Open-source network intrusion detection compared

Which open-source network intrusion detection engine fits your team

If your team needs network detection that feeds a SIEM, the sensor you pick decides what evidence reaches the analysts. Suricata is the default choice for signature-driven IDS and IPS with protocol logging, because it matches a complete signature language and writes its EVE JSON output for downstream tools. Zeek is the better fit when you care more about behavioral network metadata and scripted analysis than about matching known signatures, since the Zeek documentation says it is not a classic signature-based IDS. Snort 3 suits teams that already run Snort rules and can accept that the latest detections sit in the Snort Subscriber Ruleset, which requires a paid subscription, while the Community Ruleset is free. Before committing, confirm which rule sets your threat model requires and whether your SIEM ingests EVE JSON or Zeek logs without custom parsing.

Security Onion is the choice when you want a bundled distribution rather than assembling a sensor, SIEM backend and packet capture yourself. It combines Suricata or Zeek with Strelka, full packet capture and Elastic components in one product. Its license is the Elastic License 2.0, which is not OSI approved and forbids offering the software as a managed service, so legal review should happen before you adopt it. Confirm the license terms against your own use case with Security Onion Solutions and check the hardware guidance in the Security Onion documentation, which suggests 32GB of RAM or more even for modest traffic volumes.

Whichever engine you choose, plan for the sensor hardware and the log pipeline together. Suricata is multi-threaded and can inspect multi-gigabit traffic in a single instance, but actual throughput depends on your traffic mix, capture method and rule load, so test on representative traffic. Ask your team to confirm the current stable versions and support windows on the project sites before writing the deployment plan, because Suricata 7 is end of life and Zeek releases on a different LTS cadence.

Which network IDS fits each scenario

This table maps common team situations to the network IDS most likely to fit, with the condition that makes each fit hold. Read each row against your own detection goals, staffing and whether you need packets blocked inline, then confirm the open items before you shortlist.

Scenario	Best fit	Why and what to confirm
Signature-based alerting with inline prevention	Suricata or Snort 3	Both match signatures and can run inline to drop traffic. Suricata is GPL-2.0 and Snort 3 is GPL V2. Confirm that your rule source (Emerging Threats, VRT, Community or Subscriber) covers the threats you care about and who will tune false positives.
SIEM-first team that wants protocol logs	Suricata with EVE JSON, or Zeek	Suricata writes EVE JSON alerts and events for Logstash-style pipelines. Zeek writes connection and protocol logs by default. Confirm whether you need signature alerts, metadata or both, since Zeek is not a classic signature IDS.
Investigators who need searchable connection and transcript records	Zeek	Zeek logs connections, HTTP sessions, DNS, SSL certificates and SMTP content as TSV or JSON for hunting. Confirm storage sizing for your traffic volume and whether your SIEM ingests Zeek logs without heavy parsing.
Team already committed to Snort rules	Snort 3	Snort 3 runs the community and subscriber rule sets. Confirm whether you need the Subscriber Rule Set for the newest detections, since that requires a paid subscription from Cisco Talos.
Team that wants one bundled distribution	Security Onion	Security Onion bundles Suricata or Zeek, Strelka, packet capture and Elastic components. Confirm its Elastic-2.0 license terms fit your use, and plan for the recommended manager, search and sensor layout at scale.
Multi-gigabit links with a single sensor	Suricata	Suricata is multi-threaded and supports hardware acceleration and PF_RING or AF_PACKET capture. Confirm the vendor acceleration and capture method your NICs support, and test throughput on representative traffic.
Large sites that need horizontal sensor scaling	Zeek	Zeek clusters use a load balancer, back-end workers and a manager, with ZeekControl support. Confirm the operator effort to run the cluster and how you will consolidate logs centrally.

