Server Security Init Skill

Server Security Init Skill

Skill by ara.so — Security Skills collection.

This skill guides you through safely initializing and hardening fresh Ubuntu/Debian SSH servers. It implements a staged bootstrap flow: selecting or generating local SSH keys, requiring public-key login before remote changes, creating a non-root sudo user, changing the SSH port, disabling root/password login, configuring UFW firewall, setting up fail2ban, and verifying no lockout conditions exist.

What This Skill Does

The server-security-init skill provides guidance for:

• Public-key SSH bootstrap without exposing root passwords in commands, logs, or config files
• Staged SSH hardening with verified non-root sudo login before disabling old access paths
• UFW firewall setup with default-deny inbound policy and explicit port allowances
• Fail2ban jail configuration for SSH brute-force protection with management IP exceptions
• Debian 12/systemd socket checks to prevent ssh.socket from silently keeping SSH on the old port
• Server-side verification using ss -ltnp, sshd -T, systemctl, and ufw status
• Recovery checks for fail2ban bans, firewall mistakes, and SSH listener mismatches

Installation

This skill should be installed by copying the server-security-init/ directory to your AI agent's user-level skills directory.

For AI agents:

• Install only the server-security-init/ directory, NOT the repository root
• Do not execute server initialization during skill installation
• After installation, inform the user if their agent needs to restart or reload skills

For humans using npx:

npx skills add https://github.com/DeerYang/server-security-init-skill/tree/main/server-security-init -g

Target specific agents:

npx skills add https://github.com/DeerYang/server-security-init-skill/tree/main/server-security-init -g -a codex

Staged Security Initialization Flow

Stage 1: Pre-Flight Checks and Bootstrap

Before making any changes, gather and verify:

# Example bootstrap facts to collect
bootstrap_info = {
    "server_ip": "203.0.113.42",
    "current_user": "root",
    "ssh_port": 22,
    "management_ips": ["198.51.100.5"],  # Your current IP
    "local_ssh_key": "~/.ssh/id_ed25519",
    "target_user": "admin",
    "target_ssh_port": 2222
}

Verify existing SSH key or generate:

# Check for existing key
ls -la ~/.ssh/id_ed25519.pub

# Generate if needed (DO NOT automate this without user confirmation)
ssh-keygen -t ed25519 -C "server-bootstrap-$(date +%Y%m%d)" -f ~/.ssh/id_ed25519

Test password-based root SSH access (initial):

ssh -p 22 root@203.0.113.42 'echo "SSH accessible"'

Stage 2: Install Public Key

Copy your public key to the server:

# Using ssh-copy-id (prompts for password)
ssh-copy-id -i ~/.ssh/id_ed25519.pub -p 22 root@203.0.113.42

# Or manually
ssh -p 22 root@203.0.113.42 'mkdir -p ~/.ssh && chmod 700 ~/.ssh'
cat ~/.ssh/id_ed25519.pub | ssh -p 22 root@203.0.113.42 'cat >> ~/.ssh/authorized_keys && chmod 600 ~/.ssh/authorized_keys'

Verify public-key login works:

ssh -i ~/.ssh/id_ed25519 -p 22 root@203.0.113.42 'echo "Public key works"'

CRITICAL: Do not proceed until public-key authentication is verified.

Stage 3: Create Non-Root Sudo User

# Create user and grant sudo
ssh -i ~/.ssh/id_ed25519 -p 22 root@203.0.113.42 << 'EOF'
adduser --disabled-password --gecos "Admin User" admin
usermod -aG sudo admin
echo "admin ALL=(ALL) NOPASSWD:ALL" > /etc/sudoers.d/admin
chmod 440 /etc/sudoers.d/admin
EOF

Copy SSH key to new user:

ssh -i ~/.ssh/id_ed25519 -p 22 root@203.0.113.42 << 'EOF'
mkdir -p /home/admin/.ssh
cp /root/.ssh/authorized_keys /home/admin/.ssh/
chown -R admin:admin /home/admin/.ssh
chmod 700 /home/admin/.ssh
chmod 600 /home/admin/.ssh/authorized_keys
EOF

Verify new user sudo access:

ssh -i ~/.ssh/id_ed25519 -p 22 admin@203.0.113.42 'sudo echo "Sudo works"'

CRITICAL: Do not proceed until the new user can SSH in and sudo without password.

