man/systemd-journald.service.xml

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   5
   6 <refentry id="systemd-journald.service"
   7           xmlns:xi="http://www.w3.org/2001/XInclude">
   8
   9   <refentryinfo>
  10     <title>systemd-journald.service</title>
  11     <productname>systemd</productname>
  12   </refentryinfo>
  13
  14   <refmeta>
  15     <refentrytitle>systemd-journald.service</refentrytitle>
  16     <manvolnum>8</manvolnum>
  17   </refmeta>
  18
  19   <refnamediv>
  20     <refname>systemd-journald.service</refname>
  21     <refname>systemd-journald.socket</refname>
  22     <refname>systemd-journald-dev-log.socket</refname>
  23     <refname>systemd-journald-audit.socket</refname>
  24     <refname>systemd-journald@.service</refname>
  25     <refname>systemd-journald@.socket</refname>
  26     <refname>systemd-journald-varlink@.socket</refname>
  27     <refname>systemd-journald</refname>
  28     <refpurpose>Journal service</refpurpose>
  29   </refnamediv>
  30
  31   <refsynopsisdiv>
  32     <para><filename>systemd-journald.service</filename></para>
  33     <para><filename>systemd-journald.socket</filename></para>
  34     <para><filename>systemd-journald-dev-log.socket</filename></para>
  35     <para><filename>systemd-journald-audit.socket</filename></para>
  36     <para><filename>systemd-journald@.service</filename></para>
  37     <para><filename>systemd-journald@.socket</filename></para>
  38     <para><filename>systemd-journald-varlink@.socket</filename></para>
  39     <para><filename>/usr/lib/systemd/systemd-journald</filename></para>
  40   </refsynopsisdiv>
  41
  42   <refsect1>
  43     <title>Description</title>
  44
  45     <para><filename>systemd-journald</filename> is a system service
  46     that collects and stores logging data. It creates and maintains
  47     structured, indexed journals based on logging information that is
  48     received from a variety of sources:</para>
  49
  50     <itemizedlist>
  51       <listitem><para>Kernel log messages, via kmsg</para></listitem>
  52
  53       <listitem><para>Simple system log messages, via the <filename>libc</filename> <citerefentry
  54       project='man-pages'><refentrytitle>syslog</refentrytitle><manvolnum>3</manvolnum></citerefentry>
  55       call</para></listitem>
  56
  57       <listitem><para>Structured system log messages via the native Journal API, see
  58       <citerefentry><refentrytitle>sd_journal_print</refentrytitle><manvolnum>3</manvolnum></citerefentry>
  59       and <ulink url="https://systemd.io/JOURNAL_NATIVE_PROTOCOL">Native Journal
  60       Protocol</ulink></para></listitem>
  61
  62       <listitem><para>Standard output and standard error of service units. For further details see
  63       below.</para></listitem>
  64
  65       <listitem><para>Audit records, originating from the kernel audit subsystem</para></listitem>
  66     </itemizedlist>
  67
  68     <para>The daemon will implicitly collect numerous metadata fields
  69     for each log messages in a secure and unfakeable way. See
  70     <citerefentry><refentrytitle>systemd.journal-fields</refentrytitle><manvolnum>7</manvolnum></citerefentry>
  71     for more information about the collected metadata.