How signature and behavioral analysis differ in practice

Suricata and Snort are rule-driven detection engines. Both match packets against a signature language that describes known threats, policy violations and malicious behavior, and both alert on or, when run inline, block matching traffic. The practical question is who writes, tunes and retires those rules. Suricata can pull the Emerging Threats Suricata ruleset or the VRT ruleset, and Snort uses the Community Ruleset or, for the latest detections, the Snort Subscriber Ruleset delivered to paying subscribers. Confirm the license terms of each rule source with its provider before you build a detection program around it, since the rule sets differ in access and cost.

Suricata also goes beyond pure signatures. It documents detection of traffic anomalies alongside its signature language, and it supports Lua scripting for logic the rule syntax cannot express. Zeek sits at the other end of the spectrum. The Zeek documentation states that it is not a classic signature-based IDS and is not optimized for byte matching, and it points users who need signatures toward systems such as Suricata. Its default analyses, including logging, are implemented as scripts rather than hard-coded in the core. Behavioral detection with Zeek therefore depends on the scripts your team writes or adopts, which means the detection logic is a maintained software asset, not a vendor update you receive.

The choice comes down to what your team will write and maintain: signature sets with a tuning and suppression process, scripts with their own testing and version control, or both. Ask each vendor or integrator how rule updates are tested before production, who owns false-positive review, and how a custom script is versioned and rolled back. Many deployments run more than one engine, for example a signature engine for known threats alongside a passive analyzer that produces metadata for hunting. Plan for that overlap and the alert deduplication it requires in your SIEM from the start.

Suricata, Snort 3 and Zeek in brief, with license terms

Suricata, Snort 3 and Zeek each cover a different part of network monitoring, and each ships under a license the OSI recognizes as open source. The cards below give each tool's primary role, its license and the condition under which that role matters most for your environment.

Suricata: signature detection with NSM output

Suricata is the GPL-2.0 network IDS and IPS engine owned and supported by the non-profit Open Information Security Foundation (OISF). It matches a complete signature language against traffic, detects anomalies and writes all events as EVE JSON that a SIEM can ingest. The role matters most when you need multi-threaded signature inspection at multi-gigabit rates. Check that your capture method (PF_RING or AF_PACKET) and hardware acceleration suit your traffic, and plan your upgrade path, because the Suricata 7 line has reached end of life.

Snort 3: inline intrusion prevention

Snort 3 is an open source intrusion prevention system licensed under GPL V2. It matches rules against packets, generates alerts and can run inline to stop matching traffic, and it also works as a packet sniffer or logger. The role matters most when you intend to block traffic inline and need a scriptable configuration in LuaJIT. Check the ruleset before you commit: the Community Ruleset is free and quality checked by Cisco Talos, while the latest detections require a Subscriber Rule Set subscription, which is a recurring cost to plan for.

Zeek: passive analysis and rich logs

Zeek is a passive network traffic analyzer released under the permissive BSD license. Corelight serves as custodian, providing financial backing and code without owning the project. By default Zeek writes connection records and application transcripts for HTTP, DNS, SSL certificates and SMTP as TSV or JSON, which suits investigations and SIEM ingestion. The role matters most when you need scriptable network evidence rather than signature matching. Zeek is not a classic signature IDS, so pair it with Suricata or Snort where byte-level matches are required. Check the LTS maintenance window for your chosen line.

Open source licenses across all three

GPL-2.0 for Suricata, GPL V2 for Snort 3 and BSD for Zeek are all OSI open source licenses, so the engines themselves carry no source-available restrictions. The license covers the engine code, but it does not decide what rules or scripts you may use. The rule source terms apply separately, including Snort's subscriber rules and any commercial feed you add to Suricata. Ask your legal and procurement teams to review those terms on their own track before deployment, and record which feed each sensor loads so audits can trace every alert to its source.

Open-source network intrusion detection capabilities side by side

This table sets Suricata, Snort 3 and Zeek against the same capability rows, from license and detection method to output format and cluster support. Read each row as a question about your own sensors: where a cell is a question, confirm the answer with the project documentation before you commit to a design.

