We like AWStats for general log monitoring, but AWStats doesn't do security log analysis -- for that we use
Splunk. If you haven't heard of Splunk, stop reading and check it out; it is simply unmatched as a security log analysis tool. Grab traps, alerts, and syslog and SNMP data, and once you have the data, Splunk lets you graph and search it quickly via a simple, browser-like interface. In addition to helping you spot potential threats and dangerous trends, Splunk can aid compliance efforts, get alerts on thresholds you set, and generate nice reports of your findings.
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