man/systemd-journald.service.xml

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   5
   6 <refentry id="systemd-journald.service">
   7
   8   <refentryinfo>
   9     <title>systemd-journald.service</title>
  10     <productname>systemd</productname>
  11   </refentryinfo>
  12
  13   <refmeta>
  14     <refentrytitle>systemd-journald.service</refentrytitle>
  15     <manvolnum>8</manvolnum>
  16   </refmeta>
  17
  18   <refnamediv>
  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>
  28   </refnamediv>
  29
  30   <refsynopsisdiv>
  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>
  39   </refsynopsisdiv>
  40
  41   <refsect1>
  42     <title>Description</title>
  43
  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>
  48
  49     <itemizedlist>
  50       <listitem><para>Kernel log messages, via kmsg</para></listitem>
  51
  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>
  55
  56       <listitem><para>Structured system log messages via the native
  57       Journal API, see
  58       <citerefentry><refentrytitle>sd_journal_print</refentrytitle><manvolnum>3</manvolnum></citerefentry></para></listitem>
  59
  60       <listitem><para>Standard output and standard error of service units. For further details see
  61       below.</para></listitem>
  62
  63       <listitem><para>Audit records, originating from the kernel audit subsystem</para></listitem>
  64     </itemizedlist>
  65
  66     <para>The daemon will implicitly collect numerous metadata fields
  67     for each log messages in a secure and unfakeable way. See
  68     <citerefentry><refentrytitle>systemd.journal-fields</refentrytitle><manvolnum>7</manvolnum></citerefentry>
  69     for more information about the collected metadata.
  70     </para>
  71
  72     <para>Log data collected by the journal is primarily text-based but can also include binary data where
  73     necessary. Individual fields making up a log record stored in the journal may be up to 2^64-1 bytes in size.</para>
  74
  75     <para>The journal service stores log data either persistently below <filename>/var/log/journal</filename> or in a
  76     volatile way below <filename>/run/log/journal/</filename> (in the latter case it is lost at reboot). By default, log
  77     data is stored persistently if <filename>/var/log/journal/</filename> exists during boot, with an implicit fallback
  78     to volatile storage otherwise. Use <varname>Storage=</varname> in
  79     <citerefentry><refentrytitle>journald.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry> to configure
  80     where log data is placed, independently of the existence of <filename>/var/log/journal/</filename>.</para>
  81
  82     <para>On systems where <filename>/var/log/journal/</filename> does not exist yet but where persistent logging is
  83     desired (and the default <filename>journald.conf</filename> is used), it is sufficient to create the directory, and
  84     ensure it has the correct access modes and ownership:</para>
  85
  86     <programlisting>mkdir -p /var/log/journal
  87 systemd-tmpfiles --create --prefix /var/log/journal</programlisting>
  88
  89     <para>See
  90     <citerefentry><refentrytitle>journald.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>
  91     for information about the configuration of this service.</para>
  92   </refsect1>
  93
  94   <refsect1>
  95     <title>Stream logging</title>
  96
  97     <para>The systemd service manager invokes all service processes with standard output and standard error connected
  98     to the journal by default. This behaviour may be altered via the
  99     <varname>StandardOutput=</varname>/<varname>StandardError=</varname> unit file settings, see
 100     <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry> for details. The
 101     journal converts the log byte stream received this way into individual log records, splitting the stream at newline
 102     (<literal>\n</literal>, ASCII <constant>10</constant>) and <constant>NUL</constant> bytes.</para>
 103
 104     <para>If <filename>systemd-journald.service</filename> is stopped, the stream connections associated with all
 105     services are terminated. Further writes to those streams by the service will result in <constant>EPIPE</constant>
 106     errors. In order to react gracefully in this case it is recommended that programs logging to standard output/error
 107     ignore such errors. If the <constant>SIGPIPE</constant> UNIX signal handler is not blocked or turned off, such
 108     write attempts will also result in such process signals being generated, see
 109     <citerefentry><refentrytitle>signal</refentrytitle><manvolnum>7</manvolnum></citerefentry>. To mitigate this issue,
 110     systemd service manager explicitly turns off the <constant>SIGPIPE</constant> signal for all invoked processes by
