Tune Intrusion Policies Using Rules

This chapter provides information on custom rules in Snort 3, intrusion rule action, intrusion event notification filters in an intrusion policy, converting Snort 2 custom rules to Snort 3, and adding rule groups with custom rules to an intrusion policy.

Overview of Tuning Intrusion Rules

You can configure rule states and other settings for shared object rules, standard text rules, and inspector rules.

You enable a rule by setting its rule state to Alert or to Block. Enabling a rule causes the system to generate events on traffic matching the rule. Disabling a rule stops processing of the rule. You can also set your intrusion policy so that a rule set to Block generates events on, and drops, matching traffic.

You can filter rules to display a subset of rules, enabling you to select the exact set of rules where you want to change rule states or rule settings.

When an intrusion rule or rule argument requires a disabled inspector, the system automatically uses it with its current configuration even though it remains disabled in the network analysis policy’s web interface.

Intrusion Rule Types

An intrusion rule is a specified set of keywords and arguments that the system uses to detect attempts to exploit vulnerabilities in your network. As the system analyzes network traffic, it compares packets against the conditions specified in each rule, and triggers the rule if the data packet meets all the conditions specified in the rule.

An intrusion policy contains:

  • intrusion rules, which are subdivided into shared object rules and standard text rules

  • inspector rules, which are associated with a detection option of the packet decoder or with one of the inspectors included with the system

The following table summarizes attributes of these rule types:

Table 1. Intrusion Rule Types

Type

Generator ID (GID)

Snort ID (SID)

Source

Can Copy?

Can Edit?

shared object rule

3

lower than 1000000

Cisco Talos Intelligence Group (Talos)

yes

limited

standard text rule

1

(Global domain or legacy GID)

lower than 1000000

Talos

yes

limited

1000 - 2000

(descendant domain)

1000000 or higher

Created or imported by user

yes

yes

preprocessor rule

decoder- or preprocessor- specific

lower than 1000000

Talos

no

no

1000000 or higher

Generated by the system during option configuration

no

no

You cannot save changes to any rule created by Talos, but you can save a copy of a modified rule as a custom rule. You can modify either variables used in the rule or rule header information (such as source and destination ports and IP addresses). In a multidomain deployment, rules created by Talos belong to the Global domain. Administrators in descendant domains can save local copies of the rules, which they can then edit.

For the rules it creates, Talos assigns default rule states in each default intrusion policy. Most preprocessor rules are disabled by default and must be enabled if you want the system to generate events for preprocessor rules and, in an inline deployment, drop offending packets.

Prerequisites for Network Analysis and Intrusion Policies

To allow the Snort inspection engine to process traffic for intrusion and malware analysis, you must have the IPS license enabled for the FTD device.

You must be an Admin user to manage network analysis, intrusion policies, and perform migration tasks.

Custom Rules in Snort 3

You can create a custom intrusion rule by importing a local rule file. The rule file can either have a .txt or .rules extension. The system saves the custom rule in the local rule category, regardless of the method you used to create it. A custom rule must belong to a rule group. However, a custom rule can be a part of two or more groups as well.

When you create a custom intrusion rule, the system assigns it a unique rule number, which has the format GID:SID:Rev. The elements of this number are:

  • GID—Generator ID. For custom rules, it is not necessary to specify the GID. The system automatically generates the GID based on whether you are in the Global domain or a sub-domain while uploading the rules. For all standard text rules, this value is 2000 for a Global domain.

  • SID—Snort ID. Indicates whether the rule is a local rule of a system rule. When you create a new rule, assign a unique SID to the rule.

    SID numbers for local rules start at 1000000, and the SID for each new local rule is incremented by one.

  • Rev—The revision number. For a new rule, the revision number is one. Each time you modify a custom rule the revision number should be incremented by one.

In a custom standard text rule, you set the rule header settings and the rule keywords and arguments. You can use the rule header settings to focus the rule to only match traffic using a specific protocol and traveling to or from specific IP addresses or ports.


