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</refname>
  24     <refpurpose>Journal service</refpurpose>
  25   </refnamediv>
  26
  27   <refsynopsisdiv>
  28     <para><filename>systemd-journald.service</filename></para>
  29     <para><filename>systemd-journald.socket</filename></para>
  30     <para><filename>systemd-journald-dev-log.socket</filename></para>
  31     <para><filename>systemd-journald-audit.socket</filename></para>
  32     <para><filename>/usr/lib/systemd/systemd-journald</filename></para>
  33   </refsynopsisdiv>
  34
  35   <refsect1>
  36     <title>Description</title>
  37
  38     <para><filename>systemd-journald</filename> is a system service
  39     that collects and stores logging data. It creates and maintains
  40     structured, indexed journals based on logging information that is
  41     received from a variety of sources:</para>
  42
  43     <itemizedlist>
  44       <listitem><para>Kernel log messages, via kmsg</para></listitem>
  45
  46       <listitem><para>Simple system log messages, via the <filename>libc</filename> <citerefentry
  47       project='man-pages'><refentrytitle>syslog</refentrytitle><manvolnum>3</manvolnum></citerefentry>
  48       call</para></listitem>
  49
  50       <listitem><para>Structured system log messages via the native
  51       Journal API, see
  52       <citerefentry><refentrytitle>sd_journal_print</refentrytitle><manvolnum>3</manvolnum></citerefentry></para></listitem>
  53
  54       <listitem><para>Standard output and standard error of service units. For further details see
  55       below.</para></listitem>
  56
  57       <listitem><para>Audit records, originating from the kernel audit subsystem</para></listitem>
  58     </itemizedlist>
  59
  60     <para>The daemon will implicitly collect numerous metadata fields
  61     for each log messages in a secure and unfakeable way. See
  62     <citerefentry><refentrytitle>systemd.journal-fields</refentrytitle><manvolnum>7</manvolnum></citerefentry>
  63     for more information about the collected metadata.
  64     </para>
  65
  66     <para>Log data collected by the journal is primarily text-based but can also include binary data where
  67     necessary. Individual fields making up a log record stored in the journal may be up to 2^64-1 bytes in size.</para>
  68
  69     <para>The journal service stores log data either persistently below <filename>/var/log/journal</filename> or in a
  70     volatile way below <filename>/run/log/journal/</filename> (in the latter case it is lost at reboot). By default, log
  71     data is stored persistently if <filename>/var/log/journal/</filename> exists during boot, with an implicit fallback
  72     to volatile storage otherwise. Use <varname>Storage=</varname> in
  73     <citerefentry><refentrytitle>journald.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry> to configure
  74     where log data is placed, independently of the existence of <filename>/var/log/journal/</filename>.</para>
  75
  76     <para>On systems where <filename>/var/log/journal/</filename> does not exist yet but where persistent logging is
  77     desired (and the default <filename>journald.conf</filename> is used), it is sufficient to create the directory, and
  78     ensure it has the correct access modes and ownership:</para>
  79
  80     <programlisting>mkdir -p /var/log/journal
  81 systemd-tmpfiles --create --prefix /var/log/journal</programlisting>
  82
  83     <para>See
  84     <citerefentry><refentrytitle>journald.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>
  85     for information about the configuration of this service.</para>
  86   </refsect1>
  87
  88   <refsect1>
  89     <title>Stream logging</title>
  90
  91     <para>The systemd service manager invokes all service processes with standard output and standard error connected
  92     to the journal by default. This behaviour may be altered via the
  93     <varname>StandardOutput=</varname>/<varname>StandardError=</varname> unit file settings, see
  94     <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry> for details. The
  95     journal converts the log byte stream received this way into individual log records, splitting the stream at newline
  96     (<literal>\n</literal>, ASCII <constant>10</constant>) and <constant>NUL</constant> bytes.</para>
  97
  98     <para>If <filename>systemd-journald.service</filename> is stopped, the stream connections associated with all
  99     services are terminated. Further writes to those streams by the service will result in <constant>EPIPE</constant>
 100     errors. In order to react gracefully in this case it is recommended that programs logging to standard output/error
 101     ignore such errors. If the <constant>SIGPIPE</constant> UNIX signal handler is not blocked or turned off, such
 102     write attempts will also result in such process signals being generated, see
 103     <citerefentry><refentrytitle>signal</refentrytitle><manvolnum>7</manvolnum></citerefentry>. To mitigate this issue,
