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2 <!DOCTYPE refentry PUBLIC
"-//OASIS//DTD DocBook XML V4.5//EN"
3 "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
4 <!-- SPDX-License-Identifier: LGPL-2.1-or-later -->
6 <refentry id=
"systemd-journald.service">
9 <title>systemd-journald.service
</title>
10 <productname>systemd
</productname>
14 <refentrytitle>systemd-journald.service
</refentrytitle>
15 <manvolnum>8</manvolnum>
19 <refname>systemd-journald.service
</refname>
20 <refname>systemd-journald.socket
</refname>
21 <refname>systemd-journald-dev-log.socket
</refname>
22 <refname>systemd-journald-audit.socket
</refname>
23 <refname>systemd-journald@.service
</refname>
24 <refname>systemd-journald@.socket
</refname>
25 <refname>systemd-journald-varlink@.socket
</refname>
26 <refname>systemd-journald
</refname>
27 <refpurpose>Journal service
</refpurpose>
31 <para><filename>systemd-journald.service
</filename></para>
32 <para><filename>systemd-journald.socket
</filename></para>
33 <para><filename>systemd-journald-dev-log.socket
</filename></para>
34 <para><filename>systemd-journald-audit.socket
</filename></para>
35 <para><filename>systemd-journald@.service
</filename></para>
36 <para><filename>systemd-journald@.socket
</filename></para>
37 <para><filename>systemd-journald-varlink@.socket
</filename></para>
38 <para><filename>/usr/lib/systemd/systemd-journald
</filename></para>
42 <title>Description
</title>
44 <para><filename>systemd-journald
</filename> is a system service
45 that collects and stores logging data. It creates and maintains
46 structured, indexed journals based on logging information that is
47 received from a variety of sources:
</para>
50 <listitem><para>Kernel log messages, via kmsg
</para></listitem>
52 <listitem><para>Simple system log messages, via the
<filename>libc
</filename> <citerefentry
53 project='man-pages'
><refentrytitle>syslog
</refentrytitle><manvolnum>3</manvolnum></citerefentry>
54 call
</para></listitem>
56 <listitem><para>Structured system log messages via the native Journal API, see
57 <citerefentry><refentrytitle>sd_journal_print
</refentrytitle><manvolnum>3</manvolnum></citerefentry>
58 and
<ulink url=
"https://systemd.io/JOURNAL_NATIVE_PROTOCOL">Native Journal
59 Protocol
</ulink></para></listitem>
61 <listitem><para>Standard output and standard error of service units. For further details see
62 below.
</para></listitem>
64 <listitem><para>Audit records, originating from the kernel audit subsystem
</para></listitem>
67 <para>The daemon will implicitly collect numerous metadata fields
68 for each log messages in a secure and unfakeable way. See
69 <citerefentry><refentrytitle>systemd.journal-fields
</refentrytitle><manvolnum>7</manvolnum></citerefentry>
70 for more information about the collected metadata.
73 <para>Log data collected by the journal is primarily text-based but can also include binary data where
74 necessary. Individual fields making up a log record stored in the journal may be up to
2^
64-
1 bytes in size.
</para>
76 <para>The journal service stores log data either persistently below
<filename>/var/log/journal
</filename> or in a
77 volatile way below
<filename>/run/log/journal/
</filename> (in the latter case it is lost at reboot). By default, log
78 data is stored persistently if
<filename>/var/log/journal/
</filename> exists during boot, with an implicit fallback
79 to volatile storage otherwise. Use
<varname>Storage=
</varname> in
80 <citerefentry><refentrytitle>journald.conf
</refentrytitle><manvolnum>5</manvolnum></citerefentry> to configure
81 where log data is placed, independently of the existence of
<filename>/var/log/journal/
</filename>.
</para>
83 <para>On systems where
<filename>/var/log/journal/
</filename> does not exist yet but where persistent logging is
84 desired (and the default
<filename>journald.conf
</filename> is used), it is sufficient to create the directory, and
85 ensure it has the correct access modes and ownership:
</para>
87 <programlisting>mkdir -p /var/log/journal
88 systemd-tmpfiles --create --prefix /var/log/journal
</programlisting>
91 <citerefentry><refentrytitle>journald.conf
</refentrytitle><manvolnum>5</manvolnum></citerefentry>
92 for information about the configuration of this service.