Note

Snort 3 custom rules cannot be edited. Ensure custom rules have a valid classification message for classtype within the rule text. If you import a rule without a classification or wrong classification, then delete and recreate the rule.

View Snort 3 Intrusion Rules in an Intrusion Policy

You can adjust how rules are displayed in the intrusion policy. You can also display the details for a specific rule to see rule settings, rule documentation, and other rule specifics.

Procedure

Step 1

Choose Policies > Intrusion.

Step 2

Click Snort 3 Version next to the policy.

Step 3

While viewing the rules, you can:

  • Filter the rules.
  • Choose a rule group to see rules related to that group.
  • View an intrusion rule’s details.
  • View rule comments.
  • View rule documentation.

See Edit Snort 3 Intrusion Policies for details on performing these tasks.

Intrusion Rule Action

Intrusion rule action allows you to enable or disable the rule within an individual intrusion policy, as well as specify which action the system takes if monitored conditions trigger the rule.

The Cisco Talos Intelligence Group (Talos) sets the default action of each intrusion and inspector rule in each default policy. For example, a rule may be enabled in the Security over Connectivity default policy and disabled in the Connectivity over Security default policy. Talos sometimes uses a rule update to change the default action of one or more rules in a default policy. If you allow rule updates to update your base policy, you also allow the rule update to change the default action of a rule in your policy when the default action changes in the default policy you used to create your policy (or in the default policy it is based on). Note, however, that if you have changed the rule action, the rule update does not override your change.

When you create an intrusion rule, it inherits the default actions of the rules in the default policy you use to create your policy.

Intrusion Rule Action Options

In an intrusion policy, you can set a rule’s action to the following values:

Alert

You want the system to detect a specific intrusion attempt and generate an intrusion event when it finds matching traffic. When a malicious packet crosses your network and triggers the rule, the packet is sent to its destination and the system generates an intrusion event. The malicious packet reaches its target, but you are notified through the event logging.

Block

You want the system to detect a specific intrusion attempt, drop the packet containing the attack, and generate an intrusion event when it finds matching traffic. The malicious packet never reaches its target, and you are notified through the event logging.

Disable

You do not want the system to evaluate matching traffic.


Note

Choosing either the Alert or Block options enables the rule. Choosing Disable disables the rule.

We strongly recommend that you do not enable all the intrusion rules in an intrusion policy. The performance of your managed device is likely to degrade if all rules are enabled. Instead, tune your rule set to match your network environment as closely as possible.

Set Intrusion Rule Action

Intrusion rule actions are policy-specific.

Procedure

Step 1

Choose Policies > Intrusion.

Step 2

Click Snort 3 Version next to the policy you want to edit.

Tip

 
This page shows the total number of:

  • disabled rules

  • enabled rules set to Alert

  • enabled rules set to Block

  • overridden rules

Step 3

Choose the rule or rules where you want to set the rule action.

Step 4

Choose one of the rule actions from the Rule Action drop-down list. See Edit Snort 3 Intrusion Policies for more information about the different rule actions.

Step 5

Click Save.

What to do next

Deploy configuration changes; see Deploy Configuration Changes.

Intrusion Event Notification Filters in an Intrusion Policy

The importance of an intrusion event can be based on frequency of occurrence, or on source or destination IP address. In some cases you may not care about an event until it has occurred a certain number of times. For example, you may not be concerned if someone attempts to log into a server until they fail a certain number of times. In other cases, you may only need to see a few occurrences to know there is a widespread problem. For example, if a DoS attack is launched against your web server, you may only need to see a few occurrences of an intrusion event to know that you need to address the situation. Seeing hundreds of the same event only overwhelms your system.

Intrusion Event Thresholds

You can set thresholds for individual rules to limit the number of times the system logs and displays an intrusion event based on how many times the event is generated within a specified time period. This can prevent you from being overwhelmed with a large number of identical events. You can set thresholds per shared object rule, standard text rule, or inspector rule.

Set Intrusion Event Thresholds

To set a threshold, first specify the thresholding type.

