Clever hacker fits 537,000 domains in a tiny $5 ESP32 ad-blocking dongle — firmware uses only around 50KB of RAM and can answer blocked lookups in 10 milliseconds

How cheap can you build a hardware-based ad-blocking DNS filter? “Free,” if you’re willing to salvage some used hardware that’s being thrown away. What if you aren’t so lucky? In the era of the RAMageddon, even a Raspberry Pi will cost you a couple hundred bucks. But you know, you don’t even need something that powerful. In fact, you can use a $5 microcontroller to build a fully functional ad-blocking filter with over 500,000 domains blocked and around 10ms latency.

We know that’s possible because Egyptian full-stack developer ZedAxis (@M-Abozaid on GitHub) has already built one. Using an ESP32-C3 “SuperMini” board, he’s created a backup DNS for his home network that still provides ad blocking. His primary router is a Pi-hole, which is a Raspberry Pi running specialized software to manage DHCP addressing and DNS resolution with integrated ad-blocking. The SuperMini serves as a backup when the Pi-Hole is rebooting or otherwise unavailable.

Strictly speaking, we haven’t seen ZedAxis’ creation in action, but there’s no reason to believe it doesn’t work. The ESP32 family of embedded Wi-Fi-enabled microcontrollers is a well-known quantity, and this functionality is by no means outside the realm of its capabilities. In fact, this specific trick allows the use of the ESP32-C3, which is a cost-reduced version of the chip that doesn’t have the 8MB of PSRAM found on some higher-end models. (There are MANY variants of the ESP32.)

Latest Videos From

Instead, the ESP32-C3 used by ZedAxis has just 400KB of RAM, and 4MB of flash memory. With these limited specifications, he wasn’t able to store a plaintext blocklist of any real size; it’s simply too much data. So, he did what any enterprising hacker would do: he started hashing the data to reduce its size. Using 40-bit FNV-1a hashes, because 32-bit would give too many collisions and 64-bit wastes too much space, he can store some 537,000 domains in the flash memory of the device.

The clever part isn’t really the hashing, though. Rather than storing the domains themselves, the build process downloads one or more public blocklists, strips out duplicate entries and comments, hashes each remaining domain into a 40-bit value, sorts the resulting list, and writes the finished database into the ESP32’s flash memory. When a DNS query arrives, the device hashes the requested hostname the same way and performs a binary search against the sorted hash table. If it finds a match, the request is blocked. Otherwise, the query is forwarded to an upstream resolver. According to ZedAxis, the finished firmware uses only around 50KB of RAM while answering blocked lookups in roughly 10 milliseconds.

I built a $2 ad-blocker for my whole network – YouTube

Watch On