What Is an IDS: Types, IPS Differences and Snort vs Suricata

An IDS detects intrusions on network or host and generates alerts without blocking the traffic by itself. The acronym stands for Intrusion Detection System. Monitors network traffic, system events or both, looking for patterns that indicate an intrusion attempt or policy violation. The name has been in use for several decades and, although the category has evolved toward modern products like NDR (Network Detection and Response) and XDR, the principles stay the same: observe, compare against rules or models, alert. In 2026 the classic IDS doesn't die; it coexists with NDR and integrates as a layer of a broader defensive programme.

This guide explains what an IDS actually is, the two classic categories (NIDS network-based and HIDS host-based), the precise differences with IPS, firewall and SIEM, practical comparison of the three platforms that appear most in real production (Snort, Suricata, Zeek), how it fits the SIEM and SOC ecosystem, common deployment mistakes and when an IDS makes sense versus modern NDR or integrated XDR.

What an IDS is

An IDS is a passive monitoring system that detects suspicious activity and generates alerts, without blocking the traffic or activity by itself. The four defining ideas:

1. Passive. Observes, doesn't intervene. That's what distinguishes it from IPS.
2. Rule-based, anomaly-based or both. Rules detect known patterns (signatures); anomaly analysis detects deviations from the normal baseline.
3. Generates events. The alert lands in a SIEM, on a SOC team or in a mailbox. Action is taken by someone afterwards.
4. Covers network, host or both. NIDS observes traffic; HIDS observes system events.

What it gives operationally:

• Visibility of internal and external traffic without requiring an agent on every endpoint.
• Lateral movement detection that a perimeter firewall doesn't see.
• Forensic evidence after an incident.
• Documentable compliance in NIS2, DORA, ISO 27001, ENS, PCI DSS.
• Low cost on well-operated open source platforms.

What limits it:

• Only alerts, doesn't block. Without a team to act, the alert is decoration.
• False positives. Without tuning, it drowns the SOC and ends up ignored.
• Encryption. TLS 1.3 and QUIC reduce what a NIDS can inspect without TLS termination.
• Traffic encrypted by VPN or modern applications stays out of reach.

NIDS vs HIDS

The classic division that remains valid.

NIDS (Network-based IDS)

Sensor that captures network traffic at a strategic point (SPAN port, network TAP, equivalent cloud layer) and analyses it. Rules comparing packets against signatures, engines detecting statistical anomalies, protocol decoders.

Open source products: Snort, Suricata, Zeek. Commercial: Cisco Firepower, Palo Alto WildFire, Check Point IPS, Trellix Network Security Platform.

Advantage: a single install covers many hosts. Inconvenience: encrypted TLS limits it, VPN or modern application encapsulation too.

HIDS (Host-based IDS)

Agent installed on the endpoint that monitors operating system events: logs, processes, connections, file integrity (FIM), syscalls.

Open source products: OSSEC (now discontinued in favour of Wazuh), Wazuh agent, AIDE for pure FIM. Commercial: most modern EDRs include HIDS functionality (CrowdStrike Falcon, SentinelOne, Microsoft Defender for Endpoint).

Advantage: sees decrypted content on the host, captures events without depending on the network. Inconvenience: requires agent, maintaining coverage, per-endpoint weight.

In modern practice, the two coexist. The NIDS and HIDS separation has blurred inside NDR, EDR and XDR platforms; the principle remains valid for defensive architecture.

IDS, IPS, firewall and SIEM

Confusion between these terms is common. Practical differences:

• Firewall. Blocks or allows traffic based on L3-L7 rules. Preventive. Modern ones (Next-Generation Firewall) integrate IDS/IPS capabilities and application filtering.
• IDS. Detects and alerts on suspicious traffic. Passive. Doesn't cut the connection.
• IPS (Intrusion Prevention System). Detects and blocks. The active evolution of IDS. In modern products usually appears as a module inside the NGFW.
• SIEM. Platform that ingests logs from IDS, firewall, EDR, identity and many other sources, correlates them and generates richer alerts. Detail in what is a SIEM.
• NDR (Network Detection and Response). Modern category extending the classic IDS with ML, threat intelligence and integrated response capabilities.
• EDR / XDR. Endpoint equivalents. On paper they cover the HIDS role and go beyond.

In a typical architecture: the firewall blocks the known bad, the IDS/NDR observes what passes, the SIEM correlates, the SOC acts. Each piece has a specific function; none replaces the other.

