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           CONFIG_NET_SCHED
 102           CONFIG_NET_SCH_FQ_CODEL
 103
 104         Required for CPUShares= in resource control unit settings:
 105           CONFIG_CGROUP_SCHED
 106           CONFIG_FAIR_GROUP_SCHED
 107
 108         Required for CPUQuota= in resource control unit settings:
 109           CONFIG_CFS_BANDWIDTH
 110
 111         Required for IPAddressDeny=, IPAddressAllow=, IPIngressFilterPath=,
 112         IPEgressFilterPath= in resource control unit settings unit settings:
 113           CONFIG_BPF
 114           CONFIG_BPF_SYSCALL
 115           CONFIG_BPF_JIT
 116           CONFIG_HAVE_EBPF_JIT
 117           CONFIG_CGROUP_BPF
 118
 119         Required for SocketBind{Allow|Deny}=, RestrictNetworkInterfaces= in
 120         resource control unit settings:
 121           CONFIG_BPF
 122           CONFIG_BPF_SYSCALL
 123           CONFIG_BPF_JIT
 124           CONFIG_HAVE_EBPF_JIT
 125           CONFIG_CGROUP_BPF
 126
 127         For UEFI systems:
 128           CONFIG_EFIVAR_FS
 129           CONFIG_EFI_PARTITION
 130
 131         Required for signed Verity images support:
 132           CONFIG_DM_VERITY_VERIFY_ROOTHASH_SIG
 133         Required to verify signed Verity images using keys enrolled in the MoK
 134         (Machine-Owner Key) keyring:
 135           CONFIG_DM_VERITY_VERIFY_ROOTHASH_SIG_SECONDARY_KEYRING
 136           CONFIG_IMA_ARCH_POLICY
 137           CONFIG_INTEGRITY_MACHINE_KEYRING
 138
 139         Required for reading credentials from SMBIOS:
 140           CONFIG_DMI
 141           CONFIG_DMI_SYSFS
 142
 143         Required for RestrictFileSystems= in service units:
 144           CONFIG_BPF
 145           CONFIG_BPF_SYSCALL
 146           CONFIG_BPF_LSM
 147           CONFIG_DEBUG_INFO_BTF
 148           CONFIG_LSM="...,bpf" or kernel booted with lsm="...,bpf".
 149
 150         We recommend to turn off Real-Time group scheduling in the kernel when
 151         using systemd. RT group scheduling effectively makes RT scheduling
 152         unavailable for most userspace, since it requires explicit assignment of
 153         RT budgets to each unit whose processes making use of RT. As there's no
 154         sensible way to assign these budgets automatically this cannot really be
 155         fixed, and it's best to disable group scheduling hence:
 156            CONFIG_RT_GROUP_SCHED=n
 157
 158         It's a good idea to disable the implicit creation of networking bonding
 159         devices by the kernel networking bonding module, so that the
 160         automatically created "bond0" interface doesn't conflict with any such
 161         device created by systemd-networkd (or other tools). Ideally there would
 162         be a kernel compile-time option for this, but there currently isn't. The
 163         next best thing is to make this change through a modprobe.d drop-in.
 164         This is shipped by default, see modprobe.d/systemd.conf.
 165
 166         Required for systemd-nspawn:
 167           CONFIG_DEVPTS_MULTIPLE_INSTANCES or Linux kernel >= 4.7
 168
 169         Required for systemd-oomd:
 170           CONFIG_PSI
 171
 172         Note that kernel auditing is broken when used with systemd's container
 173         code. When using systemd in conjunction with containers, please make
 174         sure to either turn off auditing at runtime using the kernel command
 175         line option "audit=0", or turn it off at kernel compile time using:
 176           CONFIG_AUDIT=n
 177
 178         If systemd is compiled with libseccomp support on architectures which do
 179         not use socketcall() and where seccomp is supported (this effectively
 180         means x86-64 and ARM, but excludes 32-bit x86!), then nspawn will now
 181         install a work-around seccomp filter that makes containers boot even
 182         with audit being enabled. This works correctly only on kernels 3.14 and
 183         newer though. TL;DR: turn audit off, still.
