Core Code
Core Code Modern Dev Handbook
LINUX-PRODUCTION Contents
Linux Fundamentals for Production
Filesystem Layout (FHS) in Practice Users, Groups, and Service Accounts Permissions & ACLs (chmod/chown/setuid) Sudoers Safe Defaults Packages: apt/dnf, Repos, and Pinning Shell & Environment Hygiene Editing Configs Safely Time, Timezones, and NTP System Introspection Toolkit
Processes & Signals
Linux Process Model (PID, PPID, Fork/Exec) ps/top/htop: Finding What Hurts Signals in Practice (TERM/KILL/HUP) nice/renice and Scheduling Basics ulimits and Resource Limits Background Jobs (nohup, tmux, systemd-run) /proc and What It Tells You cgroups Basics (CPU/Memory Isolation) Zombies, Orphans, and Stuck Processes
systemd & Services
systemd Unit Types Overview Service Unit Best Practices Dependencies & Ordering (After/Wants/Requires) Restart Policies and Backoff Environment Files and Secrets Handling journald Basics (Reading, Filtering, Retention) Logging Practices with systemd Timers vs Cron Debugging systemd Services
Networking Basics for Production
ip, routes, and interfaces DNS Debugging (dig, resolvectl, hosts) Ports and Sockets with ss curl for HTTP Debugging TCP Basics (Handshake, States, Retries) Firewall Overview (nftables/iptables/ufw) NAT and Port Forwarding Concepts TLS and Certificates Basics Network Troubleshooting Playbook
Storage & Filesystems
Disk Layout and Partitioning (lsblk/fdisk) Filesystems (ext4/xfs) Basics Mounts and /etc/fstab Safety Disk Usage: df vs du (and Why They Differ) Inodes and “Disk Full” Mysteries Swap Strategy in Production Log Rotation (logrotate) Basics Temporary and Runtime Storage Hygiene Backup Storage Considerations
Security Basics
SSH Hardening in Production Users and Least Privilege in Production Firewall Minimum Rules for Production Linux Patching Strategy in Production Managing Secrets on Linux Hosts Linux Audit and Basic Logging in Production Fail2ban and Rate Limiting in Production SELinux and AppArmor Overview in Production Linux Security Baseline Checklist for Production
Observability on Linux
Linux Log Sources Map in Production journalctl Power Usage in Production Log Structure and Correlation in Production Metrics Hooks Overview in Production Health Checks and Readiness in Production Tracing Mindset for Linux and Production Systems Core Dumps Basics in Production Debug Packages and Symbols in Production Linux Observability Checklist for Production
Performance & Debugging
Load Average Explained for Production Debugging CPU Profiling Basics in Linux Production Memory Pressure Basics in Linux Production IO Bottlenecks and iostat in Production lsof and Open Files in Production Debugging Practical strace Usage in Production Sockets Debugging Tools in Linux Production dmesg and Kernel Signals for Production Debugging Production Debugging Workflow for Linux Systems
Deployment Patterns
Environment and Configuration Management in Production Release Directory Pattern for Safe Linux Deployments Zero-Downtime Restarts in Production Running Migrations Safely in Production Secrets Deploy Patterns in Production Backups and Restore Drills in Production Rollback Strategies for Production Deployments Maintenance Windows in Production Operations Production Deployment Checklist for Linux Systems
Production Checklists
Writing Effective Runbooks for Production Systems Failure Modes Mapping for Linux Production Systems Incident Response Basics for Linux Production Break-Glass Access in Production Capacity Planning Basics for Linux Production Systems Monitoring and Alerting Baseline for Linux Production Production Readiness Review for Linux Systems Configuration Drift in Linux Production Systems Go-Live Checklist

SSH Hardening in Production

Production-grade SSH hardening: eliminate password authentication, disable root login, restrict network access, enforce strong key policy, and reduce real-world attack surface.
On this page

Why SSH Is the Primary Attack Surface

In production infrastructure, SSH is usually the only administrative entry point. Attackers do not begin with kernel exploits — they begin with credentials. Weak SSH configuration means full system exposure.

Symptom

  • Hundreds or thousands of failed login attempts
  • Unknown IP addresses targeting root
  • Increased CPU usage during brute-force waves
  • Security alerts for authentication anomalies

Root Cause

  • PasswordAuthentication enabled
  • PermitRootLogin allowed
  • Port 22 open to the public internet
  • No rate limiting
  • Poor SSH key hygiene

Investigation

Inspect SSH configuration:

cat /etc/ssh/sshd_config

Count failed attempts:

grep "Failed password" /var/log/auth.log | wc -l

Top attacking IP addresses:

grep "Failed password" /var/log/auth.log | awk "{print $(NF-3)}" | sort | uniq -c | sort -nr | head

Verify SSH listener:

ss -tnlp | grep ssh

Mitigation

Disable password authentication:

PasswordAuthentication no

Disable root login:

PermitRootLogin no

Validate configuration before restart:

sshd -t

Restart safely:

systemctl restart sshd

Network Restriction

Restrict SSH by trusted IP:

ufw allow from 1.2.3.4 to any port 22

Cloud security group restriction is preferred over relying only on host firewall.

Key Strategy

Generate modern key:

ssh-keygen -t ed25519 -a 100
  • No shared keys
  • Immediate removal of former employee keys
  • Key rotation policy enforced

Hardening Strategy

  • Bastion host architecture
  • Multi-factor authentication
  • Disable legacy ciphers
  • Centralized logging
  • Rate limiting layer

Why This Matters

SSH compromise enables lateral movement and full infrastructure takeover. Most real-world breaches start with weak authentication controls. Hardening SSH is foundational, not optional.

Verification

sshd -T
ssh -v user@server

If password login is still possible, the hardening failed.

Users and Least Privilege in Production →