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.) -Dsplit-usr=false (which is the default
 265         and does not need to be specified) is the recommended setting.
 266         -Dsplit-usr=true can be used to give a semblance of support for systems
 267         with programs installed split between / and /usr. Moving everything
 268         under /usr is strongly encouraged.
 269
 270         Additional packages are necessary to run some tests:
 271         - nc                 (used by test/TEST-12-ISSUE-3171)
 272         - python             (test-udev which is installed is in python)
 273         - python-pyparsing
 274         - python-evdev       (used by hwdb parsing tests)
 275         - strace             (used by test/test-functions)
 276         - capsh              (optional, used by test-execute)
 277
 278 POLICY FOR SUPPORT OF DISTRIBUTIONS AND ARCHITECTURES:
 279         systemd main branch and latest major or stable releases are generally
 280         expected to compile on current versions of popular distributions (at
 281         least all non-EOL versions of Fedora, Debian unstable/testing/stable,
 282         latest Ubuntu LTS and non-LTS releases, openSUSE Tumbleweed/Leap,
 283         CentOS Stream 8 and 9, up-to-date Arch, etc.)  We will generally
 284         attempt to support also other non-EOL versions of various distros.
 285         Features which would break compilation on slightly older distributions
 286         will only be introduced if there are significant reasons for this
 287         (i.e. supporting them interferes with development or requires too many
 288         resources to support). In some cases backports of specific libraries or
 289         tools might be required.
 290
 291         The policy is similar for architecture support. systemd is regularly
 292         tested on popular architectures (currently amd64, i386, arm64, ppc64el,
 293         and s390x), but should compile and work also on other architectures, for
 294         which support has been added. systemd will emit warnings when
 295         architecture-specific constants are not defined.
 296
 297 STATIC COMPILATION AND "STANDALONE" BINARIES:
 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. Note that mixing & matching different
 310         versions of libsystemd and systemd is generally not recommended, since
 311         various of its APIs wrap internal state and protocols of systemd
 312         (e.g. logind and udev databases), which are not considered
 313         stable. Hence, using static libraries is not recommended since it
 314         generally means that version of the static libsystemd linked into
 315         applications and the host systemd are not in sync, and will thus create
 316         compatibility problems.
 317
 318         In addition, it is possible to disable the use of
 319         libsystemd-shared-<suffix>.so for various components (via the
 320         configuration options '-Dlink-*-shared'). In this mode, the libsystemd
 321         and libsystemd-shared code is linked statically into selected
 322         binaries. This option is intended for systems where some of the
 323         components are intended to be delivered independently of the main
 324         systemd package. Finally, some binaries can be compiled in a second
 325         version (via the configuration option '-Dstandalone-binaries'). The
 326         version suffixed with ".standalone" has the libsystemd and
 327         libsystemd-shared code linked statically. Those binaries are intended
 328         as replacements to be used in limited installations where the full
 329         systemd suite is not installed. Yet another option is to rebuild
 330         systemd with a different '-Dshared-lib-tag' setting, allowing different
 331         systemd binaries to be linked to instances of the private shared
 332         library that can be installed in parallel.
 333
 334         Again: Using the default shared linking is recommended, building static
 335         or "standalone" versions is not. Mixing versions of systemd components
 336         that would normally be built and used together (in particular various
 337         daemons and the manager) is not recommended: we do not test such
 338         combinations upstream and cannot provide support. Distributors making
 339         use of those options are responsible if things do not work as expected.
 340
 341 USERS AND GROUPS:
 342         Default udev rules use the following standard system group names, which
 343         need to be resolvable by getgrnam() at any time, even in the very early
 344         boot stages, where no other databases and network are available:
 345
 346         audio, cdrom, dialout, disk, input, kmem, kvm, lp, render, tape, tty, video
 347
 348         During runtime, the journal daemon requires the "systemd-journal" system
 349         group to exist. New journal files will be readable by this group (but
 350         not writable), which may be used to grant specific users read access. In
 351         addition, system groups "wheel" and "adm" will be given read-only access
 352         to journal files using systemd-tmpfiles-setup.service.
 353
 354         The journal remote daemon requires the "systemd-journal-remote" system
 355         user and group to exist. During execution this network facing service
 356         will drop privileges and assume this uid/gid for security reasons.
 357
 358         Similarly, the network management daemon requires the "systemd-network"
 359         system user and group to exist.
 360
 361         Similarly, the name resolution daemon requires the "systemd-resolve"
 362         system user and group to exist.
 363
 364         Similarly, the coredump support requires the "systemd-coredump" system
 365         user and group to exist.
 366
 367 GLIBC NSS:
 368         systemd ships with four glibc NSS modules:
 369
 370         nss-myhostname resolves the local hostname to locally configured IP
 371         addresses, as well as "localhost" to 127.0.0.1/::1.
