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