Firewall

Configure firewalls on servers and cloud providers with security best practices.

Firewall Rules

Critical First Steps

• Allow SSH/remote access before enabling any firewall — enabling first locks you out
• Test access in a second session before closing the first — verify the rule actually works
• Know how to access provider console — it's the only way back if locked out

Default Stance

• Default deny all incoming traffic — only open what you explicitly need
• Default allow outgoing traffic — most apps need to reach the internet
• Every open port is attack surface — question each one before adding

Essential Ports

• SSH (22 or custom): Always needed for remote access — consider limiting to your IP only
• HTTP (80): Only if serving web traffic — also needed for Let's Encrypt HTTP challenge
• HTTPS (443): For production web services
• Don't open database ports (3306, 5432, 27017) to the internet — access via SSH tunnel or private network

Provider Firewalls (Hetzner, DigitalOcean, AWS, etc.)

• Provider firewall applies before traffic reaches your server — faster, less server load
• Changes usually apply immediately — no reload command needed
• Stateful by default — allow inbound, responses automatically allowed outbound
• Apply to server groups for consistency — easier than per-server rules
• Provider firewall + OS firewall = defense in depth — use both when possible

IP Restrictions

• Limit SSH to known IPs when possible — dramatically reduces attack surface
• Your home IP may change — use a VPN with static IP or update rules when it changes
• Allow IP ranges with CIDR notation — /32 is single IP, /24 is 256 IPs
• Some providers support dynamic DNS in rules — check before building complex solutions

Common Services to Consider

• VPN (WireGuard: 51820/UDP, OpenVPN: 1194) — allows secure access without exposing other ports
• Mail (25, 465, 587) — only if running mail server
• DNS (53 TCP/UDP) — only if running DNS server
• Monitoring agents may need outbound access to specific IPs

Docker Warning

• Docker bypasses most OS firewalls by default — containers expose ports regardless of UFW/iptables
• Solution: bind containers to localhost only and use reverse proxy for public access
• Or configure Docker to respect firewall rules — requires additional setup
• Provider-level firewalls still work — they block before traffic reaches Docker

IPv6

• Firewalls often have separate IPv4 and IPv6 rules — configure both
• Provider firewalls may handle both together — check their documentation
• Attackers probe IPv6 when IPv4 is locked down — don't neglect it

Debugging

• Test from outside your network — rules may look correct but not work
• Provider dashboards often show blocked traffic logs
• "Connection refused" = port closed properly; "Connection timeout" = firewall dropping silently
• Online port scanners verify what's actually open from the internet

Common Mistakes

• Opening ports "temporarily" and forgetting to close them
• Opening 80/443 when no web server runs — unnecessary exposure
• Forgetting UDP for services that need it — DNS, VPN, game servers
• Assuming firewall is active — verify it's actually running/applied
• Only configuring IPv4 — leaving IPv6 wide open
• Trusting "security through obscurity" — non-standard ports slow attackers, don't stop them