Fluent Bit Config Validator

Overview

This skill provides a comprehensive validation workflow for Fluent Bit configurations, combining syntax validation, semantic checks, security auditing, best practice enforcement, and dry-run testing. Validate Fluent Bit configs with confidence before deploying to production.

Validation Workflow

Follow this sequential validation workflow. Each stage catches different types of issues.

Recommended: For comprehensive validation, use --check all which runs all validation stages in sequence:

python3 scripts/validate_config.py --file <config-file> --check all

Validation Stages Summary

Individual check usage (for debugging specific issues):

python3 scripts/validate_config.py --file <config-file> --check <stage-type>

Detailed section validation rules: See references/SECTION-RULES.md for comprehensive requirements, valid plugins, field specifications, and best practices for SERVICE, INPUT, FILTER, OUTPUT, and PARSER sections

Tag Consistency Check

Validates: INPUT tags match FILTER Match patterns; FILTER tags match OUTPUT Match patterns; no orphaned filters or outputs; wildcard usage is correct.

Example:

[INPUT]
    Tag    kube.*     # Produces: kube.var.log.containers.pod.log

[FILTER]
    Match  kube.*     # Matches: ✅

[OUTPUT]
    Match  app.*      # Matches: ❌ No logs will reach this output

Security Audit

Checks:

1. Hardcoded credentials: HTTP_User/Passwd, AWS keys, API tokens in plain text
2. TLS configuration: TLS disabled; tls.verify Off; missing certificate files
3. File permissions: DB and parser files readable/writable
4. Network exposure: INPUTs listening on 0.0.0.0 without auth; HTTP_Server exposed without auth

Auto-fix pattern:

# Before (insecure)
[OUTPUT]
    HTTP_User     admin
    HTTP_Passwd   password123

# After (secure)
[OUTPUT]
    HTTP_User     ${ES_USER}
    HTTP_Passwd   ${ES_PASSWORD}

Performance Analysis

Key checks:

• Mem_Buf_Limit set on all tail inputs
• storage.total_limit_size set on outputs
• Flush interval appropriate (1–5s)
• Skip_Long_Lines On; compression on network outputs
• Kubernetes: Buffer_Size 0 for kubernetes filter recommended

Dry-Run Testing

fluent-bit -c <config-file> --dry-run

Catches: config parsing errors, plugin loading errors, parser syntax errors, file permission issues, missing dependencies.

If fluent-bit binary is not available: skip this stage, document that dry-run was skipped, and recommend testing in a development environment.

Documentation Lookup

Try context7 MCP first:

Use mcp__context7__resolve-library-id with "fluent-bit"
Then use mcp__context7__get-library-docs with:
- context7CompatibleLibraryID: /fluent/fluent-bit-docs
- topic: "<plugin-type> <plugin-name> configuration"
- page: 1

Fallback to WebSearch:

Search query: "fluent-bit <plugin-type> <plugin-name> configuration parameters site:docs.fluentbit.io"

Report and Fix Issues

1. Summarize all issues:

Validation Report for fluent-bit.conf
=====================================

Errors (3):
  - [Line 15] OUTPUT elasticsearch missing required parameter 'Host'
  - [Line 25] FILTER Match pattern 'app.*' doesn't match any INPUT tags
  - [Line 8] INPUT tail missing Mem_Buf_Limit (OOM risk)

Warnings (2):
  - [Line 30] OUTPUT elasticsearch has hardcoded password (security risk)
  - [Line 12] INPUT tail missing DB file (no crash recovery)

Info (1):
  - [Line 3] SERVICE Flush interval is 10s (consider reducing for lower latency)

Best Practices (2):
  - Consider enabling HTTP_Server for health checks
  - Consider enabling compression on OUTPUT elasticsearch

2. Categorize by severity:

• Errors (must fix): Configuration won't work, Fluent Bit won't start
• Warnings (should fix): Configuration works but has issues
• Info (consider): Optimization opportunities
• Best Practices: Recommended improvements

3. Propose specific fixes:

# Fix 1: Add missing Host parameter
[OUTPUT]
    Name  es
    Match *
    Host  elasticsearch.logging.svc  # Added
    Port  9200