 372
 373         nss-resolve enables DNS resolution via the systemd-resolved DNS/LLMNR
 374         caching stub resolver "systemd-resolved".
 375
 376         nss-mymachines enables resolution of all local containers registered
 377         with machined to their respective IP addresses.
 378
 379         nss-systemd enables resolution of users/group registered via the
 380         User/Group Record Lookup API (https://systemd.io/USER_GROUP_API),
 381         including all dynamically allocated service users. (See the
 382         DynamicUser= setting in unit files.)
 383
 384         To make use of these NSS modules, please add them to the "hosts:",
 385         "passwd:" and "group:" lines in /etc/nsswitch.conf. The "resolve" module
 386         should replace the glibc "dns" module in this file (and don't worry, it
 387         chain-loads the "dns" module if it can't talk to resolved).
 388
 389         The four modules should be used in the following order:
 390
 391                 passwd: compat systemd
 392                 group: compat systemd
 393                 hosts: files mymachines resolve [!UNAVAIL=return] dns myhostname
 394
 395 SYSV INIT.D SCRIPTS:
 396         When calling "systemctl enable/disable/is-enabled" on a unit which is a
 397         SysV init.d script, it calls /usr/lib/systemd/systemd-sysv-install;
 398         this needs to translate the action into the distribution specific
 399         mechanism such as chkconfig or update-rc.d. Packagers need to provide
 400         this script if you need this functionality (you don't if you disabled
 401         SysV init support).
 402
 403         Please see src/systemctl/systemd-sysv-install.SKELETON for how this
 404         needs to look like, and provide an implementation at the marked places.
 405
 406 WARNINGS and TAINT FLAGS:
 407         systemd will warn during early boot if /usr is not already mounted at
 408         this point (that means: either located on the same file system as / or
 409         already mounted in the initrd). While in systemd itself very little
 410         will break if /usr is on a separate late-mounted partition, many of its
 411         dependencies very likely will break sooner or later in one form or
 412         another. For example, udev rules tend to refer to binaries in /usr,
 413         binaries that link to libraries in /usr, or binaries that refer to data
 414         files in /usr. Since these breakages are not always directly visible,
 415         systemd will warn about this. Such setups are not really supported by
 416         the basic set of Linux OS components. Taint flag 'split-usr' will be
 417         set when this condition is detected.
 418
 419         For more information on this issue consult
 420         https://www.freedesktop.org/wiki/Software/systemd/separate-usr-is-broken
 421
 422         systemd will warn if the filesystem is not usr-merged (i.e.: /bin, /sbin
 423         and /lib* are not symlinks to their counterparts under /usr). Taint flag
 424         'unmerged-usr' will be set when this condition is detected.
 425
 426         For more information on this issue consult
 427         https://www.freedesktop.org/wiki/Software/systemd/TheCaseForTheUsrMerge
 428
 429         systemd requires that the /run mount point exists. systemd also
 430         requires that /var/run is a symlink to /run. Taint flag 'var-run-bad'
 431         will be set when this condition is detected.
 432
 433         Systemd will also warn when the cgroup support is unavailable in the
 434         kernel (taint flag 'cgroups-missing'), the system is using the old
 435         cgroup hierarchy (taint flag 'cgroupsv1'), the hardware clock is
 436         running in non-UTC mode (taint flag 'local-hwclock'), the kernel
 437         overflow UID or GID are not 65534 (taint flags 'overflowuid-not-65534'
 438         and 'overflowgid-not-65534'), the UID or GID range assigned to the
 439         running systemd instance covers less than 0…65534 (taint flags
 440         'short-uid-range' and 'short-gid-range').
 441
 442         Taint conditions are logged during boot, but may also be checked at any
 443         time with:
 444
 445           busctl get-property org.freedesktop.systemd1 /org/freedesktop/systemd1 org.freedesktop.systemd1.Manager Tainted
 446
 447         See org.freedesktop.systemd1(5) for more information.
 448
 449 VALGRIND:
 450         To run systemd under valgrind, compile systemd with the valgrind
 451         development headers available (i.e. valgrind-devel or equivalent).
 452         Otherwise, false positives will be triggered by code which violates
 453         some rules but is actually safe. Note that valgrind generates nice
 454         output only on exit(), hence on shutdown we don't execve()
 455         systemd-shutdown.
 456
 457 STABLE BRANCHES AND BACKPORTS:
 458         Stable branches with backported patches are available in the
 459         systemd-stable repo at https://github.com/systemd/systemd-stable.
 460
 461         Stable branches are started for certain releases of systemd and named
 462         after them, e.g. v238-stable. Stable branches are managed by
 463         distribution maintainers on an as needed basis. See
 464         https://www.freedesktop.org/wiki/Software/systemd/Backports for some
 465         more information and examples.