Snort IDS / IPS Complete Practical Guide | TryHackme

We covered configuring snort as an IDS/IPS open-source solution. Snort operates as sniffer, packet logger and IPS/IDS. This was part of TryHackMe Snort.

Introduction to Snort and IDS/IPS Concepts:

• Snort can operate both as an IDS and IPS, depending on its configuration. IDS (Intrusion Detection System) and IPS (Intrusion Prevention System) work by analyzing network traffic against a set of rules.
• IDS: Monitors and alerts when traffic matches rules but does not block the traffic.
• IPS: Monitors, alerts, and can actively block or reject malicious traffic.

Explanation of IDS and IPS:

• IDS: Logs and alerts when traffic matches pre-configured rules but does not interfere with the network traffic.
• IPS: In addition to logging and alerting, it can drop or reject packets or entire connections based on the detected threat.
• Difference between drop and reject: Dropping only removes the identified packet, while rejecting cuts off the entire connection.
• Both IDS and IPS work based on a set of rules that define acceptable and unacceptable network traffic patterns.

NIDS and HIDS (Network and Host Intrusion Detection Systems):

• NIDS (Network Intrusion Detection System): Operates on a network level and monitors traffic across an entire subnet or range of IPs.
• HIDS (Host Intrusion Detection System): Works on a single host or endpoint, monitoring traffic only for that specific machine.

Detection and Prevention Techniques:

• Signature-based detection: Matches known patterns (like malware or attack signatures) with network traffic.
• Behavior-based detection: Learns normal traffic behavior over time and flags abnormal behavior, even if it doesn’t match a known signature.
• Policy-based detection: Uses system configurations or security policies to define what traffic or actions are allowed or blocked.

Snort Overview:

• Snort is a flexible IDS/IPS tool capable of real-time traffic analysis and packet logging.
• It operates in three main modes:
  • Sniffer mode: Captures and displays network traffic in real-time.
  • Packet Logger mode: Logs packets for future analysis.
  • NIDS/NIPS mode: Acts as either a network IDS or IPS, analyzing and acting on traffic in real-time.

Snort Configuration:

• Configuration file: Defines key settings such as the path to rules, variables for home and external networks, and plugins. By default, the configuration file is located in /etc/snort/snort.conf.
• Rules file: Located in /etc/snort/local.rules, where specific traffic patterns are defined for Snort to detect and respond to.

The snort.conf file is the central nervous system for Snort’s operation. It’s where you define how Snort understands your network:

• Key Purpose: Defines critical network variables (IP addresses, network ranges) and port variables.
• Why Variables Matter: These variables (e.g., $HOME_NET, $EXTERNAL_NET, $HTTP_PORTS) are used in Snort rules, making them more readable, manageable, and adaptable to different environments.
• Descriptive Naming: The tutorial emphasizes using clear, descriptive names for variables.
• Core Variables:
  • $HOME_NET: Defines your internal network’s IP range that you want to protect.
  • $EXTERNAL_NET: Represents all networks outside your $HOME_NET.

Editing Snort.config

Editing the File: Typically done using a command-line editor like nano.

Defining $HOME_NET: Modifying this variable to accurately reflect your local network’s IP address and subnet mask (e.g., ipvar HOME_NET 192.168.94.0/24).

Defining $EXTERNAL_NET: Often set to any or !$HOME_NET due to the impracticality of listing all external IPs.

Other Network Variables: You can also define variables for specific servers like DNS_SERVERS, SMTP_SERVERS, or SQL_SERVERS using the ipvar keyword (e.g., ipvar DMZ_NET [10.0.0.0/24]).

Defining Port Variables: Similar to IP variables, port variables like $HTTP_PORTS (e.g., portvar HTTP_PORTS 80) or $SSH_PORTS (e.g., portvar SSH_PORTS 22) simplify rule writing.

Saving is Key: Always remember to save your changes to snort.conf!

Configuring Alerts

An IDS is only useful if it tells you what it finds. Configuring alert logging is essential:

• Location: Also configured within snort.conf.
• Output Plugins: Find the “configure output plugins” section (often labeled as step six).
• Enabling Fast Alerts: Add or uncomment a line like output alert_fast: fast_alert.log (the video mentions fast_packet, but alert_fast is common for a summarized log format). This ensures that when Snort detects suspicious activity, an alert is logged for review and incident response.

Snort Rules

Understanding the structure of a Snort rule is fundamental:

Rule Action Protocol Source_IP Source_Port Direction_Operator Destination_IP Destination_Port (Rule_Body_Options)

1. Rule Action: What Snort does (e.g., alert, log, pass, drop).
2. Protocol: The network protocol (tcp, udp, icmp, ip).
3. Source IP & Port: Origin of the traffic (can use variables like $EXTERNAL_NET).
4. Direction Operator: -> (unidirectional) or <> (bidirectional).
5. Destination IP & Port: Target of the traffic (can use variables like $HOME_NET).
6. Rule Body (Options): Enclosed in parentheses; contains keywords for detailed inspection, messages, SIDs, etc.

While Snort comes with community rule sets, you’ll often need to write your own:

• Purpose: The local.rules file is the designated place for users to add their custom Snort rules or make modifications without altering the main rule files.

Understanding Rule Actions

• alert: Generates an alert and logs the packet (primary for IDS).
• log: Logs the packet.
• pass: Ignores the packet.
• drop: (IPS mode) Blocks the packet silently.
• reject: (IPS mode) Blocks and sends a TCP reset/ICMP error.
• sdrop: (IPS mode) Silently blocks the packet, no logging.

Crafting Snort Rules

• Basic Alert:alert tcp any any -> $HOME_NET any (msg:"Incoming TCP Traffic to HOME_NET"; sid:1000001; rev:1;)
  • Alerts on any TCP traffic from any external source to your home network.
• Bidirectional Alert: alert tcp $EXTERNAL_NET any <> $HOME_NET any (msg:"Bidirectional TCP Traffic"; sid:1000002; rev:1;)
• Drop Rule (IPS example):drop tcp $HOME_NET any -> $EXTERNAL_NET any (msg:"BLOCKING Outbound Internet Access"; sid:1000003; rev:1;)
  • This would severely restrict internet access.
• Snort ID (SID) [00:20:43 – 00:21:45]: A unique identifier for each rule. Custom rules usually start from a high number (e.g., 1,000,000) to avoid clashes with official rule sets.
• Commenting [00:21:56 – 00:24:30]: Always add comments (#) to your local.rules to explain the purpose of each custom rule.

Deep Packet Inspection with Content Matching

Snort’s true power lies in its ability to inspect the payload of packets:

• The content Keyword: Allows searching for specific byte sequences, strings, or patterns within packet data.
• Precision: Snort can use offsets and depth parameters to look at specific parts of the payload.
• Detection Capabilities:
  • File Hashes: Match against known malicious file hashes.
  • Hex Values: Detect specific byte sequences (e.g., shellcode).
  • Keywords/Strings: Find suspicious words in web pages, commands, etc.
  • URLs: Inspect URLs for malicious patterns.
• Combined with Actions: When a content match occurs, the defined rule action (alert, drop, etc.) is triggered.

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