Configuring High-Availability Clusters with CARP on FreeBSD

Introduction to High-Availability Clustering

High-availability (HA) clustering is a critical infrastructure strategy for organizations seeking to minimize downtime and ensure continuous service availability. FreeBSD offers a robust solution for creating redundant network configurations through the Common Address Redundancy Protocol (CARP), which provides a flexible and efficient method for implementing failover and load-balancing scenarios.

Understanding CARP

CARP is a open-source protocol designed to allow multiple hosts to share a set of IP addresses, creating a highly available and fault-tolerant network infrastructure. Unlike proprietary solutions, CARP provides a transparent and reliable mechanism for creating redundant systems with minimal configuration complexity.

Prerequisites for CARP Implementation

Before diving into the configuration, ensure you have the following:

1. FreeBSD Servers

   • Minimum of two FreeBSD servers (can be physical or virtual machines)
   • Identical or comparable hardware configurations
   • Network interfaces capable of supporting CARP

2. Network Requirements

   • Dedicated network interfaces for CARP synchronization
   • Static IP addresses for each server
   • Shared virtual IP address for failover
   • Network switch supporting VLAN (recommended)

3. Software Preparations

   • Updated FreeBSD installation (recommended 12.x or 13.x)
   • Root or sudo access
   • Basic networking and system administration knowledge

Kernel Configuration

CARP requires specific kernel modules to be loaded. Begin by ensuring the necessary kernel support is available:

# Add these lines to /boot/loader.conf
if_carp_load="YES"
net.inet.carp.allow=1

Alternatively, you can compile a custom kernel with CARP support:

# In your kernel configuration file
options         INET
options         CARP

Network Interface Configuration

Configure the network interfaces for CARP redundancy. This typically involves setting up both physical interfaces and CARP virtual interfaces.

Sample /etc/rc.conf Configuration

# Physical interface configuration
ifconfig_igb0="inet 192.168.1.10/24"
ifconfig_igb1="inet 192.168.1.11/24"

# CARP virtual interface
ifconfig_carp0="inet 192.168.1.100/24 vhid 1 pass secretpassword"

Key Configuration Parameters

• vhid: Virtual Host ID (unique identifier for the CARP group)
• pass: Shared secret for authentication
• Virtual IP address: The floating IP that will failover between servers

Configuring CARP Behavior

CARP allows fine-tuning of failover and synchronization behaviors through several important parameters:

1. Preemption Determine whether a recovered node should immediately take over as the primary:

   sysctl net.inet.carp.preempt=1
   

2. Synchronization Interval Control how frequently CARP monitors node status:

   sysctl net.inet.carp.advskew=100
   

3. Advertisement Interval Adjust how often CARP sends status updates:

   sysctl net.inet.carp.ad_timeout=240
   

Firewall Configuration

When using CARP, configure your firewall to allow CARP traffic. For PF (Packet Filter), add these rules:

# /etc/pf.conf
carp_if = "carp0"
pass quick on $carp_if

For IPFW:

# Allow CARP protocol
ipfw add 00100 allow carp from any to any

Monitoring and Troubleshooting

Checking CARP Status

Use these commands to verify CARP configuration:

# View CARP interface status
ifconfig carp0

# Detailed CARP information
sysctl net.inet.carp

Common Troubleshooting Scenarios

1. No Failover Occurring

   • Verify matching VHID and password
   • Check network connectivity
   • Ensure CARP protocol is allowed through firewalls

2. Unexpected Failbacks

   • Adjust advskew to prevent rapid switching
   • Implement additional health checks

Advanced CARP Configurations

Multiple Virtual IP Scenarios

You can create multiple CARP groups for different services:

# Web service CARP group
ifconfig_carp1="inet 192.168.1.200/24 vhid 2 pass webservice"

# Database service CARP group
ifconfig_carp2="inet 192.168.1.201/24 vhid 3 pass database"

Security Considerations

1. Use strong, unique passwords for CARP groups
2. Limit physical network access
3. Implement additional authentication mechanisms
4. Regularly update and patch FreeBSD systems

Performance Optimization

• Use dedicated, low-latency network interfaces for CARP
• Match hardware specifications between nodes
• Implement proper load-balancing strategies
• Monitor system resources during failover events

Conclusion

CARP on FreeBSD provides a powerful, flexible solution for creating high-availability network infrastructures. By understanding its configuration nuances and implementing best practices, organizations can achieve robust, fault-tolerant system designs.

Recommended Next Steps

1. Develop comprehensive failover testing procedures
2. Create detailed documentation of your specific configuration
3. Implement comprehensive monitoring solutions
4. Conduct regular disaster recovery drills

References

• FreeBSD Handbook: CARP Documentation
• Official FreeBSD CARP Manual Pages
• Network Redundancy Protocols RFC Documents