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