</para>
96 <title>Stream logging
</title>
98 <para>The systemd service manager invokes all service processes with standard output and standard error connected
99 to the journal by default. This behaviour may be altered via the
100 <varname>StandardOutput=
</varname>/
<varname>StandardError=
</varname> unit file settings, see
101 <citerefentry><refentrytitle>systemd.exec
</refentrytitle><manvolnum>5</manvolnum></citerefentry> for details. The
102 journal converts the log byte stream received this way into individual log records, splitting the stream at newline
103 (
<literal>\n
</literal>, ASCII
<constant>10</constant>) and
<constant>NUL
</constant> bytes.
</para>
105 <para>If
<filename>systemd-journald.service
</filename> is stopped, the stream connections associated with all
106 services are terminated. Further writes to those streams by the service will result in
<constant>EPIPE
</constant>
107 errors. In order to react gracefully in this case it is recommended that programs logging to standard output/error
108 ignore such errors. If the
<constant>SIGPIPE
</constant> UNIX signal handler is not blocked or turned off, such
109 write attempts will also result in such process signals being generated, see
110 <citerefentry project='man-pages'
><refentrytitle>signal
</refentrytitle><manvolnum>7</manvolnum></citerefentry>.
111 To mitigate this issue, systemd service manager explicitly turns off the
<constant>SIGPIPE
</constant>
112 signal for all invoked processes by default (this may be changed for each unit individually via the
113 <varname>IgnoreSIGPIPE=
</varname> option, see
114 <citerefentry><refentrytitle>systemd.exec
</refentrytitle><manvolnum>5</manvolnum></citerefentry> for
115 details). After the standard output/standard error streams have been terminated they may not be recovered
116 until the services they are associated with are restarted. Note that during normal operation,
117 <filename>systemd-journald.service
</filename> stores copies of the file descriptors for those streams in
118 the service manager. If
<filename>systemd-journald.service
</filename> is restarted using
119 <command>systemctl restart
</command> or equivalent operation instead of a pair of separate
120 <command>systemctl stop
</command> and
<command>systemctl start
</command> commands (or equivalent
121 operations), these stream connections are not terminated and survive the restart. It is thus safe to
122 restart
<filename>systemd-journald.service
</filename>, but stopping it is not recommended.
</para>
124 <para>Note that the log record metadata for records transferred via such standard output/error streams reflect the
125 metadata of the peer the stream was originally created for. If the stream connection is passed on to other
126 processes (such as further child processes forked off the main service process), the log records will not reflect
127 their metadata, but will continue to describe the original process. This is different from the other logging
128 transports listed above, which are inherently record based and where the metadata is always associated with the
129 individual record.
</para>
131 <para>In addition to the implicit standard output/error logging of services, stream logging is also available
132 via the
<citerefentry><refentrytitle>systemd-cat
</refentrytitle><manvolnum>1</manvolnum></citerefentry> command
135 <para>Currently, the number of parallel log streams
<filename>systemd-journald
</filename> will accept is limited to
136 4096. When this limit is reached further log streams may be established but will receive
137 <constant>EPIPE
</constant> right from the beginning.
</para>
141 <title>Journal Namespaces
</title>
143 <para>Journal 'namespaces' are both a mechanism for logically isolating the log stream of projects
144 consisting of one or more services from the rest of the system and a mechanism for improving
145 performance. Multiple journal namespaces may exist simultaneously, each defining its own, independent log
146 stream managed by its own instance of
<command>systemd-journald
</command>. Namespaces are independent of
147 each other, both in the data store and in the IPC interface. By default only a single 'default' namespace
148 exists, managed by
<filename>systemd-journald.service
</filename> (and its associated socket
149 units). Additional namespaces are created by starting an instance of the
150 <filename>systemd-journald@.service
</filename> service template. The instance name is the namespace
151 identifier, which is a short string used for referencing the journal namespace. Service units may be
152 assigned to a specific journal namespace through the
<varname>LogNamespace=
</varname> unit file setting,
153 see
<citerefentry><refentrytitle>systemd.exec
</refentrytitle><manvolnum>5</manvolnum></citerefentry> for
154 details. The
<option>--namespace=
</option> switch of
155 <citerefentry><refentrytitle>journalctl
</refentrytitle><manvolnum>1</manvolnum></citerefentry> may be
156 used to view the log stream of a specific namespace. If the switch is not used the log stream of the
157 default namespace is shown, i.e. log data from other namespaces is not visible.
