man/systemd-journald.service.xml

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   9 <refentry id="systemd-journald.service">
  10
  11   <refentryinfo>
  12     <title>systemd-journald.service</title>
  13     <productname>systemd</productname>
  14   </refentryinfo>
  15
  16   <refmeta>
  17     <refentrytitle>systemd-journald.service</refentrytitle>
  18     <manvolnum>8</manvolnum>
  19   </refmeta>
  20
  21   <refnamediv>
  22     <refname>systemd-journald.service</refname>
  23     <refname>systemd-journald.socket</refname>
  24     <refname>systemd-journald-dev-log.socket</refname>
  25     <refname>systemd-journald-audit.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>/usr/lib/systemd/systemd-journald</filename></para>
  36   </refsynopsisdiv>
  37
  38   <refsect1>
  39     <title>Description</title>
  40
  41     <para><filename>systemd-journald</filename> is a system service
  42     that collects and stores logging data. It creates and maintains
  43     structured, indexed journals based on logging information that is
  44     received from a variety of sources:</para>
  45
  46     <itemizedlist>
  47       <listitem><para>Kernel log messages, via kmsg</para></listitem>
  48
  49       <listitem><para>Simple system log messages, via the <filename>libc</filename> <citerefentry
  50       project='man-pages'><refentrytitle>syslog</refentrytitle><manvolnum>3</manvolnum></citerefentry>
  51       call</para></listitem>
  52
  53       <listitem><para>Structured system log messages via the native
  54       Journal API, see
  55       <citerefentry><refentrytitle>sd_journal_print</refentrytitle><manvolnum>4</manvolnum></citerefentry></para></listitem>
  56
  57       <listitem><para>Standard output and standard error of service units. For further details see
  58       below.</para></listitem>
  59
  60       <listitem><para>Audit records, originating from the kernel audit subsystem</para></listitem>
  61     </itemizedlist>
  62
  63     <para>The daemon will implicitly collect numerous metadata fields
  64     for each log messages in a secure and unfakeable way. See
  65     <citerefentry><refentrytitle>systemd.journal-fields</refentrytitle><manvolnum>7</manvolnum></citerefentry>
  66     for more information about the collected metadata.
  67     </para>
  68
  69     <para>Log data collected by the journal is primarily text-based but can also include binary data where
  70     necessary. Individual fields making up a log record stored in the journal may be up to 2^64-1 bytes in size.</para>
  71
  72     <para>The journal service stores log data either persistently below <filename>/var/log/journal</filename> or in a
  73     volatile way below <filename>/run/log/journal/</filename> (in the latter case it is lost at reboot). By default, log
  74     data is stored persistently if <filename>/var/log/journal/</filename> exists during boot, with an implicit fallback
  75     to volatile storage otherwise. Use <varname>Storage=</varname> in
  76     <citerefentry><refentrytitle>journald.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry> to configure
  77     where log data is placed, independently of the existence of <filename>/var/log/journal/</filename>.</para>
  78
  79     <para>On systems where <filename>/var/log/journal/</filename> does not exist yet but where persistent logging is
  80     desired (and the default <filename>journald.conf</filename> is used), it is sufficient to create the directory, and
  81     ensure it has the correct access modes and ownership:</para>
  82
  83     <programlisting>mkdir -p /var/log/journal
  84 systemd-tmpfiles --create --prefix /var/log/journal</programlisting>
  85
  86     <para>See
  87     <citerefentry><refentrytitle>journald.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>
  88     for information about the configuration of this service.</para>
  89   </refsect1>
  90
  91   <refsect1>
  92     <title>Stream logging</title>
  93
  94     <para>The systemd service manager invokes all service processes with standard output and standard error connected
  95     to the journal by default. This behaviour may be altered via the
  96     <varname>StandardOutput=</varname>/<varname>StandardError=</varname> unit file settings, see
  97     <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry> for details. The
  98     journal converts the log byte stream received this way into individual log records, splitting the stream at newline
  99     (<literal>\n</literal>, ASCII <constant>10</constant>) and <constant>NUL</constant> bytes.</para>
 100
 101     <para>If <filename>systemd-journald.service</filename> is stopped, the stream connections associated with all
 102     services are terminated. Further writes to those streams by the service will result in <constant>EPIPE</constant>
 103     errors. In order to react gracefully in this case it is recommended that programs logging to standard output/error
 104     ignore such errors. If the <constant>SIGPIPE</constant> UNIX signal handler is not blocked or turned off, such