 111     default (this may be changed for each unit individually via the <varname>IgnoreSIGPIPE=</varname> option, see
 112     <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry> for
 113     details). After the standard output/standard error streams have been terminated they may not be recovered until the
 114     services they are associated with are restarted. Note that during normal operation,
 115     <filename>systemd-journald.service</filename> stores copies of the file descriptors for those streams in the
 116     service manager. If <filename>systemd-journald.service</filename> is restarted using <command>systemctl
 117     restart</command> or equivalent operation instead of a pair of separate <command>systemctl stop</command> and
 118     <command>systemctl start</command> commands (or equivalent operations), these stream connections are not terminated
 119     and survive the restart. It is thus safe to restart <filename>systemd-journald.service</filename>, but stopping it
 120     is not recommended.</para>
 121
 122     <para>Note that the log record metadata for records transferred via such standard output/error streams reflect the
 123     metadata of the peer the stream was originally created for. If the stream connection is passed on to other
 124     processes (such as further child processes forked off the main service process), the log records will not reflect
 125     their metadata, but will continue to describe the original process. This is different from the other logging
 126     transports listed above, which are inherently record based and where the metadata is always associated with the
 127     individual record.</para>
 128
 129     <para>In addition to the implicit standard output/error logging of services, stream logging is also available
 130     via the <citerefentry><refentrytitle>systemd-cat</refentrytitle><manvolnum>1</manvolnum></citerefentry> command
 131     line tool.</para>
 132
 133     <para>Currently, the number of parallel log streams <filename>systemd-journald</filename> will accept is limited to
 134     4096. When this limit is reached further log streams may be established but will receive
 135     <constant>EPIPE</constant> right from the beginning.</para>
 136   </refsect1>
 137
 138   <refsect1>
 139     <title>Journal Namespaces</title>
 140
 141     <para>Journal 'namespaces' are both a mechanism for logically isolating the log stream of projects
 142     consisting of one or more services from the rest of the system and a mechanism for improving
 143     performance. Multiple journal namespaces may exist simultaneously, each defining its own, independent log
 144     stream managed by its own instance of <command>systemd-journald</command>. Namespaces are independent of
 145     each other, both in the data store and in the IPC interface. By default only a single 'default' namespace
 146     exists, managed by <filename>systemd-journald.service</filename> (and its associated socket
 147     units). Additional namespaces are created by starting an instance of the
 148     <filename>systemd-journald@.service</filename> service template. The instance name is the namespace
 149     identifier, which is a short string used for referencing the journal namespace. Service units may be
 150     assigned to a specific journal namespace through the <varname>LogNamespace=</varname> unit file setting,
 151     see <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry> for
 152     details. The <option>--namespace=</option> switch of
 153     <citerefentry><refentrytitle>journalctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> may be
 154     used to view the log stream of a specific namespace. If the switch is not used the log stream of the
 155     default namespace is shown, i.e. log data from other namespaces is not visible.</para>
 156
 157     <para>Services associated with a specific log namespace may log via syslog, the native logging protocol
 158     of the journal and via stdout/stderr; the logging from all three transports is associated with the
 159     namespace.</para>
 160
 161     <para>By default only the default namespace will collect kernel and audit log messages.</para>
 162
 163     <para>The <command>systemd-journald</command> instance of the default namespace is configured through
 164     <filename>/etc/systemd/journald.conf</filename> (see below), while the other instances are configured
 165     through <filename>/etc/systemd/journald@<replaceable>NAMESPACE</replaceable>.conf</filename>. The journal
 166     log data for the default namespace is placed in
 167     <filename>/var/log/journal/<replaceable>MACHINE_ID</replaceable></filename> (see below) while the data
 168     for the other namespaces is located in
 169     <filename>/var/log/journal/<replaceable>MACHINE_ID</replaceable>.<replaceable>NAMESPACE</replaceable></filename>.</para>
