ioc2rpz: Building DNS-Layer Security with Erlang and Response Policy Zones
Hook
According to Cisco’s 2016 security report, 91.3% of malware uses DNS for command and control. While DNS is the internet’s control plane, most organizations lack an automated way to weaponize threat intelligence at the DNS layer—until now.
Context
Response Policy Zones (RPZ) have been available in ISC BIND since version 9.8, giving DNS administrators a powerful mechanism to block malicious domains at the DNS resolution level. The concept is elegant: instead of maintaining firewall rules or proxy blacklists, you configure your DNS server to return controlled responses (like NXDOMAIN or a sinkhole IP) for known-bad domains. The challenge? Operationalizing this capability at scale.
Most threat intelligence feeds distribute Indicators of Compromise (IOCs) in various text formats—CSV files, JSON arrays, plain lists with different delimiters. Converting these into RPZ-formatted zone files, managing updates across multiple feeds, handling indicator expiration, and distributing zones to production DNS servers traditionally required significant manual scripting or commercial solutions. ioc2rpz bridges this gap by acting as a specialized authoritative DNS server that ingests IOCs from arbitrary sources, transforms them into RPZ feeds automatically, and serves these zones via standard DNS protocols. Written in Erlang, it leverages the language’s concurrency and fault-tolerance capabilities to handle millions of indicators without performance degradation.
Technical Insight
At its core, ioc2rpz operates as a custom DNS server, but one with a singular focus: transforming threat intelligence into Response Policy Zones and distributing them efficiently. The architecture relies on Erlang’s ETS (Erlang Term Storage) for in-memory indicator storage, enabling fast lookups even with massive datasets. When a downstream DNS server requests a zone transfer via AXFR or IXFR, ioc2rpz generates the RPZ from its ETS tables.
The ingestion pipeline supports multiple source types—HTTP/HTTPS/FTP endpoints, local files, and shell scripts for custom integrations. The key to handling diverse feed formats is regex-based parsing. In your configuration file (ioc2rpz.conf), you define sources with custom regular expressions to extract domains or IPs from arbitrary text formats. The system supports any file format where indicators are separated by newline or carriage return characters, as long as you can write a regex to extract them.
What makes ioc2rpz particularly powerful is its support for both cached and non-cached (“live”) zones. Live zones are generated on-demand when a DNS server requests them, which conserves memory when serving multiple RPZ feeds that aren’t frequently accessed. The system also respects IOC expiration times, automatically rebuilding zones as indicators age out, ensuring your DNS security posture doesn’t degrade with stale data.
One standout feature is DNS-over-TLS (DoT) support for RPZ distribution. In traditional setups, zone transfers happen over plaintext DNS, meaning an adversary monitoring your network traffic could observe which threat indicators you’re tracking. With ioc2rpz’s DoT implementation on port 853, zone transfers are encrypted. The current implementation has limitations—TLS 1.2 only, single request per session, and unencrypted NOTIFY messages—but it’s still a significant security improvement over plaintext AXFR. The DoT service activates automatically when you provide a certificate in the configuration. Erlang’s certificate handling means renewals don’t require service restarts—simply replace the certificate file and the system picks up the new cert within two minutes due to caching.
From a deployment perspective, ioc2rpz expects a configuration file mounted at /opt/ioc2rpz/cfg/ioc2rpz.conf and exposes several ports: 53/UDP for SOA requests, 53/TCP for zone transfers and management, 853/TCP for encrypted DoT transfers, and 8443/TCP for REST API. The Docker deployment model makes it straightforward to run alongside existing DNS infrastructure—your RPZ-capable recursive resolvers (ISC BIND 9.8+, PowerDNS Recursor 4.0+, and partial support on Knot DNS) point to ioc2rpz as an authoritative source for RPZ zones, pulling updates automatically.
A sample bind configuration file (named.conf) is provided in the cfg folder to help with integration. You’d configure BIND to use ioc2rpz as an RPZ source with standard zone transfer syntax, essentially treating it like any other authoritative DNS server. The beauty of this approach is that ioc2rpz handles all the complexity of feed aggregation, parsing, deduplication, and expiration—your existing DNS infrastructure just sees clean, standards-compliant RPZ data.
Gotcha
The DoT implementation, while valuable, has notable constraints that may impact security-conscious deployments. Supporting only TLS 1.2 means you’re not getting modern protocol protections from TLS 1.3, and the single-request-per-session limitation could impact transfer efficiency for large zones. More critically, DNS NOTIFY messages—which signal zone updates to secondary servers—remain unencrypted, potentially leaking information about when your threat intelligence gets updated.
Memory is your primary scaling bottleneck. Since ioc2rpz holds indicator sets in RAM via ETS tables, deploying it on memory-constrained systems with millions of indicators will cause problems. The README explicitly states that system memory is the only limitation, even for Raspberry Pi deployments. There’s no mention of disk-backed storage options for indicators, so you need to plan memory capacity based on your aggregate feed size. Additionally, the Erlang runtime requirement means you’re committing to managing an Erlang-based service in your infrastructure stack. If your team lacks Erlang expertise, troubleshooting production issues or customizing the solution becomes significantly harder. The ecosystem assumes you’re comfortable with Erlang’s deployment model and debugging tools, which is a non-trivial operational consideration for many teams.
Verdict
Use if you’re running RPZ-capable DNS infrastructure (ISC BIND 9.8+, PowerDNS Recursor 4.0+, or Knot DNS with partial support) and need to operationalize multiple threat intelligence feeds at scale—especially if you’re dealing with millions of indicators and want automated feed management without manual zone file wrangling. It’s particularly compelling if you value encrypted RPZ distribution and have the system memory to support large in-memory indicator sets. The Erlang foundation makes it a solid choice for high-availability environments where DNS uptime is critical. Skip if your DNS servers don’t support RPZ (eliminating the entire value proposition), you’re looking for a simple single-feed solution where manual zone management is acceptable, or your team lacks Erlang operational experience and can’t justify the learning curve. Also skip if you need cutting-edge TLS 1.3 support or require disk-backed indicator storage for memory-constrained deployments. For small networks or home labs, Pi-hole offers simpler DNS-based blocking, while enterprises with budget may prefer turnkey commercial DNS security platforms that bundle threat intelligence with support contracts.
