How Intrusion Detection Systems (IDS) Work

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What is an Intrusion Detection System (IDS)?​

An Intrusion Detection System (IDS) is a software or hardware solution designed to monitor network traffic, system events, or user actions to identify suspicious activity or potential attacks. An IDS does not prevent attacks directly, but it alerts administrators to potential threats so they can take action.

How does IDS work?​

IDS analyzes data in real time and compares it to known attack patterns or deviations from normal behavior. IDS work based on two main approaches:

1. Signature analysis:
   • Compares network traffic or system events against a database of known threats (signatures).
   • Example: If a packet matching the SQL injection signature is detected in traffic, the IDS generates an alert.
2. Behavior (anomaly) analysis:
   • Monitors deviations from "normal" behavior of a system or network.
   • Example: If a server that normally sends 1GB of data per day suddenly starts sending 100GB, it could be a sign of an attack.

Main types of IDS​

1. NIDS (Network-based Intrusion Detection System):
   • Analyzes network traffic.
   • Installed at key network points (e.g. routers or firewalls).
   • Example: Snort, Suricata.
2. HIDS (Host-based Intrusion Detection System):
   • Monitors activity on a specific device (host).
   • Analyzes logs, file system changes, processes and system calls.
   • Example: OSSEC, Tripwire.
3. Hybrid IDS:
   • Combines the functionality of NIDS and HIDS.
   • Analyzes both network traffic and host events.
4. Wireless IDS (WIDS):
   • Specializes in monitoring wireless networks.
   • Detects attacks such as Wi-Fi traffic interception or creation of fake access points.
5. Cloud IDS:
   • Designed to protect cloud infrastructures.
   • Analyzes traffic and events in cloud services (e.g. AWS, Azure).

Stages of IDS work​

1. Data collection​

• IDS collects data from various sources:
  • Network traffic packets.
  • Operating system or application logs.
  • System calls, processes and files.

2. Data Analysis​

• The data is analyzed using one of two approaches:
  • Signature Analysis: Searches for matches with known threats.
  • Anomaly Analysis: Finding deviations from normal behavior.

3. Generating alerts​

• If suspicious activity is detected, the IDS generates an alert.
• Notifications can be presented in the form of:
  • Messages in the console.
  • Logs.
  • Notifications via email or SMS.

4. Reaction​

• IDS itself does not prevent attacks, but provides information to take action:
  • The administrator can block the IP address.
  • The intrusion prevention system (IPS) may be activated.

Advantages of IDS​

1. Early detection of threats:
   • Helps identify attacks at early stages.
2. Real-time monitoring:
   • Continuously monitors network or system activity.
3. Flexibility:
   • Suitable for various environments: corporate networks, cloud infrastructures, individual devices.
4. Integration with other systems:
   • Can work with SIEM (Security Information and Event Management) for data analysis.

Disadvantages of IDS​

1. False positives:
   • IDS can generate a lot of false alarms, especially when analyzing anomalies.
2. Signature dependency:
   • Signature analysis is ineffective against new (zero) threats.
3. Resource intensity:
   • Analyzing large amounts of data can require significant computing resources.
4. Does not prevent attacks:
   • IDS only detects threats, but does not block them (unlike IPS).

Example of IDS operation​

Scenario: Detecting a DDoS Attack​

1. Data collection:
   • NIDS records a sharp increase in incoming traffic to the web server.
2. Data Analysis:
   • Signature analysis identifies packets characteristic of a DDoS attack.
   • Anomaly analysis shows that traffic is significantly higher than normal.
3. Generating alerts:
   • IDS sends a notification to the administrator: "Potential DDoS attack detected."
4. Reaction:
   • The administrator activates firewall rules to block suspicious IP addresses.

Difference between IDS and IPS​

• IDS (Intrusion Detection System):
  • Only detects threats.
  • Does not take automatic actions.
• IPS (Intrusion Prevention System):
  • Detects and prevents threats in real time.
  • Can block suspicious traffic or actions.

Popular IDS tools​

1. Snort:
   • Open NIDS/HIDS with signature and behavioral analysis support.
2. Meerkat:
   • Modern NIDS with support for multi-threading and advanced analysis.
3. OSSEC:
   • HIDS with an emphasis on log and system event analysis.
4. Zeek (formerly Bro):
   • A powerful tool for analyzing network traffic and identifying complex threats.
5. Darktrace:
   • Advanced machine learning based system for anomaly detection.

Conclusion​

Intrusion detection systems (IDS) play an important role in ensuring the security of networks and systems. They help to identify attacks at an early stage and provide information for decision-making. However, it is important to understand that IDS does not replace other means of protection (such as firewalls or antiviruses), but complements them.

If you have additional questions about how IDS works or examples of their use, please ask!

 

[Man]

IDS (Intrusion Detection System) is a software or hardware solution that monitors network traffic or activity on hosts, identifying suspicious activity and potential attacks.

1. Types of IDS​

1.1. By location​

Type	Where does it work?	What does it analyze?
Network (NIDS)	At the edge of the network (eg router)	All traffic (packets, DDoS attacks, port scanning).
Host (HIDS)	On a specific server/PC	Logs, file changes, processes.

1.2. By detection method​

Type	Operating principle	Pros and cons
Signature	Compares with a database of known attacks (like an antivirus).	Accuracy against known threats. Does not catch zero-day.
Anomaly (AI)	Looks for deviations from the "norm" (machine learning).	Catches new threats. False positives.

2. How exactly does IDS detect attacks?​

2.1. Examples of triggers​

• Network attacks:
  • Suspicious packets (eg SYN flood).
  • Attempts to exploit vulnerabilities (SQL injections in HTTP requests).
• Host attacks:
  • Modifying system files (eg /etc/passwd).
  • Launching malicious processes (cryptominers).

2.2. Stages of work​

1. Data collection:
   • NIDS: copies traffic via port mirroring (SPAN) .
   • HIDS: reads logs (/var/log/), checks file integrity.
2. Analysis:
   • Signature-based: looks for patterns (for example, "../../etc/passwd" in the URL).
   • Behavioral: detects anomalies (sharp increase in outgoing traffic).
3. Reaction:
   • Sending an alert to the administrator.
   • IP blocking (if IDS is integrated with firewall).

3. Popular IDS​

Name	Type	Peculiarities
Snort	NIDS (signature)	Free, flexible rules.
Meerkat	NIDS (hybrid)	Multithreading support.
OSSEC	HIDS	Log analysis, rootkit detection.
Sea (Bro)	NESTS	Deep traffic analysis (L7).

4. IDS Limitations​

• False Positives: False positives (e.g. a legitimate vulnerability scanner).
• Does not block attacks: Only detects (unlike IPS - Intrusion Prevention System).
• Difficulty of setup: Rules for signature-based IDS need to be updated manually.

5. How to strengthen protection?​

1. Combine NIDS + HIDS for complete coverage.
2. Use IPS (eg Suricata in IPS mode).
3. Integrate with SIEM (Splunk, Wazuh) for event correlation.
4. Set up automatic responses:
   • IP blocking via iptables/fail2ban.
   • Stopping suspicious processes.

Conclusion​

1. An IDS is a "watchdog" that only warns of attacks.
2. NIDS monitors the network, HIDS monitors individual servers.
3. Signature systems are accurate but blind to new threats, while anomaly systems are the opposite.

For maximum protection, use IDS + IPS + SIEM . Need help with setup? I'm ready to explain the details!