How to Reset Kernel Settings to Defaults on FreeBSD

FreeBSD, one of the most stable and high-performance Unix-like operating systems, provides administrators with extensive control over kernel behavior through various tunable parameters. While this flexibility is powerful, it can sometimes lead to problematic configurations or performance issues that require restoring kernel settings to their default values. This comprehensive guide explores the process of resetting kernel parameters to their defaults on FreeBSD systems.

Understanding FreeBSD Kernel Configuration

Before diving into reset procedures, it’s important to understand how FreeBSD manages kernel configurations and which files control these settings.

Key Configuration Files

FreeBSD’s kernel behavior is controlled through several configuration mechanisms:

1. /boot/loader.conf - Contains boot-time kernel tunable parameters
2. /etc/sysctl.conf - Contains runtime kernel parameters
3. /etc/rc.conf - Contains system configuration settings that may affect kernel behavior
4. Custom kernel configuration - If you’ve built a custom kernel

These files allow administrators to modify various aspects of kernel behavior, from memory management to network stack settings.

Identifying Current Kernel Settings

Before resetting, it’s useful to understand your current configuration to determine what needs to be restored to defaults.

Checking Loader Settings

To view the current loader settings:

# Display all loader settings
sysctl -a | grep "loader"

# Alternative: view boot-time loader tunables
grep -v "^#" /boot/loader.conf

Checking Sysctl Parameters

To view current kernel parameters:

# View all kernel settings
sysctl -a

# Filter for specific subsystems, e.g., network settings
sysctl -a | grep net

Resetting Loader Configuration

The /boot/loader.conf file and its associated /boot/loader.conf.local contain boot-time kernel tunables that are applied during system initialization.

Creating Backup Copies

Always create backups before making changes:

cp /boot/loader.conf /boot/loader.conf.bak
cp /boot/loader.conf.local /boot/loader.conf.local.bak 2>/dev/null

Removing Custom Settings

To reset to defaults, you can either:

1. Comment out all custom settings by prefixing lines with #
2. Remove the file contents entirely

# Option 1: Comment out all non-comment lines
sed -i '' 's/^[^#]/#&/' /boot/loader.conf

# Option 2: Clear the file (preserving defaults)
echo "# Restored to defaults on $(date)" > /boot/loader.conf

For /boot/loader.conf.local, if it exists:

rm /boot/loader.conf.local

Resetting Sysctl Parameters

The sysctl utility allows viewing and modifying kernel parameters on a running system. Settings in /etc/sysctl.conf persist across boots.

Backing Up Current Settings

cp /etc/sysctl.conf /etc/sysctl.conf.bak

Removing Custom Settings

To reset to defaults:

# Comment out all non-comment lines
sed -i '' 's/^[^#]/#&/' /etc/sysctl.conf

# Alternatively, clear the file
echo "# Restored to defaults on $(date)" > /etc/sysctl.conf

Applying Default Values to Running System

For some parameters, you may need to explicitly reset them on the running system:

# Example: Reset specific parameter to default
sysctl kern.ipc.somaxconn=128  # Replace with actual default

# For network settings
sysctl net.inet.tcp.delayed_ack=1  # Example default

Finding the default values can be challenging. You can:

• Consult FreeBSD documentation
• Boot from installation media and check values
• Consult /usr/src/sys/*/NOTES files for documented defaults

Resetting System RC Configuration

Some kernel-related settings are controlled through /etc/rc.conf:

Backing Up RC Configuration

cp /etc/rc.conf /etc/rc.conf.bak

Removing Kernel-Related Settings

Edit /etc/rc.conf and remove or comment out lines that affect kernel behavior:

# Common kernel-related settings to check
# firewall_enable="YES"
# gateway_enable="YES"
# ipfw_load="YES"
# pf_enable="YES"
# kern_securelevel_enable="YES"

Handling Custom Kernels

If you’ve built and installed a custom kernel, resetting to defaults means reverting to the GENERIC kernel:

Identifying Current Kernel

uname -i  # Shows kernel identifier

Restoring GENERIC Kernel

If your system boots from the GENERIC kernel but uses a custom one:

1. Edit /boot/loader.conf and remove or comment out any lines setting kernel= to a custom value

2. Alternatively, add or modify:

   kernel="GENERIC"
   

Rebuilding and Installing GENERIC Kernel

If you don’t have the GENERIC kernel available:

# Navigate to source directory (if installed)
cd /usr/src

# Configure and build GENERIC kernel
make buildkernel KERNCONF=GENERIC
make installkernel KERNCONF=GENERIC

If source is not installed:

# Install source first
pkg install git
git clone https://git.FreeBSD.org/src.git /usr/src
cd /usr/src
git checkout releng/$(uname -r | cut -f1,2 -d'.')

# Then build and install GENERIC kernel
make buildkernel KERNCONF=GENERIC
make installkernel KERNCONF=GENERIC

Handling Kernel Modules

Kernel modules can also modify system behavior:

Listing Loaded Modules

kldstat

Unloading Non-Default Modules

kldunload module_name

Preventing Modules from Loading at Boot

Edit /boot/loader.conf and comment out or remove lines that load custom modules.

Special Consideration: ZFS Settings

If you’re using ZFS, there are specific kernel parameters that might need attention:

Viewing ZFS Parameters

sysctl -a | grep vfs.zfs

Resetting ZFS Parameters

ZFS parameters often have their defaults listed in the output of:

sysctl -d vfs.zfs.some.parameter

To reset:

# Example
sysctl vfs.zfs.prefetch_disable=0  # Default is usually 0

Testing Your Changes

After making changes, testing is crucial:

Immediate Testing

For sysctl changes that don’t require a reboot:

# Example: Test network stack
ping -c 3 localhost
ping -c 3 1.1.1.1

Reboot Testing

For loader.conf and other boot-time changes:

# Reboot the system
shutdown -r now

After rebooting, verify settings:

sysctl -a > after_reset.txt

Troubleshooting After Reset

Common Issues

1. Boot Failures: If you encounter boot failures:

   • Boot into single-user mode by selecting this option at the boot menu
   • Mount the root filesystem: mount -u /; mount -a -t ufs
   • Restore your backup: cp /boot/loader.conf.bak /boot/loader.conf

2. Performance Issues: If you experience performance degradation:

   • Some defaults may not be optimal for your hardware
   • Consider tuning specific parameters based on documented best practices

3. Network Connectivity Problems:

   • Check network interface configuration: ifconfig
   • Restore specific network parameters if needed

Verifying Reset Success

To confirm your reset was successful:

# Compare current settings with defaults
# If you have access to another default FreeBSD installation:
diff -y current_settings.txt default_settings.txt

Best Practices for Kernel Configuration Management

Documentation

Always document your changes:

# Add comments in configuration files
# Example:
# Modified on 2025-03-20 by admin for high-traffic web server

Configuration Tracking

Consider using version control for configuration files:

# Initialize a Git repository for tracking
cd /etc
git init
git add rc.conf sysctl.conf
git commit -m "Initial configuration state"

Incremental Changes

Rather than making multiple changes at once:

1. Make one change at a time
2. Test thoroughly
3. Document the effect
4. Move to the next parameter

This methodology makes troubleshooting much easier.

Conclusion

Resetting FreeBSD kernel settings to their defaults can resolve various system issues and provide a clean slate for optimization. The process involves understanding the configuration files, creating proper backups, carefully removing custom settings, and methodically testing the system after changes.

While default settings are designed to work well for general-purpose use, remember that production systems often benefit from tuning specific parameters based on their workload. After resetting to defaults, consider employing a structured approach to performance tuning by making documented, incremental changes guided by system monitoring and FreeBSD best practices.

By following this guide, system administrators can confidently restore FreeBSD systems to their default kernel configurations, resolving issues and creating a solid foundation for future optimization.