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NAME | SYNOPSIS | DESCRIPTION | STREAM LOGGING | JOURNAL NAMESPACES | SIGNALS | CREDENTIALS | KERNEL COMMAND LINE | ACCESS CONTROL | FILES | SEE ALSO | NOTES | COLOPHON |
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SYSTEMD-....SERVICE(8) systemd-journald.service SYSTEMD-....SERVICE(8)
systemd-journald.service, systemd-journald.socket, systemd-
journald-dev-log.socket, systemd-journald-audit.socket, systemd-
[email protected], [email protected], systemd-journald-
[email protected], systemd-journald - Journal service
systemd-journald.service
systemd-journald.socket
systemd-journald-dev-log.socket
systemd-journald-audit.socket
[email protected]
[email protected]
[email protected]
/usr/lib/systemd/systemd-journald
systemd-journald is a system service that collects and stores
logging data. It creates and maintains structured, indexed
journals based on logging information that is received from a
variety of sources:
• Kernel log messages, via kmsg
• Simple system log messages, via the libc syslog(3) call
• Structured system log messages via the native Journal API,
see sd_journal_print(3) and Native Journal Protocol[1]
• Standard output and standard error of service units. For
further details see below.
• Audit records, originating from the kernel audit subsystem
The daemon will implicitly collect numerous metadata fields for
each log messages in a secure and unfakeable way. See
systemd.journal-fields(7) for more information about the
collected metadata.
Log data collected by the journal is primarily text-based but can
also include binary data where necessary. Individual fields
making up a log record stored in the journal may be up to 2⁶⁴-1
bytes in size.
The journal service stores log data either persistently below
/var/log/journal or in a volatile way below /run/log/journal/ (in
the latter case it is lost at reboot). By default, log data is
stored persistently if /var/log/journal/ exists during boot, with
an implicit fallback to volatile storage otherwise. Use Storage=
in journald.conf(5) to configure where log data is placed,
independently of the existence of /var/log/journal/.
Note that journald will initially use volatile storage, until a
call to journalctl --flush (or sending SIGUSR1 to journald) will
cause it to switch to persistent logging (under the conditions
mentioned above). This is done automatically on boot via
"systemd-journal-flush.service".
On systems where /var/log/journal/ does not exist yet but where
persistent logging is desired (and the default journald.conf is
used), it is sufficient to create the directory, and ensure it
has the correct access modes and ownership:
mkdir -p /var/log/journal
systemd-tmpfiles --create --prefix /var/log/journal
See journald.conf(5) for information about the configuration of
this service.
The systemd service manager invokes all service processes with
standard output and standard error connected to the journal by
default. This behaviour may be altered via the
StandardOutput=/StandardError= unit file settings, see
systemd.exec(5) for details. The journal converts the log byte
stream received this way into individual log records, splitting
the stream at newline ("\n", ASCII 10) and NUL bytes.
If systemd-journald.service is stopped, the stream connections
associated with all services are terminated. Further writes to
those streams by the service will result in EPIPE errors. In
order to react gracefully in this case it is recommended that
programs logging to standard output/error ignore such errors. If
the SIGPIPE UNIX signal handler is not blocked or turned off,
such write attempts will also result in such process signals
being generated, see signal(7). To mitigate this issue, systemd
service manager explicitly turns off the SIGPIPE signal for all
invoked processes by default (this may be changed for each unit
individually via the IgnoreSIGPIPE= option, see systemd.exec(5)
for details). After the standard output/standard error streams
have been terminated they may not be recovered until the services
they are associated with are restarted. Note that during normal
operation, systemd-journald.service stores copies of the file
descriptors for those streams in the service manager. If
systemd-journald.service is restarted using systemctl restart or
equivalent operation instead of a pair of separate systemctl stop
and systemctl start commands (or equivalent operations), these
stream connections are not terminated and survive the restart. It
is thus safe to restart systemd-journald.service, but stopping it
is not recommended.
Note that the log record metadata for records transferred via
such standard output/error streams reflect the metadata of the
peer the stream was originally created for. If the stream
connection is passed on to other processes (such as further child
processes forked off the main service process), the log records
will not reflect their metadata, but will continue to describe
the original process. This is different from the other logging
transports listed above, which are inherently record based and
where the metadata is always associated with the individual
record.
