         time with:
 424
 425           busctl get-property org.freedesktop.systemd1 /org/freedesktop/systemd1 org.freedesktop.systemd1.Manager Tainted
 426
 427         See org.freedesktop.systemd1(5) for more information.
 428
 429 VALGRIND:
 430         To run systemd under valgrind, compile systemd with the valgrind
 431         development headers available (i.e. valgrind-devel or equivalent).
 432         Otherwise, false positives will be triggered by code which violates
 433         some rules but is actually safe. Note that valgrind generates nice
 434         output only on exit(), hence on shutdown we don't execve()
 435         systemd-shutdown.
 436
 437 STABLE BRANCHES AND BACKPORTS:
 438         Stable branches with backported patches are available in the
 439         systemd-stable repo at https://github.com/systemd/systemd-stable.
 440
 441         Stable branches are started for certain releases of systemd and named
 442         after them, e.g. v238-stable. Stable branches are managed by
 443         distribution maintainers on an as needed basis. See
 444         https://www.freedesktop.org/wiki/Software/systemd/Backports for some
 445         more information and examples.