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 <filename>/run/</filename> is flushed to
 140         <filename>/var/</filename> in order to make it persistent (if this is enabled). This must be used
 141         after <filename>/var/</filename> is mounted, as otherwise log data from <filename>/run</filename> is
 142         never flushed to <filename>/var</filename> regardless of the configuration. Use the
 143         <command>journalctl --flush</command> command to request flushing of the journal files, and wait for
 144         the operation to complete. See
 145         <citerefentry><refentrytitle>journalctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> for
 146         details.</para></listitem>
 147       </varlistentry>
 148
 149       <varlistentry>
 150         <term>SIGUSR2</term>
 151
 152         <listitem><para>Request immediate rotation of the journal files. Use the <command>journalctl
 153         --rotate</command> command to request journal file rotation, and wait for the operation to
 154         complete.</para></listitem>
 155       </varlistentry>
 156
 157       <varlistentry>
 158         <term>SIGRTMIN+1</term>
 159
 160         <listitem><para>Request that all unwritten log data is written to disk. Use the <command>journalctl
 161         --sync</command> command to trigger journal synchronization, and wait for the operation to
 162         complete.</para></listitem>
 163       </varlistentry>
 164     </variablelist>
 165   </refsect1>
 166
 167   <refsect1>
 168     <title>Kernel Command Line</title>
 169
 170     <para>A few configuration parameters from
 171     <filename>journald.conf</filename> may be overridden on the kernel
 172     command line:</para>
 173
 174     <variablelist class='kernel-commandline-options'>
 175       <varlistentry>
 176         <term><varname>systemd.journald.forward_to_syslog=</varname></term>
 177         <term><varname>systemd.journald.forward_to_kmsg=</varname></term>
 178         <term><varname>systemd.journald.forward_to_console=</varname></term>
 179         <term><varname>systemd.journald.forward_to_wall=</varname></term>
 180
 181         <listitem><para>Enables/disables forwarding of collected log
 182         messages to syslog, the kernel log buffer, the system console
 183         or wall.
 184         </para>
 185
 186         <para>See
 187         <citerefentry><refentrytitle>journald.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 188         for information about these settings.</para>
 189         </listitem>
 190
 191       </varlistentry>
 192     </variablelist>
 193   </refsect1>
 194
 195   <refsect1>
 196     <title>Access Control</title>
 197
 198     <para>Journal files are, by default, owned and readable by the
 199     <literal>systemd-journal</literal> system group but are not
 200     writable. Adding a user to this group thus enables them to read
 201     the journal files.</para>
 202
 203     <para>By default, each user, with a UID outside the range of system users,
 204     dynamic service users, and the nobody user, will get their own set of
 205     journal files in <filename>/var/log/journal/</filename>. See
 206     <ulink url="https://systemd.io/UIDS-GIDS">Users, Groups, UIDs and GIDs on systemd systems</ulink>
 207     for more details about UID ranges. These journal
 208     files will not be owned by the user, however, in order to avoid
 209     that the user can write to them directly. Instead, file system
 210     ACLs are used to ensure the user gets read access only.</para>
 211
 212     <para>Additional users and groups may be granted access to journal
 213     files via file system access control lists (ACL). Distributions
 214     and administrators may choose to grant read access to all members
 215     of the <literal>wheel</literal> and <literal>adm</literal> system
 216     groups with a command such as the following:</para>
 217
 218     <programlisting># setfacl -Rnm g:wheel:rx,d:g:wheel:rx,g:adm:rx,d:g:adm:rx /var/log/journal/</programlisting>
 219
 220     <para>Note that this command will update the ACLs both for
 221     existing journal files and for future journal files created in the
 222     <filename>/var/log/journal/</filename> directory.</para>
 223   </refsect1>
 224
 225   <refsect1>
 226     <title>Files</title>
 227
 228     <variablelist>
 229       <varlistentry>
 230         <term><filename>/etc/systemd/journald.conf</filename></term>
 231
 232         <listitem><para>Configure <command>systemd-journald</command> behavior. See
 233         <citerefentry><refentrytitle>journald.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
 234         </para></listitem>
 235       </varlistentry>
 236
 237       <varlistentry>
 238         <term><filename>/run/log/journal/<replaceable>machine-id</replaceable>/*.journal</filename></term>
 239         <term><filename>/run/log/journal/<replaceable>machine-id</replaceable>/*.journal~</filename></term>
 240         <term><filename>/var/log/journal/<replaceable>machine-id</replaceable>/*.journal</filename></term>
 241         <term><filename>/var/log/journal/<replaceable>machine-id</replaceable>/*.journal~</filename></term>
 242
 243         <listitem><para><command>systemd-journald</command> writes entries to files in
 244         <filename>/run/log/journal/<replaceable>machine-id</replaceable>/</filename>
 245         or
 246         <filename>/var/log/journal/<replaceable>machine-id</replaceable>/</filename>
 247         with the <literal>.journal</literal> suffix. If the daemon is
 248         stopped uncleanly, or if the files are found to be corrupted,
 249         they are renamed using the <literal>.journal~</literal>
 250         suffix, and <command>systemd-journald</command> starts writing
 251         to a new file. <filename>/run</filename> is used when
 252         <filename>/var/log/journal</filename> is not available, or
 253         when <option>Storage=volatile</option> is set in the
 254         <citerefentry><refentrytitle>journald.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 255         configuration file.</para>
 256
 257         <para>When <filename>systemd-journald</filename> ceases writing to a journal file,
 258         it will be renamed to <literal><replaceable>original-name</replaceable>@<replaceable>suffix.journal</replaceable></literal>
 259         (or <literal><replaceable>original-name</replaceable>@<replaceable>suffix.journal~</replaceable></literal>).
 260         Such files are "archived" and will not be written to any more.</para>
 261
 262         <para>In general, it is safe to read or copy any journal file (active or archived).
 263         <citerefentry><refentrytitle>journalctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 264         and the functions in the
 265         <citerefentry><refentrytitle>sd-journal</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 266         library should be able to read all entries that have been fully written.</para>
 267
 268         <para><filename>systemd-journald</filename> will automatically remove the oldest
 269         archived journal files to limit disk use. See <varname>SystemMaxUse=</varname>
 270         and related settings in
 271         <citerefentry><refentrytitle>journald.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
 272         </para></listitem>
 273       </varlistentry>
 274
 275       <varlistentry>
 276         <term><filename>/dev/kmsg</filename></term>
 277         <term><filename>/dev/log</filename></term>
 278         <term><filename>/run/systemd/journal/dev-log</filename></term>
 279         <term><filename>/run/systemd/journal/socket</filename></term>
 280         <term><filename>/run/systemd/journal/stdout</filename></term>
 281
 282         <listitem><para>Sockets and other paths that
 283         <command>systemd-journald</command> will listen on that are
 284         visible in the file system. In addition to these, journald can
 285         listen for audit events using netlink.</para></listitem>
 286       </varlistentry>
 287     </variablelist>
 288   </refsect1>
 289
 290   <refsect1>
 291     <title>See Also</title>
 292     <para>
 293       <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 294       <citerefentry><refentrytitle>journalctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 295       <citerefentry><refentrytitle>journald.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 296       <citerefentry><refentrytitle>systemd.journal-fields</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
 297       <citerefentry><refentrytitle>sd-journal</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 298       <citerefentry><refentrytitle>systemd-coredump</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 299       <citerefentry project='die-net'><refentrytitle>setfacl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 300       <citerefentry><refentrytitle>sd_journal_print</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 301       <command>pydoc systemd.journal</command>
 302     </para>
 303   </refsect1>
 304
 305 </refentry>