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Kernel Hardening Guide

Kernel Hardening Guide

Last Updated: 2026-02-09 Server: Hetzner Production (46.224.40.5) Status: ✅ Active (20+ sysctl rules)

Overview

Kernel hardening uses sysctl parameters to protect against network-level attacks, prevent IP spoofing, and restrict access to sensitive kernel information.

Current Configuration

Configuration File

Location: /etc/sysctl.d/99-security-hardening.conf

# =============================================================================
# Kernel Security Hardening - iSu Technologies
# Applied: 2026-02-09
# =============================================================================

# IP Spoofing protection
net.ipv4.conf.all.rp_filter = 1
net.ipv4.conf.default.rp_filter = 1

# Ignore ICMP broadcast requests
net.ipv4.icmp_echo_ignore_broadcasts = 1

# Disable source packet routing
net.ipv4.conf.all.accept_source_route = 0
net.ipv4.conf.default.accept_source_route = 0
net.ipv6.conf.all.accept_source_route = 0
net.ipv6.conf.default.accept_source_route = 0

# Ignore send redirects
net.ipv4.conf.all.send_redirects = 0
net.ipv4.conf.default.send_redirects = 0

# Block SYN attacks
net.ipv4.tcp_syncookies = 1
net.ipv4.tcp_max_syn_backlog = 2048
net.ipv4.tcp_synack_retries = 2
net.ipv4.tcp_syn_retries = 5

# Log Martians (packets with impossible addresses)
net.ipv4.conf.all.log_martians = 1
net.ipv4.icmp_ignore_bogus_error_responses = 1

# Ignore ICMP redirects
net.ipv4.conf.all.accept_redirects = 0
net.ipv4.conf.default.accept_redirects = 0
net.ipv6.conf.all.accept_redirects = 0
net.ipv6.conf.default.accept_redirects = 0

# Disable secure redirects
net.ipv4.conf.all.secure_redirects = 0
net.ipv4.conf.default.secure_redirects = 0

# Increase system file descriptor limit
fs.file-max = 65535

# Protect against symlink attacks
fs.protected_symlinks = 1
fs.protected_hardlinks = 1

# Restrict dmesg access
kernel.dmesg_restrict = 1

# Restrict access to kernel pointers
kernel.kptr_restrict = 2

Protection Summary

Protection	Parameter	Effect
IP Spoofing	rp_filter = 1	Rejects packets with spoofed source
SYN Flood	tcp_syncookies = 1	Mitigates SYN flood attacks
ICMP Attacks	icmp_echo_ignore_broadcasts = 1	Prevents Smurf attacks
Redirects	accept_redirects = 0	Prevents ICMP redirect attacks
Martian Logging	log_martians = 1	Logs suspicious packets
Symlink Attacks	protected_symlinks = 1	Prevents symlink races
Kernel Info	kptr_restrict = 2	Hides kernel pointers

Quick Commands

Check Current Values

# View all security sysctl values
sysctl -a | grep -E "rp_filter|syncookies|icmp_echo|accept_redirects|log_martians"

# Check specific value
sysctl net.ipv4.tcp_syncookies

# View entire hardening config
cat /etc/sysctl.d/99-security-hardening.conf

Apply Changes

# Apply all sysctl configurations
sysctl --system

# Apply specific file
sysctl -p /etc/sysctl.d/99-security-hardening.conf

# Temporarily change a value (testing)
sysctl -w net.ipv4.tcp_syncookies=1

Monitoring

Check for Martian Packets

# View kernel logs for martian packets
dmesg | grep -i martian

# Live monitoring
dmesg -w | grep -i martian

Network Attack Indicators

# Check for SYN_RECV connections (potential SYN flood)
ss -tnp state syn-recv | wc -l

# View connection states
ss -s

# Check for high numbers of TIME_WAIT
ss -tnp state time-wait | wc -l

Troubleshooting

Settings Not Applied?

# Check if config file exists
ls -la /etc/sysctl.d/99-security-hardening.conf

# Check for syntax errors
sysctl -p /etc/sysctl.d/99-security-hardening.conf 2>&1

# Verify settings loaded
sysctl net.ipv4.tcp_syncookies
# Should return: net.ipv4.tcp_syncookies = 1

Revert Changes

# Remove hardening file
mv /etc/sysctl.d/99-security-hardening.conf /tmp/

# Reset to defaults
sysctl --system

# Or reboot
reboot

Last hardening: 2026-02-09 Maintainer: Nhlanhla Mnyandu (nhlanhla@isutech.co.za)