Introduction to syslog

syslogd is a ubiquitous component of UNIX systems that performs system logging activities. syslogd reads from a set of log sources such as /dev/log and /dev/klog and processes the log messages as instructed in /etc/syslog.conf. Applications log messages to syslog using the syslog() call (see syslog(3C)).

For more information on syslogd, see syslogd(1M).

syslog Message Format

A syslog message has a standard format that includes an optional priority level and facility. The priority level indicates the urgency of the message. The facility indicates the subsystem that posted the message. Table 3-1 lists the priority level and facilities defined in /usr/include/syslog.h.

Table 3-1 syslog Priority Levels

Message	Description
LOG_ALERT	Take action immediately,
LOG_CRIT	Critical conditions have occurred.
LOG_DEBUG	Debug-level message.
LOG_EMERG	System is unusable.
LOG_ERR	Error conditions.
LOG_INFO	Informational message.
LOG_NOTICE	Normal but significant conditions that warrant attention.
LOG_WARNING	Warning conditions.

Table 3-2 describes syslog Facilities Messages.

Table 3-2 syslog Facilities Messages

Message	Description
LOG_AUTH	Security and authorization messages (DEPRECATED; use LOG_AUTHPRIV instead).
LOG_AUTHPRIV	Security and authorization messages (private).
LOG_CRON	Clock daemon (cron and at).
LOG_DAEMON	System daemons without separate facility values.
LOG_FTP	Ftp daemon.
LOG_KERN	Kernel messages.
LOG_LOCAL0 through LOC_LOCAL7	Reserved for local use.
LOG_LPR	Line printer subsystem.
LOG_MAIL	Mail subsystem.
LOG_NEWS	USENET news subsystem.
LOG_SYSLOG	Messages generated internally by syslogd.
LOG_USER (default)	Generic user-level messages.
LOG_UUCP	UUCP subsystem.

Message Filtering

Using /etc/syslog.conf, messages can be filtered based on their priority level and facility. Messages can be directed to:

Specific log files
The console
A specified user. The message is sent to the user's terminal if the user is logged in.
All logged-in users
Forwarded to remote systems. For more information, see the “Log Consolidation Overview”.

For more information on configuring message filters, see the syslogd(8) manpage.

Introduction to syslog

Technical documentation

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syslog Message Format

Message Filtering