# CIS Benchmark Kernel Hardening for Debian
# Implements CIS Debian Benchmark controls related to kernel parameters

# ============================================================================
# Network Parameters
# ============================================================================

# Disable IP packet forwarding (not a router)
net.ipv4.ip_forward = 0
net.ipv6.conf.all.forwarding = 0

# Disable source 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

# Disable ICMP redirects
net.ipv4.conf.all.accept_redirects = 0
net.ipv4.conf.default.accept_redirects = 0
net.ipv4.conf.all.send_redirects = 0
net.ipv4.conf.default.send_redirects = 0

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

# Ignore bogus ICMP error responses
net.ipv4.icmp_ignore_bogus_error_responses = 1

# Enable TCP SYN cookies protection
net.ipv4.tcp_syncookies = 1

# Enable TCP reverse path filtering (source verification)
net.ipv4.conf.all.rp_filter = 1
net.ipv4.conf.default.rp_filter = 1

# Log martian packets
net.ipv4.conf.all.log_martians = 1

# Disable IPv6 if not used (enabled but strictly controlled via firewall)
# net.ipv6.conf.all.disable_ipv6 = 1

# ============================================================================
# System Hardening
# ============================================================================

# Disable magic sysrq key
kernel.sysrq = 0

# Address space layout randomization
kernel.randomize_va_space = 2

# Restrict core dumps
kernel.core_pattern = |/bin/false

# Enable hard link and symlink protection
fs.protected_hardlinks = 1
fs.protected_symlinks = 1

# Enable ptrace restrictions (only allow ptrace from descendants)
kernel.yama.ptrace_scope = 1

# Disable unprivileged BPF
kernel.unprivileged_bpf_disabled = 1

# Disable user namespaces
user.max_user_namespaces = 0

# ============================================================================
# Network Stack Hardening
# ============================================================================

# Enable TCP timestamps for protection against spoofing
net.ipv4.tcp_timestamps = 1

# Reduce TCP timeout for closing connections
net.ipv4.tcp_fin_timeout = 30

# Maximum number of SYN backlog queue
net.ipv4.tcp_max_syn_backlog = 2048

# Minimum, default, and maximum TCP buffer sizes
net.ipv4.tcp_rmem = 4096 87380 16777216
net.ipv4.tcp_wmem = 4096 65536 16777216

# ============================================================================
# ARP Hardening
# ============================================================================

# Ignore ARP requests on all interfaces
net.ipv4.conf.all.arp_ignore = 1
net.ipv4.conf.default.arp_ignore = 1

# Enable ARP filtering
net.ipv4.conf.all.arp_filter = 1
net.ipv4.conf.default.arp_filter = 1

# ============================================================================
# ICMP Rate Limiting
# ============================================================================

# Rate limit ICMP messages
net.ipv4.icmp_ratelimit = 100

# ============================================================================
# Memory Overcommit
# ============================================================================

# Disable overcommit (conservative memory allocation)
vm.overcommit_memory = 2
vm.overcommit_ratio = 80