In addition to the implicit standard output/error logging of
services, stream logging is also available via the systemd-cat(1)
command line tool.
Currently, the number of parallel log streams systemd-journald
will accept is limited to 4096. When this limit is reached
further log streams may be established but will receive EPIPE
right from the beginning.
Journal 'namespaces' are both a mechanism for logically isolating
the log stream of projects consisting of one or more services
from the rest of the system and a mechanism for improving
performance. Multiple journal namespaces may exist
simultaneously, each defining its own, independent log stream
managed by its own instance of systemd-journald. Namespaces are
independent of each other, both in the data store and in the IPC
interface. By default only a single 'default' namespace exists,
managed by systemd-journald.service (and its associated socket
units). Additional namespaces are created by starting an instance
of the [email protected] service template. The instance
name is the namespace identifier, which is a short string used
for referencing the journal namespace. Service units may be
assigned to a specific journal namespace through the
LogNamespace= unit file setting, see systemd.exec(5) for details.
The --namespace= switch of journalctl(1) may be used to view the
log stream of a specific namespace. If the switch is not used the
log stream of the default namespace is shown, i.e. log data from
other namespaces is not visible.
Services associated with a specific log namespace may log via
syslog, the native logging protocol of the journal and via
stdout/stderr; the logging from all three transports is
associated with the namespace.
By default only the default namespace will collect kernel and
audit log messages.
The systemd-journald instance of the default namespace is
configured through /etc/systemd/journald.conf (see below), while
the other instances are configured through
/etc/systemd/journald@NAMESPACE.conf. The journal log data for
the default namespace is placed in /var/log/journal/MACHINE_ID
(see below) while the data for the other namespaces is located in
/var/log/journal/MACHINE_ID.NAMESPACE.
SIGUSR1
Request that journal data from /run/ is flushed to /var/ in
order to make it persistent (if this is enabled). This must
be used after /var/ is mounted, as otherwise log data from
/run/ is never flushed to /var/ regardless of the
configuration. Use the journalctl --flush command to request
flushing of the journal files, and wait for the operation to
complete. See journalctl(1) for details.
Added in version 186.
SIGUSR2
Request immediate rotation of the journal files. Use the
journalctl --rotate command to request journal file rotation,
and wait for the operation to complete.
Added in version 186.
SIGRTMIN+1
Request that all unwritten log data is written to disk. Use
the journalctl --sync command to trigger journal
synchronization, and wait for the operation to complete.
Added in version 228.
systemd-journald supports the service credentials logic as
implemented by ImportCredential=/LoadCredential=/SetCredential=
(see systemd.exec(5) for details). The following credentials are
used when passed in:
journal.forward_to_socket
May contain a socket address to which logs should be
forwarded. See ForwardToSocket= in journald.conf(5).
Added in version 256.
journal.storage
May be used to specify where journal files should be stored.
See Storage= in journald.conf(5).
Added in version 256.
A few configuration parameters from journald.conf may be
overridden on the kernel command line:
systemd.journald.forward_to_syslog=,
systemd.journald.forward_to_kmsg=,
systemd.journald.forward_to_console=,
systemd.journald.forward_to_wall=
Enables/disables forwarding of collected log messages to
syslog, the kernel log buffer, the system console or wall.
See journald.conf(5) for information about these settings.
Added in version 186.
systemd.journald.max_level_store=,
systemd.journald.max_level_syslog=,
systemd.journald.max_level_kmsg=,
systemd.journald.max_level_console=,
systemd.journald.max_level_wall=,
systemd.journald.max_level_socket=
Controls the maximum log level of messages that are stored in
the journal, forwarded to syslog, kmsg, the console, the
wall, or a socket. This kernel command line options override
the settings of the same names in the journald.conf(5) file.
Added in version 232.
Note that these kernel command line options are only honoured by
the default namespace, see above.
Journal files are, by default, owned and readable by the
"systemd-journal" system group but are not writable. Adding a
user to this group thus enables them to read the journal files.
