systemd System and Service Manager

DETAILS:
        http://0pointer.de/blog/projects/systemd.html

WEB SITE:
        https://www.freedesktop.org/wiki/Software/systemd

GIT:
        git@github.com:systemd/systemd.git
        https://github.com/systemd/systemd

MAILING LIST:
        https://lists.freedesktop.org/mailman/listinfo/systemd-devel

IRC:
        #systemd on irc.libera.chat

BUG REPORTS:
        https://github.com/systemd/systemd/issues

AUTHOR:
        Lennart Poettering
        Kay Sievers
        ...and many others

LICENSE:
        LGPLv2.1+ for all code
        - except src/basic/MurmurHash2.c which is Public Domain
        - except src/basic/siphash24.c which is CC0 Public Domain
        - except src/journal/lookup3.c which is Public Domain
        - except src/udev/* which is (currently still) GPLv2, GPLv2+
        - except tools/chromiumos/* which is BSD-style

REQUIREMENTS:
        Linux kernel >= 3.13
        Linux kernel >= 4.2 for unified cgroup hierarchy support
        Linux kernel >= 4.10 for cgroup-bpf egress and ingress hooks
        Linux kernel >= 4.15 for cgroup-bpf device hook
        Linux kernel >= 4.17 for cgroup-bpf socket address hooks
        Linux kernel >= 5.3 for bounded-loops in BPF program
        Linux kernel >= 5.4 for signed Verity images support

        Kernel Config Options:
          CONFIG_DEVTMPFS
          CONFIG_CGROUPS (it is OK to disable all controllers)
          CONFIG_INOTIFY_USER
          CONFIG_SIGNALFD
          CONFIG_TIMERFD
          CONFIG_EPOLL
          CONFIG_UNIX (it requires CONFIG_NET, but every other flag in it is not necessary)
          CONFIG_SYSFS
          CONFIG_PROC_FS
          CONFIG_FHANDLE (libudev, mount and bind mount handling)

        Kernel crypto/hash API
          CONFIG_CRYPTO_USER_API_HASH
          CONFIG_CRYPTO_HMAC
          CONFIG_CRYPTO_SHA256

        udev will fail to work with the legacy sysfs layout:
          CONFIG_SYSFS_DEPRECATED=n

        Legacy hotplug slows down the system and confuses udev:
          CONFIG_UEVENT_HELPER_PATH=""

        Userspace firmware loading is not supported and should
        be disabled in the kernel:
          CONFIG_FW_LOADER_USER_HELPER=n

        Some udev rules and virtualization detection relies on it:
          CONFIG_DMIID

        Support for some SCSI devices serial number retrieval, to
        create additional symlinks in /dev/disk/ and /dev/tape:
          CONFIG_BLK_DEV_BSG

        Required for PrivateNetwork= in service units:
          CONFIG_NET_NS
        Note that systemd-localed.service and other systemd units use
        PrivateNetwork so this is effectively required.

        Required for PrivateUsers= in service units:
          CONFIG_USER_NS

        Optional but strongly recommended:
          CONFIG_IPV6
          CONFIG_AUTOFS_FS
          CONFIG_TMPFS_XATTR
          CONFIG_{TMPFS,EXT4_FS,XFS,BTRFS_FS,...}_POSIX_ACL
          CONFIG_SECCOMP
          CONFIG_SECCOMP_FILTER (required for seccomp support)
          CONFIG_CHECKPOINT_RESTORE (for the kcmp() syscall)

        Required for CPUShares= in resource control unit settings
          CONFIG_CGROUP_SCHED
          CONFIG_FAIR_GROUP_SCHED

        Required for CPUQuota= in resource control unit settings
          CONFIG_CFS_BANDWIDTH

        Required for IPAddressDeny=, IPAddressAllow=, IPIngressFilterPath=,
        IPEgressFilterPath= in resource control unit settings
        unit settings
          CONFIG_BPF
          CONFIG_BPF_SYSCALL
          CONFIG_BPF_JIT
          CONFIG_HAVE_EBPF_JIT
          CONFIG_CGROUP_BPF

        Required for SocketBind{Allow|Deny}= in resource control unit settings
          CONFIG_BPF
          CONFIG_BPF_SYSCALL
          CONFIG_BPF_JIT
          CONFIG_HAVE_EBPF_JIT
          CONFIG_CGROUP_BPF

        For UEFI systems:
          CONFIG_EFIVAR_FS
          CONFIG_EFI_PARTITION

        Required for signed Verity images support:
          CONFIG_DM_VERITY_VERIFY_ROOTHASH_SIG

        We recommend to turn off Real-Time group scheduling in the
        kernel when using systemd. RT group scheduling effectively
        makes RT scheduling unavailable for most userspace, since it
        requires explicit assignment of RT budgets to each unit whose
        processes making use of RT. As there's no sensible way to
        assign these budgets automatically this cannot really be
        fixed, and it's best to disable group scheduling hence.
           CONFIG_RT_GROUP_SCHED=n

        It's a good idea to disable the implicit creation of networking bonding
        devices by the kernel networking bonding module, so that the
        automatically created "bond0" interface doesn't conflict with any such
        device created by systemd-networkd (or other tools). Ideally there
        would be a kernel compile-time option for this, but there currently
        isn't. The next best thing is to make this change through a modprobe.d
        drop-in. This is shipped by default, see modprobe.d/systemd.conf.

