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SYSTEMD-JOURNALD.SERVICE(8) systemd-journald.service SYSTEMD-JOURNALD.SERVICE(8)
NAME
systemd-journald.service, systemd-journald.socket, systemd-journald-dev-log.socket,
systemd-journald-audit.socket, systemd-journald@.service, systemd-journald@.socket,
systemd-journald-varlink@.socket, systemd-journald - Journal service
SYNOPSIS
systemd-journald.service
systemd-journald.socket
systemd-journald-dev-log.socket
systemd-journald-audit.socket
systemd-journald@.service
systemd-journald@.socket
systemd-journald-varlink@.socket
/lib/systemd/systemd-journald
DESCRIPTION
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:
o Kernel log messages, via kmsg
o Simple system log messages, via the libc syslog(3) call
o Structured system log messages via the native Journal API, see sd_journal_print(3) and
Native Journal Protocol[1]
o Standard output and standard error of service units. For further details see below.
o 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 2^64-1 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.
STREAM LOGGING
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
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 systemd-journald@.service 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 AT 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.
SIGNALS
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.
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.
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.
KERNEL COMMAND LINE
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.
Note that these kernel command line options are only honoured by the default namespace,
see above.
ACCESS CONTROL
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.
FILES
/etc/systemd/journald.conf
Configure systemd-journald behavior. See journald.conf(5).
/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 AT suffix.journal" (or "original-name AT 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).
/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).
If journal namespacing is used these paths are slightly altered to include a namespace
identifier, see above.
SEE ALSO
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
NOTES
1. Native Journal Protocol
https://systemd.io/JOURNAL_NATIVE_PROTOCOL
2. Users, Groups, UIDs and GIDs on systemd systems
https://systemd.io/UIDS-GIDS
systemd 249 SYSTEMD-JOURNALD.SERVICE(8)
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