Splunk or Security Onion: which fits your SIEM scope
If your Splunk renewal is coming up and you are weighing alternatives, the short answer is this: Splunk fits teams that need a broad SIEM, custom search across many log sources and a large app ecosystem, while Security Onion fits teams whose priority is network-centric monitoring, threat hunting and full packet capture on self-hosted Linux. Security Onion is not a general log platform. It bundles the Security Onion Console, Elasticsearch, Suricata, Zeek, Elastic Fleet agents and Suricata full packet capture into one distribution, so its strengths are sensor-level visibility and analyst workflows around network traffic. Splunk's strengths sit in search language, correlation content and integrations across identity, cloud and application logs.
The decision turns on five questions. First, detection scope: how much of your alerting comes from endpoints, identity and cloud logs versus network traffic, and whether you need Splunk Enterprise Security content or can build detections in Sigma, Suricata and Security Onion's Detections interface. Second, network visibility: whether you need Zeek metadata and packet capture for retrospective hunting, and how much storage that implies. Third, licensing: Security Onion's repository is under the Elastic License 2.0, which is source-available rather than OSI open source, so have legal review the terms against how you plan to use, modify and redistribute it. Fourth, air-gap requirements: Security Onion supports full installs without internet access, but you should confirm the update path for your environment. Fifth, in-house capacity: a production Distributed deployment with manager, sensor and search nodes is the recommended design, and someone must own upgrades, rule tuning and capacity planning.
Before you commit, confirm which Security Onion release line fits your plans. Security Onion 2.4 reached end of life on October 1, 2026, and 3.0.0 supports only Oracle Linux 9, so check the Security Onion end-of-life documentation against your operating system standard. Then size a pilot against the published hardware minimums and your real traffic volume, and test your top ten Splunk detections against Suricata, Sigma and Security Onion's tuning options. Where a detection depends on Splunk-only apps or SPL, record it as a gap to close before the renewal date rather than assuming it will carry over.
Which option fits which scenario
This table maps common operating conditions to the platform that usually fits better, with the reason and the conditions that would change the answer. Read each row against your own staffing, network visibility needs and licensing position, since the right choice depends on what your team can run and what your detections must cover.
| Scenario | Better fit | Why and when the answer changes |
|---|---|---|
| Network-heavy threat hunting with packet-level evidence | Security Onion | It bundles Suricata, Zeek and full packet capture in one console, so hunters pivot from alerts to network metadata and captured traffic. Splunk can ingest network logs, but it does not provide this capture stack out of the box. |
| Broad multi-source SIEM with custom correlation across many applications | Splunk Enterprise Security | Splunk's search language, app ecosystem and correlation content cover wide, heterogeneous estates. Security Onion is network and endpoint focused, so custom correlation across business applications needs more engineering on the Elastic side. |
| Air-gapped or disconnected sites | Security Onion | Security Onion supports full installs without internet access, with the ISO carrying packages and NIDS, YARA and Sigma rules, and updates applied from newer ISO media. Confirm that your Splunk licensing and app updates can also be run offline before comparing. |
| Teams that need a commercial support contract with a single vendor | Splunk Enterprise Security | Splunk offers established vendor support across its product line. Security Onion Pro adds professional support, so confirm the scope, response terms and escalation path in writing before deciding. |
| Limited Linux operations staff | Splunk Enterprise Security | Security Onion is a Linux distribution with a multi-node Distributed design for production, and it runs only on x86-64 with Oracle Linux 9 for version 3.0.0. Teams without Linux depth should plan for this or use a managed partner. |
| Strict licensing needs for an open-source only policy | Splunk Enterprise Security (neutral to Security Onion) | Security Onion is source-available under the Elastic License 2.0, not OSI-approved open source. If your policy requires OSI-approved licenses, choose a component such as Wazuh or OpenSearch instead and confirm with your legal team. |
| Organizations whose 2.4 installs need an upgrade path | Security Onion (version 3 path) | Security Onion 2.4 reached end of life on October 1, 2026. Installations on Oracle Linux 9 can upgrade to 3 after moving full packet capture to Suricata, while other distributions need a fresh install, so plan the migration window early. |
Where the data lives and who controls the platform
Splunk Enterprise, Splunk Cloud Platform and a Security Onion deployment differ most in where security logs sit, who operates the software and who holds the keys. The cards below cover those decisions, the identity and encryption options in each model, and how compliance evidence gets produced, which is work your own team and auditors carry out.