By default, each user, with a UID outside the range of system
users, dynamic service users, and the nobody user, will get their
own set of journal files in /var/log/journal/. See Users, Groups,
UIDs and GIDs on systemd systems[2] for more details about UID
ranges. These journal files will not be owned by the user,
however, in order to avoid that the user can write to them
directly. Instead, file system ACLs are used to ensure the user
gets read access only.
Additional users and groups may be granted access to journal
files via file system access control lists (ACL). Distributions
and administrators may choose to grant read access to all members
of the "wheel" and "adm" system groups with a command such as the
following:
# setfacl -Rnm g:wheel:rx,d:g:wheel:rx,g:adm:rx,d:g:adm:rx /var/log/journal/
Note that this command will update the ACLs both for existing
journal files and for future journal files created in the
/var/log/journal/ directory.
/etc/systemd/journald.conf
Configure systemd-journald behavior. See journald.conf(5).
Added in version 206.
/run/log/journal/machine-id/*.journal,
/run/log/journal/machine-id/*.journal~,
/var/log/journal/machine-id/*.journal,
/var/log/journal/machine-id/*.journal~
systemd-journald writes entries to files in
/run/log/journal/machine-id/ or /var/log/journal/machine-id/
with the ".journal" suffix. If the daemon is stopped
uncleanly, or if the files are found to be corrupted, they
are renamed using the ".journal~" suffix, and
systemd-journald starts writing to a new file. /run/ is used
when /var/log/journal is not available, or when
Storage=volatile is set in the journald.conf(5) configuration
file.
When systemd-journald ceases writing to a journal file, it
will be renamed to "original-name@suffix.journal" (or
"original-name@suffix.journal~"). Such files are "archived"
and will not be written to any more.
In general, it is safe to read or copy any journal file
(active or archived). journalctl(1) and the functions in the
sd-journal(3) library should be able to read all entries that
have been fully written.
systemd-journald will automatically remove the oldest
archived journal files to limit disk use. See SystemMaxUse=
and related settings in journald.conf(5).
Added in version 206.
/dev/kmsg, /dev/log, /run/systemd/journal/dev-log,
/run/systemd/journal/socket, /run/systemd/journal/stdout
Sockets and other file node paths that systemd-journald will
listen on and are visible in the file system. In addition to
these, systemd-journald can listen for audit events using
netlink(7), depending on whether
"systemd-journald-audit.socket" is enabled or not.
Added in version 228.
If journal namespacing is used these paths are slightly altered
to include a namespace identifier, see above.
systemd(1), journalctl(1), journald.conf(5),
systemd.journal-fields(7), sd-journal(3), systemd-coredump(8),
setfacl(1), sd_journal_print(3), pydoc systemd.journal
1. Native Journal Protocol
https://systemd.io/JOURNAL_NATIVE_PROTOCOL
2. Users, Groups, UIDs and GIDs on systemd systems
https://systemd.io/UIDS-GIDS
This page is part of the systemd (systemd system and service
manager) project. Information about the project can be found at
⟨http://www.freedesktop.org/wiki/Software/systemd⟩. If you have
a bug report for this manual page, see
⟨http://www.freedesktop.org/wiki/Software/systemd/#bugreports⟩.
This page was obtained from the project's upstream Git repository
⟨https://github.com/systemd/systemd.git⟩ on 2024-06-14. (At that
time, the date of the most recent commit that was found in the
repository was 2024-06-13.) If you discover any rendering
problems in this HTML version of the page, or you believe there
is a better or more up-to-date source for the page, or you have
corrections or improvements to the information in this COLOPHON
(which is not part of the original manual page), send a mail to
[email protected]
systemd 257~devel SYSTEMD-....SERVICE(8)
Pages that refer to this page: coredumpctl(1), journalctl(1), systemctl(1), systemd-cat(1), sd-journal(3), sd_journal_open(3), sd_journal_print(3), sd_journal_stream_fd(3), coredump.conf(5), journald.conf(5), journal-remote.conf(5), journal-upload.conf(5), org.freedesktop.LogControl1(5), pstore.conf(5), systemd.exec(5), daemon(7), kernel-command-line(7), systemd.directives(7), systemd.index(7), systemd.journal-fields(7), systemd.system-credentials(7), systemd-coredump(8), systemd-journal-gatewayd.service(8), systemd-journal-remote.service(8), systemd-journal-upload.service(8)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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