Network Access Control Lists and Filtering

Access Control Lists (ACLs) provide first-line defense against DDoS attacks. Configure router and firewall ACLs to block traffic from known attack sources. Implement geographic filtering to block traffic from regions where you have no legitimate users. Maintain dynamic blacklists that automatically block identified attackers.

Protocol filtering reduces attack surface by limiting allowed traffic types. Block unnecessary protocols and restrict others to required ports. Implement strict UDP filtering, as UDP floods represent common attack vectors. Configure ICMP limiting to prevent ping floods while allowing necessary diagnostic traffic.

Source address validation prevents spoofed packet attacks. Implement ingress filtering (BCP 38) to block packets with spoofed source addresses. Configure Reverse Path Forwarding (RPF) checks on routers to verify packet sources. These measures prevent your network from being used in reflection attacks while blocking some incoming attacks.

Application-layer filtering provides granular protection against sophisticated attacks. Implement Web Application Firewalls (WAF) to filter malicious HTTP/HTTPS traffic. Configure rules to block known attack patterns, suspicious user agents, and malformed requests. Use positive security models that explicitly allow known good traffic rather than trying to block all bad traffic.