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Websocket Server for Arduino

This library implements a Websocket server running on an Arduino. It's based on the proposed standard published December 2011 which is supported in the current versions (June 2012) of Firefox, Chrome, and Safari 6.0 beta (not older Safari, unfortunately) and thus is quite usable.

The implementation in this library has restrictions as the Arduino platform resources are very limited:

  • The server only handles TXT frames.
  • The server only handles single byte chars. The Arduino just can't handle UTF-8 to it's full.
  • The server only accepts final frames with maximum payload length of 64 bytes. No fragmented data, in other words.
  • For now, the server silently ignores all frames except TXT and CLOSE.
  • The amount of simultaneous connections may be limited by RAM or hardware. (Each connection takes 16 bytes of RAM, and the W5100 shield is hardware-limited to 4 simultaneous connections.)
  • There's no keep-alive logic implemented.

Required headers (example):

GET /whatever HTTP/1.1
Upgrade: websocket
Connection: Upgrade
Sec-WebSocket-Key: dGhlIHNhbXBsZSBub25jZQ==
Sec-WebSocket-Version: 13

The server checks that all of these headers are present, but only cares that the version is 13.

Response example:

HTTP/1.1 101 Switching Protocols
Upgrade: websocket
Connection: Upgrade
Sec-WebSocket-Accept: s3pPLMBiTxaQ9kYGzzhZRbK+xOo=

The last line is the Base64 encoded SHA-1 hash of the key with a concatenated GUID, as specified by the standard.

Daniel O'Neill: Compared to the original, this library consumes significantly less RAM and provides additional functionality.


  • Arduino IDE 1.0.1 or greater. You should not use 1.0 since it has a bug in the Ethernet library that will affect this library.
  • An Arduino Duemilanove or greater with Ethernet shield. An Arduino Ethernet should work too, but it has not been tested.
  • A Websocket client that conforms to version 13 of the protocol.

Getting started

Install the library to "libraries" folder in your Arduino sketchbook folder. For example, on a mac that's ~/Documents/Arduino/libraries.

Try the supplied echo example together with the web based test client to ensure that things work.

Start playing with your own code and enjoy!


I'm a pretty lousy programmer, at least when it comes to Arduino, and it's been 15 years since I last touched C++, so do file issues for every opportunity for improvement.

Oh by the way, quoting myself:

Don't forget to place a big fat disclaimer in the README. There is most certainly bugs in the code and I may well have misunderstood some things in the specification which I have only skimmed through and not slept with. So please do not use this code in appliancies where living things could get hurt, like space shuttles, dog tread mills and Large Hadron Colliders.


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