Table 2. Thresholding Options

Option

Description

Limit

Logs and displays events for the specified number of packets (specified by the Count argument) that trigger the rule during the specified time period. For example, if you set the type to Limit, the Count to 10, and the Seconds to 60, and 14 packets trigger the rule, the system stops logging events for the rule after displaying the first 10 that occur within the same minute.

Threshold

Logs and displays a single event when the specified number of packets (specified by the Count argument) trigger the rule during the specified time period. Note that the counter for the time restarts after you hit the threshold count of events and the system logs that event. For example, you set the type to Threshold, Count to 10, and Seconds to 60, and the rule triggers 10 times by second 33. The system generates one event, then resets the Seconds and Count counters to 0. The rule then triggers another 10 times in the next 25 seconds. Because the counters reset to 0 at second 33, the system logs another event.

Both

Logs and displays an event once per specified time period, after the specified number (count) of packets trigger the rule. For example, if you set the type to Both, Count to two, and Seconds to 10, the following event counts result:

  • If the rule is triggered once in 10 seconds, the system does not generate any events (the threshold is not met)

  • If the rule is triggered twice in 10 seconds, the system generates one event (the threshold is met when the rule triggers the second time)

  • If the rule is triggered four times in 10 seconds, the system generates one event (the threshold is met when the rule triggers the second time, and following events are ignored)

Secondly, specify tracking, which determines whether the event threshold is calculated per source or destination IP address.

Table 3. Thresholding IP Options

Option

Description

Source

Calculates event instance count per source IP address.

Destination

Calculates event instance count per destination IP address.

Finally, specify the number of instances and time period that define the threshold.

Table 4. Thresholding Instance/Time Options

Option

Description

Count

The number of event instances per specified time period per tracking IP address required to meet the threshold.

Seconds

The number of seconds that elapse before the count resets. If you set the threshold type to limit, the tracking to Source IP, the count to 10, and the seconds to 10, the system logs and displays the first 10 events that occur in 10 seconds from a given source port. If only 7 events occur in the first 10 seconds, the system logs and displays those; if 40 events occur in the first 10 seconds, the system logs and displays 10, then begins counting again when the 10-second time period elapses.

Note that you can use intrusion event thresholding alone or in any combination with rate-based attack prevention, the detection_filter keyword, and intrusion event suppression.


Tip

You can also add thresholds from within the packet view of an intrusion event.

Set Threshold for an Intrusion Rule in Snort 3

You can set a single threshold for a rule from the Rule Detail page. Adding a threshold overwrites any existing threshold for the rule.

Procedure

Step 1

Choose Objects > Intrusion Rules .

Step 2

Click Snort 3 All Rules tab.

Step 3

From an intrusion rule’s Alert Configuration column, click the None link.

Step 4

Click Edit (edit icon).

Step 5

In the Alert Configuration window, click the Threshold tab.

Step 6

From the Type drop-down list, choose the type of threshold you want to set:

  • Choose Limit to limit notification to the specified number of event instances per time period.

  • Choose Threshold to provide notification for each specified number of event instances per time period.

  • Choose Both to provide notification once per time period after a specified number of event instances.

Step 7

Choose Source or Destination in the Track By field to indicate whether you want the event instances tracked by source or destination IP address.

Step 8

Enter the number of event instances you want to use as your threshold in the Count field.

Step 9

Enter a number that specifies the time period, in seconds, for which event instances are tracked in the Seconds field.

Step 10

Click Save.

Refer to the video Snort 3 Suppression and Threshold for additional support and information.

What to do next

Deploy configuration changes; see Deploy Configuration Changes.

View and Delete Intrusion Event Thresholds

To view or delete an existing threshold setting for a rule, use the Rules Details view to display the configured settings for a threshold and see if they are appropriate for your system. If they are not, you can add a new threshold to overwrite the existing values.

Procedure

Step 1

Choose Objects > Intrusion Rules.

Step 2

Click Snort 3 All Rules tab.

