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: vlan
config binding: net
config binding: policy_id
: Refers to the absolute or relative path to the snort.conf
for specific configuration.
: Refers to the comma separated 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.
: 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
: Refers to the comma separated list of 16bit policyIds
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:
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.
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.
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
A higher revision of a rule in one configuration will override other revisions
of the same rule in other configurations.
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 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
Events and Output:
An unique policy id can be assigned by user, to each configuration using the
following config line:
: 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
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 , vlan_event_types (true unified logging)
: 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.
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.