Where events are stored
Splunk Enterprise keeps indexed events on servers you run, while Splunk Cloud Platform keeps them in Splunk-operated infrastructure. Security Onion stores Elasticsearch indices on search nodes inside your network, so every raw event, packet capture and alert stays under your control. Confirm with your architecture team which regions, subnets and backup targets will hold each data class before the design is signed off.
Who operates the platform
With Splunk Cloud Platform, Splunk operates the search tier and indexers under its service terms. With Security Onion, your team installs, patches and upgrades the console, manager, sensors and search nodes, so operating duties and their evidence move in house. Ask who holds administrator access to the console and who gets root on the underlying hosts.
Identity and single sign-on
Security Onion Pro adds OIDC single sign-on to the Security Onion Console, so analysts can authenticate through your existing identity provider. Confirm which OIDC provider you will use, how group claims map to console roles, and how accounts are removed when staff leave. Splunk deployments integrate with SSO through their own configuration, which should be compared with the same identity controls.
Encryption and FIPS mode
Security Onion Pro includes LUKS data-at-rest encryption and a FIPS mode option, which are relevant where cryptographic module requirements apply. Check whether your Splunk tier provides equivalent encryption controls for indexed data and backups, and whether FIPS validation is required for the specific components you run. Key custody and rotation are your responsibility in the self-hosted model, so document them before go-live.
Air-gapped and restricted networks
Security Onion supports full installs on networks without internet access, with the ISO carrying packages and NIDS, YARA and Sigma rules. Updates in airgap mode come from newer ISO media through the soup tool. This keeps detection content and telemetry on premises, but it also means your team must source and verify each update package and record its provenance.
Compliance evidence and audit scope
Compliance evidence in either model comes from configuration records, access logs, change history and retention settings that you export and retain. In the self-hosted model, your team produces that evidence directly and can show it to assessors. DeployOpen prepares controls and evidence for compliance assessments but does not issue audit reports, attestations or certifications; independent auditors and certification bodies do that work.
What Elastic License 2.0 means for your internal use
The Security Onion repository is licensed under the Elastic License 2.0, a source-available license rather than an OSI-approved open-source license. For an internal security team that runs Security Onion in its own network, the license lets you read, modify and operate the code. The practical question is not whether you can run it but whether your intended use stays inside the license terms. Before you commit to a design, read the license text in the repository's LICENSE file and have legal counsel confirm that your planned use fits it, including any plan to offer monitoring to other organizations, host Security Onion for customers, or resell access to it. Treat that review as part of the architecture decision, because a managed-service or resale plan changes the answer.
Security Onion Pro is a separate, paid tier sold on 1 to 5 year terms. It adds OIDC single sign-on to the Security Onion Console, LUKS data-at-rest encryption, FIPS mode, OS STIG compliance, the Security Onion API, external notifications, Manager of Managers, an MCP server, the Onion AI assistant and professional support. Ask the vendor which of these features your environment needs, how the term length, renewal and price are set, and what happens to the features and support if the subscription lapses. Those answers belong in the quote, alongside the question of whether the open-source edition alone meets your monitoring requirements.
Keep the licensing review and the Pro decision on the same timeline as the Splunk renewal, so the decision does not get made under contract pressure. Confirm with your legal and procurement teams whether the license terms affect your internal monitoring, your contractors and your managed service providers, and record the answer in your third-party inventory. If DeployOpen implements the platform for you, the scope document should state which components are used, which rules and packages are customized, and who holds responsibility for license compliance at each stage.
What each option costs to run, by component
Splunk and Security Onion costs fall into different buckets: a commercial license and its add-ons on one side, and hardware, storage and staff time on the other. Both sides carry recurring costs, so the comparison depends on volume, retention and how much detection work your team will own.