Step 3

Choose the rule with a configured threshold as shown in the Alert Configuration column (the Alert Configuration column displays Threshold as a link for the rule).

Step 4

To remove the threshold for the rule, click Threshold link in the Alert Configuration column.

Step 5

Click Edit (edit icon).

Step 6

Click Threshold tab.

Step 7

Click Reset.

Step 8

Click Save.

What to do next

Deploy configuration changes; see Deploy Configuration Changes.

Intrusion Policy Suppression Configuration

You can suppress intrusion event notification when a specific IP address or range of IP addresses triggers a specific rule or inspector. This is useful for eliminating false positives. For example, if you have a mail server that transmits packets that look like a specific exploit, you might suppress event notification for that event when it is triggered by your mail server. The rule triggers for all packets, but you only see events for legitimate attacks.

Intrusion Policy Suppression Types

Note that you can use intrusion event suppression alone or in any combination with rate-based attack prevention, the detection_filter keyword, and intrusion event thresholding.


Tip

You can add suppressions from within the packet view of an intrusion event. You can also access suppression settings by using the Alert Configuration column on the intrusion rules editor page (Objects > Intrusion Rules > Snort 3 All Rules).

Set Suppression for an Intrusion Rule in Snort 3

You can set one or more suppressions for a rule in your intrusion policy.

Before you begin

Ensure you create the required network objects to be added for source or destination suppression.

Procedure

Step 1

Choose Objects > Intrusion Rules.

Step 2

Click Snort 3 All Rules tab.

Step 3

Click the None link in the intrusion rule’s Alert Configuration column,.

Step 4

Click Edit (edit icon).

Step 5

From the Suppressions tab, click the add icon (+) next to any of the following options:

  • Choose Source Networks to suppress events generated by packets originating from a specified source IP address.

  • Choose Destination Networks to suppress events generated by packets going to a specified destination IP address.

Step 6

Select any of the preset networks in the Network drop-down list.

Step 7

Click Save.

Step 8

(Optional) Repeat the last three steps if required.

Step 9

Click Save in the Alert Configuration window.

What to do next

Deploy configuration changes; see Deploy Configuration Changes.

View and Delete Suppression Conditions

You may want to view or delete an existing suppression condition. For example, you can suppress event notification for packets originating from a mail server IP address because the mail server normally transmits packets that look like exploits. If you then decommission that mail server and reassign the IP address to another host, you should delete the suppression conditions for that source IP address.

Procedure

Step 1

Choose Objects > Intrusion Rules.

Step 2

Click Snort 3 All Rules tab.

Step 3

Choose the rule for which you want to view or delete suppressions.

Step 4

Click Suppression in the Alert Configuration column.

Step 5

Click Edit (edit icon).

Step 6

Click Suppressions tab.

Step 7

Remove the suppression by clicking Clear (clear icon) next to the suppression.

Step 8

Click Save.

What to do next

Deploy configuration changes; see Deploy Configuration Changes.

Add Intrusion Rule Comments

You can add comments to rules in your intrusion policy. Comments added this way are policy-specific; that is, comments you add to a rule in one intrusion policy are not visible in other intrusion policies.

Procedure

Step 1

Choose Policies > Intrusion.

Step 2

Click Snort 3 Version next to the policy you want to edit.

Step 3

In the right side of the page where all the rules are listed, choose the rule where you want to add a comment.

Step 4

Click Comment (comment icon) under the Comments column.

Step 5

In the Comments field, enter the rule comment.

Step 6

Click Add Comment.

Step 7

Click Save.

Tip

 

The system displays a Comment (comment icon) next to the rule in the Comments column.

What to do next

Deploy configuration changes; see Deploy Configuration Changes.

Snort 2 Custom Rules Conversion to Snort 3

If you are using custom rules, make sure you are prepared to manage that rule set for Snort 3 prior to conversion from Snort 2 to Snort 3. If you are using a rule set from a third-party vendor, contact that vendor to confirm that their rules will successfully convert to Snort 3 or to obtain a replacement rule set written natively for Snort 3. If you have custom rules that you have written yourself, familiarize with writing Snort 3 rules prior to conversion, so you can update your rules to optimize Snort 3 detection after conversion. See the links below to learn more about writing rules in Snort 3.

