Why Run a Linux OS Just to Drop Packets? Build a Bare-Metal DNS Sinkhole on an $8 ESP32-S3

Running a full general-purpose Linux stack just to intercept UDP port 53 and drop ad domains to 0.0.0.0 is pure architectural overkill. Millions of home lab users spin up Pi-hole on a Raspberry Pi 4, Raspberry Pi 5, or dedicated x86 mini PC. It works, but allocating gigabytes of RAM, an entire OS kernel, and a power-hungry board to a task consisting almost entirely of string lookups and pointer matching is the definition of using a sledgehammer on a thumbtack.

Developer s60sc demonstrated that you can strip away the Linux operating system entirely with ESP32_AdBlocker. By running a bare-metal DNS sinkhole on an $8 microcontroller, you get sub-50 microsecond lookups, instant recovery, milliwatt power draw, and total immunity to SD card filesystem corruption.

How Microcontroller-Level DNS Sinkholing Works

The operational mechanics of ESP32_AdBlocker mirror standard DNS sinkhole architecture without the overhead of user-space daemon scheduling.

+-------------------------------------------------------------+
|                     Client DNS Request                      |
+-------------------------------------------------------------+
                               |
                               v
+-------------------------------------------------------------+
|               ESP32-S3 Bare-Metal Ingress                   |
+-------------------------------------------------------------+
                               |
                   [ Matches Blocklist? ]
                               |
                +--------------+--------------+
                |                             |
             [ YES ]                       [ NO ]
                |                             |
                v                             v
   +-------------------------+   +-------------------------+
   |   Drop Query / Return   |   |   Forward Upstream to   |
   |        0.0.0.0          |   | Google, Cloudflare,     |
   |  (Lookup Latency <50µs) |   | Quad9, or NextDNS       |
   +-------------------------+   +-------------------------+

Query Ingestion: The ESP32 intercepts incoming IPv4 DNS queries sent from local clients or your gateway router.
In-Memory Matching: The query is parsed and evaluated against local blocklists loaded in high-speed PSRAM. Matching hits return 0.0.0.0 instantly.
Upstream Forwarding: Clean queries route directly to your chosen upstream resolver, such as Google, Cloudflare, Quad9, or NextDNS.
Data Ingestion: The system handles raw hosts-file lists and AdBlock-formatted sources, including aggregated collections like Steven Black.

Configuration, live allow/block domain injection, and runtime state are managed through an on-chip web server backed by local non-volatile storage.

Hardware Selection: Why the ESP32-S3-N16R8 is Mandatory

Do not attempt to run this setup on standard legacy microcontrollers. A base ESP32 with 4MB PSRAM and a 1.5MB default filesystem will truncate aggregated lists and force higher lookup times.

The ESP32-S3-N16R8 is the definitive reference design for this workload.

Feature	Standard ESP32 (4MB PSRAM)	ESP32-S3-N16R8 (8MB PSRAM)
Blocklist Capacity	Likely truncated	Handles massive aggregated lists
Lookup Latency	~100 µs	< 50 µs
Flash Storage	Default 1.5MB FS	16MB Flash (enables 12.5MB FATFS)
Availability / Cost	Common	Easy to find, ~$8

The 8MB of Octal PSRAM enables the board to load extensive blocklists into volatile memory without running out of heap space. This cuts lookup latency down to under 50 microseconds.

To build this appliance, you only need three physical items:

An ESP32-S3-N16R8 development board (~$8).
A verified data-capable USB-C cable (ensure it is not a charge-only cable).
A basic 5V/1A USB power source, such as the accessory port on your home router.

Partitioning Flash and Firmware Flashing

Standard partition schemes allocate only 1.5MB to the onboard flash filesystem. This is inadequate for modern aggregated blocklists. Utilizing the 16MB onboard flash of the N16R8 requires creating a 12.5MB FATFS partition.

1. Toolchain and Core Configuration

Clone the project repository from GitHub into your local sketch directory, ensuring the root folder is renamed to remove any -master branch suffix. You must use ESP32 Arduino Core v3.1.1 or newer with PSRAM support activated.

2. Custom Partition Scheme

Modify your boards.txt or select the appropriate profile to expose the 16M Flash (2MB APP/12.5MB FATFS) partition mapping. This reserves 2MB for the binary runtime while dedicating the remaining 12.5MB purely to list structures and web static assets.

3. Flash Settings (Arduino IDE Tools Menu)

Set your flash parameters precisely to avoid runtime memory errors:

USB CDC on Boot: Enabled
Erase All Flash Before Sketch Upload: Enabled (first flash only)
Flash Mode: DIO 80MHz
Partition Scheme: 16M Flash (2MB APP/12.5MB FATFS)

4. Initial Provisioning

Flash the firmware over USB. Open the Serial Monitor at boot to capture the fallback Access Point IP, typically http://192.168.4.1.