Splunk license and workload terms
Splunk is licensed on terms agreed in your contract, and the exact basis (ingest, workload or another model) decides how growth in data volume affects the bill. This is the largest recurring line for most buyers and it renews on a schedule you set. Ask the vendor or reseller to state the licensing basis in writing, the term length, the renewal uplift and what happens if daily volume exceeds the contracted level.
Splunk premium apps and support
Enterprise Security and other premium apps are separate line items on top of the platform license, and they often come up at renewal. Support tiers are also priced separately. Confirm which apps your detections depend on, whether they are included in the quote, and which support entitlements are bundled. Security Onion has no equivalent license line, so this cost has no direct counterpart on that side.
Security Onion hardware for each node role
Security Onion is free to download, but you pay for the servers it runs on. The supported design separates manager, search and sensor nodes, and each has its own minimum. The hardware docs list 4 cores and 16 GB RAM for a manager or search node, and 4 cores, 12 GB RAM and 2 NICs for a sensor. Standalone nodes need 4 cores, 24 GB RAM, 200 GB storage and 2 NICs. Production sizing will exceed these minimums, so price the hardware, or the cloud instances, against your traffic and retention targets.
Storage and retention for both platforms
Storage is the cost that scales fastest in either product. On Splunk it is shaped by ingest volume and index retention. On Security Onion, the search nodes hold Elasticsearch data and full packet capture consumes disk separately, so retention periods for logs and packets must be set independently. Ask for projected daily volume, the retention period per data type, and the storage tier behind each node before comparing totals.
Staff time for detections and upgrades
Security Onion moves detection tuning to your team: NIDS rules in Suricata, Sigma rules in ElastAlert and YARA rules in Strelka are managed through the Console, and each needs review, suppression and threshold work. Upgrades also consume time, since 2.4 installations on other distributions need a fresh install before reaching end of life. Estimate the engineer hours for tuning, patching and upgrades, and compare them with the Splunk content and administration work your team does today.
Security Onion Pro and outside services
Security Onion Pro is a paid edition sold on 1 to 5 year terms. It adds OIDC single sign-on, LUKS encryption, FIPS mode, OS STIG compliance, the Security Onion API and professional support. Those features are recurring, and the term length drives the annual figure. Confirm which Pro features your controls require before buying, and treat outside services such as implementation or testing as separate, scoped fees agreed in writing.
What a move from Splunk to Security Onion involves
The sequence turns a Splunk deployment into a Security Onion deployment that has been validated against the same traffic and events, so cutover is a decision backed by evidence rather than a date on a calendar.
Confirm operating system and hardware fit
Confirm that the target hardware is x86-64, because Security Onion supports only that processor type. Check the distribution requirement too: Security Onion 3.0.0 supports only Oracle Linux 9, and 2.x installs on other distributions cannot move to 3 in place. Your infrastructure team owns this check, and the output is a written hardware and OS list signed off by the security and platform leads. The 2.3 and 2.4 end-of-life dates in the Security Onion lifecycle docs also shape the timeline, so if you run 2.x today, decide early whether you upgrade to 3 or perform a fresh install.
Size nodes against the published minimums
Map your planned sensors, search nodes and manager to the minimum hardware figures in the Security Onion hardware documentation: a standalone node needs 4 cores, 24 GB RAM, 200 GB storage and 2 NICs, a manager or search node needs 4 cores and 16 GB RAM, and a sensor needs 4 cores, 12 GB RAM and 2 NICs. Treat these as floors, not production sizing. Storage for indexed events and full packet capture depends on your daily volume and retention target, which you must measure in your own Splunk license or index data before sizing. The output is a node inventory with storage calculated from your measured volume.
Deploy a non-production environment first
Install Security Onion in Evaluation mode to learn the Console, the detection workflow and the sensor placement on a lab network, or use Distributed mode if you intend to build the production design now. Distributed is the recommended design for production and consists of a manager node, one or more sensor nodes and one or more search nodes. Because the architecture docs note that search nodes stop indexing if they cannot reach the manager, plan manager availability and receiver redundancy from the start. The step is complete when a test sensor sees traffic and events reach the Console.
