Bare Metal Web Hosting: Running a Website on an 8-Bit AVR Microcontroller

In the world of modern cloud computing, where we measure server capacity in vCPUs and gigabytes of RAM, the idea of hosting a website on an 8-bit microcontroller seems like an exercise in absurdity. However, for the embedded engineer, these "dumb things to do" are often the best way to explore the fundamental layers of the internet protocol suite.

Recently, developer Maurycy demonstrated a project that pushes the limits of the AVR64DD32—a low-cost, 8-bit microcontroller—by turning it into a fully functional (albeit slow) web server. This project serves as a masterclass in stripping away abstractions to understand how data actually moves from a wire to a browser.

The Hardware: AVR64DD32

The choice of hardware is critical when working with extreme constraints. The AVR64DD32 is a modern relative of the famous ATmega328 (the heart of the Arduino Uno), but it offers several advantages for this specific use case:

• CPU: Single 8-bit AVR core running at 24 MHz.
• Memory: 8 kB of SRAM and 64 kB of Flash.
• Cost: Approximately $1.
• Peripherals: Improved programming pins and more efficient peripherals compared to older ATmega chips.

While 64 kB of Flash might seem spacious for a simple script, implementing a networking stack from scratch quickly consumes available resources.

The Connectivity Challenge: Why Not Ethernet?

For most embedded projects, the immediate instinct is to use Ethernet. However, the author highlights a significant physical layer hurdle: Manchester encoding.

Standard 10BASE-T Ethernet runs at 10 Mbps. Because it uses Manchester encoding—where a '0' is represented by a transition from high to low and a '1' from low to high—the actual signal frequency on the wire is 20 MHz. With the AVR's I/O pins maxing out at 12 MHz, the microcontroller simply cannot toggle its pins fast enough to generate a native Ethernet signal.

To avoid the delay of ordering a dedicated Ethernet controller chip (like the ENC28J60, which was a staple of early PIC18 demos), the author opted for a more nostalgic approach: SLIP (Serial Line Internet Protocol).

Implementing SLIP (RFC 1055)

SLIP is a legacy standard designed to run networks over serial lines. It is remarkably simple: it wraps packets in 0xC0 bytes and uses a basic escape sequence (0xDB 0xDC and 0xDB 0xDD) to handle data that conflicts with the control characters.

By using a USB-to-Serial adapter and the slattach utility in Linux, the microcontroller can be treated as a standard network interface. This effectively turns a modern PC into a gateway for the 8-bit chip.

Building the Network Stack

Once the physical link was established, the author had to implement the logic to handle internet traffic. This was broken down into three layers:

1. The IP Layer

Implementing IPv4 was surprisingly straightforward because modern operating systems have largely abandoned packet fragmentation. By ignoring fragmentation and focusing on the core header, the author could generate response headers simply by swapping the source and destination IP addresses and resetting the Time-to-Live (TTL) counter.

2. The TCP Layer

TCP is where the complexity resides. Unlike the stateless nature of IP, TCP requires the microcontroller to:

• Track connection states (SYN, ACK, FIN).
• Manage sequence numbers to ensure packets arrive in order.
• Handle retransmissions for lost packets.

This required several days of development and remains the most fragile part of the implementation.

3. The HTTP Layer

To keep the footprint minimal, the author bypassed a full HTTP parser. Instead, the server is programmed to send a hardcoded response regardless of the request. As long as the site only contains a single URL, this "shortcut" is indistinguishable from a real web server to the end user.

Routing and Public Access

Since the microcontroller lacks a public IPv4 address and sits behind a NAT, the author utilized a WireGuard VPN to connect a local Linux router to a VPS in a Helsinki datacenter.

To avoid disrupting the VPS's primary website, the author configured a reverse proxy. Any requests hitting the VPS under the /mcu path are forwarded to the microcontroller's local IP address. This architecture not only provides public access but also adds a layer of protection against basic SYN-flood DDoS attacks, as the VPS handles the initial TCP handshake.

Technical Reflections and Community Insights

The project sparked a discussion among the hardware community regarding the evolution of the AVR line. While the AVR DD series is praised for its peripherals, some observers noted the emergence of the PIC32 CM, which offers an ARM Cortex-M0+ core while maintaining 5V operation and similar peripherals, potentially rendering 8-bit architectures obsolete for these types of tasks.

However, as one commenter noted, the charm of such projects lies in the constraints. The result is a website that loads with the nostalgic, top-to-bottom rendering speed of the dial-up era—a testament to the fact that the client (the browser) does the heavy lifting, while the server merely needs to deliver the bytes.