 184
 185         glibc >= 2.16
 186         libcap
 187         libmount >= 2.30 (from util-linux)
 188                 (util-linux *must* be built without --enable-libmount-support-mtab)
 189         libseccomp >= 2.3.1 (optional)
 190         libblkid >= 2.24 (from util-linux) (optional)
 191         libkmod >= 15 (optional)
 192         PAM >= 1.1.2 (optional)
 193         libcryptsetup (optional), >= 2.3.0 required for signed Verity images support
 194         libaudit (optional)
 195         libacl (optional)
 196         libbpf >= 0.1.0 (optional)
 197         libfdisk >= 2.32 (from util-linux) (optional)
 198         libselinux (optional)
 199         liblzma (optional)
 200         liblz4 >= 1.3.0 / 130 (optional)
 201         libzstd >= 1.4.0 (optional)
 202         libgcrypt (optional)
 203         libqrencode (optional)
 204         libmicrohttpd (optional)
 205         libidn2 or libidn (optional)
 206         gnutls >= 3.1.4 (optional, >= 3.6.0 is required to support DNS-over-TLS with gnutls)
 207         openssl >= 1.1.0 (optional, required to support DNS-over-TLS with openssl)
 208         elfutils >= 158 (optional)
 209         polkit (optional)
 210         tzdata >= 2014f (optional)
 211         pkg-config
 212         gperf
 213         docbook-xsl (optional, required for documentation)
 214         xsltproc    (optional, required for documentation)
 215         python >= 3.7 (required by meson too, >= 3.9 is required for ukify)
 216         python-jinja2
 217         python-pefile (optional, required for ukify)
 218         python-lxml (optional, required to build the indices)
 219         pyelftools (optional, required for systemd-boot)
 220         meson >= 0.60.0
 221         ninja
 222         gcc >= 8.4
 223         awk, sed, grep, and similar tools
 224         clang >= 10.0, llvm >= 10.0 (optional, required to build BPF programs
 225                 from source code in C)
 226
 227         During runtime, you need the following additional
 228         dependencies:
 229
 230         util-linux >= v2.27.1 required (including but not limited to: mount,
 231                                         umount, swapon, swapoff, sulogin,
 232                                         agetty, fsck)
 233         dbus >= 1.4.0 (strictly speaking optional, but recommended)
 234                 NOTE: If using dbus < 1.9.18, you should override the default
 235                 policy directory (--with-dbuspolicydir=/etc/dbus-1/system.d).
 236         polkit (optional)
 237
 238         To build in directory build/:
 239           meson setup build/ && ninja -C build/
 240
 241         Any configuration options can be specified as -Darg=value... arguments
 242         to meson. After the build directory is initially configured, meson will
 243         refuse to run again, and options must be changed with:
 244           meson configure -Darg=value build/
 245         meson configure without any arguments will print out available options and
 246         their current values.
 247
 248         Useful commands:
 249           ninja -C build -v some/target
 250           meson test -C build/
 251           sudo meson install -C build/ --no-rebuild
 252           DESTDIR=... meson install -C build/
 253
 254         A tarball can be created with:
 255           v=250 && git archive --prefix=systemd-$v/ v$v | zstd >systemd-$v.tar.zstd
 256
 257         When systemd-hostnamed is used, it is strongly recommended to install
 258         nss-myhostname to ensure that, in a world of dynamically changing
 259         hostnames, the hostname stays resolvable under all circumstances. In
 260         fact, systemd-hostnamed will warn if nss-myhostname is not installed.
 261
 262         nss-systemd must be enabled on systemd systems, as that's required for
 263         DynamicUser= to work. Note that we ship services out-of-the-box that
 264         make use of DynamicUser= now, hence enabling nss-systemd is not
 265         optional.
 266
 267         Note that the build prefix for systemd must be /usr/. (Moreover, packages
 268         systemd relies on — such as D-Bus — really should use the same prefix,
 269         otherwise you are on your own.) Split-usr and unmerged-usr systems are no
 270         longer supported, and moving everything under /usr/ is required. Systems
 271         with a separate /usr/ partition must mount it before transitioning into it
 272         (i.e.: from the initrd). For more information see:
 273         https://www.freedesktop.org/wiki/Software/systemd/separate-usr-is-broken
 274         https://www.freedesktop.org/wiki/Software/systemd/TheCaseForTheUsrMerge
 275
 276         Additional packages are necessary to run some tests:
 277         - nc                 (used by test/TEST-12-ISSUE-3171)
 278         - python             (test-udev which is installed is in python)
 279         - python-pyparsing
 280         - python-evdev       (used by hwdb parsing tests)
 281         - strace             (used by test/test-functions)
 282         - capsh              (optional, used by test-execute)
 283
 284 POLICY FOR SUPPORT OF DISTRIBUTIONS AND ARCHITECTURES:
 285         systemd main branch and latest major or stable releases are generally
 286         expected to compile on current versions of popular distributions (at
 287         least all non-EOL versions of Fedora, Debian unstable/testing/stable,
 288         latest Ubuntu LTS and non-LTS releases, openSUSE Tumbleweed/Leap,
 289         CentOS Stream 8 and 9, up-to-date Arch, etc.)  We will generally
 290         attempt to support also other non-EOL versions of various distros.
