curl / Docs / Protocols / WebSocket

WebSocket in curl


WebSocket communication with libcurl is done by setting up a transfer to a URL using the ws:/ or wss:// URL schemes. The latter one being the secure version done over HTTPS.

When using wss:// to do WebSocket over HTTPS, the standard TLS and HTTPS options will be acknowledged for the CA, verification of server certificate etc.

WebSocket communication is done by upgrading a connection from either HTTP or HTTPS. When given a WebSocket URL to work with, libcurl considers it a transfer failure if the upgrade procedure fails. This means that a plain HTTP 200 response code is considered an error for this work.


The WebSocket API is described in the individual man pages for the new API.

WebSocket with libcurl can be done two ways.

  1. Get the WebSocket frames from the server sent to the write callback. You can then respond with curl_ws_send() from within the callback (or outside of it).

  2. Set CURLOPT_CONNECT_ONLY to 2L (new for WebSocket), which makes libcurl do an HTTP GET + Upgrade: request plus response in the curl_easy_perform() call before it returns and then you can use curl_ws_recv() and curl_ws_send() to receive and send WebSocket frames from and to the server.

The new options to curl_easy_setopt():

CURLOPT_WS_OPTIONS - to control specific behavior. CURLWS_RAW_MODE makes libcurl provide all WebSocket traffic raw in the callback.

The new function calls:

curl_ws_recv() - receive a WebSocket frame

curl_ws_send() - send a WebSocket frame

curl_ws_meta() - return WebSocket metadata within a write callback

Max frame size

The current implementation only supports frame sizes up to a max (64K right now). This is because the API delivers full frames and it then cannot manage the full 2^63 bytes size.

If we decide we need to support (much) larger frames than 64K, we need to adjust the API accordingly to be able to deliver partial frames in both directions.


If the given WebSocket URL (using ws:// or wss://) fails to get upgraded via a 101 response code and instead gets another response code back from the HTTP server - the transfer will return CURLE_HTTP_RETURNED_ERROR for that transfer. Note then that even 2xx response codes are then considered error since it failed to provide a WebSocket transfer.

Test suite

I looked for an existing small WebSocket server implementation with maximum flexibility to dissect and cram into the test suite but I ended up deciding that extending the existing test suite server sws to deal with WebSocket might be the better way.

Command line tool WebSocket

The plan is to make curl do WebSocket similar to telnet/nc. That part of the work has not been started.


Future work

Why not libWebSocket

libWebSocket is said to be a solid, fast and efficient WebSocket library with a vast amount of users. My plan was originally to build upon it to skip having to implement the low level parts of WebSocket myself.

Here are the reasons why I have decided to move forward with WebSocket in curl without using libWebSocket: