How to get the VLAN ID in snort alert?

Question

I am trying to parse snort alerts and filter necessary information such as vlan id where a malicious internal machine belong to! However, I only can get the message, source and destination IPs, I need to get the VLAN ID too.

Thanks

Answer 1

(Assuming you're using Unified2 log format)

You can get the VLAN ID logged by using the vlan_event_types parameter when specifying the alert configuration:

output alert_unified2: \
    filename <base filename> [, <limit <size in MB>] [, nostamp] [, mpls_event_types] \
    [, vlan_event_types]

When enabled and the packet contains a VLAN header, a log item will contain the following record with vlan id :

E. Unified2 IDS Event (Version 2)

 sensor id 4 bytes event id 4 bytes event second 4 bytes event microsecond 4 bytes signature id 4 bytes generator id 4 bytes signature revision 4 bytes classification id 4 bytes priority id 4 bytes ip source 4 bytes ip destination 4 bytes source port/icmp type 2 bytes dest. port/icmp code 2 bytes protocol 1 byte impact flag 1 byte impact 1 byte blocked 1 byte mpls label 4 bytes vlan id 2 bytes padding 2 bytes 

Unified2 IDS Event (Version 2) are logged for IPv4 packets which contain either MPLS or VLAN headers. Otherwise a Unified2 IDS Event is logged.

• Note that you'll need to pass –enable-mpls to configure in order to have Snort fill in the mpls label field.

• Note that you'll need to configure unified2 logging with either mpls_event_types or vlan_event_types to get this record type.

[ https://www.snort.org/faq/readme-unified2]