man/systemd-journald.service.xml

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   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 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>
  60
  61       <listitem><para>Standard output and standard error of service units. For further details see
  62       below.</para></listitem>
  63
  64       <listitem><para>Audit records, originating from the kernel audit subsystem</para></listitem>
  65     </itemizedlist>
  66
  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.
  71     </para>
  72
  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>
  75
  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>
  82
  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>
  86
  87     <programlisting>mkdir -p /var/log/journal
  88 systemd-tmpfiles --create --prefix /var/log/journal</programlisting>
  89
  90     <para>See
  91     <citerefentry><refentrytitle>journald.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>
  92     for information about the configuration of this service.</para>
  93   </refsect1>
  94
  95   <refsect1>
  96     <title>Stream logging</title>
  97
  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>
 104
 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>
 123
 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>
 130
 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
 133     line tool.</para>
 134
 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>
 138   </refsect1>
 139
 140   <refsect1>
 141     <title>Journal Namespaces</title>
 142
 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>
 158
 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
 161     namespace.</para>
 162
 163     <para>By default only the default namespace will collect kernel and audit log messages.</para>
 164
 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>
 172   </refsect1>
 173
 174   <refsect1>
 175     <title>Signals</title>
 176
 177     <variablelist>
 178       <varlistentry>
 179         <term>SIGUSR1</term>
 180
 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>
 189       </varlistentry>
 190
 191       <varlistentry>
 192         <term>SIGUSR2</term>
 193
 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>
 197       </varlistentry>
 198
 199       <varlistentry>
 200         <term>SIGRTMIN+1</term>
 201
 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>
 205       </varlistentry>
 206     </variablelist>
 207   </refsect1>
 208
 209   <refsect1>
 210     <title>Kernel Command Line</title>
 211
 212     <para>A few configuration parameters from
 213     <filename>journald.conf</filename> may be overridden on the kernel
 214     command line:</para>
 215
 216     <variablelist class='kernel-commandline-options'>
 217       <varlistentry>
 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>
 222
 223         <listitem><para>Enables/disables forwarding of collected log
 224         messages to syslog, the kernel log buffer, the system console
 225         or wall.
 226         </para>
 227
 228         <para>See
 229         <citerefentry><refentrytitle>journald.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 230         for information about these settings.</para>
 231         </listitem>
 232
 233       </varlistentry>
 234     </variablelist>
 235
 236     <para>Note that these kernel command line options are only honoured by the default namespace, see
 237     above.</para>
 238   </refsect1>
 239
 240   <refsect1>
 241     <title>Access Control</title>
 242
 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>
 247
 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>
 256
 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>
 262
 263     <programlisting># setfacl -Rnm g:wheel:rx,d:g:wheel:rx,g:adm:rx,d:g:adm:rx /var/log/journal/</programlisting>
 264
 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>
 268   </refsect1>
 269
 270   <refsect1>
 271     <title>Files</title>
 272
 273     <variablelist>
 274       <varlistentry>
 275         <term><filename>/etc/systemd/journald.conf</filename></term>
 276
 277         <listitem><para>Configure <command>systemd-journald</command> behavior. See
 278         <citerefentry><refentrytitle>journald.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
 279         </para></listitem>
 280       </varlistentry>
 281
 282       <varlistentry>
 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>
 287
 288         <listitem><para><command>systemd-journald</command> writes entries to files in
 289         <filename>/run/log/journal/<replaceable>machine-id</replaceable>/</filename>
 290         or
 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>
 301
 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>
 306
 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>
 312
 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>.
 317         </para></listitem>
 318       </varlistentry>
 319
 320       <varlistentry>
 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>
 326
 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>
 331       </varlistentry>
 332     </variablelist>
 333
 334     <para>If journal namespacing is used these paths are slightly altered to include a namespace identifier, see above.</para>
 335   </refsect1>
 336
 337   <refsect1>
 338     <title>See Also</title>
 339     <para>
 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>
 349     </para>
 350   </refsect1>
 351
 352 </refentry>