1systemd System and Service Manager
2
3WEB SITE:
4 https://systemd.io
5
6GIT:
7 git@github.com:systemd/systemd.git
8 https://github.com/systemd/systemd
9
10MAILING LIST:
11 https://lists.freedesktop.org/mailman/listinfo/systemd-devel
12
13IRC:
14 #systemd on irc.libera.chat
15
16BUG REPORTS:
17 https://github.com/systemd/systemd/issues
18
19OLDER DOCUMENTATION:
20
21 http://0pointer.de/blog/projects/systemd.html
22 https://www.freedesktop.org/wiki/Software/systemd
23
24AUTHOR:
25 Lennart Poettering
26 Kay Sievers
27 ...and many others
28
29LICENSE:
30 LGPL-2.1-or-later for all code, exceptions noted in LICENSES/README.md
31
32REQUIREMENTS:
33 Linux kernel ≥ 3.15
34 ≥ 4.3 for ambient capabilities
35 ≥ 4.5 for pids controller in cgroup v2
36 ≥ 4.6 for cgroup namespaces
37 ≥ 4.9 for RENAME_NOREPLACE support in vfat
38 ≥ 4.10 for cgroup-bpf egress and ingress hooks
39 ≥ 4.15 for cgroup-bpf device hook and cpu controller in cgroup v2
40 ≥ 4.17 for cgroup-bpf socket address hooks
41 ≥ 4.20 for PSI (used by systemd-oomd)
42 ≥ 5.3 for bounded loops in BPF program
43 ≥ 5.4 for signed Verity images
44 ≥ 5.7 for BPF links and the BPF LSM hook
45
46 Kernel versions below 4.15 have significant gaps in functionality and
47 are not recommended for use with this version of systemd. Taint flag
48 'old-kernel' will be set. Systemd will most likely still function, but
49 upstream support and testing are limited.
50
51 Kernel Config Options:
52 CONFIG_DEVTMPFS
53 CONFIG_CGROUPS (it is OK to disable all controllers)
54 CONFIG_INOTIFY_USER
55 CONFIG_SIGNALFD
56 CONFIG_TIMERFD
57 CONFIG_EPOLL
58 CONFIG_UNIX (it requires CONFIG_NET, but every other flag in it is not necessary)
59 CONFIG_SYSFS
60 CONFIG_PROC_FS
61 CONFIG_FHANDLE (libudev, mount and bind mount handling)
62
63 Kernel crypto/hash API:
64 CONFIG_CRYPTO_USER_API_HASH
65 CONFIG_CRYPTO_HMAC
66 CONFIG_CRYPTO_SHA256
67
68 udev will fail to work with the legacy sysfs layout:
69 CONFIG_SYSFS_DEPRECATED=n
70
71 Legacy hotplug slows down the system and confuses udev:
72 CONFIG_UEVENT_HELPER_PATH=""
73
74 Userspace firmware loading is not supported and should be disabled in
75 the kernel:
76 CONFIG_FW_LOADER_USER_HELPER=n
77
78 Some udev rules and virtualization detection relies on it:
79 CONFIG_DMIID
80
81 Support for some SCSI devices serial number retrieval, to create
82 additional symlinks in /dev/disk/ and /dev/tape:
83 CONFIG_BLK_DEV_BSG
84
85 Required for PrivateNetwork= in service units:
86 CONFIG_NET_NS
87 Note that systemd-localed.service and other systemd units use
88 PrivateNetwork so this is effectively required.
89
90 Required for PrivateUsers= in service units:
91 CONFIG_USER_NS
92
93 Optional but strongly recommended:
94 CONFIG_IPV6
95 CONFIG_AUTOFS_FS
96 CONFIG_TMPFS_XATTR
97 CONFIG_{TMPFS,EXT4_FS,XFS,BTRFS_FS,...}_POSIX_ACL
98 CONFIG_SECCOMP
99 CONFIG_SECCOMP_FILTER (required for seccomp support)
100 CONFIG_KCMP (for the kcmp() syscall, used to be under
101 CONFIG_CHECKPOINT_RESTORE before ~5.12)
102
103 Required for CPUShares= in resource control unit settings:
104 CONFIG_CGROUP_SCHED
105 CONFIG_FAIR_GROUP_SCHED
106
107 Required for CPUQuota= in resource control unit settings:
108 CONFIG_CFS_BANDWIDTH
109
110 Required for IPAddressDeny=, IPAddressAllow=, IPIngressFilterPath=,
111 IPEgressFilterPath= in resource control unit settings unit settings:
112 CONFIG_BPF
113 CONFIG_BPF_SYSCALL
114 CONFIG_BPF_JIT
115 CONFIG_HAVE_EBPF_JIT
116 CONFIG_CGROUP_BPF
117
118 Required for SocketBind{Allow|Deny}=, RestrictNetworkInterfaces= in
119 resource control unit settings:
120 CONFIG_BPF
121 CONFIG_BPF_SYSCALL
122 CONFIG_BPF_JIT
123 CONFIG_HAVE_EBPF_JIT
124 CONFIG_CGROUP_BPF
125
126 For UEFI systems:
127 CONFIG_EFIVAR_FS
128 CONFIG_EFI_PARTITION
129
130 Required for signed Verity images support:
131 CONFIG_DM_VERITY_VERIFY_ROOTHASH_SIG
132
133 Required for RestrictFileSystems= in service units:
134 CONFIG_BPF
135 CONFIG_BPF_SYSCALL
136 CONFIG_BPF_LSM
137 CONFIG_DEBUG_INFO_BTF
138 CONFIG_LSM="...,bpf" or kernel booted with lsm="...,bpf".
139
140 We recommend to turn off Real-Time group scheduling in the kernel when
141 using systemd. RT group scheduling effectively makes RT scheduling
142 unavailable for most userspace, since it requires explicit assignment of
143 RT budgets to each unit whose processes making use of RT. As there's no
144 sensible way to assign these budgets automatically this cannot really be
145 fixed, and it's best to disable group scheduling hence:
146 CONFIG_RT_GROUP_SCHED=n
147
148 It's a good idea to disable the implicit creation of networking bonding
149 devices by the kernel networking bonding module, so that the
150 automatically created "bond0" interface doesn't conflict with any such
151 device created by systemd-networkd (or other tools). Ideally there would
152 be a kernel compile-time option for this, but there currently isn't. The
153 next best thing is to make this change through a modprobe.d drop-in.
154 This is shipped by default, see modprobe.d/systemd.conf.
155
156 Required for systemd-nspawn:
157 CONFIG_DEVPTS_MULTIPLE_INSTANCES or Linux kernel >= 4.7
158
159 Required for systemd-oomd:
160 CONFIG_PSI
161
162 Note that kernel auditing is broken when used with systemd's container
163 code. When using systemd in conjunction with containers, please make
164 sure to either turn off auditing at runtime using the kernel command
165 line option "audit=0", or turn it off at kernel compile time using:
166 CONFIG_AUDIT=n
167 If systemd is compiled with libseccomp support on architectures which do
168 not use socketcall() and where seccomp is supported (this effectively
169 means x86-64 and ARM, but excludes 32-bit x86!), then nspawn will now
170 install a work-around seccomp filter that makes containers boot even
171 with audit being enabled. This works correctly only on kernels 3.14 and
172 newer though. TL;DR: turn audit off, still.