 170   </refsect1>
 171
 172   <refsect1>
 173     <title>Signals</title>
 174
 175     <variablelist>
 176       <varlistentry>
 177         <term>SIGUSR1</term>
 178
 179         <listitem><para>Request that journal data from <filename>/run/</filename> is flushed to
 180         <filename>/var/</filename> in order to make it persistent (if this is enabled). This must be used
 181         after <filename>/var/</filename> is mounted, as otherwise log data from <filename>/run</filename> is
 182         never flushed to <filename>/var</filename> regardless of the configuration. Use the
 183         <command>journalctl --flush</command> command to request flushing of the journal files, and wait for
 184         the operation to complete. See
 185         <citerefentry><refentrytitle>journalctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> for
 186         details.</para></listitem>
 187       </varlistentry>
 188
 189       <varlistentry>
 190         <term>SIGUSR2</term>
 191
 192         <listitem><para>Request immediate rotation of the journal files. Use the <command>journalctl
 193         --rotate</command> command to request journal file rotation, and wait for the operation to
 194         complete.</para></listitem>
 195       </varlistentry>
 196
 197       <varlistentry>
 198         <term>SIGRTMIN+1</term>
 199
 200         <listitem><para>Request that all unwritten log data is written to disk. Use the <command>journalctl
 201         --sync</command> command to trigger journal synchronization, and wait for the operation to
 202         complete.</para></listitem>
 203       </varlistentry>
 204     </variablelist>
 205   </refsect1>
 206
 207   <refsect1>
 208     <title>Kernel Command Line</title>
 209
 210     <para>A few configuration parameters from
 211     <filename>journald.conf</filename> may be overridden on the kernel
 212     command line:</para>
 213
 214     <variablelist class='kernel-commandline-options'>
 215       <varlistentry>
 216         <term><varname>systemd.journald.forward_to_syslog=</varname></term>
 217         <term><varname>systemd.journald.forward_to_kmsg=</varname></term>
 218         <term><varname>systemd.journald.forward_to_console=</varname></term>
 219         <term><varname>systemd.journald.forward_to_wall=</varname></term>
 220
 221         <listitem><para>Enables/disables forwarding of collected log
 222         messages to syslog, the kernel log buffer, the system console
 223         or wall.
 224         </para>
 225
 226         <para>See
 227         <citerefentry><refentrytitle>journald.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 228         for information about these settings.</para>
 229         </listitem>
 230
 231       </varlistentry>
 232     </variablelist>
 233
 234     <para>Note that these kernel command line options are only honoured by the default namespace, see
 235     above.</para>
 236   </refsect1>
 237
 238   <refsect1>
 239     <title>Access Control</title>
 240
 241     <para>Journal files are, by default, owned and readable by the
 242     <literal>systemd-journal</literal> system group but are not
 243     writable. Adding a user to this group thus enables them to read
 244     the journal files.</para>
 245
 246     <para>By default, each user, with a UID outside the range of system users,
 247     dynamic service users, and the nobody user, will get their own set of
 248     journal files in <filename>/var/log/journal/</filename>. See
 249     <ulink url="https://systemd.io/UIDS-GIDS">Users, Groups, UIDs and GIDs on systemd systems</ulink>
 250     for more details about UID ranges. These journal
 251     files will not be owned by the user, however, in order to avoid
 252     that the user can write to them directly. Instead, file system
 253     ACLs are used to ensure the user gets read access only.</para>
 254
 255     <para>Additional users and groups may be granted access to journal
 256     files via file system access control lists (ACL). Distributions
 257     and administrators may choose to grant read access to all members
 258     of the <literal>wheel</literal> and <literal>adm</literal> system
 259     groups with a command such as the following:</para>
 260
 261     <programlisting># setfacl -Rnm g:wheel:rx,d:g:wheel:rx,g:adm:rx,d:g:adm:rx /var/log/journal/</programlisting>
 262
 263     <para>Note that this command will update the ACLs both for
 264     existing journal files and for future journal files created in the
 265     <filename>/var/log/journal/</filename> directory.</para>
 266   </refsect1>
 267
 268   <refsect1>
 269     <title>Files</title>
 270
 271     <variablelist>
 272       <varlistentry>
 273         <term><filename>/etc/systemd/journald.conf</filename></term>
 274
 275         <listitem><para>Configure <command>systemd-journald</command> behavior. See