 291         Features which would break compilation on slightly older distributions
 292         will only be introduced if there are significant reasons for this
 293         (i.e. supporting them interferes with development or requires too many
 294         resources to support). In some cases backports of specific libraries or
 295         tools might be required.
 296
 297         The policy is similar for architecture support. systemd is regularly
 298         tested on popular architectures (currently amd64, i386, arm64, ppc64el,
 299         and s390x), but should compile and work also on other architectures, for
 300         which support has been added. systemd will emit warnings when
 301         architecture-specific constants are not defined.
 302
 303 STATIC COMPILATION AND "STANDALONE" BINARIES:
 304         systemd provides a public shared libraries libsystemd.so and
 305         libudev.so. The latter is deprecated, and the sd-device APIs in
 306         libsystemd should be used instead for new code. In addition, systemd is
 307         built with a private shared library, libsystemd-shared-<suffix>.so,
 308         that also includes the libsystemd code, and by default most systemd
 309         binaries are linked to it. Using shared libraries saves disk space and
 310         memory at runtime, because only one copy of the code is needed.
 311
 312         It is possible to build static versions of systemd public shared
 313         libraries (via the configuration options '-Dstatic-libsystemd' and
 314         '-Dstatic-libudev'). This allows the libsystemd and libudev code to be
 315         linked statically into programs. Note that mixing & matching different
 316         versions of libsystemd and systemd is generally not recommended, since
 317         various of its APIs wrap internal state and protocols of systemd
 318         (e.g. logind and udev databases), which are not considered
 319         stable. Hence, using static libraries is not recommended since it
 320         generally means that version of the static libsystemd linked into
 321         applications and the host systemd are not in sync, and will thus create
 322         compatibility problems.
 323
 324         In addition, it is possible to disable the use of
 325         libsystemd-shared-<suffix>.so for various components (via the
 326         configuration options '-Dlink-*-shared'). In this mode, the libsystemd
 327         and libsystemd-shared code is linked statically into selected
 328         binaries. This option is intended for systems where some of the
 329         components are intended to be delivered independently of the main
 330         systemd package. Finally, some binaries can be compiled in a second
 331         version (via the configuration option '-Dstandalone-binaries'). The
 332         version suffixed with ".standalone" has the libsystemd and
 333         libsystemd-shared code linked statically. Those binaries are intended
 334         as replacements to be used in limited installations where the full
 335         systemd suite is not installed. Yet another option is to rebuild
 336         systemd with a different '-Dshared-lib-tag' setting, allowing different
 337         systemd binaries to be linked to instances of the private shared
 338         library that can be installed in parallel.
 339
 340         Again: Using the default shared linking is recommended, building static
 341         or "standalone" versions is not. Mixing versions of systemd components
 342         that would normally be built and used together (in particular various
 343         daemons and the manager) is not recommended: we do not test such
 344         combinations upstream and cannot provide support. Distributors making
 345         use of those options are responsible if things do not work as expected.
 346
 347 USERS AND GROUPS:
 348         Default udev rules use the following standard system group names, which
 349         need to be resolvable by getgrnam() at any time, even in the very early
 350         boot stages, where no other databases and network are available:
 351
 352         audio, cdrom, dialout, disk, input, kmem, kvm, lp, render, tape, tty, video
 353
 354         During runtime, the journal daemon requires the "systemd-journal" system
 355         group to exist. New journal files will be readable by this group (but
 356         not writable), which may be used to grant specific users read access. In
 357         addition, system groups "wheel" and "adm" will be given read-only access
 358         to journal files using systemd-tmpfiles-setup.service.
