Introduction to syslog-ng

syslog-ng reference manual
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# Chapter 1. Introduction to syslog-ng

One of the most neglected area of Unix is handling system events. Daily checks for system messages is crucial for the security and health conditions of a computer system.

System logs contain much "noise" - messages which have no importance - and on the contrary important events, which should not be lost in the load of messages. With current tools it's difficult to select which messages we are interested in.

A message is sent to different destinations based on the assigned facility/priority pair. There are 12+8 (12 real and 8 local) predefined facilities (mail, news, auth etc.), and 8 different priorities (ranging from alert to debug).

One problem is that there are facilities which are too general (daemon), and these facilities are used by many programs, even if they do not relate each other. It is difficult to find the interesting bits from the enourmous amount of messages.

A second problem is that there are very few programs which allow setting their "facility code" to log under. It's at best a compile time parameter.

So using facilities as a means of filtering is not the best way. For it to be a good solution would require runtime option for all applications, which specifies the log facility to log under, and the ability to create new facilities in syslogd. Neither of these are available, and the first is neither feasible.

One of the design principles of syslog-ng was to make message filtering much more finegrained. syslog-ng is able to filter messages based on the contents of messages in addition to the priority/facility pair. This way only the messages we are really interested in get to a specific destination. Another design principle was to make logforwarding between firewalled segments easier: long hostname format, which makes it easy to find the originating and chain of forwarding hosts even if a log message traverses several computers. And last principle was a clean and powerful configuration file format.

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