Description:
===========
Snort now supports multiple configurations based on VLAN Id or IP subnet within 
a single instance of Snort. This will allow administrators to specify multiple 
snort configuration files and bind each configuration to one or more VLANs or 
subnets rather than running one Snort for each configuration required. Each 
unique snort configuration file will create a new configuration instance within 
snort. VLANs/Subnets not bound to any specific configuration will use the 
default configuration. Each configuration can have different preprocessor 
settings and detection rules.

Creating Multiple Configurations:
================================
Default configuration for snort is specified using the existing -c option. A 
default configuration binds multiple vlans or networks to non-default 
configurations, using the following configuration line:

config binding: <path_to_snort.conf> vlan <vlanIdList>
config binding: <path_to_snort.conf> net <ipList>

<path_to_snort.conf> : Refers to the absolute or relative path to the snort.conf
for specific configuration.

<vlanIdList>         : Refers to the comma seperated list of vlandIds and vlanId
ranges.  The format for ranges is two vlanId separated by a "-". Spaces are 
allowed within ranges.  Valid vlanId is any number in 0-4095 range. Negative 
vland Ids and alphanumeric are not supported.

<ipList>	     : Refers to ip subnets. Subnets can be CIDR blocks for IPV6
or IPv4.  A maximum of 512 individual IPv4 or IPv6 addresses or CIDRs can be
specified.

NOTE: Vlan and Subnets can not be used in the same line. Configurations can be 
applied based on either Vlans or Subnets not both. 

Configuration Specific Elements:
===============================

Config Options:
==============

Generally config options defined within the default configuration are global by 
default i.e. their value applies to all other configurations. The following 
config options are specific to each configuration.
policy_id
policy_mode
policy_version  

The following config options are specific to each configuration. If not defined 
in a configuration, the default values of the option (not the default 
configuration values) take effect.

config checksum_drop
config disable_decode_alerts
config disable_decode_drops
config disable_ipopt_alerts
config disable_ipopt_drops
config disable_tcpopt_alerts
config disable_tcpopt_drops
config disable_tcpopt_experimental_alerts
config disable_tcpopt_experimental_drops
config disable_tcpopt_obsolete_alerts
config disable_tcpopt_obsolete_drops
config disable_ttcp_alerts
config disable_tcpopt_ttcp_alerts
config disable_ttcp_drops

Rules:
=====
Rules are specific to configurations but only some parts of a rule can be 
customized for performance reasons. If a rule is not specified in a 
configuration then the rule will never raise an event for the configuration. 
A rule shares all parts of the rule options, including the general options, 
payload detection options, non-payload detection options, and post-detection 
options.  Parts of the rule header can be specified differently across 
configurations, limited to: Source IP address and port
    Destination IP address and port
    Action

A higher revision of a rule in one configuration will override other revisions 
of the same rule in other configurations.

Variables:
==========
Variables defined using "var", "portvar" and "ipvar" are specific to 
configurations.  If the rules in a configuration use variables, those variables 
must be defined in that configuration. 

Preprocessors:
==============
Preprocessors configurations can be defined within each vlan or subnet specific 
configuration.  Options controlling specific preprocessor memory usage, through 
specific limit on memory usage or number of instances, are processed only in 
default policy. The options control total memory usage for a preprocessor across
all policies.  These options are ignored in non-default policies without raising
an error. A preprocessor must be configured in default configuration before it 
can be configured in non-default configuration. This is required as some 
mandatory preprocessor configuration options are processed only in default 
configuration.

Events and Output:
==================
An unique policy id can be assigned by user, to each configuration using the 
following config line:

config policy_id: <id>

<id>		     : Refers to a 16-bit unsigned value. This policy id will 
be used to identify alerts from a specific configuration in the unified2 records.

NOTE: If no policy id is specified, snort assigns 0 (zero) value to the 
configuration.

To enable vlanId logging in unified2 records the following 
options can be used.

output alert_unified2: vlan_event_types (alert logging only)
output unified2: filename <filename>, vlan_event_types (true unified logging)

<filename>	     : Refers to the absolute or relative filename.

vlan_event_types     : When this option is set, snort will use unified2 event 
type 104 and 105 for IPv4 and IPv6 respectively. 

Events 

NOTE: Each event logged will have the vlanId from the packet if vlan headers 
are present otherwise 0 will be used.

How Configuration is applied?
============================
Snort assigns every incoming packet to a unique configuration based on the 
following criteria.  If VLANID is present, then the innermost VLANID is used 
to find bound configuration. If the bound configuration is the default 
configuration, then destination IP address is searched to the most specific 
subnet that is bound to a non-default  configuration. The packet is assigned 
non-default configuration if found otherwise the check is repeated using 
source IP address.  In the end, default configuration is used if no other 
matching configuration is found.

For addressed based configuration binding, this can lead to conflicts between 
configurations if source address is bound to one configuration and destination 
address is bound to another.  In this case, snort will use the first 
configuration in the order of definition, that can be applied to the packet.