GSLB Switcher

A Global Server Load Balancing (GSLB) switcher that monitors the health of a primary server and automatically switches DNS records between primary and secondary IP addresses based on health check results.

Overview

This tool provides automated failover for DNS-based load balancing by continuously monitoring a primary server's health and updating DNS records in OpnSense Unbound DNS when the primary becomes unavailable or recovers. It ensures high availability by seamlessly switching traffic to a secondary server or load balancer when the primary fails health checks.

Features

• Health Monitoring: Continuously checks the primary server's availability
• Failover: Automatically switches to secondary when primary is unhealthy, and back to primary when it recovers
• OpnSense Integration: Works with OpnSense Unbound DNS Host Override records
• HTTP Health Checks: Monitors HTTP/HTTPS endpoints for availability

Currently supported DNS providers:

• OpnSense Unbound DNS (Host Override records)

Currently supported health checks:

• Basic HTTP/HTTPS endpoint check (status code 200-299 = healthy)
  • Includes automatic retry mechanism: 3 attempts with 10-second delays between retries
  • 5-second timeout per request attempt

How It Works

Switching Logic

The GSLB switcher follows a simple yet effective decision-making process:

1. Health Check: Every 60 seconds, the tool performs an HTTP GET request to the configured primary health check URL

2. Evaluate Health Status:

   • Healthy: HTTP status code 200-299
   • Unhealthy: Any other status code, timeout, or connection error (after all retries are exhausted)

3. Decision Making:

   • If primary is healthy AND DNS points to secondary → Switch to primary IP
   • If primary is unhealthy AND DNS points to primary → Switch to secondary IP
   • If DNS already points to the correct IP → No action taken

This logic ensures that:

• Traffic always flows to the healthy server
• Unnecessary DNS updates are avoided
• Primary server is preferred when healthy (automatic failback)

Configuration

The application is configured entirely through environment variables:

Required Environment Variables

Variable	Description	Example
GSLB_HOST	The hostname/FQDN managed by GSLB	api.example.com
GSLB_PRIMARY_IP	IP address of the primary server	10.0.0.101
GSLB_PRIMARY_CHECK	HTTP(S) URL to check primary health	https://10.0.0.101:443/health
GSLB_SECONDARY_IP	IP address of the secondary/failover server	10.0.0.102
OPNSENSE_HOST	OpnSense API endpoint base URL	https://firewall.example.com
OPNSENSE_AUTH	OpnSense API authentication credentials	key:secret

Optional Environment Variables

Variable	Description	Default	Example
GSLB_PRIMARY_CHECK_SKIP_TLS_VERIFY	Skip TLS certificate verification for health checks	false	true

Configuration Example:

export GSLB_HOST="k8s-apiserver.local"
export GSLB_PRIMARY_IP="10.0.0.201"
export GSLB_PRIMARY_CHECK="https://10.0.0.201:6443/healthz"
export GSLB_SECONDARY_IP="10.0.0.202"
export OPNSENSE_HOST="https://opnsense.local"
export OPNSENSE_AUTH="your-api-key:your-api-secret"

# Optional: Skip TLS verification for health checks
# export GSLB_PRIMARY_CHECK_SKIP_TLS_VERIFY="true"

Prerequisites

OpnSense Setup

1. Enable Unbound DNS in OpnSense
2. Create a Host Override for your managed hostname:
   • Navigate to Services → Unbound DNS → Overrides
   • Add Host Override with your hostname and initial IP address
   • Must be an A (IPv4) or AAAA (IPv6) record
3. Generate API Credentials:
   • Navigate to System → Access → Users
   • Create or edit a user
   • Generate API key and secret
   • Ensure the user has permissions to access Unbound API endpoints

Health Check Endpoint

Your primary server should expose an HTTP(S) endpoint that returns:

• Status code 200-299 when healthy
• Any other status code, timeout, or connection refused when unhealthy