 104     systemd service manager explicitly turns off the <constant>SIGPIPE</constant> signal for all invoked processes by
 105     default (this may be changed for each unit individually via the <varname>IgnoreSIGPIPE=</varname> option, see
 106     <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry> for
 107     details). After the standard output/standard error streams have been terminated they may not be recovered until the
 108     services they are associated with are restarted. Note that during normal operation,
 109     <filename>systemd-journald.service</filename> stores copies of the file descriptors for those streams in the
 110     service manager. If <filename>systemd-journald.service</filename> is restarted using <command>systemctl
 111     restart</command> or equivalent operation instead of a pair of separate <command>systemctl stop</command> and
 112     <command>systemctl start</command> commands (or equivalent operations), these stream connections are not terminated
 113     and survive the restart. It is thus safe to restart <filename>systemd-journald.service</filename>, but stopping it
 114     is not recommended.</para>
 115
 116     <para>Note that the log record metadata for records transferred via such standard output/error streams reflect the
 117     metadata of the peer the stream was originally created for. If the stream connection is passed on to other
 118     processes (such as further child processes forked off the main service process), the log records will not reflect
 119     their metadata, but will continue to describe the original process. This is different from the other logging
 120     transports listed above, which are inherently record based and where the metadata is always associated with the
 121     individual record.</para>
 122
 123     <para>In addition to the implicit standard output/error logging of services, stream logging is also available
 124     via the <citerefentry><refentrytitle>systemd-cat</refentrytitle><manvolnum>1</manvolnum></citerefentry> command
 125     line tool.</para>
 126
 127     <para>Currently, the number of parallel log streams <filename>systemd-journald</filename> will accept is limited to
 128     4096. When this limit is reached further log streams may be established but will receive
 129     <constant>EPIPE</constant> right from the beginning.</para>
 130   </refsect1>
 131
 132   <refsect1>
 133     <title>Signals</title>
 134
 135     <variablelist>
 136       <varlistentry>
 137         <term>SIGUSR1</term>
 138
 139         <listitem><para>Request that journal data from
 140         <filename>/run/</filename> is flushed to
 141         <filename>/var/</filename> in order to make it persistent (if
 142         this is enabled). This must be used after
 143         <filename>/var/</filename> is mounted, as otherwise log data
 144         from <filename>/run</filename> is never flushed to
 145         <filename>/var</filename> regardless of the configuration. The
 146         <command>journalctl --flush</command> command uses this signal
 147         to request flushing of the journal files, and then waits for
 148         the operation to complete. See
 149         <citerefentry><refentrytitle>journalctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 150         for details.</para></listitem>
 151       </varlistentry>
 152
 153       <varlistentry>
 154         <term>SIGUSR2</term>
 155
 156         <listitem><para>Request immediate rotation of the journal
 157         files. The <command>journalctl --rotate</command> command uses
 158         this signal to request journal file
 159         rotation.</para></listitem>
 160       </varlistentry>
 161
 162       <varlistentry>
 163         <term>SIGRTMIN+1</term>
 164
 165         <listitem><para>Request that all unwritten log data is written
 166         to disk. The <command>journalctl --sync</command> command uses
 167         this signal to trigger journal synchronization, and then waits
 168         for the operation to complete.</para></listitem>
 169       </varlistentry>
 170     </variablelist>
 171   </refsect1>
 172
 173   <refsect1>
 174     <title>Kernel Command Line</title>
 175
 176     <para>A few configuration parameters from
 177     <filename>journald.conf</filename> may be overridden on the kernel
 178     command line:</para>
 179
 180     <variablelist class='kernel-commandline-options'>
 181       <varlistentry>
 182         <term><varname>systemd.journald.forward_to_syslog=</varname></term>
 183         <term><varname>systemd.journald.forward_to_kmsg=</varname></term>
 184         <term><varname>systemd.journald.forward_to_console=</varname></term>
 185         <term><varname>systemd.journald.forward_to_wall=</varname></term>
 186
 187         <listitem><para>Enables/disables forwarding of collected log
 188         messages to syslog, the kernel log buffer, the system console
 189         or wall.
