renatocaliari/cali-server-security

Server Security Audit & Hardening

6-layer security model for private servers behind Tailscale.

Security Layers

Internet → CloudFlare → UFW → Tailscale → SSH → Docker → App

| Layer | Purpose | Tool | |---|---|---| | 1. CloudFlare | CDN, DDoS, SSL, WAF, hide real IP | CloudFlare DNS + Tunnel | | 2. UFW | Port filtering, default deny | UFW firewall | | 3. Tailscale | WireGuard tunnel, private network | Tailscale | | 4. SSH | Authentication, access control | OpenSSH / Tailscale SSH | | 5. Docker | Container isolation, socket security | Docker Engine | | 6. Application | Process isolation, env vars, volumes | Docker Compose |

Required Stack

# Server packages
tailscale          # WireGuard tunnel
ufw                # Firewall
docker             # Container runtime
docker-compose     # Multi-container apps
fail2ban           # (optional) Brute-force protection

# SSH config
openssh-server     # SSH daemon

Audit Commands

Layer 2: UFW

sudo ufw status verbose
# Check:
#   Status: active
#   Default: deny (incoming), allow (outgoing)
#   Only tailscale0 ports allowed (22, 2222)
#   80/tcp and 443/tcp allowed (for web)

Layer 3: Tailscale

tailscale status
# Check:
#   Server listed with correct IP
#   SSH enabled
#   Correct tags assigned

tailscale set --ssh=true  # ensure SSH enabled

Layer 4: SSH

# Check critical settings
grep -E "PermitRootLogin|PasswordAuthentication|X11Forwarding|MaxAuthTries|LoginGraceTime" /etc/ssh/sshd_config

# Expected:
#   PermitRootLogin no
#   PasswordAuthentication no
#   X11Forwarding no
#   MaxAuthTries 3
#   LoginGraceTime 30

# Check deploy user
id deploy
cat /home/deploy/.ssh/authorized_keys
# Verify: command= restricts to docker compose only
# Verify: no-agent-forwarding, no-port-forwarding, no-X11-forwarding

Layer 5: Docker

# Socket permissions
ls -la /var/run/docker.sock
# Expected: srw-rw---- root docker

# No insecure registries
cat /etc/docker/daemon.json | grep insecure
# Expected: nothing or empty

# Containers not exposing ports on 0.0.0.0
sudo ss -tlnp | grep -E "0\.0\.0\.0:(3001|3002|4040|9122|9111)"
# Expected: nothing

Layer 6: Application

# Check container isolation
docker ps --format "{{.Names}}: {{.Ports}}"
# Expected: no 0.0.0.0 bindings for internal services

# Check volumes
docker inspect <container> | grep -A5 "Mounts"
# Expected: named volumes or specific host paths, not /

Vulnerability Categories

Critical (fix immediately)

| Issue | Risk | Fix | |---|---|---| | PermitRootLogin yes | Root SSH access | Set to no | | PasswordAuthentication yes | Brute force risk | Set to no |

Medium (fix soon)

| Issue | Risk | Fix | |---|---|---| | X11Forwarding yes | Display hijacking | Set to no | | Processes on 0.0.0.0 | Port exposure | Bind to 127.0.0.1 or tailscale0 | | No fail2ban | No brute-force protection | Install + configure |

Low (consider)

| Issue | Risk | Fix | |---|---|---| | No AllowUsers | Any user can SSH | Add AllowUsers deploy | | No log monitoring | Missed intrusion attempts | Set up log alerts |

Hardening Checklist

SSH Hardening

# Add to /etc/ssh/sshd_config
PermitRootLogin no
PasswordAuthentication no
X11Forwarding no
MaxAuthTries 3
LoginGraceTime 30
AllowUsers deploy

# Restart SSH
sudo systemctl restart ssh

UFW Hardening

# Verify rules
sudo ufw status numbered

# Remove any overly permissive rules
sudo ufw delete <rule-number>

# Rate limiting (optional)
sudo ufw limit 22/tcp

Docker Hardening

# Restrict socket access
sudo chmod 660 /var/run/docker.sock
sudo chown root:docker /var/run/docker.sock

# No privileged containers
docker inspect <container> | grep Privileged
# Expected: false

# Read-only rootfs (optional)
docker run --read-only ...

fail2ban (optional)

sudo apt install fail2ban
sudo systemctl enable fail2ban

# Configure
cat > /etc/fail2ban/jail.local << 'EOF'
[sshd]
enabled = true
port = 22,2222
filter = sshd
logpath = /var/log/auth.log
maxretry = 3
bantime = 3600
EOF

sudo systemctl restart fail2ban

Examples

Example 1: Full audit

# Run all checks
echo "=== UFW ===" && sudo ufw status verbose
echo "=== SSH ===" && grep -E "PermitRootLogin|PasswordAuthentication|X11Forwarding" /etc/ssh/sshd_config
echo "=== Tailscale ===" && tailscale status
echo "=== Docker ===" && docker ps --format "{{.Names}}: {{.Ports}}"
echo "=== Ports ===" && sudo ss -tlnp | grep -E "0\.0\.0\.0"

Example 2: Fix critical vulnerabilities

# Fix SSH (run as root)
sudo sed -i 's/PermitRootLogin yes/PermitRootLogin no/' /etc/ssh/sshd_config
sudo sed -i 's/PasswordAuthentication yes/PasswordAuthentication no/' /etc/ssh/sshd_config
sudo sed -i 's/X11Forwarding yes/X11Forwarding no/' /etc/ssh/sshd_config
sudo systemctl restart ssh

Example 3: Check for exposed ports

# Find processes listening on 0.0.0.0
sudo ss -tlnp | grep "0\.0\.0\.0" | awk '{print $NF, $4}'
# Should only show: sshd (22), caddy (80,443)

Edge Cases

Server is public (no Tailscale)

• UFW must allow SSH on standard port (22) from specific IPs
• Consider fail2ban for brute-force protection
• Use key-based auth only (no passwords)

Multiple deploy users

• Each user needs separate command= in authorized_keys
• Use AllowUsers to whitelist all deploy users
• Verify each user's Docker group membership

Root access needed temporarily

• Use sudo from deploy user instead of PermitRootLogin
• Add deploy user to sudoers: deploy ALL=(ALL) NOPASSWD: /usr/bin/docker
• Never leave PermitRootLogin yes permanently

Container exposes ports

• Check docker-compose.yml for ports: section
• Bind to 127.0.0.1:host_port instead of host_port:host_port
• Use UFW to block if needed

Test Cases

Should

• [ ] UFW active with default deny incoming
• [ ] SSH only accessible via Tailscale (port 22, 2222)
• [ ] PermitRootLogin set to no
• [ ] PasswordAuthentication set to no
• [ ] Deploy user is non-root with docker group
• [ ] Deploy SSH key has command= restriction
• [ ] Docker socket owned by root:docker
• [ ] No processes listening on 0.0.0.0 for internal services

Should NOT

• [ ] Root should NOT be able to SSH directly
• [ ] Password auth should NOT be enabled
• [ ] Internal services should NOT bind to 0.0.0.0
• [ ] Deploy user should NOT have shell access
• [ ] Docker socket should NOT be world-readable