Run sensors and Splunk in parallel
Mirror or tap the same network segments that feed your current Splunk inputs into the new sensors, and keep existing Splunk forwarding running. Both platforms then receive the same traffic, which lets you compare alerts from each. Confirm which log sources Security Onion can ingest through Elastic Fleet agents and which must stay on Splunk during the overlap. The output is a source inventory that marks each feed as parallel, moved or retired.
Map correlation searches to Security Onion detections
Inventory your Splunk correlation searches and alerts, then sort them by type. Network-based detections map to Suricata NIDS rules, which the Security Onion Console manages with modify, suppress and threshold tuning. Log-based detections map to Sigma rules loaded into ElastAlert, using custom filters. File-based detections map to YARA rules loaded into Strelka. Searches that rely on SPL features, lookups or multi-event correlation without a Sigma or NIDS equivalent need a redesign, and some may have no direct equivalent, so confirm each one with your detection engineers. The output is a signed mapping sheet with each Splunk search marked as rebuilt, replaced or dropped.
Export historical search results
Splunk supports exporting search results through Splunk Web, the CLI, the REST API, SDKs, the dump search command and data forwarding, and Splunk describes the CLI as its most stable retrieval method. Pick the method that matches your volume, then export the retention window your analysts or auditors need, in raw, CSV or JSON form. Load what you need for reference into a separate archive, because Security Onion will not hold Splunk's index data. Confirm the export format and integrity with the people who will query the archive. The output is an archived set of historical results with a manifest of time ranges and record counts.
Validate detections and plan cutover
Run both platforms in parallel until the rebuilt detections fire on the same test events and analysts confirm the alert quality is acceptable. Record every divergence and close it or accept it in writing. Set the cutover criteria, the rollback path and the date you will stop Splunk ingestion, and agree them with the CISO and the platform owner. If you are on Security Onion Pro, confirm whether the features you need, such as OIDC single sign-on or the Security Onion API, fit your identity and integration plan before cutover. The output is a signed cutover record and a decommissioning plan for Splunk, including the export archive location and access controls.
Questions buyers ask before choosing Security Onion over Splunk
How much does Security Onion cost compared with Splunk?
Security Onion itself has no license fee for the free distribution, so the cost sits in hardware, storage, staff time and any outside help. Splunk's cost depends on its licensing model and your data volume, which you should confirm in your own contract. Your Security Onion budget will be driven by sensor count, traffic volume, retention period and how many search nodes you need. Security Onion Pro is a separate paid tier sold on 1 to 5 year terms, so ask for its current pricing directly. Request a written quote that separates one-time build work from recurring operations and support.
How long does it take to deploy Security Onion in production?
The timeline depends on hardware readiness, network design and how many sensors you deploy, so no single duration applies. The documentation recommends the Distributed deployment type for production, which means planning a manager node, sensor nodes and search nodes before installation. Hardware must meet the published minimums, and the platform supports only x86-64 processors. Confirm lead times for hardware and network taps or span ports early. A fixed-scope engagement lets you agree deliverables and a timeline in writing before work starts.
What work does our team have to do after installation?
Your team owns detection tuning, alert triage, upgrades and retention management after go-live. The Detections interface in the Console manages Suricata NIDS rules, Sigma rules loaded into ElastAlert and YARA rules loaded into Strelka, with modify, suppress and threshold tuning. Expect ongoing work to suppress noisy NIDS alerts and to write Sigma filters for your environment. Version currency also matters: Security Onion 2.4 reaches end of life on October 1, 2026, and 3.0.0 supports only Oracle Linux 9, so plan platform upgrades as recurring work.
Can we run Security Onion ourselves without a partner?
Yes, the platform is free to install and operate in your own infrastructure, but a self-run deployment depends on in-house skills in Linux, Suricata, Zeek and Elastic-based search. Teams with a small security staff often find the first build and the detection tuning take the longest. A partner can plan, implement, harden, document and support the deployment on a fixed scope, or provide retainer support afterward. Ask any provider to describe its scope in writing, including who handles upgrades and backups.