Capability	Suricata	Snort 3	Zeek
License	GPL-2.0, maintained by the non-profit OISF	GPL V2 (Snort 3, per its LICENSE file)	BSD license, with Corelight as project custodian
Primary role	Network IDS, IPS and network security monitoring engine	Intrusion prevention system, also usable as a packet sniffer or logger	Passive network traffic analyzer used for network security monitoring
Detection method	Complete signature language, plus traffic anomaly detection and Lua scripting	Rule-based detection with sticky buffers and service autodetection	Not a classic signature IDS; detection is written as scripts
Rule or script source	Emerging Threats Suricata ruleset and VRT ruleset	Community Ruleset (Cisco Talos QA) and Subscriber Ruleset for paying users	Turing-complete scripting language; default analyses are scripts
Protocol and file logging	HTTP, TLS certificates, all DNS queries, file extraction and full pcap capture	Not stated in supplied facts; confirm log coverage in the Snort 3 documentation	Connection records, HTTP, DNS, SSL certificates and SMTP content by default
Output format	EVE JSON for events and alerts, suited to Logstash and similar tools	Alert output format not covered by supplied facts; confirm the format and fields with the Snort project	TSV or JSON log files that external databases or SIEM products can consume
Multi-threading and capture	Multi-threaded; one instance can inspect multi-gigabit traffic; PF_RING and AF_PACKET capture	Multiple packet processing threads; packet I/O through the DAQ library	Not covered by supplied facts; confirm threading and capture options in the Zeek documentation
Cluster support	Not covered by supplied facts; confirm multi-sensor scaling approach with the project	Not covered by supplied facts; confirm clustering options with the Snort project	Clusters with a front-end load balancer, back-end instances and a central manager, supported by ZeekControl
Current release	8.0.7 (September 15, 2026), with Suricata 7 end of life	3.12.2.0 (April 23, 2026)	9.0.0 LTS, with 8.0.10 as the previous LTS release

Where rules and scripts come from, and who pays for them

Suricata matches traffic against a complete signature language that covers known threats, policy violations and malicious behavior, and it also detects traffic anomalies. Most teams do not write those signatures from scratch. The Suricata features page names two rule sources it can use: the Emerging Threats Suricata ruleset and the VRT ruleset. Each set has its own publisher, update cadence and terms, so the content you depend on is a separate decision from the engine itself.

Emerging Threats ruleset: Confirm the edition you pull, the license terms for your use, and the update channel before you automate downloads.
VRT ruleset: Confirm access, redistribution and update terms directly with its publisher, since they may differ from the open ruleset.
Lua scripting: Logic that the rule syntax cannot express can be written in Lua. Your team owns those scripts, so review and testing recur with each Suricata upgrade.
The engine: Suricata is GPL-2.0 and owned and supported by OISF, a 501(c)3 non-profit. The supplied facts list no paid rule tier for the engine.
Tuning effort: Enabling a large ruleset without a tuning pass creates alert volume that analysts must triage. Staff time for tuning is a recurring cost even when the rules are free.

Before relying on any ruleset, confirm its license for your use, how often it updates, who approves changes, and whether it matches the syntax of the Suricata line you run. Suricata 7 is end of life, so validate against the current 8.0.7 release or your chosen line.

Which licenses are OSI open source and which are not

Suricata (GPL-2.0), Snort 3 (GPL V2) and Zeek (BSD) are all open source engines under licenses the Open Source Initiative recognizes, so the engine itself carries no license restriction on internal use. The licenses differ in obligations, though. GPL-2.0 and GPL V2 are copyleft: if you distribute a modified engine, you owe source under the same terms. Zeek's BSD license is permissive and has fewer distribution obligations. Suricata is owned and supported by the Open Information Security Foundation, a non-profit, which is a governance point to weigh alongside the license. Snort's rule sets are a separate matter. The Community Ruleset is free, but the Snort Subscriber Ruleset, which carries the latest Cisco Talos detections, is a paid content tier. Paying for it does not change the GPL V2 license of the Snort 3 engine, so budget for it as content, not as a license upgrade.