Connect a client to the temporary ESP32_AdBlocker wireless network, navigate to the Web UI, and supply your local network credentials. Once the board joins your local network and establishes an internet connection, it automatically pulls required web layout assets and configuration files directly from GitHub into its SDCARD/html local directory.

Managing Blocklists Within Memory Constraints

Because embedded firmware architectures optimize for lookup speed over dynamic aggregation, the system accepts one primary remote blocklist URL.

Handling multi-source blocking requires specific strategies:

Consolidation: If you rely on multiple blocklist maintainers, merge and deduplicate them offline into a single aggregated file before pointing the firmware to your custom URL.
Local Overrides (custom_domains.txt): Keep your custom allow and deny entries in a local file. Uploading custom_domains.txt directly to the onboard storage through the Web UI keeps private domain lists local without pushing internal network traffic to external servers.
Automated Sync: The internal scheduler executes daily updates at your designated time. Incremental delta updates process rapidly, though the initial compile-and-write cycle on first boot will take a few minutes depending on the target list size.

Dashboard Architecture and Real-Time Controls

The native web interface provides direct control over core operational registers and file systems:

Metrics Engine: Real-time telemetry displaying running totals of Allowed and Blocked queries since the last boot cycle.
Blocklist Orchestration: A dedicated interface to update the target list URL, execute ad-hoc sync operations, or kill an in-flight sync using the Stop Download interrupt. Leaving the URL blank triggers a rebuild of the currently cached list.
Runtime Rules: Input fields for granular Block, Unblock, and Check operations to modify routing rules instantly without triggering a filesystem rewrite.
Factory Sanitation: The Clear Custom routine purges all manual whitelist and blacklist adjustments with a single command.

+-------------------------------------------------------------+
|                ESP32_AdBlocker Web Console                  |
+-------------------------------------------------------------+
|  [Configure]   - Static IP, Gateway, Core Network Settings  |
|  [Settings]    - Blocklist Parsing Logic & Environment Vars |
|  [Network]     - Wi-Fi, Ethernet, W5500 SPI Bus Config      |
|  [Log]         - Real-Time Query Audit & Verbose Telemetry  |
+-------------------------------------------------------------+

All modifications are written directly to non-volatile storage by hitting the Save button.

Network Integration and Protocol Constraints

To sinkhole your entire network, assign the ESP32 a static IPv4 address and set your router's DHCP options to distribute this address as the primary DNS server.

Critical IPv6 Warning

ESP32_AdBlocker does not support IPv6. If IPv6 DNS remains enabled on your local router or client adapters, modern operating systems will silently fall back to your IPv6 DNS path, completely bypassing the IPv4 sinkhole. You must explicitly disable IPv6 DNS routing across your network.

Client-Level Configuration Example (Windows CLI)

Assign the ESP32 node (example: 192.168.1.10) directly to your physical interface:

bash
netsh interface ipv4 set dnsservers "Ethernet" static 192.168.1.10 primary

Roll back to upstream defaults:

bash
netsh interface ipv4 set dnsservers "Ethernet" static 8.8.8.8 primary

Neutralizing Encrypted DNS Bypasses

Modern browsers leverage DNS-over-HTTPS (DoH) and DNS-over-TLS (DoT) by default, bypassing local port 53 listeners. To guarantee complete capture:

Disable Secure DNS in the settings of Chrome, Firefox, and Edge.
Implement router-level firewall drops on outbound DoH endpoint targets.

Architectural Trade-offs: ESP32 vs. Linux-Based Solutions

Selecting bare-metal firmware over a standard Linux stack involves clear operational trade-offs.

                       SYSTEM SELECTION MATRIX
                       
  ESP32-S3 BARE METAL                    LINUX / PI-HOLE / DOCKER
+-----------------------------+        +-----------------------------+
| * Sub-50µs lookup latency   |        | * Dynamic multi-URL engines |
| * $8 BOM cost               |        | * Regex pattern routing     |
| * Zero OS maintenance       |        | * Per-device client ACLs    |
| * Flash partition limits    |        | * Multi-service hosting     |
| * Single remote list URL    |        | * Heavy OS overhead & wear  |
+-----------------------------+        +-----------------------------+

Filesystem Constraints: You must carefully size your custom FATFS partition. Pushing past available flash breaks list compilation.
Ruleset Flexibility: You trade complex Regex filters, per-client conditional forwarding, and historical database logging for raw execution speed and absolute appliance stability.

Final Assessment

If your infrastructure relies on deep multi-tenant client policies, active directory forwarding, and long-term analytical queries, run Pi-hole inside Docker on a dedicated host.

However, if your objective is to build a reliable, inexpensive ad-blocking appliance that runs independently without system updates, SSH management, or SD card wear, the ESP32-S3 is an ideal platform. Running this bare-metal firmware delivers a compact, high-efficiency network sinkhole for just $8.