A DNSBL (DNS-based Blocklist) is a method for publishing lists of IP addresses in a format that can be queried using the DNS protocol. It's one of the most efficient ways to check IP reputation at scale and forms the backbone of email security worldwide.
Understanding DNSBLs and Blocklists
DNSBL stands for DNS-based Blocklist (also called RBL - Realtime Blocklist). It's a clever technique that uses the existing DNS infrastructure to distribute blocklist data in real-time. An IP blocklist is a curated database of IP addresses identified as sources of malicious activity.
Instead of downloading and maintaining local copies of blocklists, applications can simply perform a DNS lookup to check if an IP is listed. Modern blocklists are dynamic, continuously updated databases maintained by security researchers, community reports, and automated detection systems.
Types of IP Blocklists
Different blocklists serve different purposes. Understanding the categories helps you choose the right protection for your use case:
Spam Blocklists
Focus on IPs sending unsolicited email or comment spam. Essential for mail server protection.
Malware/Botnet Blocklists
Track IPs associated with malware distribution, C2 servers, or botnet activity.
Attack Source Blocklists
Identify IPs performing web attacks, vulnerability scanning, or brute force attempts.
How DNSBL Lookups Work
The lookup process is surprisingly simple and leverages the existing DNS infrastructure:
- Reverse the IP - The IP address is reversed (e.g., 1.2.3.4 becomes 4.3.2.1)
- Append the zone - The DNSBL zone is appended (e.g., 4.3.2.1.bl.fraudcache.com)
- Perform DNS lookup - A standard DNS A record query is performed
- Check the response - If listed, you get a response (usually 127.0.0.x). No response means not listed.
Example lookup command:
dig +short 1.2.3.4.bl.fraudcache.com
This returns 127.0.0.2 if listed, or NXDOMAIN if not listed.
How Blocklists Are Used in Practice
When your system receives a connection request, it queries one or more blocklists to check if the source IP is listed. If the IP appears on the list, the connection can be rejected or flagged for additional scrutiny.
Blocklists can be queried via DNS (DNSBL), REST APIs, or downloaded as files for local processing. DNSBL is preferred for high-volume, low-latency scenarios like email filtering, while file downloads work better for firewalls that need offline access.
Benefits of DNSBL
Lightning Fast
DNS lookups are extremely fast, typically under 50ms, with no local database to maintain.
Always Current
No need to download updates - every query returns real-time data reflecting the latest threats.
Universal Support
Almost every mail server (Postfix, Exim, Exchange) and many security applications support DNSBL queries natively.
Fraudcache DNSBL Service
Fraudcache provides a free DNSBL service that integrates seamlessly with popular mail servers and security tools.
Our DNSBL includes response codes that indicate the threat category (spam, web attacks, botnet, etc.), allowing for more nuanced filtering decisions. The service is updated continuously based on our global threat intelligence network.
Related Articles
Deepen your understanding with these related topics:
- What is IP Reputation? - Understand the fundamentals of how IP reputation is calculated
- How IP Scoring Works - Learn how Fraudcache's confidence scoring system works
- Postfix Spam Filtering with DNSBL - Step-by-step guide to integrating DNSBL with Postfix mail servers
Conclusion
DNSBLs and IP blocklists are essential tools for protecting your infrastructure from known threats. By leveraging real-time DNS queries, you can block malicious traffic at the network edge before it reaches your applications. Fraudcache's DNSBL service provides comprehensive coverage across spam, malware, and attack sources, with the speed and reliability needed for production environments.
Set Up DNSBL Integration
Learn how to integrate Fraudcache DNSBL with your mail server or application.