 276         <citerefentry><refentrytitle>journald.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
 277         </para></listitem>
 278       </varlistentry>
 279
 280       <varlistentry>
 281         <term><filename>/run/log/journal/<replaceable>machine-id</replaceable>/*.journal</filename></term>
 282         <term><filename>/run/log/journal/<replaceable>machine-id</replaceable>/*.journal~</filename></term>
 283         <term><filename>/var/log/journal/<replaceable>machine-id</replaceable>/*.journal</filename></term>
 284         <term><filename>/var/log/journal/<replaceable>machine-id</replaceable>/*.journal~</filename></term>
 285
 286         <listitem><para><command>systemd-journald</command> writes entries to files in
 287         <filename>/run/log/journal/<replaceable>machine-id</replaceable>/</filename>
 288         or
 289         <filename>/var/log/journal/<replaceable>machine-id</replaceable>/</filename>
 290         with the <literal>.journal</literal> suffix. If the daemon is
 291         stopped uncleanly, or if the files are found to be corrupted,
 292         they are renamed using the <literal>.journal~</literal>
 293         suffix, and <command>systemd-journald</command> starts writing
 294         to a new file. <filename>/run</filename> is used when
 295         <filename>/var/log/journal</filename> is not available, or
 296         when <option>Storage=volatile</option> is set in the
 297         <citerefentry><refentrytitle>journald.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 298         configuration file.</para>
 299
 300         <para>When <filename>systemd-journald</filename> ceases writing to a journal file,
 301         it will be renamed to <literal><replaceable>original-name</replaceable>@<replaceable>suffix.journal</replaceable></literal>
 302         (or <literal><replaceable>original-name</replaceable>@<replaceable>suffix.journal~</replaceable></literal>).
 303         Such files are "archived" and will not be written to any more.</para>
 304
 305         <para>In general, it is safe to read or copy any journal file (active or archived).
 306         <citerefentry><refentrytitle>journalctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 307         and the functions in the
 308         <citerefentry><refentrytitle>sd-journal</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 309         library should be able to read all entries that have been fully written.</para>
 310
 311         <para><filename>systemd-journald</filename> will automatically remove the oldest
 312         archived journal files to limit disk use. See <varname>SystemMaxUse=</varname>
 313         and related settings in
 314         <citerefentry><refentrytitle>journald.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
 315         </para></listitem>
 316       </varlistentry>
 317
 318       <varlistentry>
 319         <term><filename>/dev/kmsg</filename></term>
 320         <term><filename>/dev/log</filename></term>
 321         <term><filename>/run/systemd/journal/dev-log</filename></term>
 322         <term><filename>/run/systemd/journal/socket</filename></term>
 323         <term><filename>/run/systemd/journal/stdout</filename></term>
 324
 325         <listitem><para>Sockets and other file node paths that <command>systemd-journald</command> will
 326         listen on and are visible in the file system. In addition to these,
 327         <command>systemd-journald</command> can listen for audit events using <citerefentry
 328         project='man-pages'><refentrytitle>netlink</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para></listitem>
 329       </varlistentry>
 330     </variablelist>
 331
 332     <para>If journal namespacing is used these paths are slightly altered to include a namespace identifier, see above.</para>
 333   </refsect1>
 334
 335   <refsect1>
 336     <title>See Also</title>
 337     <para>
 338       <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 339       <citerefentry><refentrytitle>journalctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 340       <citerefentry><refentrytitle>journald.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 341       <citerefentry><refentrytitle>systemd.journal-fields</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
 342       <citerefentry><refentrytitle>sd-journal</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 343       <citerefentry><refentrytitle>systemd-coredump</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 344       <citerefentry project='man-pages'><refentrytitle>setfacl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 345       <citerefentry><refentrytitle>sd_journal_print</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 346       <command>pydoc systemd.journal</command>
 347     </para>
 348   </refsect1>
 349
 350 </refentry>