</para>
159 <para>Services associated with a specific log namespace may log via syslog, the native logging protocol
160 of the journal and via stdout/stderr; the logging from all three transports is associated with the
163 <para>By default only the default namespace will collect kernel and audit log messages.
</para>
165 <para>The
<command>systemd-journald
</command> instance of the default namespace is configured through
166 <filename>/etc/systemd/journald.conf
</filename> (see below), while the other instances are configured
167 through
<filename>/etc/systemd/journald@
<replaceable>NAMESPACE
</replaceable>.conf
</filename>. The journal
168 log data for the default namespace is placed in
169 <filename>/var/log/journal/
<replaceable>MACHINE_ID
</replaceable></filename> (see below) while the data
170 for the other namespaces is located in
171 <filename>/var/log/journal/
<replaceable>MACHINE_ID
</replaceable>.
<replaceable>NAMESPACE
</replaceable></filename>.
</para>
175 <title>Signals
</title>
181 <listitem><para>Request that journal data from
<filename>/run/
</filename> is flushed to
182 <filename>/var/
</filename> in order to make it persistent (if this is enabled). This must be used
183 after
<filename>/var/
</filename> is mounted, as otherwise log data from
<filename>/run/
</filename> is
184 never flushed to
<filename>/var/
</filename> regardless of the configuration. Use the
185 <command>journalctl --flush
</command> command to request flushing of the journal files, and wait for
186 the operation to complete. See
187 <citerefentry><refentrytitle>journalctl
</refentrytitle><manvolnum>1</manvolnum></citerefentry> for
188 details.
</para></listitem>
194 <listitem><para>Request immediate rotation of the journal files. Use the
<command>journalctl
195 --rotate
</command> command to request journal file rotation, and wait for the operation to
196 complete.
</para></listitem>
200 <term>SIGRTMIN+
1</term>
202 <listitem><para>Request that all unwritten log data is written to disk. Use the
<command>journalctl
203 --sync
</command> command to trigger journal synchronization, and wait for the operation to
204 complete.
</para></listitem>
210 <title>Kernel Command Line
</title>
212 <para>A few configuration parameters from
213 <filename>journald.conf
</filename> may be overridden on the kernel
216 <variablelist class='kernel-commandline-options'
>
218 <term><varname>systemd.journald.forward_to_syslog=
</varname></term>
219 <term><varname>systemd.journald.forward_to_kmsg=
</varname></term>
220 <term><varname>systemd.journald.forward_to_console=
</varname></term>
221 <term><varname>systemd.journald.forward_to_wall=
</varname></term>
223 <listitem><para>Enables/disables forwarding of collected log
224 messages to syslog, the kernel log buffer, the system console
229 <citerefentry><refentrytitle>journald.conf
</refentrytitle><manvolnum>5</manvolnum></citerefentry>
230 for information about these settings.
</para>
236 <para>Note that these kernel command line options are only honoured by the default namespace, see
241 <title>Access Control
</title>
243 <para>Journal files are, by default, owned and readable by the
244 <literal>systemd-journal
</literal> system group but are not
245 writable. Adding a user to this group thus enables them to read
246 the journal files.
</para>
248 <para>By default, each user, with a UID outside the range of system users,
249 dynamic service users, and the nobody user, will get their own set of
250 journal files in
<filename>/var/log/journal/
</filename>. See
251 <ulink url=
"https://systemd.io/UIDS-GIDS">Users, Groups, UIDs and GIDs on systemd systems
</ulink>
252 for more details about UID ranges. These journal
253 files will not be owned by the user, however, in order to avoid
254 that the user can write to them directly. Instead, file system
255 ACLs are used to ensure the user gets read access only.
</para>
257 <para>Additional users and groups may be granted access to journal
258 files via file system access control lists (ACL). Distributions
259 and administrators may choose to grant read access to all members
260 of the
<literal>wheel
</literal> and
<literal>adm
</literal> system
261 groups with a command such as the following:
</para>
263 <programlisting># setfacl -Rnm g:wheel:rx,d:g:wheel:rx,g:adm:rx,d:g:adm:rx /var/log/journal/
</programlisting>
265 <para>Note that this command will update the ACLs both for
266 existing journal files and for future journal files created in the
267 <filename>/var/log/journal/
</filename> directory.