        Required for systemd-nspawn:
          CONFIG_DEVPTS_MULTIPLE_INSTANCES or Linux kernel >= 4.7

        Required for systemd-oomd:
          CONFIG_PSI

        Note that kernel auditing is broken when used with systemd's
        container code. When using systemd in conjunction with
        containers, please make sure to either turn off auditing at
        runtime using the kernel command line option "audit=0", or
        turn it off at kernel compile time using:
          CONFIG_AUDIT=n
        If systemd is compiled with libseccomp support on
        architectures which do not use socketcall() and where seccomp
        is supported (this effectively means x86-64 and ARM, but
        excludes 32-bit x86!), then nspawn will now install a
        work-around seccomp filter that makes containers boot even
        with audit being enabled. This works correctly only on kernels
        3.14 and newer though. TL;DR: turn audit off, still.

        glibc >= 2.16
        libcap
        libmount >= 2.30 (from util-linux)
                (util-linux *must* be built without --enable-libmount-support-mtab)
        libseccomp >= 2.3.1 (optional)
        libblkid >= 2.24 (from util-linux) (optional)
        libkmod >= 15 (optional)
        PAM >= 1.1.2 (optional)
        libcryptsetup (optional), >= 2.3.0 required for signed Verity images support
        libaudit (optional)
        libacl (optional)
        libbpf >= 0.2.0 (optional)
        libfdisk >= 2.33 (from util-linux) (optional)
        libselinux (optional)
        liblzma (optional)
        liblz4 >= 1.3.0 / 130 (optional)
        libzstd >= 1.4.0 (optional)
        libgcrypt (optional)
        libqrencode (optional)
        libmicrohttpd (optional)
        libpython (optional)
        libidn2 or libidn (optional)
        gnutls >= 3.1.4 (optional, >= 3.6.0 is required to support DNS-over-TLS with gnutls)
        openssl >= 1.1.0 (optional, required to support DNS-over-TLS with openssl)
        elfutils >= 158 (optional)
        polkit (optional)
        tzdata >= 2014f (optional)
        pkg-config
        gperf
        docbook-xsl (optional, required for documentation)
        xsltproc    (optional, required for documentation)
        python-jinja2
        python-lxml (optional, required to build the indices)
        python >= 3.5
        meson >= 0.46 (>= 0.49 is required to build position-independent executables)
        ninja
        gcc, awk, sed, grep, and similar tools
        clang >= 10.0, llvm >= 10.0 (optional, required to build BPF programs
                from source code in C)

        During runtime, you need the following additional
        dependencies:

        util-linux >= v2.27.1 required
        dbus >= 1.4.0 (strictly speaking optional, but recommended)
                NOTE: If using dbus < 1.9.18, you should override the default
                policy directory (--with-dbuspolicydir=/etc/dbus-1/system.d).
        dracut (optional)
        polkit (optional)

        To build in directory build/:
          meson setup build/ && meson compile -C build/

        Any configuration options can be specified as -Darg=value... arguments
        to meson. After the build directory is initially configured, meson will
        refuse to run again, and options must be changed with:
          meson configure -Darg=value build/
        meson configure without any arguments will print out available options and
        their current values.

        Useful commands:
          meson compile -v -C build/ some/target
          meson test -C build/
          sudo meson install -C build/
          DESTDIR=... meson install -C build/

        A tarball can be created with:
          git archive --format=tar --prefix=systemd-222/ v222 | xz > systemd-222.tar.xz

        When systemd-hostnamed is used, it is strongly recommended to
        install nss-myhostname to ensure that, in a world of
        dynamically changing hostnames, the hostname stays resolvable
        under all circumstances. In fact, systemd-hostnamed will warn
        if nss-myhostname is not installed.

        nss-systemd must be enabled on systemd systems, as that's required for
        DynamicUser= to work. Note that we ship services out-of-the-box that
        make use of DynamicUser= now, hence enabling nss-systemd is not
        optional.

