Cisco ASA with FirePOWER Services Local Management Configuration Guide, Version 6.6
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DNS-based Security Intelligence allows you to block traffic, or exempt it from blocking, based on the domain name requested
by a client. Cisco provides domain name intelligence you can use to filter your traffic; you can also configure custom lists
and feeds of domain names tailored to your deployment. DNS-based Security Intelligence filtering takes place after hardware-level
handling and traffic decryption, and before most other policy-based inspection, analysis, or traffic handling.
Traffic blocked by a DNS policy is immediately blocked and therefore is not subject to any further inspection—not for intrusions,
exploits, malware, and so on. You can override blocking by adding it to a whitelist, to force access control rule evaluation. Also, recommended in passive deployments, you can use a “monitor-only” setting
for Security Intelligence filtering. This allows the ASA FirePOWER module to analyze connections that would have been blocked,
but also logs the match to the blacklist and generates an end-of-connection security intelligence event.
You configure DNS-based Security Intelligence using a DNS policy and associated DNS rules. To deploy it, you must associate
your DNS policy with an access control policy, then deploy your configuration.
DNS Policy Components
A DNS policy allows you to block (or exempt from blocking) domain name-based connections. The following list describes the
configurations you can change after creating a DNS policy.
Name and Description
Each DNS policy must have a unique name. A description is optional.
Rules provide a granular method of handling network traffic based on the domain name. Rules in a DNS policy are numbered,
starting at 1. The ASA FirePOWER module matches traffic to DNS rules in top-down order by ascending rule number.
When you create a DNS policy, the ASA FirePOWER module populates it with a default Global DNS Whitelist rule, and a default Global DNS Blacklist rule. Each rule is fixed to the first position in their respective categories. You cannot modify these rules, but you can
disable them. The module evaluates rules in the following order:
Global DNS Whitelist rule (if enabled)
Global DNS Blacklist rule (if enabled)
Blacklist and monitor rules
Usually, the module handles domain name-based network traffic according to the first DNS rule where all the rule’s conditions
match the traffic. If no DNS rules match the traffic, the module continues evaluating the traffic based on the associated
access control policy's rules. DNS rule conditions can be simple or complex.
Editing a DNS Policy
Only one person should edit a DNS policy at a time, using a single browser window. If multiple users attempt to save the same
policy, only the first set of saved changes are retained.
To protect the privacy of your session, after thirty minutes of inactivity on the policy editor, a warning appears. After
sixty minutes, the module discards your changes.
DNS rules handle traffic based on the domain name requested by a host. As part of Security Intelligence, this evaluation happens
after any traffic decryption, and before access control evaluation.
The ASA FirePOWER module matches traffic to DNS rules in the order you specify. In most cases, the module handles network
traffic according to the first DNS rule where all the rule’s conditions match the traffic. When you create DNS rules, the
module places whitelist rules before monitor and blacklist rules, and evaluates traffic against whitelist rules first.
In addition to its unique name, each DNS rule has the following basic components:
By default, rules are enabled. If you disable a rule, the ASA FirePOWER module does not use it to evaluate network traffic,
and stops generating warnings and errors for that rule.
Rules in a DNS policy are numbered, starting at 1. The ASA FirePOWER module matches traffic to rules in top-down order by
ascending rule number. With the exception of Monitor rules, the first rule that traffic matches is the rule that handles that
Conditions specify the specific traffic the rule handles. A DNS rule must contain a DNS feed or list condition, and can also
match traffic by security zone or network.
A rule’s action determines how the ASA FirePOWER module handles matching traffic:
Traffic on a whitelist is allowed, subject to further access control inspection.
Monitored traffic is subject to further evaluation by remaining DNS blacklist rules. If the traffic does not match a DNS blacklist rule, it is inspected with access control rules. The module logs a Security Intelligence event for the traffic.
Blocked traffic is dropped without further inspection. You can also return a Domain Not Found response, or redirect the DNS
query to a sinkhole server.
Creating and Editing DNS Rules
In a DNS policy, you can add up to a total of 32767 DNS lists to the whitelist and blacklist rules. That is, the number of lists in the DNS policy cannot exceed 32767.
The Rules tab of the DNS policy editor allows you to add, edit, move, enable, disable, delete, and otherwise manage DNS rules within
For each rule, the policy editor displays its name, a summary of its conditions, and the rule action. Other icons represent
warnings, errors, and other important information. Disabled rules are dimmed and marked (
) beneath the rule name.
Enabling and Disabling DNS Rules
When you create a DNS rule, it is enabled by default. If you disable a rule, the ASA FirePOWER module does not use it to evaluate
network traffic and stops generating warnings and errors for that rule. When viewing the list of rules in a DNS policy, disabled
rules are dimmed, although you can still modify them. Note that you can also enable or disable a DNS rule using the DNS rule
Rules in a DNS policy are numbered, starting at 1. The ASA FirePOWER module matches traffic to DNS rules in top-down order
by ascending rule number. In most cases, the module handles network traffic according to the first DNS rule where all the
rule’s conditions match the traffic:
For Monitor rules, the module logs the traffic, then continues evaluating traffic against lower-priority DNS blacklist rules.
For non-Monitor rules, the module does not continue to evaluate traffic against additional, lower-priority DNS rules after
that traffic matches a rule.
Note the following regarding rule order:
The Global Whitelist is always first, and takes precedence over all other rules.
The Whitelist section precedes the Blacklist section; whitelist rules always take precedence over other rules.