Security Onion is the exception to check first. Its components moved from the GPL to the Elastic License 2.0 starting with Security Onion 2.4, announced on August 31, 2022. The Elastic License 2.0 is not OSI approved, and it forbids offering the software as a managed service. If you plan to run Security Onion for internal teams, that restriction may not matter. If you plan to operate it for customers, through a managed security provider or as a hosted service, it does. Security Onion Pro is a separate paid license that adds OIDC single sign-on, FIPS and STIG hardening, notifications, the Security Onion API, Manager of Managers and reports. Confirm which of these you need before you design around them, because they are the features most teams ask for after the first deployment.

Have counsel confirm the license obligations for your own distribution model before you commit. The questions are specific: whether you modify the engine or only configure it, whether you redistribute binaries to other entities or to subsidiaries, whether a third party operates the stack for you, and whether any rule or content bundle you ship carries its own terms. Ask Security Onion Solutions and the Snort and Zeek projects for written confirmation where your answer depends on their documentation rather than on the license text alone. Security Onion license documentation is the natural starting point for the Elastic License 2.0 questions.

What drives sensor hardware and how far each engine scales

Sensor sizing depends on how much traffic crosses the monitored link, how that traffic is captured, and whether one sensor or a cluster does the work. The supplied facts give hardware figures only for the Security Onion bundle, so every engine's sizing must come from a traffic test in your own environment.

Traffic rate sets the baseline

Sustained throughput, packet rate and connection count decide how much inspection work a sensor performs. Measure peak and average bits per second on each tap or span port, not a single snapshot, and include seasonal peaks such as month-end processing. Ask each vendor or project for the traffic profile behind any throughput claim, since the same link can produce very different load depending on protocol mix and rule count. No sizing figures for Suricata, Snort or Zeek alone are supplied here, so run a test with your own traffic.

Capture method and acceleration

Suricata supports capture through PF_RING and AF_PACKET and hardware acceleration from several vendors, so the capture layer can limit throughput before the detection engine does. Confirm which capture method your Linux kernel, NIC and driver support, and whether your NIC vendor's acceleration path is compatible with the Suricata version you plan to run. Snort 3 and Zeek have their own packet I/O choices, and Snort 3 uses the DAQ library, so verify each one against your hardware.

Threads and single-instance limits

Suricata is multi-threaded, and the project states that a single instance can inspect multi-gigabit traffic. Snort 3 also runs multiple packet processing threads. Thread count should match the cores you reserve for capture and detection, while leaving headroom for logging and the agent that ships EVE JSON or other logs. Ask for thread tuning guidance for your link speed and rule set, and test with the rules you intend to enable in production.

Clustered design for Zeek

Zeek is commonly scaled out rather than up. Large sites place a front-end load balancer across back-end Zeek instances coordinated by a central manager, and ZeekControl supports this layout out of the box. Plan the number of workers, the load balancing method and the manager host as separate items. Clustering adds operational work, including log collection from every worker, so ask who will maintain the cluster and how node failure is handled.

Storage for logs and full pcap

Metadata logs, alert records and full packet capture consume storage at different rates. Full pcap grows with traffic volume and is the largest consumer when enabled, while EVE JSON or Zeek logs grow with event volume and protocol mix. Decide how long you must keep raw packets for investigation, where they are written, and whether the SIEM index will hold the same data. Storage sizing belongs in the traffic test, and retention rules should be written down before the sensor goes live.

Bundled minimum from Security Onion

Security Onion publishes bare minimum hardware of 4 CPU cores, 24GB RAM and 200GB storage for a standalone install, and suggests 32GB of RAM or more even when monitoring a small amount of traffic. Treat these as the floor for a bundled stack that includes Suricata or Zeek plus Elasticsearch, not as a sizing for a sensor alone. A standalone install that also runs the search backend will need more capacity as data grows, so confirm the distributed layout before buying hardware.

How to choose and pilot a network IDS

This sequence moves a team from a stated detection goal to a licensed production choice, with each step producing a document that the next step depends on. Done in order, it shows whether Suricata, Snort or Zeek fits the traffic, the SIEM and the team that will run it.