You can refer to other blogs at https://blog.snort.org/ to learn more about Snort 3 rules.

To use the system-provided tool to convert Snort 2 rules to Snort 3 rules, see Snort 2 Custom Rules Conversion to Snort 3.


Important

Snort 2 network analysis policy (NAP) settings cannot be copied to Snort3 automatically. NAP settings have to be manually replicated in Snort 3.

Convert all Snort 2 Custom Rules across all Intrusion Policies to Snort 3

Procedure

Step 1

Choose Objects > Intrusion Rules .

Step 2

Click Snort 3 All Rules tab.

Step 3

Ensure All Rules is selected in the left pane.

Step 4

Click the Tasks drop-down list and choose:

  • —To automatically convert all the Snort 2 custom rules across all the intrusion policies to Snort 3 and import them into FMC as Snort 3 custom rules.

  • —To automatically convert all the Snort 2 custom rules across all the intrusion policies to Snort 3 and download them into your local system.

Step 5

Click OK.

Note

 

  • If you selected Convert and import in the previous step, then all the converted rules are saved under a newly created rule group All Snort 2 Converted Global under Local Rules.

  • If you selected Convert and download in the previous step, then save the rules file locally. You can review the converted rules in the downloaded file and later upload them by following the steps in Add Custom Rules to Rule Groups.

Refer to the video Converting Snort 2 Rules to Snort 3 for additional support and information.

What to do next

Deploy configuration changes; see Deploy Configuration Changes.

Convert Snort 2 Custom Rules of a Single Intrusion Policy to Snort 3

Procedure

Step 1

Choose Policies > Intrusion.

Step 2

In the Intrusion Policies tab, click Show Snort 3 Sync status.

Step 3

Click the Sync icon of the intrusion policy.

Note

 

If the Snort 2 and the Snort 3 versions of the intrusion policy are synchronized, then the Sync icon is in green . It indicates that there are no custom rules to be converted.

Step 4

Read through the summary and click the Custom Rules tab.

Step 5

Choose:

  • Import converted rules to this policy—To convert the Snort 2 custom rules in the intrusion policy to Snort 3 and import them into FMC as Snort 3 custom rules.

  • Download converted rules—To convert the Snort 2 custom rules in the intrusion policy to Snort 3 and download them into your local system. You can review the converted rules in the downloaded file and later upload the file by clicking the upload icon.

Step 6

Click Re-Sync.

What to do next

Deploy configuration changes; see Deploy Configuration Changes.

Add Custom Rules to Rule Groups

Uploading custom rules in the FMC adds the custom rules that you have created locally to the list of all the Snort 3 rules.

Procedure

Step 1

Choose Objects > Intrusion Rules.

Step 2

Click Snort 3 All Rules tab.

Step 3

Click the Tasks drop-down list.

Step 4

Drag and drop the .txt or .rules file that contains the Snort 3 custom rules that you have created.

Step 5

Click OK.

Note

 
If there are any errors in the selected file, then you cannot proceed further. You can download the error file and Replace File link to upload version 2 of the file, after fixing the errors.

Step 6

Associate rules to a rule group to add the new rules to that group.

You can also create a new custom rule group (by clicking the Create New Custom Rule Group link) and then add the rules to the new group.

Note

 
If there are no existing local rule groups, then proceed by clicking Create New Custom Rule Group to proceed. Enter a Name for the new rule group and click Save.

Step 7

Choose either of the following:

  • Merge Rules to merge the new rules that you are adding with the existing rules in the rule group.

  • Replace all rules in the group with file contents to replace all the exiting rules with the new rules that you are adding.

Note

 

If you chose more than one rule group in the previous step, then only the Merge Rules option is available.

Step 8

Click Next.

Review the summary to know the new rule IDs that are being added and optionally download it.

Step 9

Click Finish.