The Global Blacklist is always first in the Blacklist section, and takes precedence over all other Monitor and blacklist rules.
The Blacklist section contains Monitor and blacklist rules.
When you first create a DNS rule, the module positions it last in the Whitelist section if you assign a Whitelist action, or last in the Blacklist section if you assign any other action.
You can drag and drop rules to reorder them, and change the evaluation order.
DNS Rule Actions
Every DNS rule has an action
that determines the following for matching traffic:
handling—foremost, the rule action governs whether the module will monitor or block traffic that matches the rule’s conditions,
or simply allow it to pass to the next step in processing.
logging—the rule action determines when and how you can log details about matching traffic
Keep in mind that only devices deployed inline can block traffic. Devices deployed passively can pass and log, but not affect,
The Whitelist action allows matching traffic to pass. If you choose this option, traffic is subject to further inspection either by a matching
access control rule, or by the access control policy's default action.
The module does not log whitelist matches. Logging of these connections depends on their eventual disposition.
The Monitor action does not affect traffic flow; matching traffic is neither immediately passed nor blocked. Rather, traffic is matched
against additional rules to determine whether to permit or deny it. The first non-Monitor DNS rule matched determines whether
the module blocks the traffic. If there are no additional matching rules, the traffic is subject to access control evaluation.
For connections monitored by a DNS policy, the ASA FirePOWER module logs end-of-connection Security Intelligence and connection
These actions block traffic without further inspection of any kind:
The Drop action drops the traffic.
The Domain Not Found action returns a non-existent internet domain response to the DNS query, which prevents the client from resolving the DNS
The Sinkhole action returns a sinkhole object's IPv4 or IPv6 address in response to the DNS query. The sinkhole server can log, or log
and block, follow-on connections to the IP address. If you configure a Sinkhole action, you must also configure a sinkhole object.
For a connection blocked based on the Drop or Domain Not Found actions, the module logs beginning-of-connection Security Intelligence and connection events. Because blocked traffic is
immediately denied without further inspection, there is no unique end of connection to log.
For a connection blocked based on the Sinkhole action, logging depends on the sinkhole object configuration. If you configure your sinkhole object to only log sinkhole
connections, the module logs end-of-connection connection events for the follow-on connection. If you configure your sinkhole
object to log and block sinkhole connections, the module logs beginning-of-connection connection events for the follow-on
connection, then blocks that connection.
DNS Rule Conditions
A DNS rule’s conditions identify the type of traffic that rule handles. Conditions can be simple or complex. You must define
a DNS feed or list condition. You can additionally control traffic by security zone or network.
When adding conditions to a DNS rule:
If you do not configure a particular condition for a rule, the module does not match traffic based on that criterion.
You can configure multiple conditions per rule. Traffic must match all the conditions in the rule for the rule to apply to traffic.
For each condition in a rule, you can add up to 50 criteria. Traffic that matches any of a condition’s criteria satisfies the condition. For example, you can use a single rule to block traffic based on up to
50 DNS lists and feeds.
Controlling Traffic Based on DNS and Security Zone
Zone conditions in DNS rules allow you to control traffic by its source and destination security zones. A security zone is
a grouping of one or more interfaces. An option you choose during a device’s initial setup, called its detection mode, determines
how the module initially configures the device’s interfaces, and whether those interfaces belong to a security zone.
To control traffic based on DNS and security zone:
In the DNS rule editor, click the Zones tab.
Find and select the zones you want to add from the Available Zones. To search for zones to add, click the Search by name prompt above the Available Zones list, then type a zone name. The list updates as you type to display matching zones.
Click to select a zone, or right-click and then select Select All.
Network conditions in DNS rules allow you to control traffic by its source IP address. You can explicitly specify the source
IP addresses for the traffic you want to control.
To control traffic based on DNS and network:
In the DNS rule editor, click the Networks tab.
Find and select the networks you want to add from the Available Networks, as follows:
To add a network object on the fly, which you can then add to the condition, click the add icon above the Available Networks list and proceed as described in Working with Network Objects.
To search for network objects to add, click the Search by name or value prompt above the Available Networks list, then type an object name or the value of one of the object’s components. The list updates as you type to display matching
Click Add to Source.
You can also drag and drop selected objects.
Add any source IP addresses or address blocks that you want to specify manually. Click the Enter an IPaddress prompt below the Source Networks list; then type an IP address or address block and click Add.
Controlling Traffic Based on DNS List, Feed, or Category
DNS conditions in DNS rules allow you to control traffic if a DNS list, feed, or category contains the domain name requested
by the client. You must define a DNS condition in a DNS rule.
Regardless of whether you add a global or custom whitelist or blacklist to a DNS condition, the ASA FirePOWER module applies the configured rule action to the traffic. For example, if you add the
Global Whitelist to a rule, and configure a Drop action, the module blocks all traffic that should have been passed for further assessment.
To control traffic based on DNS list, feed, or category:
In the DNS rule editor, click the DNS tab.
Find and select the DNS lists and feeds you want to add from the DNS Lists and Feeds, as follows:
To add a DNS list or feed on the fly, which you can then add to the condition, click the add icon above the DNS Lists and Feeds list and proceed as described in Working with the Intelligence Feed.
To search for DNS lists, feeds, or categories to add, click the Search by name or value prompt above the DNS Lists and Feeds list, then type an object name or the value of one of the object’s components. The list updates as you type to display matching