Snort, Suricata and Zeek: practical comparison

The three open source platforms that appear most in real production, especially in Spanish mid-market and the public sector.

Snort

The oldest and best-known open source IDS. Created by Marty Roesch (Sourcefire, now Cisco). Rules in .rules format comparing patterns against packets. Active community, community rule set (Snort Community Rules) and commercial (Talos Subscriber). Snort 3 (2021+) redesigned the architecture for multi-threading.

• Best for: signature-based detection, legacy integrations, organisations with prior Sourcefire/Cisco experience.
• Limitations: limited anomaly detection, ageing rule syntax, less prepared for high throughput without tuning.

Suricata

Launched in 2009 by OISF (Open Information Security Foundation). Rules compatible with Snort plus its own extensions. Native multi-threading, protocol parsing, writing relevant traffic to disk, file extraction, Lua scripting. Direct output to EVE JSON, ideal for SIEM integration.

• Best for: high throughput (10G/40G/100G with appropriate hardware), modern use, integration with Elastic, Splunk or Wazuh stacks.
• Limitations: requires fine tuning to avoid false positives, medium-high operational complexity.

Has become the modern default when an organisation chooses open source IDS/NDR from scratch.

Zeek (formerly Bro)

Launched in 1994 by Vern Paxson (LBNL). Different philosophy: not a rule engine, it's a deep protocol analyser generating very rich structured logs (conn.log, http.log, dns.log, ssl.log, files.log). Scripts in its own Bro/Zeek scripting language to define custom detection logic.

• Best for: forensic analysis, threat hunting, organisations with a team capable of writing scripts. Generates the richest dataset of the triad.
• Limitations: high learning curve, requires SIEM or ELK behind it to exploit logs, doesn't detect "out of the box" like Snort/Suricata.

Typical stacks combining the three

Realistic patterns in Spanish clients:

• Suricata only. The most common choice in mid-market. Sufficient coverage with community or commercial rules (ET Open, ET Pro from Proofpoint, ANY.RUN).
• Suricata + Zeek parallel. Double pipeline: Suricata for immediate alerts, Zeek for forensic enrichment. Everything goes to the SIEM for correlation.
• Wazuh with network IDS module. For organisations already running Wazuh as SIEM.
• Commercial NDR on top of Suricata. Products like Stamus Networks, Corelight, Vectra build on open source engines and add ML, UI, cloud integration and support.

Integration with SIEM and SOC

An isolated IDS doesn't deliver real value. Correct integration:

1. Structured output from the IDS to the SIEM (Splunk, Elastic, Wazuh, Microsoft Sentinel). Suricata writes native EVE JSON; Zeek emits structured logs.
2. Correlation in SIEM with other sources: firewall, EDR, identity, DNS, proxy.
3. Detection rules that combine IDS events with context. An isolated IDS alert may be a false positive; correlated with an anomalous login at the same time on another system, it rises in priority.
4. Playbooks the SOC team executes after each type of IDS alert: isolation, forensic capture, communication.
5. Feedback loop. The SOC feeds back rule tuning to reduce false positives.

An IDS without a SOC or response process is a noise generator. The IDS only delivers if the defensive team operates it.

Common deployment mistakes

What gets seen in audits and projects with clients having an IDS deployed but poorly operated.

Default rules without tuning. Generates thousands of false positives in the first hours. The team desensitises, alerts get ignored, the important ones get lost in the noise.

Wrong sensor placement. Sensor only at the perimeter leaves internal lateral movement blind. Sensor only in the datacenter leaves outbound traffic blind. The right approach is multiple sensors, with east-west and north-south visibility.

Insufficient capacity. Sensor receiving 10 Gbps with hardware dimensioned for 1 Gbps leaves packets uninspected. The packet loss figure in production should be zero or close to it.

TLS without termination. 95% of modern traffic is encrypted. Without authorised TLS termination at the perimeter or without complementary endpoint visibility, the IDS only sees handshakes.

Outdated rules. A rule set without updates for months falls behind real threats. Subscription to commercial feeds (ET Pro, Talos) or daily community feed updates.

No SIEM integration. The IDS generates its alerts in an isolated console no one watches. Every alert should go to the SIEM and enter the normal triage flow.

Confusing IDS with compliance only. Buying IDS to tick the NIS2 or PCI box without operating it. A serious audit detects the lack of real operation and fails.