  72     </para>
  73
  74     <para>Log data collected by the journal is primarily text-based but can also include binary data where
  75     necessary. Individual fields making up a log record stored in the journal may be up to 2⁶⁴-1 bytes in size.</para>
  76
  77     <para>The journal service stores log data either persistently below <filename>/var/log/journal</filename> or in a
  78     volatile way below <filename>/run/log/journal/</filename> (in the latter case it is lost at reboot). By default, log
  79     data is stored persistently if <filename>/var/log/journal/</filename> exists during boot, with an implicit fallback
  80     to volatile storage otherwise. Use <varname>Storage=</varname> in
  81     <citerefentry><refentrytitle>journald.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry> to configure
  82     where log data is placed, independently of the existence of <filename>/var/log/journal/</filename>.</para>
  83
  84     <para>Note that journald will initially use volatile storage, until a call to
  85     <command>journalctl --flush</command> (or sending <constant>SIGUSR1</constant> to journald) will cause
  86     it to switch to persistent logging (under the conditions mentioned above). This is done automatically
  87     on boot via <literal>systemd-journal-flush.service</literal>.</para>
  88
  89     <para>On systems where <filename>/var/log/journal/</filename> does not exist yet but where persistent logging is
  90     desired (and the default <filename>journald.conf</filename> is used), it is sufficient to create the directory, and
  91     ensure it has the correct access modes and ownership:</para>
  92
  93     <programlisting>mkdir -p /var/log/journal
  94 systemd-tmpfiles --create --prefix /var/log/journal</programlisting>
  95
  96     <para>See
  97     <citerefentry><refentrytitle>journald.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>
  98     for information about the configuration of this service.</para>
  99   </refsect1>
 100
 101   <refsect1>
 102     <title>Stream logging</title>
 103
 104     <para>The systemd service manager invokes all service processes with standard output and standard error connected
 105     to the journal by default. This behaviour may be altered via the
 106     <varname>StandardOutput=</varname>/<varname>StandardError=</varname> unit file settings, see
 107     <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry> for details. The
 108     journal converts the log byte stream received this way into individual log records, splitting the stream at newline
 109     (<literal>\n</literal>, ASCII <constant>10</constant>) and <constant>NUL</constant> bytes.</para>
 110
 111     <para>If <filename>systemd-journald.service</filename> is stopped, the stream connections associated with all
 112     services are terminated. Further writes to those streams by the service will result in <constant>EPIPE</constant>
 113     errors. In order to react gracefully in this case it is recommended that programs logging to standard output/error
 114     ignore such errors. If the <constant>SIGPIPE</constant> UNIX signal handler is not blocked or turned off, such
 115     write attempts will also result in such process signals being generated, see
 116     <citerefentry project='man-pages'><refentrytitle>signal</refentrytitle><manvolnum>7</manvolnum></citerefentry>.
 117     To mitigate this issue, systemd service manager explicitly turns off the <constant>SIGPIPE</constant>
 118     signal for all invoked processes by default (this may be changed for each unit individually via the
 119     <varname>IgnoreSIGPIPE=</varname> option, see
 120     <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry> for
 121     details). After the standard output/standard error streams have been terminated they may not be recovered
 122     until the services they are associated with are restarted. Note that during normal operation,
 123     <filename>systemd-journald.service</filename> stores copies of the file descriptors for those streams in
 124     the service manager. If <filename>systemd-journald.service</filename> is restarted using
 125     <command>systemctl restart</command> or equivalent operation instead of a pair of separate
 126     <command>systemctl stop</command> and <command>systemctl start</command> commands (or equivalent
 127     operations), these stream connections are not terminated and survive the restart. It is thus safe to
 128     restart <filename>systemd-journald.service</filename>, but stopping it is not recommended.</para>
 129
 130     <para>Note that the log record metadata for records transferred via such standard output/error streams reflect the
 131     metadata of the peer the stream was originally created for. If the stream connection is passed on to other
 132     processes (such as further child processes forked off the main service process), the log records will not reflect
 133     their metadata, but will continue to describe the original process. This is different from the other logging
 134     transports listed above, which are inherently record based and where the metadata is always associated with the
 135     individual record.</para>
 136
 137     <para>In addition to the implicit standard output/error logging of services, stream logging is also available
 138     via the <citerefentry><refentrytitle>systemd-cat</refentrytitle><manvolnum>1</manvolnum></citerefentry> command
 139     line tool.</para>
 140
 141     <para>Currently, the number of parallel log streams <filename>systemd-journald</filename> will accept is limited to
 142     4096. When this limit is reached further log streams may be established but will receive
 143     <constant>EPIPE</constant> right from the beginning.</para>
 144   </refsect1>
 145
 146   <refsect1>
 147     <title>Journal Namespaces</title>
 148
 149     <para>Journal 'namespaces' are both a mechanism for logically isolating the log stream of projects
 150     consisting of one or more services from the rest of the system and a mechanism for improving