173
174 glibc >= 2.16
175 libcap
176 libmount >= 2.30 (from util-linux)
177 (util-linux *must* be built without --enable-libmount-support-mtab)
178 libseccomp >= 2.3.1 (optional)
179 libblkid >= 2.24 (from util-linux) (optional)
180 libkmod >= 15 (optional)
181 PAM >= 1.1.2 (optional)
182 libcryptsetup (optional), >= 2.3.0 required for signed Verity images support
183 libaudit (optional)
184 libacl (optional)
185 libbpf >= 0.2.0 (optional)
186 libfdisk >= 2.32 (from util-linux) (optional)
187 libselinux (optional)
188 liblzma (optional)
189 liblz4 >= 1.3.0 / 130 (optional)
190 libzstd >= 1.4.0 (optional)
191 libgcrypt (optional)
192 libqrencode (optional)
193 libmicrohttpd (optional)
194 libpython (optional)
195 libidn2 or libidn (optional)
196 gnutls >= 3.1.4 (optional, >= 3.6.0 is required to support DNS-over-TLS with gnutls)
197 openssl >= 1.1.0 (optional, required to support DNS-over-TLS with openssl)
198 elfutils >= 158 (optional)
199 polkit (optional)
200 tzdata >= 2014f (optional)
201 pkg-config
202 gperf
203 docbook-xsl (optional, required for documentation)
204 xsltproc (optional, required for documentation)
205 python-jinja2
206 python-lxml (optional, required to build the indices)
207 python >= 3.5
208 meson >= 0.53.2
209 ninja
210 gcc, 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
270POLICY FOR SUPPORT OF DISTRIBUTIONS AND ARCHITECTURES:
271
272 systemd main branch and latest major or stable releases are generally
273 expected to compile on current versions of popular distributions (at
274 least all non-EOL versions of Fedora, Debian unstable/testing/stable,
275 latest Ubuntu LTS and non-LTS releases, openSUSE Tumbleweed/Leap,
276 CentOS Stream 8 and 9, up-to-date Arch, etc.) We will generally
277 attempt to support also other non-EOL versions of various distros.
278 Features which would break compilation on slightly-older distributions
279 will only be introduced if there are significant reasons for this
280 (i.e. supporting them interferes with development or requires too many
281 resources to support). In some cases backports of specific libraries or
282 tools might be required.
283
284 The policy is similar wrt. architecture support. systemd is regularly
285 tested on popular architectures (currently amd64, i386, arm64, ppc64el,
286 and s390x), but should compile and work also on other architectures, for
287 which support has been added. systemd will emit warnings when
288 architecture-specific constants are not defined.
289
290USERS AND GROUPS:
291 Default udev rules use the following standard system group names, which
292 need to be resolvable by getgrnam() at any time, even in the very early
293 boot stages, where no other databases and network are available:
294
295 audio, cdrom, dialout, disk, input, kmem, kvm, lp, render, tape, tty, video
296
297 During runtime, the journal daemon requires the "systemd-journal" system
298 group to exist. New journal files will be readable by this group (but
299 not writable), which may be used to grant specific users read access. In
300 addition, system groups "wheel" and "adm" will be given read-only access
301 to journal files using systemd-tmpfiles.service.
302
303 The journal remote daemon requires the "systemd-journal-remote" system
304 user and group to exist. During execution this network facing service
305 will drop privileges and assume this uid/gid for security reasons.
306
307 Similarly, the network management daemon requires the "systemd-network"
308 system user and group to exist.
309
310 Similarly, the name resolution daemon requires the "systemd-resolve"
311 system user and group to exist.
312
313 Similarly, the coredump support requires the "systemd-coredump" system
314 user and group to exist.
315
316NSS:
317 systemd ships with four glibc NSS modules:
318
319 nss-myhostname resolves the local hostname to locally configured IP
320 addresses, as well as "localhost" to 127.0.0.1/::1.
321
322 nss-resolve enables DNS resolution via the systemd-resolved DNS/LLMNR
323 caching stub resolver "systemd-resolved".
324
325 nss-mymachines enables resolution of all local containers registered
326 with machined to their respective IP addresses.
327
328 nss-systemd enables resolution of users/group registered via the
329 User/Group Record Lookup API (https://systemd.io/USER_GROUP_API),
330 including all dynamically allocated service users. (See the
331 DynamicUser= setting in unit files.)
332
333 To make use of these NSS modules, please add them to the "hosts:",
334 "passwd:" and "group:" lines in /etc/nsswitch.conf. The "resolve" module
335 should replace the glibc "dns" module in this file (and don't worry, it
336 chain-loads the "dns" module if it can't talk to resolved).
