NetFlow Exporters support versions IPFIX, v5, and v9. Starting in software version 5.3, the Common Event Format (CEF) version 23 is also supported. CEF is a standard format used by event collection/correlation Security Information and Event Management (SIEM) vendors. SIEMs such as Arcsight, Splunk, and QRadar accept CEF format. By supporting CEF, NetFlow metadata can integrate with and use a variety of SIEMs.
CEF is a logging format that uses the syslog message as a transport mechanism, meaning that the CEF message (header and payload) is included within the syslog message. The transport protocol that is supported is UDP and the default port number is 514.
Metadata that is generated by NetFlow can be exported in the supported formats to one or more collectors. Each exporter must have the same export type (v5, v9, IPFIX, or CEF). One CEF message is sent out per record per flow.
Also, starting in software version 5.3, IP fragmentation is supported. CEF does not allow a message to be split over multiple CEF payloads. Since CEF messages are verbose, they can be larger than the MTU.
To support CEF messages that exceed the MTU, a single IP datagram containing a CEF message will be broken up into multiple packets of smaller sizes. The reassembly of the datagram will occur at the receiving end (at the SIEMs).
For details on the CEF message format, refer to CEF Message Format.
An example of the CEF message format is as follows:
Fri Feb 23 02:25:37 2018 9/3/e1 CEF:23|Gigamon|metadata|5.3.00|4|metadatageneration|6| src=220.127.116.11 GigamonMdataDnsAdditionalType=41GigamonMdataDnsAdditionalTypeText=OPT
In the example CEF message, there is a syslog header, a CEF header, and anextension that contains the CEF payload. The fields are delimited with a vertical bar (|).
The syslog header contains the following:
|timestamp—Fri Feb 23 02:25:37 2018
|host name identifier—9/3/e1
Note: The host name identifier has the format <box ID>/<slot ID>/<engine ID>.For example, 9/3/e1 means 9 is the box ID, 3 is the slot ID, and e1 is the engine ID.
The CEF header contains the following:
The CEF extension contains key-value pairs delimited with a space. In the example CEF message, the following is the CEF payload, in plaintext:
The CEF standard specifies key-value pairs. There are some predefined standardkeys, for example, src is a predefined key for source IP address.
For keys that are not predefined in the CEF standard, such as the NetFlow metadata elements in the CEF extension, there are custom-defined keys. Custom-defined keys have the following format:
For example, GigamonMdataDnsAdditionalTypeText, is a custom-defined key that contains the following:
Another example of the CEF format is the following SSL record:
Thu Mar 1 08:21:28 2018 1/1/e1 CEF:23|Gigamon|metadata|5.3.00|4|metadata
generation|6|GigamonMdataSslIssuerName=DigiCert SHA2 High Assurance S
52053657276 6572204341 GigamonMdataSslCertSubCommonName=*.stickyadstv.com
5776865656c204d6564696120496e632f4f553d46726565776865656c2f434e3d2a2e737469636b796164737 4762e636f6d dst=10.50.22.59 src=18.104.22.168