 151     performance. Multiple journal namespaces may exist simultaneously, each defining its own, independent log
 152     stream managed by its own instance of <command>systemd-journald</command>. Namespaces are independent of
 153     each other, both in the data store and in the IPC interface. By default only a single 'default' namespace
 154     exists, managed by <filename>systemd-journald.service</filename> (and its associated socket
 155     units). Additional namespaces are created by starting an instance of the
 156     <filename>systemd-journald@.service</filename> service template. The instance name is the namespace
 157     identifier, which is a short string used for referencing the journal namespace. Service units may be
 158     assigned to a specific journal namespace through the <varname>LogNamespace=</varname> unit file setting,
 159     see <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry> for
 160     details. The <option>--namespace=</option> switch of
 161     <citerefentry><refentrytitle>journalctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> may be
 162     used to view the log stream of a specific namespace. If the switch is not used the log stream of the
 163     default namespace is shown, i.e. log data from other namespaces is not visible.</para>
 164
 165     <para>Services associated with a specific log namespace may log via syslog, the native logging protocol
 166     of the journal and via stdout/stderr; the logging from all three transports is associated with the
 167     namespace.</para>
 168
 169     <para>By default only the default namespace will collect kernel and audit log messages.</para>
 170
 171     <para>The <command>systemd-journald</command> instance of the default namespace is configured through
 172     <filename>/etc/systemd/journald.conf</filename> (see below), while the other instances are configured
 173     through <filename>/etc/systemd/journald@<replaceable>NAMESPACE</replaceable>.conf</filename>. The journal
 174     log data for the default namespace is placed in
 175     <filename>/var/log/journal/<replaceable>MACHINE_ID</replaceable></filename> (see below) while the data
 176     for the other namespaces is located in
 177     <filename>/var/log/journal/<replaceable>MACHINE_ID</replaceable>.<replaceable>NAMESPACE</replaceable></filename>.</para>
 178   </refsect1>
 179
 180   <refsect1>
 181     <title>Signals</title>
 182
 183     <variablelist>
 184       <varlistentry>
 185         <term>SIGUSR1</term>
 186
 187         <listitem><para>Request that journal data from <filename>/run/</filename> is flushed to
 188         <filename>/var/</filename> in order to make it persistent (if this is enabled). This must be used
 189         after <filename>/var/</filename> is mounted, as otherwise log data from <filename>/run/</filename> is
 190         never flushed to <filename>/var/</filename> regardless of the configuration. Use the
 191         <command>journalctl --flush</command> command to request flushing of the journal files, and wait for
 192         the operation to complete. See
 193         <citerefentry><refentrytitle>journalctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> for
 194         details.</para></listitem>
 195       </varlistentry>
 196
 197       <varlistentry>
 198         <term>SIGUSR2</term>
 199
 200         <listitem><para>Request immediate rotation of the journal files. Use the <command>journalctl
 201         --rotate</command> command to request journal file rotation, and wait for the operation to
 202         complete.</para></listitem>
 203       </varlistentry>
 204
 205       <varlistentry>
 206         <term>SIGRTMIN+1</term>
 207
 208         <listitem><para>Request that all unwritten log data is written to disk. Use the <command>journalctl
 209         --sync</command> command to trigger journal synchronization, and wait for the operation to
 210         complete.</para></listitem>
 211       </varlistentry>
 212     </variablelist>
 213   </refsect1>
 214
 215   <refsect1>
 216     <title>Kernel Command Line</title>
 217
 218     <para>A few configuration parameters from
 219     <filename>journald.conf</filename> may be overridden on the kernel
 220     command line:</para>
 221
 222     <variablelist class='kernel-commandline-options'>
 223       <varlistentry>
 224         <term><varname>systemd.journald.forward_to_syslog=</varname></term>
 225         <term><varname>systemd.journald.forward_to_kmsg=</varname></term>
 226         <term><varname>systemd.journald.forward_to_console=</varname></term>
 227         <term><varname>systemd.journald.forward_to_wall=</varname></term>
 228
 229         <listitem><para>Enables/disables forwarding of collected log
 230         messages to syslog, the kernel log buffer, the system console
 231         or wall.
 232         </para>
 233
 234         <para>See
 235         <citerefentry><refentrytitle>journald.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 236         for information about these settings.</para>
 237         </listitem>
 238
 239       </varlistentry>
 240     </variablelist>
 241
 242     <para>Note that these kernel command line options are only honoured by the default namespace, see
 243     above.</para>
 244   </refsect1>
 245
 246   <refsect1>
 247     <title>Access Control</title>
 248
 249     <para>Journal files are, by default, owned and readable by the
 250     <literal>systemd-journal</literal> system group but are not
 251     writable. Adding a user to this group thus enables them to read
 252     the journal files.</para>
 253
 254     <para>By default, each user, with a UID outside the range of system users,
 255     dynamic service users, and the nobody user, will get their own set of
 256     journal files in <filename>/var/log/journal/</filename>. See
 257     <ulink url="https://systemd.io/UIDS-GIDS">Users, Groups, UIDs and GIDs on systemd systems</ulink>
 258     for more details about UID ranges. These journal
 259     files will not be owned by the user, however, in order to avoid