Stage 4: Configure Firewall (UFW)

ssh -i ~/.ssh/id_ed25519 -p 22 admin@203.0.113.42 << 'EOF'
sudo apt-get update
sudo apt-get install -y ufw

# Default policies
sudo ufw default deny incoming
sudo ufw default allow outgoing

# Allow new SSH port BEFORE enabling firewall
sudo ufw allow 2222/tcp comment 'SSH'

# Allow other services as needed
sudo ufw allow 80/tcp comment 'HTTP'
sudo ufw allow 443/tcp comment 'HTTPS'

# Enable firewall (answer yes)
sudo ufw --force enable

# Verify
sudo ufw status numbered
EOF

Verify firewall rules:

ssh -i ~/.ssh/id_ed25519 -p 22 admin@203.0.113.42 'sudo ufw status verbose'

Expected output:

Status: active
Logging: on (low)

To                         Action      From
--                         ------      ----
2222/tcp                   ALLOW IN    Anywhere                   # SSH
80/tcp                     ALLOW IN    Anywhere                   # HTTP
443/tcp                    ALLOW IN    Anywhere                   # HTTPS

Stage 5: Change SSH Port

Edit sshd_config:

ssh -i ~/.ssh/id_ed25519 -p 22 admin@203.0.113.42 << 'EOF'
sudo cp /etc/ssh/sshd_config /etc/ssh/sshd_config.backup
sudo sed -i 's/^#Port 22$/Port 2222/' /etc/ssh/sshd_config
sudo sed -i 's/^Port 22$/Port 2222/' /etc/ssh/sshd_config
EOF

Check for ssh.socket (Debian 12+):

ssh -i ~/.ssh/id_ed25519 -p 22 admin@203.0.113.42 << 'EOF'
if systemctl is-enabled ssh.socket 2>/dev/null | grep -q enabled; then
    echo "WARNING: ssh.socket is enabled and may keep SSH on port 22"
    echo "Disabling ssh.socket and using ssh.service directly..."
    sudo systemctl disable --now ssh.socket
    sudo systemctl enable ssh.service
fi
EOF

Restart SSH service:

ssh -i ~/.ssh/id_ed25519 -p 22 admin@203.0.113.42 'sudo systemctl restart ssh'

Verify new port is listening:

ssh -i ~/.ssh/id_ed25519 -p 22 admin@203.0.113.42 'sudo ss -ltnp | grep :2222'

Expected output:

LISTEN 0      128          0.0.0.0:2222       0.0.0.0:*    users:(("sshd",pid=1234,fd=3))

Test new port:

ssh -i ~/.ssh/id_ed25519 -p 2222 admin@203.0.113.42 'echo "New port works"'

CRITICAL: Keep the old SSH session open until new port is verified.

Stage 6: Harden SSH Configuration

ssh -i ~/.ssh/id_ed25519 -p 2222 admin@203.0.113.42 << 'EOF'
sudo sed -i 's/^#PermitRootLogin .*/PermitRootLogin no/' /etc/ssh/sshd_config
sudo sed -i 's/^PermitRootLogin .*/PermitRootLogin no/' /etc/ssh/sshd_config
sudo sed -i 's/^#PasswordAuthentication .*/PasswordAuthentication no/' /etc/ssh/sshd_config
sudo sed -i 's/^PasswordAuthentication .*/PasswordAuthentication no/' /etc/ssh/sshd_config
sudo sed -i 's/^#PubkeyAuthentication .*/PubkeyAuthentication yes/' /etc/ssh/sshd_config
sudo sed -i 's/^#ChallengeResponseAuthentication .*/ChallengeResponseAuthentication no/' /etc/ssh/sshd_config
sudo sed -i 's/^ChallengeResponseAuthentication .*/ChallengeResponseAuthentication no/' /etc/ssh/sshd_config

# Test configuration
sudo sshd -t

# Restart SSH
sudo systemctl restart ssh
EOF

Verify effective configuration:

ssh -i ~/.ssh/id_ed25519 -p 2222 admin@203.0.113.42 'sudo sshd -T | grep -E "^(permitrootlogin|passwordauthentication|pubkeyauthentication|port)"'

Expected output:

port 2222
permitrootlogin no
pubkeyauthentication yes
passwordauthentication no

Stage 7: Configure Fail2ban

ssh -i ~/.ssh/id_ed25519 -p 2222 admin@203.0.113.42 << 'EOF'
sudo apt-get install -y fail2ban

# Create local jail configuration
sudo tee /etc/fail2ban/jail.local > /dev/null <<'CONFIG'
[DEFAULT]
bantime = 3600
findtime = 600
maxretry = 5
ignoreip = 127.0.0.1/8 ::1 198.51.100.5

[sshd]
enabled = true
port = 2222
logpath = /var/log/auth.log
backend = systemd
CONFIG

sudo systemctl enable fail2ban
sudo systemctl restart fail2ban
EOF

Verify fail2ban status:

ssh -i ~/.ssh/id_ed25519 -p 2222 admin@203.0.113.42 'sudo fail2ban-client status sshd'

Expected output:

Status for the jail: sshd
|- Filter
|  |- Currently failed: 0
|  |- Total failed:     0
|  `- File list:        /var/log/auth.log
`- Actions
   |- Currently banned: 0
   |- Total banned:     0
   `- Banned IP list:

