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         http://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
 132         Required for RestrictFileSystems= in service units:
 133           CONFIG_BPF
 134           CONFIG_BPF_SYSCALL
 135           CONFIG_BPF_LSM
 136           CONFIG_DEBUG_INFO_BTF
 137           CONFIG_LSM="...,bpf" or kernel booted with lsm="...,bpf".
 138
 139         We recommend to turn off Real-Time group scheduling in the kernel when
 140         using systemd. RT group scheduling effectively makes RT scheduling
 141         unavailable for most userspace, since it requires explicit assignment of
 142         RT budgets to each unit whose processes making use of RT. As there's no
 143         sensible way to assign these budgets automatically this cannot really be
 144         fixed, and it's best to disable group scheduling hence:
 145            CONFIG_RT_GROUP_SCHED=n
 146
 147         It's a good idea to disable the implicit creation of networking bonding
 148         devices by the kernel networking bonding module, so that the
 149         automatically created "bond0" interface doesn't conflict with any such
 150         device created by systemd-networkd (or other tools). Ideally there would
 151         be a kernel compile-time option for this, but there currently isn't. The
 152         next best thing is to make this change through a modprobe.d drop-in.
 153         This is shipped by default, see modprobe.d/systemd.conf.
 154
 155         Required for systemd-nspawn:
 156           CONFIG_DEVPTS_MULTIPLE_INSTANCES or Linux kernel >= 4.7
 157
 158         Required for systemd-oomd:
 159           CONFIG_PSI
 160
 161         Note that kernel auditing is broken when used with systemd's container
 162         code. When using systemd in conjunction with containers, please make
 163         sure to either turn off auditing at runtime using the kernel command
 164         line option "audit=0", or turn it off at kernel compile time using:
 165           CONFIG_AUDIT=n
 166         If systemd is compiled with libseccomp support on architectures which do
 167         not use socketcall() and where seccomp is supported (this effectively
 168         means x86-64 and ARM, but excludes 32-bit x86!), then nspawn will now
 169         install a work-around seccomp filter that makes containers boot even
 170         with audit being enabled. This works correctly only on kernels 3.14 and
 171         newer though. TL;DR: turn audit off, still.
 172
 173         glibc >= 2.16
 174         libcap
 175         libmount >= 2.30 (from util-linux)
 176                 (util-linux *must* be built without --enable-libmount-support-mtab)
 177         libseccomp >= 2.3.1 (optional)
 178         libblkid >= 2.24 (from util-linux) (optional)
 179         libkmod >= 15 (optional)
 180         PAM >= 1.1.2 (optional)
 181         libcryptsetup (optional), >= 2.3.0 required for signed Verity images support
 182         libaudit (optional)
 183         libacl (optional)
 184         libbpf >= 0.1.0 (optional)
 185         libfdisk >= 2.32 (from util-linux) (optional)
 186         libselinux (optional)
 187         liblzma (optional)
 188         liblz4 >= 1.3.0 / 130 (optional)
 189         libzstd >= 1.4.0 (optional)
 190         libgcrypt (optional)
 191         libqrencode (optional)
 192         libmicrohttpd (optional)
 193         libpython (optional)
 194         libidn2 or libidn (optional)
 195         gnutls >= 3.1.4 (optional, >= 3.6.0 is required to support DNS-over-TLS with gnutls)
 196         openssl >= 1.1.0 (optional, required to support DNS-over-TLS with openssl)
 197         elfutils >= 158 (optional)
 198         polkit (optional)
 199         tzdata >= 2014f (optional)
 200         pkg-config
 201         gperf
 202         docbook-xsl (optional, required for documentation)
 203         xsltproc    (optional, required for documentation)
 204         python-jinja2
 205         python-lxml (optional, required to build the indices)
 206         python >= 3.5
 207         meson >= 0.53.2
 208         ninja
 209         gcc >= 4.7
 210         awk, sed, grep, and similar tools
 211         clang >= 10.0, llvm >= 10.0 (optional, required to build BPF programs
 212                 from source code in C)
 213         gnu-efi >= 3.0.5 (optional, required for systemd-boot)
 214
 215         During runtime, you need the following additional
 216         dependencies:
 217
 218         util-linux >= v2.27.1 required
 219         dbus >= 1.4.0 (strictly speaking optional, but recommended)
 220                 NOTE: If using dbus < 1.9.18, you should override the default
 221                 policy directory (--with-dbuspolicydir=/etc/dbus-1/system.d).
