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⚙️ Configuration

Understanding how the Caddy Web Application Firewall (WAF) processes incoming requests, evaluates rules, and decides when to block a request is crucial for effective configuration and management. This section explains the request lifecycle within the WAF, including the order of operations and the logic behind blocking decisions.

Request Processing Flow: From Caddy to the WAF

  1. Incoming Request:
    When a client sends an HTTP request to your Caddy server, it is first handled by Caddy's core request-handling mechanism.

  2. WAF Middleware Invoked:
    If a waf block is configured in your Caddyfile, the WAF middleware is invoked as the first handler in the route.

  3. Request Context Initialization:
    The WAF initializes a WAFState struct for each incoming request. This struct holds:

    • The request's anomaly score (TotalScore).
    • Its blocking status (Blocked).
    • The response status code (StatusCode).
    • A unique log identifier for tracing the request in logs.

  4. Phase-Based Evaluation:
    The WAF processes the request in distinct phases, each representing a specific stage of the request lifecycle. Phases are executed in numerical order:

    • Phase 1: Request Headers (and Early Checks)
      This phase occurs before the request body is parsed and includes:

      • Country Blocking/Whitelisting (Optional):
        Checks the request's source IP against a configured country list. If the IP originates from a blocked country (or not from a whitelisted country), the request is immediately blocked.
      • Rate Limiting (Optional):
        Checks the rate limiter against the client IP and request path. If the request count exceeds the limit within the configured time window, the request is blocked.
      • IP Blacklisting:
        Checks the request's source IP against the configured IP blacklist. If a match is found (direct IP or CIDR range), the request is blocked.
      • DNS Blacklisting:
        Checks the request's Host header against the DNS blacklist. If a match is found, the request is blocked.
      • Rule Evaluation for Request Headers:
        Checks the request headers against the configured rules for Phase 1.

    • Phase 2: Request Body:
      Analyzes the request body against rules configured for this phase, looking for malicious payloads.

    • Phase 3: Response Headers:
      After the request is processed by the backend, the WAF analyzes the response headers before sending them to the client, looking for malicious payloads or unintended information.

    • Phase 4: Response Body:
      After the response is processed by the backend, the WAF analyzes the response body before sending it to the client, looking for malicious or unintended content.

  5. Rule Evaluation within Each Phase:

    • For each phase, the WAF iterates through the rules defined for that phase, respecting their order and priority.
    • For each rule, the WAF extracts the configured target (e.g., URL, headers, body, cookies) from the request (or response in Phases 3 and 4).
    • The extracted value is matched against the rule's regular expression (pattern).
    • If a match is found:
      • The rule's hit count is incremented.
      • The rule's score is added to the request's TotalScore.
      • The WAF checks if the rule's action is block or if the TotalScore exceeds the anomaly_threshold.
      • If either condition is met:
        • The request is marked as Blocked.
        • The response status code is set to 403 Forbidden (customizable).
        • A WARN log entry is created with details about the blocked request.
        • If a custom response is defined, it is sent to the client.
        • Request processing stops immediately, and no further WAF rules or handlers are executed.
      • If the rule's action is log, the match is logged at INFO level, and the request continues processing.

  6. Request Continues or Blocked:

    • If the request is not Blocked, it proceeds to the next handler on the route (e.g., your application's handler).
    • If the request is Blocked, a 403 error (or custom response) is returned to the client, and no further handlers are executed.

  7. Metrics Collection:
    For every request processed, the WAF updates internal counters:

    • total_requests: Incremented for all requests.
    • blocked_requests: Incremented if the request is blocked.
    • allowed_requests: Incremented if the request is allowed.

  8. Metrics Exposure:
    A /waf_metrics endpoint provides a JSON output of these metrics for monitoring and troubleshooting, offering insights into WAF activity.

Blocking Logic and Precedence

  • Early Checks Take Precedence:
    Country blocking/whitelisting, rate limiting, IP blacklisting, and DNS blacklisting in Phase 1 take precedence over rule-based checks. If a request is blocked by any of these, further rule evaluations are skipped.

  • Rule Priority:
    Within each phase, rules are evaluated in the order they appear in the configuration file, with higher priority rules evaluated first.

  • Anomaly Scoring:
    The anomaly_threshold blocks requests that trigger multiple lower-severity rules by accumulating their scores.

  • Rule Action block:
    If a rule has the block action, the request is immediately blocked, regardless of the anomaly_threshold or other rules.

  • First Match Blocks (with Exception):
    If a rule matches and the request is blocked, processing stops immediately, except for rules with the log action, which only log the match and continue processing.

  • Custom Responses:
    Custom responses for blocked requests take precedence over the default blocking message.

  • Short Circuiting:
    If a request is blocked, processing stops immediately, saving resources and ensuring a fast response.

  • Response Processing:
    In Phases 3 and 4, the WAF evaluates the response being sent to the client. If the response is blocked, the configured custom response is returned instead.

Key Takeaways

  • Ordered Processing:
    The WAF processes requests in a specific order, ensuring higher-priority checks are performed first.

  • Flexible Blocking:
    Requests can be blocked based on explicit rules, anomaly scores, country restrictions, rate limits, blacklists, or a combination of factors.

  • Customizable Behavior:
    The WAF is highly customizable through configuration options, allowing adaptation to specific security needs.

  • Efficient Handling:
    Short-circuiting ensures that once a request is blocked, no further processing is performed, improving performance.

  • Response Evaluation:
    The WAF evaluates responses and blocks them if necessary, ensuring only safe content is sent to the client.

Configuration Options

The WAF provides a variety of configuration options to control its behavior. These options are typically set in the Caddyfile. Below is a detailed table of each option:

Option Description Example
anomaly_threshold Sets the threshold for the anomaly score. Requests exceeding this score are blocked. anomaly_threshold 20
rule_file Path to the JSON file containing the WAF's ruleset. rule_file rules.json
ip_blacklist_file Path to the file containing blacklisted IP addresses and CIDR ranges. ip_blacklist_file blacklist.txt
dns_blacklist_file Path to the file containing blacklisted domain names. dns_blacklist_file domains.txt
rate_limit Configures rate limiting for incoming requests. Requires parameters like requests, window, and cleanup_interval. rate_limit { requests 100 window 1m cleanup_interval 5m paths /api/v1/.* match_all_paths false }
block_countries Blocks requests from specified countries using the MaxMind GeoIP2 database. block_countries GeoLite2-Country.mmdb RU CN
whitelist_countries Whitelists requests from specified countries. Requests from non-whitelisted countries are blocked. whitelist_countries GeoLite2-Country.mmdb US CA
log_severity Sets the minimum logging level (debug, info, warn, error). log_severity info
log_json Enables JSON format for log messages. log_json
log_path Specifies the path for the WAF log file. log_path /var/log/waf/access.log
redact_sensitive_data Redacts sensitive data from the request query string in logs. redact_sensitive_data
custom_response Defines custom HTTP responses for blocked requests. Requires status code, content type, and response content or file path. custom_response 403 application/json error.json
  • GeoIP:

Note

The request will be geo-whitelisted if both block_countries and whitelist_countries are used and the same country code is specified on both directives.

By carefully configuring these options, you can tailor the WAF's behavior to meet your specific security requirements, balancing protection with performance. Thorough testing after making changes is essential to ensure the WAF operates as expected.