systemd/README

systemd System and Service Manager

WEB SITE:
        https://systemd.io

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

OLDER DOCUMENTATION:
        https://0pointer.de/blog/projects/systemd.html
        https://www.freedesktop.org/wiki/Software/systemd

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

LICENSE:
        LGPL-2.1-or-later for all code, exceptions noted in LICENSES/README.md

REQUIREMENTS:
        Linux kernel ≥ 3.15
                     ≥ 4.3 for ambient capabilities
                     ≥ 4.5 for pids controller in cgroup v2
                     ≥ 4.6 for cgroup namespaces
                     ≥ 4.9 for RENAME_NOREPLACE support in vfat
                     ≥ 4.10 for cgroup-bpf egress and ingress hooks
                     ≥ 4.15 for cgroup-bpf device hook and cpu controller in cgroup v2
                     ≥ 4.17 for cgroup-bpf socket address hooks
                     ≥ 4.20 for PSI (used by systemd-oomd)
                     ≥ 5.3 for bounded loops in BPF program
                     ≥ 5.4 for signed Verity images
                     ≥ 5.7 for BPF links and the BPF LSM hook

        ⛔ Kernel versions below 3.15 ("minimum baseline") are not supported at
        all, and are missing required functionality (e.g. CLOCK_BOOTTIME
        support for timerfd_create()).

        ⚠️ Kernel versions below 4.15 ("recommended baseline") have significant
        gaps in functionality and are not recommended for use with this version
        of systemd (e.g. lack sufficiently comprehensive and working cgroupv2
        support). Taint flag 'old-kernel' will be set. systemd will most likely
        still function, but upstream support and testing are limited.

        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)

        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_KCMP (for the kcmp() syscall, used to be under
                       CONFIG_CHECKPOINT_RESTORE before ~5.12)

        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}=, RestrictNetworkInterfaces= 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
        Required to verify signed Verity images using keys enrolled in the MoK
        (Machine-Owner Key) keyring:
          CONFIG_DM_VERITY_VERIFY_ROOTHASH_SIG_SECONDARY_KEYRING
          CONFIG_IMA_ARCH_POLICY
          CONFIG_INTEGRITY_MACHINE_KEYRING

        Required for RestrictFileSystems= in service units:
          CONFIG_BPF
          CONFIG_BPF_SYSCALL
          CONFIG_BPF_LSM
          CONFIG_DEBUG_INFO_BTF
          CONFIG_LSM="...,bpf" or kernel booted with lsm="...,bpf".

        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 >= 0.1.0 (optional)
        libfdisk >= 2.32 (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)
        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 >= 3.7 (required by meson too, >= 3.9 is required for ukify)
        python-jinja2
        python-pefile (optional, required for ukify)
        python-lxml (optional, required to build the indices)
        pyelftools (optional, required for systemd-boot)
        meson >= 0.53.2
        ninja
        gcc >= 4.7
        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 (including but not limited to: mount,
                                        umount, swapon, swapoff, sulogin,
                                        agetty, fsck)
        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/ && ninja -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:
          ninja -C build -v some/target
          meson test -C build/
          sudo meson install -C build/ --no-rebuild
          DESTDIR=... meson install -C build/

        A tarball can be created with:
          v=250 && git archive --prefix=systemd-$v/ v$v | zstd >systemd-$v.tar.zstd

        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)
        - python             (test-udev which is installed is in python)
        - python-pyparsing
        - python-evdev       (used by hwdb parsing tests)
        - strace             (used by test/test-functions)
        - capsh              (optional, used by test-execute)

POLICY FOR SUPPORT OF DISTRIBUTIONS AND ARCHITECTURES:
        systemd main branch and latest major or stable releases are generally
        expected to compile on current versions of popular distributions (at
        least all non-EOL versions of Fedora, Debian unstable/testing/stable,
        latest Ubuntu LTS and non-LTS releases, openSUSE Tumbleweed/Leap,
        CentOS Stream 8 and 9, up-to-date Arch, etc.)  We will generally
        attempt to support also other non-EOL versions of various distros.
        Features which would break compilation on slightly-older distributions
        will only be introduced if there are significant reasons for this
        (i.e. supporting them interferes with development or requires too many
        resources to support). In some cases backports of specific libraries or
        tools might be required.

        The policy is similar wrt. architecture support. systemd is regularly
        tested on popular architectures (currently amd64, i386, arm64, ppc64el,
        and s390x), but should compile and work also on other architectures, for
        which support has been added. systemd will emit warnings when
        architecture-specific constants are not defined.

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-setup.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.

GLIBC 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 and TAINT FLAGS:
        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. Such setups are not really supported by
        the basic set of Linux OS components. Taint flag 'split-usr' will be
        set when this condition is detected.

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

        systemd will warn if the filesystem is not usr-merged (i.e.: /bin, /sbin
        and /lib* are not symlinks to their counterparts under /usr). Taint flag
        'unmerged-usr' will be set when this condition is detected.

        For more information on this issue consult
        https://www.freedesktop.org/wiki/Software/systemd/TheCaseForTheUsrMerge

        systemd requires that the /run mount point exists. systemd also
        requires that /var/run is a symlink to /run. Taint flag 'var-run-bad'
        will be set when this condition is detected.

        Systemd will also warn when the cgroup support is unavailable in the
        kernel (taint flag 'cgroups-missing'), the system is using the old
        cgroup hierarchy (taint flag 'cgroupsv1'), the hardware clock is
        running in non-UTC mode (taint flag 'local-hwclock'), the kernel
        overflow UID or GID are not 65534 (taint flags 'overflowuid-not-65534'
        and 'overflowgid-not-65534'), the UID or GID range assigned to the
        running systemd instance covers less than 0…65534 (taint flags
        'short-uid-range' and 'short-gid-range').

        Taint conditions are logged during boot, but may also be checked at any
        time with:

          busctl get-property org.freedesktop.systemd1 /org/freedesktop/systemd1 org.freedesktop.systemd1.Manager Tainted

        See org.freedesktop.systemd1(5) for more information.

VALGRIND:
        To run systemd under valgrind, compile systemd with the valgrind
        development headers available (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.