Stage 8: Update Local SSH Config

Add server entry to local ~/.ssh/config:

cat >> ~/.ssh/config << 'EOF'

Host my-server
    HostName 203.0.113.42
    User admin
    Port 2222
    IdentityFile ~/.ssh/id_ed25519
    StrictHostKeyChecking ask
    UserKnownHostsFile ~/.ssh/known_hosts
EOF

chmod 600 ~/.ssh/config

Test simplified connection:

ssh my-server 'echo "SSH config works"'

Verification Checklist

After completing all stages, verify the following server-side:

SSH Service

ssh my-server << 'EOF'
# Check SSH is listening on correct port only
sudo ss -ltnp | grep sshd

# Check effective sshd configuration
sudo sshd -T | grep -E "^(port|permitrootlogin|passwordauthentication|pubkeyauthentication)"

# Check ssh.socket is not interfering (Debian 12)
systemctl is-enabled ssh.socket
systemctl is-active ssh.socket
EOF

Firewall

ssh my-server 'sudo ufw status verbose'

Verify:

• Status is "active"
• Default incoming is "deny"
• New SSH port is allowed
• No rule allowing port 22

Fail2ban

ssh my-server << 'EOF'
sudo systemctl is-active fail2ban
sudo fail2ban-client status sshd
sudo fail2ban-client get sshd ignoreip
EOF

Verify:

• fail2ban service is active
• sshd jail is enabled
• Your management IP is in ignoreip

User and Sudo

ssh my-server << 'EOF'
whoami
sudo -l
groups
EOF

Verify:

• Logged in as non-root user
• User has NOPASSWD sudo
• User is in sudo group

Common Patterns

Adding Additional Management IPs

ssh my-server << 'EOF'
sudo sed -i 's/^ignoreip = .*/& 203.0.113.100/' /etc/fail2ban/jail.local
sudo fail2ban-client reload
sudo fail2ban-client get sshd ignoreip
EOF

Opening Additional Firewall Ports

ssh my-server << 'EOF'
sudo ufw allow 8080/tcp comment 'Application'
sudo ufw status numbered
EOF

Checking for Banned IPs

ssh my-server 'sudo fail2ban-client status sshd'

Unbanning an IP

ssh my-server 'sudo fail2ban-client set sshd unbanip 198.51.100.10'

Troubleshooting

Locked Out After Port Change

If new port doesn't work and old session is closed:

1. Use provider console/VNC access
2. Check sudo ss -ltnp | grep sshd for actual listening port
3. Check sudo systemctl status ssh.socket and ssh.service
4. Review /var/log/auth.log for SSH errors

UFW Blocked New SSH Port

If you enabled UFW before allowing new SSH port:

1. Use provider console/VNC access
2. sudo ufw disable
3. sudo ufw allow 2222/tcp
4. sudo ufw enable

Fail2ban Banned Your IP

Check ban status:

sudo fail2ban-client status sshd

Unban yourself:

sudo fail2ban-client set sshd unbanip YOUR_IP

Add to ignoreip:

sudo nano /etc/fail2ban/jail.local
# Add IP to ignoreip line
sudo fail2ban-client reload

SSH Port Reverts to 22 (Debian 12)

This happens when ssh.socket is active:

sudo systemctl disable --now ssh.socket
sudo systemctl enable --now ssh.service
sudo systemctl restart ssh
sudo ss -ltnp | grep sshd

Root Login Still Works

Check effective configuration:

sudo sshd -T | grep permitrootlogin

If it shows "yes", ensure:

• /etc/ssh/sshd_config has PermitRootLogin no (uncommented)
• No Match blocks at end of file override this
• Restart: sudo systemctl restart ssh

Password Authentication Still Accepted

sudo sshd -T | grep passwordauthentication

If it shows "yes":

• Edit /etc/ssh/sshd_config: PasswordAuthentication no
• Also set: ChallengeResponseAuthentication no
• Test: sudo sshd -t
• Restart: sudo systemctl restart ssh

Safety Notes

• Always keep an existing SSH session open when changing SSH or firewall settings
• Verify each stage before proceeding to the next
• Never disable password auth until public-key login is proven to work
• Never change SSH port until firewall allows new port
• Never disable root login until non-root sudo user is verified
• Provider console/VNC access is the ultimate recovery path
• Do not paste private keys into prompts, scripts, or logs
• Test sudo sshd -t before restarting SSH after config changes

System Requirements

• Ubuntu 20.04+ or Debian 11+
• OpenSSH server
• systemd init system
• UFW (Uncomplicated Firewall)
• fail2ban package available in apt repositories

This skill provides guidance for AI-assisted server administration. Always review generated commands before execution on production systems.