 105     write attempts will also result in such process signals being generated, see
 106     <citerefentry><refentrytitle>signal</refentrytitle><manvolnum>7</manvolnum></citerefentry>. To mitigate this issue,
 107     systemd service manager explicitly turns off the <constant>SIGPIPE</constant> signal for all invoked processes by
 108     default (this may be changed for each unit individually via the <varname>IgnoreSIGPIPE=</varname> option, see
 109     <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry> for
 110     details). After the standard output/standard error streams have been terminated they may not be recovered until the
 111     services they are associated with are restarted. Note that during normal operation,
 112     <filename>systemd-journald.service</filename> stores copies of the file descriptors for those streams in the
 113     service manager. If <filename>systemd-journald.service</filename> is restarted using <command>systemctl
 114     restart</command> or equivalent operation instead of a pair of separate <command>systemctl stop</command> and
 115     <command>systemctl start</command> commands (or equivalent operations), these stream connections are not terminated
 116     and survive the restart. It is thus safe to restart <filename>systemd-journald.service</filename>, but stopping it
 117     is not recommended.</para>
 118
 119     <para>Note that the log record metadata for records transferred via such standard output/error streams reflect the
 120     metadata of the peer the stream was originally created for. If the stream connection is passed on to other
 121     processes (such as further child processes forked off the main service process), the log records will not reflect
 122     their metadata, but will continue to describe the original process. This is different from the other logging
 123     transports listed above, which are inherently record based and where the metadata is always associated with the
 124     individual record.</para>
 125
 126     <para>In addition to the implicit standard output/error logging of services, stream logging is also available
 127     via the <citerefentry><refentrytitle>systemd-cat</refentrytitle><manvolnum>1</manvolnum></citerefentry> command
 128     line tool.</para>
 129
 130     <para>Currently, the number of parallel log streams <filename>systemd-journald</filename> will accept is limited to
 131     4096. When this limit is reached further log streams may be established but will receive
 132     <constant>EPIPE</constant> right from the beginning.</para>
 133   </refsect1>
 134
 135   <refsect1>
 136     <title>Signals</title>
 137
 138     <variablelist>
 139       <varlistentry>
 140         <term>SIGUSR1</term>
 141
 142         <listitem><para>Request that journal data from
 143         <filename>/run/</filename> is flushed to
 144         <filename>/var/</filename> in order to make it persistent (if
 145         this is enabled). This must be used after
 146         <filename>/var/</filename> is mounted, as otherwise log data
 147         from <filename>/run</filename> is never flushed to
 148         <filename>/var</filename> regardless of the configuration. The
 149         <command>journalctl --flush</command> command uses this signal
 150         to request flushing of the journal files, and then waits for
 151         the operation to complete. See
 152         <citerefentry><refentrytitle>journalctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 153         for details.</para></listitem>
 154       </varlistentry>
 155
 156       <varlistentry>
 157         <term>SIGUSR2</term>
 158
 159         <listitem><para>Request immediate rotation of the journal
 160         files. The <command>journalctl --rotate</command> command uses
 161         this signal to request journal file
 162         rotation.</para></listitem>
 163       </varlistentry>
 164
 165       <varlistentry>
 166         <term>SIGRTMIN+1</term>
 167
 168         <listitem><para>Request that all unwritten log data is written
 169         to disk. The <command>journalctl --sync</command> command uses
 170         this signal to trigger journal synchronization, and then waits
 171         for the operation to complete.</para></listitem>
 172       </varlistentry>
 173     </variablelist>
 174   </refsect1>
 175
 176   <refsect1>
 177     <title>Kernel Command Line</title>
 178
 179     <para>A few configuration parameters from
 180     <filename>journald.conf</filename> may be overridden on the kernel
 181     command line:</para>
 182
 183     <variablelist class='kernel-commandline-options'>
 184       <varlistentry>
 185         <term><varname>systemd.journald.forward_to_syslog=</varname></term>
 186         <term><varname>systemd.journald.forward_to_kmsg=</varname></term>
 187         <term><varname>systemd.journald.forward_to_console=</varname></term>
 188         <term><varname>systemd.journald.forward_to_wall=</varname></term>
 189
 190         <listitem><para>Enables/disables forwarding of collected log
 191         messages to syslog, the kernel log buffer, the system console
 192         or wall.