 190         </para>
 191
 192         <para>See
 193         <citerefentry><refentrytitle>journald.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 194         for information about these settings.</para>
 195         </listitem>
 196
 197       </varlistentry>
 198     </variablelist>
 199   </refsect1>
 200
 201   <refsect1>
 202     <title>Access Control</title>
 203
 204     <para>Journal files are, by default, owned and readable by the
 205     <literal>systemd-journal</literal> system group but are not
 206     writable. Adding a user to this group thus enables them to read
 207     the journal files.</para>
 208
 209     <para>By default, each logged in user will get their own set of
 210     journal files in <filename>/var/log/journal/</filename>. These
 211     files will not be owned by the user, however, in order to avoid
 212     that the user can write to them directly. Instead, file system
 213     ACLs are used to ensure the user gets read access only.</para>
 214
 215     <para>Additional users and groups may be granted access to journal
 216     files via file system access control lists (ACL). Distributions
 217     and administrators may choose to grant read access to all members
 218     of the <literal>wheel</literal> and <literal>adm</literal> system
 219     groups with a command such as the following:</para>
 220
 221     <programlisting># setfacl -Rnm g:wheel:rx,d:g:wheel:rx,g:adm:rx,d:g:adm:rx /var/log/journal/</programlisting>
 222
 223     <para>Note that this command will update the ACLs both for
 224     existing journal files and for future journal files created in the
 225     <filename>/var/log/journal/</filename> directory.</para>
 226   </refsect1>
 227
 228   <refsect1>
 229     <title>Files</title>
 230
 231     <variablelist>
 232       <varlistentry>
 233         <term><filename>/etc/systemd/journald.conf</filename></term>
 234
 235         <listitem><para>Configure <command>systemd-journald</command> behavior. See
 236         <citerefentry><refentrytitle>journald.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
 237         </para></listitem>
 238       </varlistentry>
 239
 240       <varlistentry>
 241         <term><filename>/run/log/journal/<replaceable>machine-id</replaceable>/*.journal</filename></term>
 242         <term><filename>/run/log/journal/<replaceable>machine-id</replaceable>/*.journal~</filename></term>
 243         <term><filename>/var/log/journal/<replaceable>machine-id</replaceable>/*.journal</filename></term>
 244         <term><filename>/var/log/journal/<replaceable>machine-id</replaceable>/*.journal~</filename></term>
 245
 246         <listitem><para><command>systemd-journald</command> writes entries to files in
 247         <filename>/run/log/journal/<replaceable>machine-id</replaceable>/</filename>
 248         or
 249         <filename>/var/log/journal/<replaceable>machine-id</replaceable>/</filename>
 250         with the <literal>.journal</literal> suffix. If the daemon is
 251         stopped uncleanly, or if the files are found to be corrupted,
 252         they are renamed using the <literal>.journal~</literal>
 253         suffix, and <command>systemd-journald</command> starts writing
 254         to a new file. <filename>/run</filename> is used when
 255         <filename>/var/log/journal</filename> is not available, or
 256         when <option>Storage=volatile</option> is set in the
 257         <citerefentry><refentrytitle>journald.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 258         configuration file.</para>
 259
 260         <para>When <filename>systemd-journald</filename> ceases writing to a journal file,
 261         it will be renamed to <literal><replaceable>original-name</replaceable>@<replaceable>suffix.journal</replaceable></literal>
 262         (or <literal><replaceable>original-name</replaceable>@<replaceable>suffix.journal~</replaceable></literal>).
 263         Such files are "archived" and will not be written to any more.</para>
 264
 265         <para>In general, it is safe to read or copy any journal file (active or archived).
 266         <citerefentry><refentrytitle>journalctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 267         and the functions in the
 268         <citerefentry><refentrytitle>sd-journal</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 269         library should be able to read all entries that have been fully written.</para>
 270
 271         <para><filename>systemd-journald</filename> will automatically remove the oldest
 272         archived journal files to limit disk use. See <varname>SystemMaxUse=</varname>
 273         and related settings in
 274         <citerefentry><refentrytitle>journald.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
 275         </para></listitem>
 276       </varlistentry>
 277
 278       <varlistentry>
 279         <term><filename>/dev/kmsg</filename></term>
 280         <term><filename>/dev/log</filename></term>
 281         <term><filename>/run/systemd/journal/dev-log</filename></term>
 282         <term><filename>/run/systemd/journal/socket</filename></term>
 283         <term><filename>/run/systemd/journal/stdout</filename></term>
 284
 285         <listitem><para>Sockets and other paths that
 286         <command>systemd-journald</command> will listen on that are
 287         visible in the file system. In addition to these, journald can
 288         listen for audit events using netlink.</para></listitem>
 289       </varlistentry>
 290     </variablelist>
 291   </refsect1>
 292
 293   <refsect1>
 294     <title>See Also</title>
 295     <para>
 296       <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 297       <citerefentry><refentrytitle>journalctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 298       <citerefentry><refentrytitle>journald.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 299       <citerefentry><refentrytitle>systemd.journal-fields</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
 300       <citerefentry><refentrytitle>sd-journal</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 301       <citerefentry><refentrytitle>systemd-coredump</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 302       <citerefentry project='die-net'><refentrytitle>setfacl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 303       <citerefentry><refentrytitle>sd_journal_print</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 304       <command>pydoc systemd.journal</command>
 305     </para>
 306   </refsect1>
 307
 308 </refentry>