yafp - Yet Another Forward Proxy

Summary

YAFP is a forwarding proxy (set it as your proxy in the browser) for various hacking purposes.

Features

• Modern promise-based middleware syntax, designed for ES7 async/await
• Support for streaming, buffered or file based request and response modification
• Only pay for what you use: if you use stream transforms, no buffering will occur
• SSL support via SNI
• Can chain to an upstream proxy (e.g. Burp Proxy)
• Supports ws:// and wss:// WebSockets and will tunnel them through upstream proxies as well
• Transparently handles chunked encoding, can decompress requests and responses with middleware
• Strongly typed, with TypeScript definitions

Requirements

• Requires node.js >= 4.0.0

Install

npm install yafp --save

Example

Simple response modification looks something like this (in typescript):

import * as http from 'http';
import * as yafp from 'yafp';

async function textReplace(buf: Buffer): Promise<Buffer> {
    let s = buf.toString('utf8').replace(/ the /gi, ' UNICORN ');
    return new Buffer(s, 'utf8');
}

function isHtml(resp: http.IncomingMessage): boolean {
    let htmlContentTypes = ['text/html', 'text/xhtml'],
        contentType = (resp.headers['content-type'] || '').toLowerCase().split(';')[0];
    return htmlContentTypes.indexOf(contentType) !== -1
}

async function main() {
    let listenPort = 6666,
        proxy = new yafp.Proxy({port: listenPort});
    proxy.addHandler(yafp.middleware.decompressor);
    proxy.addHandler((ctx: yafp.RequestContext) => {
        ctx.withResponse((resp: http.IncomingMessage) => {
            if(isHtml(resp)) {
                ctx.withResponseBuffer(textReplace);
            }
        });
    });
    proxy.on('error', (e: any) => console.log(e));  // By default, will crash if you don't handle errors
    await proxy.start();
    console.log(`Proxy listening on port: ${listenPort}`);
}

main();

Ideas

• Interesting to test https://badssl.com/ -> would be great to repro this testing in unit tests.

Credits

• Thanks to https://github.com/runk/node-thin for inspiration and original code
• TLS/SNI detection from: https://github.com/dlundquist/sniproxy/

TODO / NOTES / IDEAS

• Certificate sniffing
• Add support for upstream proxy-auth
• NTLM proxy auth?
• Maybe use proxy-agent, node-proxying-agent or http-proxy-agent instead of tunnel-agent?