 193         </para>
 194
 195         <para>See
 196         <citerefentry><refentrytitle>journald.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 197         for information about these settings.</para>
 198         </listitem>
 199
 200       </varlistentry>
 201     </variablelist>
 202   </refsect1>
 203
 204   <refsect1>
 205     <title>Access Control</title>
 206
 207     <para>Journal files are, by default, owned and readable by the
 208     <literal>systemd-journal</literal> system group but are not
 209     writable. Adding a user to this group thus enables them to read
 210     the journal files.</para>
 211
 212     <para>By default, each logged in user will get their own set of
 213     journal files in <filename>/var/log/journal/</filename>. These
 214     files will not be owned by the user, however, in order to avoid
 215     that the user can write to them directly. Instead, file system
 216     ACLs are used to ensure the user gets read access only.</para>
 217
 218     <para>Additional users and groups may be granted access to journal
 219     files via file system access control lists (ACL). Distributions
 220     and administrators may choose to grant read access to all members
 221     of the <literal>wheel</literal> and <literal>adm</literal> system
 222     groups with a command such as the following:</para>
 223
 224     <programlisting># setfacl -Rnm g:wheel:rx,d:g:wheel:rx,g:adm:rx,d:g:adm:rx /var/log/journal/</programlisting>
 225
 226     <para>Note that this command will update the ACLs both for
 227     existing journal files and for future journal files created in the
 228     <filename>/var/log/journal/</filename> directory.</para>
 229   </refsect1>
 230
 231   <refsect1>
 232     <title>Files</title>
 233
 234     <variablelist>
 235       <varlistentry>
 236         <term><filename>/etc/systemd/journald.conf</filename></term>
 237
 238         <listitem><para>Configure <command>systemd-journald</command> behavior. See
 239         <citerefentry><refentrytitle>journald.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
 240         </para></listitem>
 241       </varlistentry>
 242
 243       <varlistentry>
 244         <term><filename>/run/log/journal/<replaceable>machine-id</replaceable>/*.journal</filename></term>
 245         <term><filename>/run/log/journal/<replaceable>machine-id</replaceable>/*.journal~</filename></term>
 246         <term><filename>/var/log/journal/<replaceable>machine-id</replaceable>/*.journal</filename></term>
 247         <term><filename>/var/log/journal/<replaceable>machine-id</replaceable>/*.journal~</filename></term>
 248
 249         <listitem><para><command>systemd-journald</command> writes entries to files in
 250         <filename>/run/log/journal/<replaceable>machine-id</replaceable>/</filename>
 251         or
 252         <filename>/var/log/journal/<replaceable>machine-id</replaceable>/</filename>
 253         with the <literal>.journal</literal> suffix. If the daemon is
 254         stopped uncleanly, or if the files are found to be corrupted,
 255         they are renamed using the <literal>.journal~</literal>
 256         suffix, and <command>systemd-journald</command> starts writing
 257         to a new file. <filename>/run</filename> is used when
 258         <filename>/var/log/journal</filename> is not available, or
 259         when <option>Storage=volatile</option> is set in the
 260         <citerefentry><refentrytitle>journald.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 261         configuration file.</para>
 262
 263         <para>When <filename>systemd-journald</filename> ceases writing to a journal file,
 264         it will be renamed to <literal><replaceable>original-name</replaceable>@<replaceable>suffix.journal</replaceable></literal>
 265         (or <literal><replaceable>original-name</replaceable>@<replaceable>suffix.journal~</replaceable></literal>).
 266         Such files are "archived" and will not be written to any more.</para>
 267
 268         <para>In general, it is safe to read or copy any journal file (active or archived).
 269         <citerefentry><refentrytitle>journalctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 270         and the functions in the
 271         <citerefentry><refentrytitle>sd-journal</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 272         library should be able to read all entries that have been fully written.</para>
 273
 274         <para><filename>systemd-journald</filename> will automatically remove the oldest
 275         archived journal files to limit disk use. See <varname>SystemMaxUse=</varname>
 276         and related settings in
 277         <citerefentry><refentrytitle>journald.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
 278         </para></listitem>
 279       </varlistentry>
 280
 281       <varlistentry>
 282         <term><filename>/dev/kmsg</filename></term>
 283         <term><filename>/dev/log</filename></term>
 284         <term><filename>/run/systemd/journal/dev-log</filename></term>
 285         <term><filename>/run/systemd/journal/socket</filename></term>
 286         <term><filename>/run/systemd/journal/stdout</filename></term>
 287
 288         <listitem><para>Sockets and other paths that
 289         <command>systemd-journald</command> will listen on that are
 290         visible in the file system. In addition to these, journald can
 291         listen for audit events using netlink.</para></listitem>
 292       </varlistentry>
 293     </variablelist>
 294   </refsect1>
 295
 296   <refsect1>
 297     <title>See Also</title>
 298     <para>
 299       <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 300       <citerefentry><refentrytitle>journalctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 301       <citerefentry><refentrytitle>journald.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 302       <citerefentry><refentrytitle>systemd.journal-fields</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
 303       <citerefentry><refentrytitle>sd-journal</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 304       <citerefentry><refentrytitle>systemd-coredump</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 305       <citerefentry project='die-net'><refentrytitle>setfacl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 306       <citerefentry><refentrytitle>sd_journal_print</refentrytitle><manvolnum>4</manvolnum></citerefentry>,
 307       <command>pydoc systemd.journal</command>
 308     </para>
 309   </refsect1>
 310
 311 </refentry>