When testing kernel parameters through sysctl, many developers assume that simply reloading the default configuration file (/etc/sysctl.conf) will reset all values. However, this isn't the case because:

• Some parameters are set during boot by the kernel or init system
• Runtime changes persist until reboot unless explicitly reset
• Loading a partial config file doesn't override all existing values

Here are three effective approaches to reset sysctl parameters:

Method 1: Full System Reboot

The most thorough solution:

sudo reboot

This guarantees all parameters return to their boot-time defaults.

Method 2: Targeted Parameter Reset

For individual parameters when you know the defaults:

# Example: Reset vm.swappiness
sudo sysctl -w vm.swappiness=60

# Example: Reset net.ipv4.tcp_fin_timeout 
sudo sysctl -w net.ipv4.tcp_fin_timeout=60

Method 3: Reapply All Defaults

Create a reset script using these steps:

#!/bin/bash

# First, clear all current settings
sudo sysctl --system

# Then reapply default configs
for conf in /usr/lib/sysctl.d/*.conf /etc/sysctl.d/*.conf /etc/sysctl.conf; do
    [ -f "$conf" ] && sudo sysctl -p "$conf"
done

Always verify changes with:

# Check all parameters
sysctl -a

# Filter specific parameters
sysctl -a | grep 'net.ipv4.tcp'

Remember this distinction:

Type	Command	Persistence
Runtime	sysctl -w	Until reboot
Persistent	Edit config files	Survives reboot

To find built-in kernel defaults (before any configs are applied):

# Boot with minimal configuration
sudo sysctl --system --boot

# Then check the current values
sysctl -a > kernel_defaults.txt

---

When testing kernel parameters through sysctl, many administrators assume that simply reloading the default configuration with sysctl -p will revert all changes. However, this only affects parameters specified in /etc/sysctl.conf or your custom config file. The actual runtime values may persist through various mechanisms.

Modern Linux systems manage sysctl parameters through multiple layers:

1. Runtime values (via /proc/sys/)
2. Boot-time settings (sysctl.service)
3. Distribution defaults (/usr/lib/sysctl.d/)
4. Local administrator settings (/etc/sysctl.d/)
5. Legacy configuration (/etc/sysctl.conf)

For a full reset to distribution defaults, follow this procedure:

Step 1: Clear runtime modifications

# Find all modified parameters
sysctl -a > current_values.txt
sysctl --system > default_values.txt
diff current_values.txt default_values.txt | grep "<" | awk '{print $2}' > modified_params.txt

# Reset each parameter individually
while read param; do
  sysctl -w "$param=$(grep "^${param}=" default_values.txt | cut -d= -f2)"
done < modified_params.txt

Step 2: Purge custom configurations

# Remove all custom configs while preserving originals
mkdir ~/sysctl_backup
mv /etc/sysctl.conf ~/sysctl_backup/
mv /etc/sysctl.d/*.conf ~/sysctl_backup/ 2>/dev/null

# Reinstall base packages to restore defaults (Debian/Ubuntu example)
apt-get --reinstall install procps

Step 3: Full system reload

# Apply all system defaults
sysctl --system

# Alternative for older systems
find /usr/lib/sysctl.d/ -name "*.conf" | xargs -I {} sysctl -p {}

To confirm successful reset:

# Compare against known good state
sysctl -a | sha256sum
# Should match output from a fresh installation

Remember that:

• sysctl -w makes temporary runtime changes
• Configuration files create persistent changes
• Some parameters may be managed by other services (like networkd)

For frequent testing, save this as reset_sysctl.sh:

#!/bin/bash
# Backup current state
TIMESTAMP=$(date +%Y%m%d-%H%M%S)
mkdir -p /var/backups/sysctl/${TIMESTAMP}
sysctl -a > /var/backups/sysctl/${TIMESTAMP}/pre-reset.conf

# Reset procedure
sysctl --system
systemctl restart systemd-sysctl

# Verify
sysctl -a > /var/backups/sysctl/${TIMESTAMP}/post-reset.conf
diff /var/backups/sysctl/${TIMESTAMP}/pre-reset.conf \
     /var/backups/sysctl/${TIMESTAMP}/post-reset.conf