Define the detection goal
Security leadership and the SOC lead decide whether the sensor must alert, prevent traffic inline, produce investigation metadata, or do all three. The output is a one-page detection charter that names the threats in scope and the response each alert triggers. Suricata covers alerting and inline prevention with signatures, Zeek produces rich metadata without being a classic signature IDS, and Snort is built around rules for both detection and inline prevention. The step is done when the charter is signed and every later choice can be checked against it.
Inventory traffic and tap points
The network team lists the segments to monitor, the expected traffic volume at each point, whether encrypted traffic is common, and where a SPAN port, network tap or packet broker can be placed. The output is a sensor placement diagram with an estimated peak throughput per tap. Ask the vendor or community which capture method each candidate supports on your hardware, since Suricata documents PF_RING and AF_PACKET capture and hardware acceleration from several vendors. The step is done when each tap has an owner and a throughput figure you can test against.
Decide the SIEM destination and ingestion path
The SOC and platform team choose where alerts and metadata land, whether that is an existing SIEM, a log platform such as Graylog or OpenSearch, or a bundled stack. They document the format each engine emits, such as Suricata's EVE JSON, Zeek's tab-separated or JSON logs, or Snort alert output, and the agent or shipper that moves it. The output is a data flow diagram with field mappings for the top detection use cases. Confirm with the SIEM vendor which parsers and content already exist for each format, because that work is recurring if it is left to your team.
Select rule and script sources and check licenses
The detection engineering lead lists the rule sets, scripts and vendor feeds each candidate would use, then records the license terms for each. Snort separates a community ruleset from a subscriber ruleset delivered to paying customers, while Suricata can use the Emerging Threats and VRT rulesets. Zeek's default analyses are scripts under a BSD license. The output is a source register showing license, cost, update cadence and the owner of each feed. Ask each provider in writing whether production use, redistribution and commercial support are included.
Run a pilot on a representative segment
The platform team deploys the shortlisted engine on one segment that carries typical traffic, including the encrypted and high-volume flows you expect in production. The team runs the chosen rules or scripts in detection mode, tests a set of known benign and simulated malicious traffic, and confirms that the SIEM receives events. The output is a pilot runbook with configuration files under version control and a list of tuning changes. The step is done when the sensor runs without manual restarts across the pilot window you agreed with the network owner.
Measure drop rates and alert volume
The SOC and platform team record packet drop rates at peak load, CPU and memory use on the sensor, alerts per day by rule or script, and the share of alerts the analysts mark as actionable. The output is a measurement report that compares each candidate on the same segment and traffic sample. Drops at peak mean the sensor has to scale out or use better capture hardware, so the report should name the hardware each result came from. The step is done when the team can state the sensor count and specification needed for production.
Confirm license and paid tier for production
Procurement, legal and the engine owner confirm the license that applies to the production deployment, any paid feed or support tier, and the terms for the SIEM platform that receives the data. They verify the current release line and its support window against the vendor's release notes before committing. The output is a signed license register and a support plan that names who patches sensors and rule sets. The step is done when every component in the architecture has a documented license, a named source of updates and an owner for each recurring cost.

What do buyers ask before deploying open-source network intrusion detection?

What does open-source network intrusion detection cost to run, and who gets paid?

The software license is usually free for Suricata and Zeek, but the total cost sits in the parts around it: sensor hardware, traffic access, rule sources, integration with your SIEM, tuning and staff time. Snort 3 is GPL V2, yet its latest detections come from the Snort Subscriber Ruleset, which requires a paid subscription, while the Community Ruleset is free. Hardware and storage are one-time purchases or recurring cloud charges, and the people who tune rules and watch alerts are recurring cost. DeployOpen scopes these items in writing before work starts and does not publish a price list, so ask for each component to be listed separately in the quote.

Can our team run Suricata or Zeek without outside help?