Important

The rule action of all the uploaded rules is in the disabled state. You have to change them to the required state to ensure the rules are active.

What to do next

Add Rule Groups with Custom Rules to an Intrusion Policy

Custom rules that are uploaded in the system have to be enabled in an intrusion policy to enforce those rules on the traffic. After uploading custom rules on FMC, add the rule group with the new custom rules in the intrusion policy.

Procedure

Step 1

Choose Policies > Intrusion.

Step 2

In the Intrusion Policies tab, click the Snort 3 Version of the intrusion policy.

Step 3

Click Add (+) next to the Rule Groups search bar.

Step 4

In the Add Rule Groups window, click the > icon next to a rule group to expand the local rule group.

Step 5

Check the check box next to the uploaded custom rules group.

Step 6

Click Save.

What to do next

Deploy configuration changes; see Deploy Configuration Changes.

Manage Custom Rules in Snort 3

Custom rules that are uploaded in the system have to be added to an intrusion policy and enabled to enforce those rules on the traffic. You can enable the uploaded custom rules across all policies or selectively on individual policies.

Follow the steps to enable custom rules in one or many intrusion policies:

Procedure

Step 1

Choose Objects > Intrusion Rules.

Step 2

Click Snort 3 All Rules tab.

Step 3

Expand Local Rules.

Step 4

Select the required rule group.

Step 5

Select the rules by checking the check boxes next to them.

Step 6

Select Per Intrusion Policy from the Rule Actions drop-down list.

Step 7

Choose:

  • All Policies—to have the same rule actions for all the rules to be added.

  • Per Intrusion Policy—to have different rule actions for each intrusion policy.

Step 8

Set the rule actions:

  • If you selected All Policies in the previous step, then select the required rule action from the Select Override state drop-down list.

  • If you selected Per Intrusion Policy in the previous step, then select the Rule Action against the policy name. To add more policies, click Add Another.

Step 9

Optionally, add a comment in the Comments text box.

Step 10

Click Save.

What to do next

Deploy the changes on the device. See, Deploy Configuration Changes.

Delete Custom Rules

Procedure

Step 1

Choose Objects > Intrusion Rules.

Step 2

Click Snort 3 All Rules tab.

Step 3

Expand Local Rules in the left pane.

Step 4

Check the check boxes of the rules you want to delete.

Step 5

Ensure that the rule action for all the rules that you select is Disable.

If required, follow the steps below to disable the rule action for multiple selected rules:

  1. From the Rule Actions drop-down box, select Per Intrusion Policy.

  2. Select All Policies radio button.

  3. Select Disable from the Select Override state drop-down list.

  4. Click Save.

  5. Check the check boxes of the rules you want to delete.

Step 6

From the Rule Actions drop-down list, select Delete.

Step 7

Click Delete in the Delete Rules pop-up window.

What to do next

Deploy configuration changes; see Deploy Configuration Changes.

Delete Rule Groups

Before you begin

Exclude the rule group you want to delete from all intrusion policies where you have included it. For steps on excluding a rule group from an intrusion policy, see Edit Snort 3 Intrusion Policies.

Procedure

Step 1

Choose Objects > Intrusion Rules.

Step 2

Click Snort 3 All Rules tab.

Step 3

Expand Local Rules in the left pane.

Step 4

Select the rule group to be deleted.

Step 5

Ensure the rule action for all the rules in the group is set to Disable before proceeding.

If the rule action for any of the rules is anything other than Disable, then you cannot delete the rule group. If required, follow the steps below to disable the rule action for all the rules:

  1. Check the check box below the Rule Actions drop-down list to select all the rules in the group.

  2. From the Rule Actions drop-down box, select Per Intrusion Policy.

  3. Select All Policies radio button.

  4. Select Disable from the Select Override state drop-down list.

  5. Click Save.

Step 6

Click the Delete (delete icon) next to the rule group.

Step 7

Click OK in the Delete Rule Group pop-up window.

What to do next

Deploy configuration changes; see Deploy Configuration Changes.