# Fix 2: Add Mem_Buf_Limit to prevent OOM
[INPUT]
    Name              tail
    Tag               kube.*
    Path              /var/log/containers/*.log
    Mem_Buf_Limit     50MB  # Added

# Fix 3: Use environment variable for password
[OUTPUT]
    Name        es
    HTTP_User   admin
    HTTP_Passwd ${ES_PASSWORD}  # Changed from hardcoded

4. Get user approval via AskUserQuestion

5. Apply approved fixes using Edit tool

6. Re-run validation to confirm

7. Provide completion summary (fixed issues, per-check pass/fail status, and overall validation result)

8. Report-only summary (when user declines fixes):

📋 Validation Report Complete - No fixes applied

Summary:
  - Errors: 2 (must fix before deployment)
  - Warnings: 16 (should fix)
  - Info: 15 (optimization suggestions)

Critical Issues Requiring Attention:
  - [Line 5] Invalid Log_Level 'invalid_level'
  - [Line 52] [OUTPUT opentelemetry] missing required parameter 'Host'

Recommendations:
  - Review the errors above before deploying this configuration
  - Consider addressing warnings to improve reliability and security
  - Run validation again after manual fixes: python3 scripts/validate_config.py --file <config> --check all

Integration with fluentbit-generator

This validator is automatically invoked by the fluentbit-generator skill after generating configurations. It can also be used standalone to validate existing configurations.

Generator workflow:

1. Generate configuration using fluentbit-generator
2. Automatically validate using fluentbit-validator
3. Fix any issues found
4. Re-validate until all checks pass
5. Deploy with confidence

Anti-Patterns

NEVER validate config syntax without checking tag routing

• WHY: A configuration that parses without errors can still drop all logs silently if no OUTPUT Match pattern covers the tags produced by the INPUTs. Syntax validation alone gives false confidence.
• BAD: Confirm fluent-bit --dry-run passes and ship the configuration to production.
• GOOD: Trace every INPUT tag through all FILTER and OUTPUT Match patterns to confirm that no logs fall through without a destination.

NEVER skip TLS certificate validation in output plugins

• WHY: tls.verify Off is convenient for local testing but is frequently forgotten when promoting a config to production, leaving log data in transit exposed to interception or man-in-the-middle attacks.
• BAD: tls.verify Off present in a production output plugin targeting an external log aggregator.
• GOOD: tls.verify On with tls.ca_file /etc/ssl/certs/ca-certificates.crt (or the appropriate CA bundle for your environment).

NEVER use the same buffer path for multiple Fluent Bit instances

• WHY: Overlapping storage.path directories corrupt the backpressure state database, causing one instance to consume or delete the other's buffered records and resulting in duplicate or lost log delivery.
• BAD: Two Fluent Bit daemonsets sharing /var/log/flb-storage/ as their storage path.
• GOOD: Assign a distinct storage.path value to each Fluent Bit instance (e.g., /var/log/flb-storage-app/ and /var/log/flb-storage-infra/).

NEVER ignore pipeline tag connection warnings

• WHY: An INPUT tag that no OUTPUT Match pattern covers causes Fluent Bit to silently drop those records. This is the most common root cause of "missing logs" production incidents and is invisible without explicit tag validation.
• BAD: Dismiss unmatched tag warnings from the validator as noise and proceed with deployment.
• GOOD: Treat any unmatched tag as a P1 configuration error — every INPUT tag must be covered by at least one OUTPUT Match pattern before the config is considered valid.

References

scripts/

validate_config.py

• Main validation script with all checks integrated in a single file
• Usage: python3 scripts/validate_config.py --file <config> --check <type>
• Available check types: all, structure, syntax, sections, tags, security, performance, best-practices, dry-run
• Comprehensive 1000+ line validator covering all validation stages
• Returns detailed error messages with line numbers
• Supports JSON output format: --json

validate.sh

• Convenience wrapper script for easier invocation
• Usage: bash scripts/validate.sh <config-file>
• Automatically calls validate_config.py with proper Python interpreter

Test Fixtures

The skill includes test configuration files in references/test-fixtures/ for validating the validator itself. See references/test-fixtures.md for details on running tests.

Documentation Sources

• Fluent Bit Official Documentation
• Fluent Bit Operations and Best Practices
• Configuration File Format
• Context7 Fluent Bit documentation (/fluent/fluent-bit-docs)