When IDS fits versus modern NDR

2026 decision in mid-sized enterprise:

• Open source IDS (Suricata) + SIEM: tight budget, internal technical team or MSSP partner, organisation with predominantly flat traffic and classic architecture.
• Commercial NDR (Vectra, ExtraHop, Darktrace, Corelight): budget available, need for automatic ML detection on behaviour, organisations with complex traffic, multi-cloud, OT.
• EDR / XDR + cloud telemetry: purely cloud-native enterprise, no traditional perimeter. The IDS concept dilutes inside XDR.
• Combination of the three layers: large organisations or critical sectors (NIS2 essential, DORA, energy, health).

No product replaces a team capable of operating it. The technical decision matters less than the organisational decision to have someone watching the alerts.

Compliance fit

IDS or equivalent NDR is directly or indirectly required in current frameworks:

• NIS2 (article 21). Incident detection, continuous monitoring. Without network detection capability, non-compliance is defensible in an audit.
• DORA (article 9). ICT resilience in financial services. Formal monitoring with metrics.
• ISO 27001:2022 (controls 8.16, 8.20, 8.21). Monitoring activities, secure networks, network services.
• ENS Royal Decree 311/2022 (op.mon). Monitoring measures system.
• PCI DSS v4.0 (req. 11.5). Mandatory intrusion detection and prevention mechanisms for environments processing card data.
• GDPR. The ability to detect and notify breaches within 72 hours requires real technical visibility.

Frequently asked questions

Exact difference between IDS and IPS?

The IDS detects and alerts; the IPS detects and blocks. Functionally IPS is IDS plus inline action capability. Modern products (NGFW, NDR) usually offer both modes on the same platform. The decision depends on accidental blocking risk: an IPS that blocks legitimate critical traffic generates an operational incident, while an IDS that only alerts is safe against false positives.

Do I need IDS if I already have EDR?

They cover different things. The EDR sees the endpoint and what happens inside, even encrypted traffic decoded by the browser. The IDS sees traffic between endpoints, including agent-less systems (printers, OT, routers, IoT). Realistic coverage combines both.

Does Suricata replace Snort?

In new deployments, almost always. Suricata is compatible with most Snort rules, scales better on multi-core and emits native JSON logs. Snort remains present in organisations with legacy investment or Cisco stacks that prefer to keep the vendor's tool.

What's Zeek for if I already have Suricata?

For forensic analysis and deep threat hunting. Suricata alerts; Zeek generates the dataset that lets you reconstruct what happened during an intrusion. Serious teams deploy both.

Can IDS inspect TLS traffic?

Only with authorised TLS termination. The usual practice in enterprise is deploying a proxy or NGFW that decrypts outbound traffic with a corporate root certificate distributed to endpoints, inspects and re-encrypts. Without this model, network IDS gets limited visibility to SNI, certificate and metadata.

What about QUIC and HTTP/3?

Suricata 7+ and Zeek 6+ partially parse QUIC. HTTP/3 over QUIC complicates inspection because it goes end-to-end encrypted. The industry is adapting; in 2026 the real coverage is improvable. The complementary defence is endpoint telemetry, which sees decrypted content.

What's a reasonable false positive rate?

After initial calibration, a mature organisation maintains rates below 5-10% on high severity alerts. Above that, noise saturates the SOC and the system stops being effective. Continuous rule review is part of the operational cost.

Related resources

• What is a SIEM: platform that ingests and correlates IDS events alongside other sources.
• Wazuh open source: SIEM + HIDS open source with integrated IDS capabilities.
• What is a SOC: team that operates the IDS, reviews alerts and executes response.
• What is an EDR and what is MDR: controls complementary to IDS in the defensive stack.
• What is threat hunting: discipline that leverages Zeek's rich telemetry and IDS logs for proactive hunting.
• What is MITRE ATT&CK: catalogue where every IDS rule should be tagged with its corresponding technique.

IDS design and operation at Secra

At Secra we work on the client side with IDS in three typical situations: review of existing deployment (coverage, sensor placement, rule tuning, real false positives, SIEM integration), new stack design (choice between Suricata, Zeek or commercial NDR depending on budget, architecture and obligations) and empirical validation via Red Team that checks whether the IDS detects real TTPs. If your organisation is about to buy IDS, already has one but it generates more noise than value or has to document monitoring capability under NIS2 or DORA, get in touch via contact.