        Note that the build prefix for systemd must be /usr. (Moreover,
        packages systemd relies on — such as D-Bus — really should use the same
        prefix, otherwise you are on your own.) -Dsplit-usr=false (which is the
        default and does not need to be specified) is the recommended setting.
        -Dsplit-usr=true can be used to give a semblance of support for systems
        with programs installed split between / and /usr. Moving everything
        under /usr is strongly encouraged.

        Additional packages are necessary to run some tests:
        - busybox            (used by test/TEST-13-NSPAWN-SMOKE)
        - nc                 (used by test/TEST-12-ISSUE-3171)
        - python3-pyparsing
        - python3-evdev      (used by hwdb parsing tests)
        - strace             (used by test/test-functions)
        - capsh              (optional, used by test-execute)

USERS AND GROUPS:
        Default udev rules use the following standard system group
        names, which need to be resolvable by getgrnam() at any time,
        even in the very early boot stages, where no other databases
        and network are available:

        audio, cdrom, dialout, disk, input, kmem, kvm, lp, render, tape, tty, video

        During runtime, the journal daemon requires the
        "systemd-journal" system group to exist. New journal files will
        be readable by this group (but not writable), which may be used
        to grant specific users read access. In addition, system
        groups "wheel" and "adm" will be given read-only access to
        journal files using systemd-tmpfiles.service.

        The journal remote daemon requires the
        "systemd-journal-remote" system user and group to
        exist. During execution this network facing service will drop
        privileges and assume this uid/gid for security reasons.

        Similarly, the network management daemon requires the
        "systemd-network" system user and group to exist.

        Similarly, the name resolution daemon requires the
        "systemd-resolve" system user and group to exist.

        Similarly, the coredump support requires the
        "systemd-coredump" system user and group to exist.

NSS:
        systemd ships with four glibc NSS modules:

        nss-myhostname resolves the local hostname to locally configured IP
        addresses, as well as "localhost" to 127.0.0.1/::1.

        nss-resolve enables DNS resolution via the systemd-resolved DNS/LLMNR
        caching stub resolver "systemd-resolved".

        nss-mymachines enables resolution of all local containers registered
        with machined to their respective IP addresses.

        nss-systemd enables resolution of users/group registered via the
        User/Group Record Lookup API (https://systemd.io/USER_GROUP_API),
        including all dynamically allocated service users. (See the
        DynamicUser= setting in unit files.)

        To make use of these NSS modules, please add them to the "hosts:",
        "passwd:" and "group:" lines in /etc/nsswitch.conf. The "resolve"
        module should replace the glibc "dns" module in this file (and don't
        worry, it chain-loads the "dns" module if it can't talk to resolved).

        The four modules should be used in the following order:

                passwd: compat systemd
                group: compat systemd
                hosts: files mymachines resolve [!UNAVAIL=return] dns myhostname

SYSV INIT.D SCRIPTS:
        When calling "systemctl enable/disable/is-enabled" on a unit which is a
        SysV init.d script, it calls /usr/lib/systemd/systemd-sysv-install;
        this needs to translate the action into the distribution specific
        mechanism such as chkconfig or update-rc.d. Packagers need to provide
        this script if you need this functionality (you don't if you disabled
        SysV init support).

        Please see src/systemctl/systemd-sysv-install.SKELETON for how this
        needs to look like, and provide an implementation at the marked places.

WARNINGS:
        systemd will warn during early boot if /usr is not already mounted at
        this point (that means: either located on the same file system as / or
        already mounted in the initrd). While in systemd itself very little
        will break if /usr is on a separate, late-mounted partition, many of
        its dependencies very likely will break sooner or later in one form or
        another. For example, udev rules tend to refer to binaries in /usr,
        binaries that link to libraries in /usr or binaries that refer to data
        files in /usr. Since these breakages are not always directly visible,
        systemd will warn about this, since this kind of file system setup is
        not really supported anymore by the basic set of Linux OS components.

        systemd requires that the /run mount point exists. systemd also
        requires that /var/run is a symlink to /run.

        For more information on this issue consult
        https://www.freedesktop.org/wiki/Software/systemd/separate-usr-is-broken

        To run systemd under valgrind, compile with meson option
        -Dvalgrind=true and have valgrind development headers installed
        (i.e. valgrind-devel or equivalent). Otherwise, false positives will be
        triggered by code which violates some rules but is actually safe. Note
        that valgrind generates nice output only on exit(), hence on shutdown
        we don't execve() systemd-shutdown.

STABLE BRANCHES AND BACKPORTS:
        Stable branches with backported patches are available in the
        systemd-stable repo at https://github.com/systemd/systemd-stable.

        Stable branches are started for certain releases of systemd and named
        after them, e.g. v238-stable. Stable branches are managed by
        distribution maintainers on an as needed basis. See
        https://www.freedesktop.org/wiki/Software/systemd/Backports/ for some
        more information and examples.
