Both Snort and OSSEC are open source IDSs. Snort is a network-based intrusion detection system (NIDS) and OSSEC is a host-based intrusion detection system (HIDS). The key difference between the approaches of Snort and OSSEC is that the NIDS methods of Snort work on data as it passes through the network. The HIDS system of OSSEC examines the log files on computers around the network to look for unexpected events. Both Snort and OSSEC are leading IDSs.