337
338 The four modules should be used in the following order:
339
340 passwd: compat systemd
341 group: compat systemd
342 hosts: files mymachines resolve [!UNAVAIL=return] dns myhostname
343
344SYSV INIT.D SCRIPTS:
345 When calling "systemctl enable/disable/is-enabled" on a unit which is a
346 SysV init.d script, it calls /usr/lib/systemd/systemd-sysv-install;
347 this needs to translate the action into the distribution specific
348 mechanism such as chkconfig or update-rc.d. Packagers need to provide
349 this script if you need this functionality (you don't if you disabled
350 SysV init support).
351
352 Please see src/systemctl/systemd-sysv-install.SKELETON for how this
353 needs to look like, and provide an implementation at the marked places.
354
355WARNINGS and TAINT FLAGS:
356 systemd will warn during early boot if /usr is not already mounted at
357 this point (that means: either located on the same file system as / or
358 already mounted in the initrd). While in systemd itself very little
359 will break if /usr is on a separate late-mounted partition, many of its
360 dependencies very likely will break sooner or later in one form or
361 another. For example, udev rules tend to refer to binaries in /usr,
362 binaries that link to libraries in /usr or binaries that refer to data
363 files in /usr. Since these breakages are not always directly visible,
364 systemd will warn about this. Such setups are not really supported by
365 the basic set of Linux OS components. Taint flag 'split-usr' will be
366 set when this condition is detected.
367
368 For more information on this issue consult
369 https://www.freedesktop.org/wiki/Software/systemd/separate-usr-is-broken
370
371 systemd will warn if the filesystem is not usr-merged (i.e.: /bin, /sbin
372 and /lib* are not symlinks to their counterparts under /usr). Taint flag
373 'unmerged-usr' will be set when this condition is detected.
374
375 For more information on this issue consult
376 https://www.freedesktop.org/wiki/Software/systemd/TheCaseForTheUsrMerge
377
378 systemd requires that the /run mount point exists. systemd also
379 requires that /var/run is a symlink to /run. Taint flag 'var-run-bad'
380 will be set when this condition is detected.
381
382 Systemd will also warn when the cgroup support is unavailable in the
383 kernel (taint flag 'cgroups-missing'), the system is using the old
384 cgroup hierarchy (taint flag 'cgroupsv1'), the hardware clock is
385 running in non-UTC mode (taint flag 'local-hwclock'), the kernel
386 overflow UID or GID are not 65534 (taint flags 'overflowuid-not-65534'
387 and 'overflowgid-not-65534'), the UID or GID range assigned to the
388 running systemd instance covers less than 0…65534 (taint flags
389 'short-uid-range' and 'short-gid-range').
390
391 Taint conditions are logged during boot, but may also be checked at any
392 time with:
393
394 busctl get-property org.freedesktop.systemd1 /org/freedesktop/systemd1 org.freedesktop.systemd1.Manager Tainted
395
396VALGRIND:
397 To run systemd under valgrind, compile with meson option
398 -Dvalgrind=true and have valgrind development headers installed
399 (i.e. valgrind-devel or equivalent). Otherwise, false positives will be
400 triggered by code which violates some rules but is actually safe. Note
401 that valgrind generates nice output only on exit(), hence on shutdown
402 we don't execve() systemd-shutdown.
403
404STABLE BRANCHES AND BACKPORTS:
405 Stable branches with backported patches are available in the
406 systemd-stable repo at https://github.com/systemd/systemd-stable.
407
408 Stable branches are started for certain releases of systemd and named
409 after them, e.g. v238-stable. Stable branches are managed by
410 distribution maintainers on an as needed basis. See
411 https://www.freedesktop.org/wiki/Software/systemd/Backports/ for some
412 more information and examples.
413
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