Basic syslog messages from the ASA lack much of the data that Stealthwatch Cloud (SWC) needs to determine if events reported by the ASA indicate a threat. Netflow Secure Event Logging (NSEL) provides the SWC with that data.
"A flow is defined as a unidirectional sequence of packets with some common properties that pass through a network device. These collected flows are exported to an external device, the NetFlow collector. Network flows are highly granular; for example, flow records include details such as IP addresses, packet and byte counts, timestamps, Type of Service (ToS), application ports, input and output interfaces, etc."1
The Cisco ASA supports NetFlow Version 9 services. The ASA implementation of NSEL provides a stateful, IP flow tracking method that exports only those records that indicate significant events in a flow. In stateful flow tracking, tracked flows go through a series of state changes.
This documentation describes a straight forward approach to configuring NetFlow for your ASAs using a CDO macro. The Cisco ASA NetFlow Implementation Guide provides an extremely detailed discussion of configuring NetFlow on the ASA and you may find it a valuable resource to accompany this content.
What to do Next
Go to Configuring NSEL for ASA Devices Using a CDO Macro.
- Configuring NSEL for ASA Devices Using a CDO Macro
- Delete NetFlow Secure Event Logging (NSEL) Configuration from an ASA
- Determine the Name of an ASA Global Policy
1. ("Cisco Systems NetFlow Services Export Version 9." Internet Engineering Task Force, Network Working Group, Request for Comments: 3954, October 2004, B. Claise, Ed. https://www.ietf.org/rfc/rfc3954.txt)