IP Blocklists - Blocking Known Malicious Sources

About 2 min read

An IP blocklist is a list that registers IP addresses confirmed to be engaged in malicious activity and automatically denies access from them. By incorporating it into the rules of a firewall or a WAF, you can block communication from known attack sources right at the entrance. As of 2025, operations that update blocklists in real time through automatic integration with threat intelligence feeds have become common.

Real-World Use Cases

"When we came under a DDoS attack, we added the attack source's IP address range (/24) to the WAF IP set to block it immediately. At the same time, we obtain the IP list of related C2 servers from threat intelligence feeds and add them to the blocklist preventively."

Types of Blocklists

Public blocklists (such as Spamhaus and AbuseIPDB) are services that share the IP addresses of spam senders and malware distributors across the community. Commercial threat intelligence feeds provide more accurate IP reputation information. An in-house blocklist is one in which you register, on your own, the attack-source IPs detected through your own log analysis; operating it with automatic updates linked to a SIEM is effective.threat intelligence books (Amazon) offer a systematic way to learn.

Operational Scenarios

When a DDoS attack on a web server is detected, you add the attack source's IP address range to the blocklist and cut it off immediately. On mail servers, the IPs of spam senders are checked against a DNS-based blocklist (DNSBL), and reception is refused. In cloud environments, by combining the AWS WAF IP set with CloudFront geo-restriction, access control on a per-country basis is also possible. By using DNS security together with an IP blocklist, you can achieve a multi-layered defense.

Operational Considerations

The biggest challenge with IP blocklists is false positives. If you block a shared IP address (a NAT environment, a CDN, or a VPN), legitimate users get caught up in it as well. You need to review the blocklist periodically and operate it so that unnecessary entries are removed. In addition, because attackers frequently change their IP addresses, do not rely on the blocklist alone; combine it with rate limiting and behavioral analysis. It is also important to protect the firewall's management console with a strong random password.network defense books (Amazon) are also helpful references.