 260     that the user can write to them directly. Instead, file system
 261     ACLs are used to ensure the user gets read access only.</para>
 262
 263     <para>Additional users and groups may be granted access to journal
 264     files via file system access control lists (ACL). Distributions
 265     and administrators may choose to grant read access to all members
 266     of the <literal>wheel</literal> and <literal>adm</literal> system
 267     groups with a command such as the following:</para>
 268
 269     <programlisting># setfacl -Rnm g:wheel:rx,d:g:wheel:rx,g:adm:rx,d:g:adm:rx /var/log/journal/</programlisting>
 270
 271     <para>Note that this command will update the ACLs both for
 272     existing journal files and for future journal files created in the
 273     <filename>/var/log/journal/</filename> directory.</para>
 274   </refsect1>
 275
 276   <refsect1>
 277     <title>Files</title>
 278
 279     <variablelist>
 280       <varlistentry>
 281         <term><filename>/etc/systemd/journald.conf</filename></term>
 282
 283         <listitem><para>Configure <command>systemd-journald</command> behavior. See
 284         <citerefentry><refentrytitle>journald.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
 285         </para></listitem>
 286       </varlistentry>
 287
 288       <varlistentry>
 289         <term><filename>/run/log/journal/<replaceable>machine-id</replaceable>/*.journal</filename></term>
 290         <term><filename>/run/log/journal/<replaceable>machine-id</replaceable>/*.journal~</filename></term>
 291         <term><filename>/var/log/journal/<replaceable>machine-id</replaceable>/*.journal</filename></term>
 292         <term><filename>/var/log/journal/<replaceable>machine-id</replaceable>/*.journal~</filename></term>
 293
 294         <listitem><para><command>systemd-journald</command> writes entries to files in
 295         <filename>/run/log/journal/<replaceable>machine-id</replaceable>/</filename>
 296         or
 297         <filename>/var/log/journal/<replaceable>machine-id</replaceable>/</filename>
 298         with the <literal>.journal</literal> suffix. If the daemon is
 299         stopped uncleanly, or if the files are found to be corrupted,
 300         they are renamed using the <literal>.journal~</literal>
 301         suffix, and <command>systemd-journald</command> starts writing
 302         to a new file. <filename>/run/</filename> is used when
 303         <filename>/var/log/journal</filename> is not available, or
 304         when <option>Storage=volatile</option> is set in the
 305         <citerefentry><refentrytitle>journald.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 306         configuration file.</para>
 307
 308         <para>When <filename>systemd-journald</filename> ceases writing to a journal file,
 309         it will be renamed to <literal><replaceable>original-name</replaceable>@<replaceable>suffix.journal</replaceable></literal>
 310         (or <literal><replaceable>original-name</replaceable>@<replaceable>suffix.journal~</replaceable></literal>).
 311         Such files are "archived" and will not be written to any more.</para>
 312
 313         <para>In general, it is safe to read or copy any journal file (active or archived).
 314         <citerefentry><refentrytitle>journalctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 315         and the functions in the
 316         <citerefentry><refentrytitle>sd-journal</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 317         library should be able to read all entries that have been fully written.</para>
 318
 319         <para><filename>systemd-journald</filename> will automatically remove the oldest
 320         archived journal files to limit disk use. See <varname>SystemMaxUse=</varname>
 321         and related settings in
 322         <citerefentry><refentrytitle>journald.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
 323         </para></listitem>
 324       </varlistentry>
 325
 326       <varlistentry>
 327         <term><filename>/dev/kmsg</filename></term>
 328         <term><filename>/dev/log</filename></term>
 329         <term><filename>/run/systemd/journal/dev-log</filename></term>
 330         <term><filename>/run/systemd/journal/socket</filename></term>
 331         <term><filename>/run/systemd/journal/stdout</filename></term>
 332
 333         <listitem><para>Sockets and other file node paths that <command>systemd-journald</command> will
 334         listen on and are visible in the file system. In addition to these,
 335         <command>systemd-journald</command> can listen for audit events using <citerefentry
 336         project='man-pages'><refentrytitle>netlink</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
 337         depending on whether <literal>systemd-journald-audit.socket</literal> is enabled or
 338         not.</para></listitem>
 339       </varlistentry>
 340     </variablelist>
 341
 342     <para>If journal namespacing is used these paths are slightly altered to include a namespace identifier, see above.</para>
 343   </refsect1>
 344
 345   <refsect1>
 346     <title>See Also</title>
 347     <para>
 348       <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 349       <citerefentry><refentrytitle>journalctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 350       <citerefentry><refentrytitle>journald.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 351       <citerefentry><refentrytitle>systemd.journal-fields</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
 352       <citerefentry><refentrytitle>sd-journal</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 353       <citerefentry><refentrytitle>systemd-coredump</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 354       <citerefentry project='man-pages'><refentrytitle>setfacl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 355       <citerefentry><refentrytitle>sd_journal_print</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 356       <command>pydoc systemd.journal</command>
 357     </para>
 358   </refsect1>
 359
 360 </refentry>