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 the kernel, from user processes via the libc syslog(3) call, from standard input and standard error of system services or via its native API. It 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. All objects stored in the journal can be up to 2^64-1 bytes in size.
By default, the journal stores log data in /run/log/journal/. Since /run/ is volatile, log data is lost at reboot. To make the data persistent, it is sufficient to create /var/log/journal/ where systemd-journald will then store the data.
systemd-journald will forward all received log messages to the AF_UNIXSOCK_DGRAM socket /run/systemd/journal/syslog, if it exists, which may be used by Unix syslog daemons to process the data further.
See journald.conf(5) for information about the configuration of this service.
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=
See journald.conf(5) for information about these settings.
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 her/him to read the journal files.
By default, each logged in user will get her/his own set of journal files in /var/log/journal/. These 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/
/run/log/journal/machine-id/*.journal, /run/log/journal/machine-id/*.journal~, /var/log/journal/machine-id/*.journal, /var/log/journal/machine-id/*.journal~