Intrusion Prevention System

The Intrusion Prevention System (IPS) system of OPNsense is based on Suricata and utilizes Netmap to enhance performance and minimize CPU utilization. This deep packet inspection system is very powerful and can be used to detect and mitigate security threats at wire speed.


It is important to define the terms used in this document. An Intrustion Detection System (IDS) watches network traffic for suspicious patterns and can alert operators when a pattern matches a database of known behaviors. An Intrusion Prevention System (IPS) goes a step further by inspecting each packet as it traverses a network interface to determine if the packet is suspicious in some way. If it matches a known pattern the system can drop the packet in an attempt to mitigate a threat.

The Suricata software can operate as both an IDS and IPS system.

Choosing an interface

You can configure the system on different interfaces. One of the most commonly asked questions is which interface to choose. Considering the continued use IPv4, usually combined with Network Address Translation, it is quite important to use the correct interface. If you are capturing traffic on a WAN interface you will see only traffic after address translation. This means all the traffic is originating from your firewall and not from the actual machine behind it that is likely triggering the alert.

Rules for an IDS/IPS system usually need to have a clear understanding about the internal network; this information is lost when capturing packets behind NAT.

Without trying to explain all the details of an IDS rule (the people at Suricata are way better in doing that), a small example of one of the ET-Open rules usually helps understanding the importance of your home network.

alert tls $HOME_NET any -> $EXTERNAL_NET any (msg:"ET TROJAN Observed Glupteba CnC Domain in TLS SNI"; flow:established,to_server; tls_sni; content:""; depth:13; isdataat:!1,relative; metadata: former_category MALWARE; reference:md5,4cc43c345aa4d6e8fd2d0b6747c3d996; classtype:trojan-activity; sid:2029751; rev:2; metadata:affected_product Windows_XP_Vista_7_8_10_Server_32_64_Bit, attack_target Client_Endpoint, deployment Perimeter, signature_severity Major, created_at 2020_03_30, updated_at 2020_03_30;)

The $HOME_NET can be configured, but usually it is a static net defined in RFC 1918. Using advanced mode you can choose an external address, but bear in mind you will not know which machine was really involved in the attack and it should really be a static address or network.

$EXTERNAL_NET is defined as being not the home net, which explains why you should not select all traffic as home since likely none of the rules will match.

Since the firewall is dropping inbound packets by default it usually does not improve security to use the WAN interface when in IPS mode because it would drop the packet that would have also been dropped by the firewall.


IDS mode is available on almost all (virtual) network types. When your network card is not (fully) supported, you can set the tunable dev.netmap.admode to the value 2 in which case emulated mode will be enforced (Configurable in System ‣ Settings ‣ Tunables). A list of natively supported physical adapters is available in the FreeBSD man page.

General setup

The settings page contains the standard options to get your IDS/IPS system up and running.


Enable Suricata

IPS mode

When enabled, the system can drop suspicious packets. In order for this to work, your network card needs to support netmap. The action for a rule needs to be “drop” in order to discard the packet, this can be configured per rule or ruleset (using an input filter)

Promiscuous mode

Listen to traffic in promiscuous mode. (all packets in stead of only the ones addressed to this network interface)

Enable syslog alerts

Send alerts to syslog, using fast log format

Enable eve syslog output

Send alerts in EVE format to syslog, using log level info. This will not change the alert logging used by the product itself. Drop logs will only be send to the internal logger, due to restrictions in suricata.

Pattern matcher

Controls the pattern matcher algorithm. Aho–Corasick is the default. On supported platforms, Hyperscan is the best option. On commodity hardware if Hyperscan is not available the suggested setting is “Aho–Corasick Ken Steele variant” as it performs better than “Aho–Corasick”.


Interfaces to protect. When in IPS mode, this need to be real interfaces supporting netmap. (when using VLAN’s, enable IPS on the parent)

Rotate log

Log rotating frequency, also used for the internal event logging (see Alert tab)

Save logs

Number of logs to keep


When using an external reporting tool, you can use syslog to ship your EVE log easily. Just enable “Enable EVE syslog output” and create a target in System ‣ Settings ‣ Logging / Targets. (filter application “suricata” and level “info”)


When using IPS mode make sure all hardware offloading features are disabled in the interface settings (Interfaces ‣ Settings). Prior to version 20.7, “VLAN Hardware Filtering” was not disabled which may cause issues for some network cards.

Advanced options

Some less frequently used options are hidden under the “advanced” toggle.

Home networks

Define custom home networks, when different than an RFC1918 network. In some cases, people tend to enable IDPS on a wan interface behind NAT (Network Address Translation), in which case Suricata would only see translated addresses in stead of internal ones. Using this option, you can define which addresses Suricata should consider local.

default packet size

With this option, you can set the size of the packets on your network. It is possible that bigger packets have to be processed sometimes. The engine can still process these bigger packets, but processing it will lower the performance.

Download rulesets

When enabling IDS/IPS for the first time the system is active without any rules to detect or block malicious traffic. The download tab contains all rulesets available on the system (which can be expanded using plugins).