A team with network engineering and Linux administration skills can run either engine, but the work is more than installing a package. Suricata needs rule management, EVE JSON pipeline handling and sensor tuning, while Zeek needs script customization and cluster management through ZeekControl when traffic is large. Zeek is not a classic signature engine, so teams that expect signature alerts will need Suricata or a second tool alongside it. Be honest about who will own rule changes and alert triage after go-live, because that ownership decides whether the sensor stays useful. DeployOpen can plan, implement, document and support these systems, or hand over a runbook and train your staff.

How long does a network IDS pilot take, and what drives that time?

The timeline depends on the scope you agree in writing, not on a fixed number. The main drivers are how many network segments need sensors, whether you can get a mirror or tap port on each one, how quickly change control approves those changes, how many log destinations and SIEM integrations are involved, and how much tuning the first alerts need before they are useful. Encrypted traffic, legacy segments and multi-site routing usually add planning time. Ask for milestones tied to specific deliverables, such as sensor placement, rule baseline, SIEM ingest and an alert review, rather than a single end date.

What do we need to provide before a network IDS engagement starts?

The engagement needs four things from your side: traffic access, log destinations, change control and rule ownership. Traffic access means switch span or tap ports, permission to place sensors, and a clear map of which segments carry sensitive traffic. Log destinations means a SIEM or log platform that accepts EVE JSON or Zeek logs, with storage and retention agreed in advance. Change control covers maintenance windows for sensor and switch changes. Rule ownership names who approves Suricata or Snort rule changes, who maintains Zeek scripts, and who accepts suppressions, so that alerts do not drift into noise after launch.

Do we need Snort subscriber rules for our use case?

You need subscriber rules only if your detection requirement depends on the newest Talos signatures delivered in near real time. The Community Ruleset is free and quality checked by Cisco Talos, and all registered users get the Registered Rule Set through an Oinkcode. Many teams start with Suricata, which can use the Emerging Threats Suricata ruleset and the VRT ruleset, and compare rule sources before buying a subscription. Confirm the license terms for each ruleset with the vendor, because commercial use and redistribution rules differ by source. DeployOpen can compare the rule sources against your threat model in the written scope.

Do we need Security Onion Pro for network intrusion detection?

Security Onion Pro is needed only if your requirements include features it adds, such as OIDC single sign-on, FIPS and STIG hardening, notifications, the Security Onion API, Manager of Managers and reports. Security Onion bundles Suricata or Zeek with Strelka, packet capture and Elastic Agent, so the base distribution can cover sensor work without the Pro license. Its license is Elastic-2.0, which is not OSI approved and forbids offering the software as a managed service, so check that terms fit how you deliver it. Confirm the hardware sizing for your traffic volume, since Security Onion suggests 32GB of RAM or more when monitoring even a small amount of traffic.

How DeployOpen scopes network IDS selection and implementation

DeployOpen plans, implements, integrates, hardens, documents and supports self-hosted open-source platforms inside your own infrastructure, including your cloud accounts, data centers or air-gapped networks. For network intrusion detection, the work starts with a written scope that fits the sensor choice to your traffic, your log pipeline and your team.

How we scope

Confirm the network segments, traffic volumes and sensor placement to be monitored before any tool is chosen
Agree in writing which engine, rule sources and log outputs are in scope, and which are excluded
Record the SIEM or log destination, retention expectations and who reviews alerts day to day

How we build

Install and configure the chosen IDS engine on the agreed sensors, with rule update processes defined
Integrate EVE JSON or Zeek log output into the SIEM pipeline and check that alerts arrive as expected
Test detection with controlled traffic and document tuning decisions, including suppressed or noisy signatures

How we hand over

Deliver written runbooks covering sensor configuration, rule updates, log flow and troubleshooting steps
Provide documentation of the controls and evidence produced, prepared for your compliance assessment
Walk your operations and security staff through alert triage and the maintenance tasks they will own

How we support

Ongoing support, upgrades and continuous testing are available as a retainer after handover
Retainer scope, response expectations and upgrade cadence are agreed in writing before work starts
Ask for the quote to state which sensor versions, rule sources and log outputs the retainer covers