Processes and the operating system kernel record a log of events that happen. These logs are used to audit the system and troubleshoot problems. Many systems record logs of events in text ﬁles that are kept in the /var/log directory. These logs can be inspected using normal text utilities such as less and tail.
A standard logging system based on the Syslog protocol is built into CentOS/RHEL. Many programs use this system to record events and organize them into log ﬁles. The systemd-journald and rsyslog services handle the syslog messages in CentOS/RHEL.
The systemd-journald service is at the heart of the operating system event logging architecture. It collects event messages from many sources including the kernel, output from the early stages of the boot process, standard output, and standard error from daemons as they start up and run, and syslog events. It then restructures them into a standard format and writes them into a structured, indexed system journal. By default, this journal is stored on a ﬁle system that does not persist across reboots.
However, the rsyslog service reads syslog messages received by systemd-journald from the journal as they arrive. It then processes the syslog events, recording them to its log ﬁles or forwarding them to other services according to its own conﬁguration. The rsyslog service sorts and writes syslog messages to the log ﬁles that do persist across reboots in /var/log. The rsyslog service sorts the log messages to speciﬁc log ﬁles based on the type of program that sent each message, or facility, and the priority of each syslog message.
In addition to syslog message ﬁles, the /var/log directory contains log ﬁles from other services on the system. The following table lists some useful ﬁles in the /var/log directory.
|LOG FILE||TYPE OF MESSAGES STORED|
|/var/log/messages||Most syslog messages are logged here. Exceptions include messages related to authentication and email processing, scheduled job execution, and those which are purely debugging- related.|
|/var/log/secure||Syslog messages related to security and authentication events.|
|/var/log/maillog||Syslog messages related to the mail server.|
|/var/log/cron||Syslog messages related to scheduled job execution.|
|/var/log/boot.log||Non-syslog console messages related to system startup.|
Overview of Syslog Priorities, rsyslog rules, log rotation and basic troubleshooting using syslog
How to use journalctl to view log messages in journal for system troubleshooting
How to configure the system journal to preserve the record of events when a server is rebooted (CentOS/RHEL)