Does Security Onion work on air-gapped networks?
Yes, Security Onion supports full installs on networks without internet access. The ISO carries packages along with NIDS, YARA and Sigma rules, and the soup update tool can apply updates from newer ISO media in airgap mode. Plan for how you will bring new media across the boundary on a regular schedule, because rule and package currency depends on it. Confirm that your hardware, hypervisor and network design match the documented requirements before you commit to an air-gapped build.
Does Security Onion replace a full SIEM like Splunk Enterprise Security?
Security Onion covers network security monitoring, threat hunting, log management and alerting well, but it is not a drop-in replacement for every Splunk Enterprise Security capability. It bundles Suricata, Zeek, Elasticsearch and Elastic Fleet agents, and Splunk's app ecosystem, correlation content and SOAR features have no direct equivalent in the free distribution. Map your current correlation searches, compliance reports and response workflows before deciding. Where gaps remain, you may need custom rules or a separate orchestration tool.
What does Security Onion Pro add over the free edition?
Security Onion Pro is a paid edition that adds OIDC single sign-on to the Console, LUKS data-at-rest encryption, FIPS mode and OS STIG compliance. It also adds the Security Onion API, external notifications, Manager of Managers, an MCP server, the Onion AI assistant and professional support. Check whether your environment needs these controls at deployment time, because some, such as FIPS mode and STIG compliance, affect how you build the system. Ask the vendor to confirm the current terms and which features apply to your deployment.
Is Security Onion open source, and does that matter for our procurement?
Security Onion is free to use, but its repository is licensed under the Elastic License 2.0, which is source-available rather than an OSI-approved open-source license. Legal and procurement teams should review the license terms against their own policies before adoption. Some organizations treat source-available software differently in procurement, so confirm the terms with your counsel. The license affects how you can use and redistribute the code, not whether you can run it internally.
What to confirm with your own environment before deciding
Run a pilot on your own traffic before committing, because Security Onion's sizing depends on what your sensors actually see. Measure sustained and peak traffic at each tap or span port, not the averages from a spreadsheet, and check it against the minimum hardware for each role in the hardware documentation: a standalone node needs 4 cores, 24 GB RAM, 200 GB storage and 2 NICs, while sensors need 4 cores, 12 GB RAM and 2 NICs. Then test full packet capture under load. Security Onion 3.0.0 replaced Stenographer with Suricata for full packet capture, so the retention window you can keep on disk is the number to measure, not the number a datasheet implies. Compare that window with the retention your investigations and your regulators require, and write both figures down before you pick a deployment type.
Test detection coverage and integrations against your current Splunk content, not against a general list of rules. Export your correlation searches and map each one to a Suricata NIDS rule, a Sigma rule run through ElastAlert, or a custom rule you will need to write and maintain. Confirm which of your log sources reach Security Onion through Elastic Fleet agents, syslog or other inputs, and which need a custom parser or a feed you will build yourself. Identity requirements deserve the same rigor: Single sign-on through OIDC is listed as a Security Onion Pro feature, so if your directory requires SSO, the Pro edition, its price and its license terms belong in the same decision. Also check whether your team needs LUKS encryption, FIPS mode or a STIG-based OS baseline, since these are also Pro features and are not part of the free distribution.
Confirm the open items directly with Security Onion Solutions in writing. Ask for the current supported upgrade path for your installed version, the support window for the distribution you choose, and what the Security Onion Pro support terms cover. Ask how the architecture behaves when the manager node is down, since the architecture documentation says search nodes stop indexing without Redis access, and what your recovery time would be. Your own auditors should review the design before you rely on it for evidence: ask them which controls your assessment expects you to show for log retention, access review and monitoring, and how they will accept a platform whose license is source-available rather than OSI open source. Finally, assess your staff honestly. Someone must own rule tuning, upgrades and air-gapped updates through the soup tool, and that skill set is the cost most likely to be underestimated.