 222         dracut (optional)
 223         polkit (optional)
 224
 225         To build in directory build/:
 226           meson setup build/ && ninja -C build/
 227
 228         Any configuration options can be specified as -Darg=value... arguments
 229         to meson. After the build directory is initially configured, meson will
 230         refuse to run again, and options must be changed with:
 231           meson configure -Darg=value build/
 232         meson configure without any arguments will print out available options and
 233         their current values.
 234
 235         Useful commands:
 236           ninja -C build -v some/target
 237           meson test -C build/
 238           sudo meson install -C build/ --no-rebuild
 239           DESTDIR=... meson install -C build/
 240
 241         A tarball can be created with:
 242           v=250 && git archive --prefix=systemd-$v/ v$v | zstd >systemd-$v.tar.zstd
 243
 244         When systemd-hostnamed is used, it is strongly recommended to install
 245         nss-myhostname to ensure that, in a world of dynamically changing
 246         hostnames, the hostname stays resolvable under all circumstances. In
 247         fact, systemd-hostnamed will warn if nss-myhostname is not installed.
 248
 249         nss-systemd must be enabled on systemd systems, as that's required for
 250         DynamicUser= to work. Note that we ship services out-of-the-box that
 251         make use of DynamicUser= now, hence enabling nss-systemd is not
 252         optional.
 253
 254         Note that the build prefix for systemd must be /usr. (Moreover, packages
 255         systemd relies on — such as D-Bus — really should use the same prefix,
 256         otherwise you are on your own.) -Dsplit-usr=false (which is the default
 257         and does not need to be specified) is the recommended setting.
 258         -Dsplit-usr=true can be used to give a semblance of support for systems
 259         with programs installed split between / and /usr. Moving everything
 260         under /usr is strongly encouraged.
 261
 262         Additional packages are necessary to run some tests:
 263         - busybox            (used by test/TEST-13-NSPAWN-SMOKE)
 264         - nc                 (used by test/TEST-12-ISSUE-3171)
 265         - python3-pyparsing
 266         - python3-evdev      (used by hwdb parsing tests)
 267         - strace             (used by test/test-functions)
 268         - capsh              (optional, used by test-execute)
 269
 270 POLICY FOR SUPPORT OF DISTRIBUTIONS AND ARCHITECTURES:
 271         systemd main branch and latest major or stable releases are generally
 272         expected to compile on current versions of popular distributions (at
 273         least all non-EOL versions of Fedora, Debian unstable/testing/stable,
 274         latest Ubuntu LTS and non-LTS releases, openSUSE Tumbleweed/Leap,
 275         CentOS Stream 8 and 9, up-to-date Arch, etc.)  We will generally
 276         attempt to support also other non-EOL versions of various distros.
 277         Features which would break compilation on slightly-older distributions
 278         will only be introduced if there are significant reasons for this
 279         (i.e. supporting them interferes with development or requires too many
 280         resources to support). In some cases backports of specific libraries or
 281         tools might be required.
 282
 283         The policy is similar wrt. architecture support. systemd is regularly
 284         tested on popular architectures (currently amd64, i386, arm64, ppc64el,
 285         and s390x), but should compile and work also on other architectures, for
 286         which support has been added. systemd will emit warnings when
 287         architecture-specific constants are not defined.
 288
 289 USERS AND GROUPS:
 290         Default udev rules use the following standard system group names, which
 291         need to be resolvable by getgrnam() at any time, even in the very early
 292         boot stages, where no other databases and network are available:
 293
 294         audio, cdrom, dialout, disk, input, kmem, kvm, lp, render, tape, tty, video
 295
 296         During runtime, the journal daemon requires the "systemd-journal" system
 297         group to exist. New journal files will be readable by this group (but
 298         not writable), which may be used to grant specific users read access. In
 299         addition, system groups "wheel" and "adm" will be given read-only access
 300         to journal files using systemd-tmpfiles.service.
 301
 302         The journal remote daemon requires the "systemd-journal-remote" system
 303         user and group to exist. During execution this network facing service
 304         will drop privileges and assume this uid/gid for security reasons.
 305
 306         Similarly, the network management daemon requires the "systemd-network"
 307         system user and group to exist.
 308
 309         Similarly, the name resolution daemon requires the "systemd-resolve"
 310         system user and group to exist.
 311
 312         Similarly, the coredump support requires the "systemd-coredump" system
 313         user and group to exist.
 314
 315 GLIBC NSS:
 316         systemd ships with four glibc NSS modules:
 317
 318         nss-myhostname resolves the local hostname to locally configured IP
 319         addresses, as well as "localhost" to 127.0.0.1/::1.
 320
 321         nss-resolve enables DNS resolution via the systemd-resolved DNS/LLMNR
 322         caching stub resolver "systemd-resolved".
