Rule Design Principles

Well-designed firewall rules form the foundation of effective protection. Following consistent principles across all firewall implementations reduces errors, simplifies management, and improves security outcomes.

Principle of Least Privilege: Every firewall rule should grant the minimum access necessary for legitimate functionality. Start with a default deny stance, blocking all traffic by default, then explicitly allow only required connections. For web servers, this typically means allowing inbound HTTP/HTTPS from the internet, but restricting administrative access to specific source IPs or VPN endpoints. Avoid broad rules that allow entire port ranges or large network blocks unless absolutely necessary.

Rule Documentation and Naming: Every rule requires clear documentation explaining its purpose, who requested it, when it was implemented, and any associated change tickets. Use consistent naming conventions that indicate the rule's function:

# Good rule naming examples
allow-https-from-internet
allow-ssh-from-admin-vpc
block-scanner-user-agents
ratelimit-api-requests

# Include metadata in rule descriptions
Description: "Allow HTTPS traffic from CloudFront distributions"
Ticket: "CHANGE-2024-0145"
Owner: "WebOps Team"
Review-Date: "2024-07-01"

Rule Order Optimization: Place rules in order of frequency and specificity. Most-matched rules should appear first to minimize processing overhead. Specific deny rules must precede general allow rules to ensure blocks take effect:

Priority 100: Block known malicious IPs (specific deny)
Priority 200: Allow HTTPS from anywhere (general allow)
Priority 300: Allow SSH from admin network (specific allow)
Priority 400: Log all other traffic (monitoring)
Priority 500: Deny all (default deny)