libcurl.js

libcurl.js

This is an experimental port of libcurl to WebAssembly for use in the browser. It provides an interface compatible with the Fetch API, allowing you to proxy HTTPS requests from the browser with full TLS encryption. Unlike previous implementations, the proxy server cannot read the contents of your requests.

Features:

• Fetch compatible API
• End to end encryption between the browser and the destination server
• Support for up to TLS 1.3
• Support for tunneling HTTP/2 connections
• Support for proxying WebSockets
• Bypass CORS restrictions
• Low latency via multiplexing and reusing open connections

Building:

You can build this project by running the following commands:

git clone https://github.com/ading2210/libcurl.js --recursive
cd libcurl.js/client
./build.sh

Make sure you have emscripten, git, and the various C build tools installed. The only OS supported for building libcurl.js is Linux. On Debian-based systems, you can run the following command to install all the dependencies:

sudo apt install make cmake emscripten autoconf automake libtool pkg-config wget xxd

The build script will generate client/out/libcurl.js as well as client/out/libcurl.mjs, which is an ES6 module. You can supply the following arguments to the build script to control the build:

• release - Use all optimizations.
• single_file - Include the WASM binary in the outputted JS using base64.
• all - Build twice, once normally, and once as a single file.

Javascript API:

Importing the Library:

To import the library, follow the build instructions in the previous section, and copy client/out/libcurl.js and client/out/libcurl.wasm to a directory of your choice. After the script is loaded, call libcurl.load_wasm, specifying the url of the libcurl.wasm file.

<script defer src="./out/libcurl.js" onload="libcurl.load_wasm('/out/libcurl.wasm');"></script>

Alternatively, prebuilt versions can be found on NPM and jsDelivr. You can use the following URLs to load libcurl.js from a third party CDN.

https://cdn.jsdelivr.net/npm/libcurl.js@latest/libcurl.js
https://cdn.jsdelivr.net/npm/libcurl.js@latest/libcurl.wasm

To know when libcurl.js has finished loading, you can use the libcurl_load DOM event.

document.addEventListener("libcurl_load", ()=>{
  libcurl.set_websocket(`wss://${location.hostname}/ws/`);
  console.log("libcurl.js ready!");
});

Once loaded, there will be a window.libcurl object which includes all the API functions.

Making HTTP Requests:

To perform HTTP requests, use libcurl.fetch, which takes the same arguments as the browser's regular fetch function. Like the standard Fetch API, libcurl.fetch will also return a Response object.

let r = await libcurl.fetch("https://ading.dev");
console.log(await r.text());

Most of the standard Fetch API's features are supported, with the exception of:

• CORS enforcement
• FormData or URLSearchParams as the request body
• Sending credentials/cookies automatically
• Caching

Creating WebSocket Connections:

To use WebSockets, create a libcurl.WebSocket object, which works identically to the regular WebSocket object.

let ws = new libcurl.WebSocket("wss://echo.websocket.org");
ws.addEventListener("open", () => {
  console.log("ws connected!");
  ws.send("hello".repeat(128));
});
ws.addEventListener("message", (event) => {
  console.log(event.data);
});

Changing the Websocket URL:

You can change the URL of the websocket proxy by using libcurl.set_websocket.

libcurl.set_websocket("ws://localhost:6001/");

If the websocket proxy URL is not set and one of the other API functions is called, an error will be thrown.

Getting Libcurl's Output:

If you want more information about a connection, you can pass the _libcurl_verbose argument to the libcurl.fetch function.

await libcurl.fetch("https://example.com", {_libcurl_verbose: 1});

By default this will print the output to the browser console, but you can set libcurl.stdout and libcurl.stderr to intercept these messages. This callback will be executed on every line of text that libcurl outputs.

libcurl.stderr = (text) => {document.body.innerHTML += text};

Proxy Server:

The proxy server consists of a standard Wisp server, allowing multiple TCP connections to share the same websocket.

To host the proxy server, run the following commands:

git clone https://github.com/ading2210/libcurl.js --recursive
cd libcurl.js
STATIC=$(pwd)/client server/run.sh

You can use the HOST and PORT environment variables to control the hostname and port that the proxy server listens on.

Copyright:

This project is licensed under the GNU AGPL v3.

Copyright Notice:

ading2210/libcurl.js - A port of libcurl to WASM for the browser.
Copyright (C) 2023 ading2210

This program is free software: you can redistribute it and/or modify
it under the terms of the GNU Affero General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU Affero General Public License for more details.

You should have received a copy of the GNU Affero General Public License
along with this program.  If not, see <https://www.gnu.org/licenses/>.

Changelog

v0.2.0 (2/2/24):

• Add an option to redirect the verbose curl output.
• Use separate callbacks for stdout and stderr.