 323
 324         nss-mymachines enables resolution of all local containers registered
 325         with machined to their respective IP addresses.
 326
 327         nss-systemd enables resolution of users/group registered via the
 328         User/Group Record Lookup API (https://systemd.io/USER_GROUP_API),
 329         including all dynamically allocated service users. (See the
 330         DynamicUser= setting in unit files.)
 331
 332         To make use of these NSS modules, please add them to the "hosts:",
 333         "passwd:" and "group:" lines in /etc/nsswitch.conf. The "resolve" module
 334         should replace the glibc "dns" module in this file (and don't worry, it
 335         chain-loads the "dns" module if it can't talk to resolved).
 336
 337         The four modules should be used in the following order:
 338
 339                 passwd: compat systemd
 340                 group: compat systemd
 341                 hosts: files mymachines resolve [!UNAVAIL=return] dns myhostname
 342
 343 SYSV INIT.D SCRIPTS:
 344         When calling "systemctl enable/disable/is-enabled" on a unit which is a
 345         SysV init.d script, it calls /usr/lib/systemd/systemd-sysv-install;
 346         this needs to translate the action into the distribution specific
 347         mechanism such as chkconfig or update-rc.d. Packagers need to provide
 348         this script if you need this functionality (you don't if you disabled
 349         SysV init support).
 350
 351         Please see src/systemctl/systemd-sysv-install.SKELETON for how this
 352         needs to look like, and provide an implementation at the marked places.
 353
 354 WARNINGS and TAINT FLAGS:
 355         systemd will warn during early boot if /usr is not already mounted at
 356         this point (that means: either located on the same file system as / or
 357         already mounted in the initrd). While in systemd itself very little
 358         will break if /usr is on a separate late-mounted partition, many of its
 359         dependencies very likely will break sooner or later in one form or
 360         another. For example, udev rules tend to refer to binaries in /usr,
 361         binaries that link to libraries in /usr, or binaries that refer to data
 362         files in /usr. Since these breakages are not always directly visible,
 363         systemd will warn about this. Such setups are not really supported by
 364         the basic set of Linux OS components. Taint flag 'split-usr' will be
 365         set when this condition is detected.
 366
 367         For more information on this issue consult
 368         https://www.freedesktop.org/wiki/Software/systemd/separate-usr-is-broken
 369
 370         systemd will warn if the filesystem is not usr-merged (i.e.: /bin, /sbin
 371         and /lib* are not symlinks to their counterparts under /usr). Taint flag
 372         'unmerged-usr' will be set when this condition is detected.
 373
 374         For more information on this issue consult
 375         https://www.freedesktop.org/wiki/Software/systemd/TheCaseForTheUsrMerge
 376
 377         systemd requires that the /run mount point exists. systemd also
 378         requires that /var/run is a symlink to /run. Taint flag 'var-run-bad'
 379         will be set when this condition is detected.
 380
 381         Systemd will also warn when the cgroup support is unavailable in the
 382         kernel (taint flag 'cgroups-missing'), the system is using the old
 383         cgroup hierarchy (taint flag 'cgroupsv1'), the hardware clock is
 384         running in non-UTC mode (taint flag 'local-hwclock'), the kernel
 385         overflow UID or GID are not 65534 (taint flags 'overflowuid-not-65534'
 386         and 'overflowgid-not-65534'), the UID or GID range assigned to the
 387         running systemd instance covers less than 0…65534 (taint flags
 388         'short-uid-range' and 'short-gid-range').
 389
 390         Taint conditions are logged during boot, but may also be checked at any
 391         time with:
 392
 393           busctl get-property org.freedesktop.systemd1 /org/freedesktop/systemd1 org.freedesktop.systemd1.Manager Tainted
 394
 395         See org.freedesktop.systemd1(5) for more information.
 396
 397 VALGRIND:
 398         To run systemd under valgrind, compile with meson option
 399         -Dvalgrind=true and have valgrind development headers installed
 400         (i.e. valgrind-devel or equivalent). Otherwise, false positives will be
 401         triggered by code which violates some rules but is actually safe. Note
 402         that valgrind generates nice output only on exit(), hence on shutdown
 403         we don't execve() systemd-shutdown.
 404
 405 STABLE BRANCHES AND BACKPORTS:
 406         Stable branches with backported patches are available in the
 407         systemd-stable repo at https://github.com/systemd/systemd-stable.
 408
 409         Stable branches are started for certain releases of systemd and named
 410         after them, e.g. v238-stable. Stable branches are managed by
 411         distribution maintainers on an as needed basis. See
 412         https://www.freedesktop.org/wiki/Software/systemd/Backports for some
 413         more information and examples.