 359
 360         The journal remote daemon requires the "systemd-journal-remote" system
 361         user and group to exist. During execution this network facing service
 362         will drop privileges and assume this uid/gid for security reasons.
 363
 364         Similarly, the network management daemon requires the "systemd-network"
 365         system user and group to exist.
 366
 367         Similarly, the name resolution daemon requires the "systemd-resolve"
 368         system user and group to exist.
 369
 370         Similarly, the coredump support requires the "systemd-coredump" system
 371         user and group to exist.
 372
 373 GLIBC NSS:
 374         systemd ships with four glibc NSS modules:
 375
 376         nss-myhostname resolves the local hostname to locally configured IP
 377         addresses, as well as "localhost" to 127.0.0.1/::1.
 378
 379         nss-resolve enables DNS resolution via the systemd-resolved DNS/LLMNR
 380         caching stub resolver "systemd-resolved".
 381
 382         nss-mymachines enables resolution of all local containers registered
 383         with machined to their respective IP addresses.
 384
 385         nss-systemd enables resolution of users/group registered via the
 386         User/Group Record Lookup API (https://systemd.io/USER_GROUP_API),
 387         including all dynamically allocated service users. (See the
 388         DynamicUser= setting in unit files.)
 389
 390         To make use of these NSS modules, please add them to the "hosts:",
 391         "passwd:", "group:", "shadow:" and "gshadow:" lines in
 392         /etc/nsswitch.conf.
 393
 394         The four modules should be used in the following order:
 395
 396                 passwd: files systemd
 397                 group: files [SUCCESS=merge] systemd
 398                 shadow: files systemd
 399                 gshadow: files systemd
 400                 hosts: mymachines resolve [!UNAVAIL=return] files myhostname dns
 401
 402 SYSV INIT.D SCRIPTS:
 403         When calling "systemctl enable/disable/is-enabled" on a unit which is a
 404         SysV init.d script, it calls /usr/lib/systemd/systemd-sysv-install;
 405         this needs to translate the action into the distribution specific
 406         mechanism such as chkconfig or update-rc.d. Packagers need to provide
 407         this script if you need this functionality (you don't if you disabled
 408         SysV init support).
 409
 410         Please see src/systemctl/systemd-sysv-install.SKELETON for how this
 411         needs to look like, and provide an implementation at the marked places.
 412
 413 WARNINGS and TAINT FLAGS:
 414         systemd requires that the /run mount point exists. systemd also
 415         requires that /var/run is a symlink to /run. Taint flag 'var-run-bad'
 416         will be set when this condition is detected.
 417
 418         Systemd will also warn when the cgroup support is unavailable in the
 419         kernel (taint flag 'cgroups-missing'), the system is using the old
 420         cgroup hierarchy (taint flag 'cgroupsv1'), the hardware clock is
 421         running in non-UTC mode (taint flag 'local-hwclock'), the kernel
 422         overflow UID or GID are not 65534 (taint flags 'overflowuid-not-65534'
 423         and 'overflowgid-not-65534'), the UID or GID range assigned to the
 424         running systemd instance covers less than 0…65534 (taint flags
 425         'short-uid-range' and 'short-gid-range').
 426
 427         Taint conditions are logged during boot, but may also be checked at any
 428         time with:
 429
 430           busctl get-property org.freedesktop.systemd1 /org/freedesktop/systemd1 org.freedesktop.systemd1.Manager Tainted
 431
 432         See org.freedesktop.systemd1(5) for more information.
 433
 434 VALGRIND:
 435         To run systemd under valgrind, compile systemd with the valgrind
 436         development headers available (i.e. valgrind-devel or equivalent).
 437         Otherwise, false positives will be triggered by code which violates
 438         some rules but is actually safe. Note that valgrind generates nice
 439         output only on exit(), hence on shutdown we don't execve()
 440         systemd-shutdown.
 441
 442 STABLE BRANCHES AND BACKPORTS:
 443         Stable branches with backported patches are available in the
 444         systemd-stable repo at https://github.com/systemd/systemd-stable.
 445
 446         Stable branches are started for certain releases of systemd and named
 447         after them, e.g. v238-stable. Stable branches are managed by
 448         distribution maintainers on an as needed basis. See
 449         https://www.freedesktop.org/wiki/Software/systemd/Backports for some
 450         more information and examples.