</para>
275 <term><filename>/etc/systemd/journald.conf
</filename></term>
277 <listitem><para>Configure
<command>systemd-journald
</command> behavior. See
278 <citerefentry><refentrytitle>journald.conf
</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
283 <term><filename>/run/log/journal/
<replaceable>machine-id
</replaceable>/*.journal
</filename></term>
284 <term><filename>/run/log/journal/
<replaceable>machine-id
</replaceable>/*.journal~
</filename></term>
285 <term><filename>/var/log/journal/
<replaceable>machine-id
</replaceable>/*.journal
</filename></term>
286 <term><filename>/var/log/journal/
<replaceable>machine-id
</replaceable>/*.journal~
</filename></term>
288 <listitem><para><command>systemd-journald
</command> writes entries to files in
289 <filename>/run/log/journal/
<replaceable>machine-id
</replaceable>/
</filename>
291 <filename>/var/log/journal/
<replaceable>machine-id
</replaceable>/
</filename>
292 with the
<literal>.journal
</literal> suffix. If the daemon is
293 stopped uncleanly, or if the files are found to be corrupted,
294 they are renamed using the
<literal>.journal~
</literal>
295 suffix, and
<command>systemd-journald
</command> starts writing
296 to a new file.
<filename>/run/
</filename> is used when
297 <filename>/var/log/journal
</filename> is not available, or
298 when
<option>Storage=volatile
</option> is set in the
299 <citerefentry><refentrytitle>journald.conf
</refentrytitle><manvolnum>5</manvolnum></citerefentry>
300 configuration file.
</para>
302 <para>When
<filename>systemd-journald
</filename> ceases writing to a journal file,
303 it will be renamed to
<literal><replaceable>original-name
</replaceable>@
<replaceable>suffix.journal
</replaceable></literal>
304 (or
<literal><replaceable>original-name
</replaceable>@
<replaceable>suffix.journal~
</replaceable></literal>).
305 Such files are
"archived" and will not be written to any more.
</para>
307 <para>In general, it is safe to read or copy any journal file (active or archived).
308 <citerefentry><refentrytitle>journalctl
</refentrytitle><manvolnum>1</manvolnum></citerefentry>
309 and the functions in the
310 <citerefentry><refentrytitle>sd-journal
</refentrytitle><manvolnum>3</manvolnum></citerefentry>
311 library should be able to read all entries that have been fully written.
</para>
313 <para><filename>systemd-journald
</filename> will automatically remove the oldest
314 archived journal files to limit disk use. See
<varname>SystemMaxUse=
</varname>
315 and related settings in
316 <citerefentry><refentrytitle>journald.conf
</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
321 <term><filename>/dev/kmsg
</filename></term>
322 <term><filename>/dev/log
</filename></term>
323 <term><filename>/run/systemd/journal/dev-log
</filename></term>
324 <term><filename>/run/systemd/journal/socket
</filename></term>
325 <term><filename>/run/systemd/journal/stdout
</filename></term>
327 <listitem><para>Sockets and other file node paths that
<command>systemd-journald
</command> will
328 listen on and are visible in the file system. In addition to these,
329 <command>systemd-journald
</command> can listen for audit events using
<citerefentry
330 project='man-pages'
><refentrytitle>netlink
</refentrytitle><manvolnum>7</manvolnum></citerefentry>.
</para></listitem>
334 <para>If journal namespacing is used these paths are slightly altered to include a namespace identifier, see above.
</para>
338 <title>See Also
</title>
340 <citerefentry><refentrytitle>systemd
</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
341 <citerefentry><refentrytitle>journalctl
</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
342 <citerefentry><refentrytitle>journald.conf
</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
343 <citerefentry><refentrytitle>systemd.journal-fields
</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
344 <citerefentry><refentrytitle>sd-journal
</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
345 <citerefentry><refentrytitle>systemd-coredump
</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
346 <citerefentry project='man-pages'
><refentrytitle>setfacl
</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
347 <citerefentry><refentrytitle>sd_journal_print
</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
348 <command>pydoc systemd.journal
</command>