In this section you will find a list of rulesets provided by different parties and when (if installed) they where last downloaded on the system. In previous versions (prior to 21.1) you could select a “filter” here to alter the default behavior of installed rules from alert to block. As of 21.1 this functionality will be covered by Policies, a separate function within the IDS/IPS module, which offers more fine grained control over the rulesets.


When migrating from a version before 21.1 the filters from the download rulesets page will automatically be migrated to policies.


The policy menu item contains a grid where you can define policies to apply to installed rules. Here you can add, update or remove policies as well as disabling them. Policies help control which rules you want to use in which manner and are the prefered method to change behaviour. Although you can still update separate rules in the rules tab, adding a lot of custom overwrites there is more sensitive to change and has the risk of slowing down the user-interface.

A policy entry contains 3 different sections. First some general information, such as the description and if the rule is enabled as well as a priority. Overlapping policies are taken care of in sequence, the first match with the lowest priority number is the one to use.

Secondly there are the matching criterias, these contain the rulesets a policy applies on as well as the action configured on a rule (disabled by default, alert or drop), finally there is the rules section containing the metadata collected from the installed rules, these contain options as affected product (Android, Adobe flash, …) and deployment (datacenter, perimeter).

The last option to select is the new action to use, either disable selected rules, only alert on them or drop traffic when matched.


The options in the rules section depend on the vendor, when no metadata is provided in the source rule, none can be used at our end.

Installed rules

The rules tab offers an easy to use grid to find the installed rules and their purpose, using the selector on top one can filter rules using the same metadata properties available in the policies view.


After applying rule changes, the rule action and status (enabled/disabled) are set, to easily find the policy which was used on the rule, check the matched_policy option in the filter. Manual (single rule) changes are being marked as policy “__manual__”

User defined rules

Most of the rules being used on your IDPS system will be supplied by third party vendors like Proofpoint, but in some cases it can be convenient to build some (limited) rules yourself. The “User defined” tab offers this functionality.


OPNsense includes a very polished solution to block protected sites based on their SSL fingerprint. You can manually add rules in the “User defined” tab.

Bypassing the engine

The Bypass toggle offers the ability to skip traffic inspection, our How-tos section contains a good example to exclude local traffic passing your network and increase routing performance.


In the “Alerts” tab you can view the alerts triggered by the IDS/IPS system. Use the info button here to collect details about the detected event or threat.

Advanced configuration

OPNsense supports custom Suricata configurations in suricata.yaml format. In order to add custom options, create a template file named custom.yaml in the /usr/local/opnsense/service/templates/OPNsense/IDS/ directory.

Since this file is parsed by our template system, you are able to use template tags using the Jinja2 language.

Available rulesets

Emerging Threats

Emerging Threats (ET) has a variety of IDS/IPS rulesets. There is a free, BSD-licensed version and a paid version available.


Proofpoint offers a community portal which provides access to documentation and updates about rules, you can visit it at . The Frequently asked questions might be a good place to start reading.

ET Open

The ETOpen Ruleset is not a full coverage ruleset and may not be sufficient for many regulated environments and thus should not be used as a standalone ruleset.

OPNsense has integrated support for ETOpen rules.

ETPro Telemetry

Proofpoint offers a free alternative for the well known ET Pro Telemetry edition ruleset. offers several blacklists for protecting against fraudulent networks.

SSL Blacklist

SSL Blacklist (SSLBL) is a project maintained by The goal is to provide a list of “bad” SSL certificates identified by to be associated with malware or botnet activities. SSLBL relies on SHA1 fingerprints of malicious SSL certificates and offers various blacklists.

See for details:

Feodo Tracker

Feodo (also known as Cridex or Bugat) is a Trojan used to commit ebanking fraud and steal sensitive information from the victim’s computer, such as credit card details or credentials. At the moment, Feodo Tracker is tracking four versions of Feodo, and they are labeled by Feodo Tracker as version A, version B, version C and version D:

  • Version A Hosted on compromised webservers running an nginx proxy on port 8080 TCP forwarding all botnet traffic to a tier 2 proxy node. Botnet traffic usually directly hits these hosts on port 8080 TCP without using a domain name.

  • Version B Hosted on servers rented and operated by cybercriminals for the exclusive purpose of hosting a Feodo botnet controller. Usually taking advantage of a domain name within ccTLD .ru. Botnet traffic usually hits these domain names using port 80 TCP.

  • Version C Successor of Feodo, completely different code. Hosted on the same botnet infrastructure as Version A (compromised webservers, nginx on port 8080 TCP or port 7779 TCP, no domain names) but using a different URL structure. This Version is also known as Geodo and Emotet.

  • Version D Successor of Cridex. This version is also known as Dridex

See for details:

URLHaus List

OPNsense version 18.1.7 introduced the URLHaus List from which collects compromised sites distributing malware.

See for details:

App detection rules

OPNsense 18.1.11 introduced the app detection ruleset. Since about 80 percent of traffic are web applications these rules are focused on blocking web services and the URLs behind them.

If you want to contribute to the ruleset see: