Simon Willison's Weblog: v8

httpjail

2025-09-19T21:57:29+00:00

Here's a promising new (experimental) project in the sandboxing space from Ammar Bandukwala at Coder. httpjail provides a Rust CLI tool for running an individual process against a custom configured HTTP proxy.

The initial goal is to help run coding agents like Claude Code and Codex CLI with extra rules governing how they interact with outside services. From Ammar's blog post that introduces the new tool, Fine-grained HTTP filtering for Claude Code:

httpjail implements an HTTP(S) interceptor alongside process-level network isolation. Under default configuration, all DNS (udp:53) is permitted and all other non-HTTP(S) traffic is blocked.

httpjail rules are either JavaScript expressions or custom programs. This approach makes them far more flexible than traditional rule-oriented firewalls and avoids the learning curve of a DSL.

Block all HTTP requests other than the LLM API traffic itself:
$ httpjail --js "r.host === 'api.anthropic.com'" -- claude "build something great"

I tried it out using OpenAI's Codex CLI instead and found this recipe worked:

brew upgrade rust
cargo install httpjail # Drops it in `~/.cargo/bin`
httpjail --js "r.host === 'chatgpt.com'" -- codex

Within that Codex instance the model ran fine but any attempts to access other URLs (e.g. telling it "Use curl to fetch simonwillison.net)" failed at the proxy layer.

This is still at a really early stage but there's a lot I like about this project. Being able to use JavaScript to filter requests via the --js option is neat (it's using V8 under the hood), and there's also a --sh shellscript option which instead runs a shell program passing environment variables that can be used to determine if the request should be allowed.

At a basic level it works by running a proxy server and setting HTTP_PROXY and HTTPS_PROXY environment variables so well-behaving software knows how to route requests.

It can also add a bunch of other layers. On Linux it sets up nftables rules to explicitly deny additional network access. There's also a --docker-run option which can launch a Docker container with the specified image but first locks that container down to only have network access to the httpjail proxy server.

It can intercept, filter and log HTTPS requests too by generating its own certificate and making that available to the underlying process.

I'm always interested in new approaches to sandboxing, and fine-grained network access is a particularly tricky problem to solve. This looks like a very promising step in that direction - I'm looking forward to seeing how this project continues to evolve.

Via Fine-grained HTTP filtering for Claude Code

Tags: http, javascript, proxies, sandboxing, security, v8, rust, claude-code, codex-cli

Directive prologues and JavaScript dark matter

2025-06-02T18:30:31+00:00

Directive prologues and JavaScript dark matter

Tom MacWright does some archaeology and describes the three different magic comment formats that can affect how JavaScript/TypeScript files are processed:

"a directive"; is a directive prologue, most commonly seen with "use strict";.

/** @aPragma */ is a pragma for a transpiler, often used for /** @jsx h */.

//# aMagicComment is usually used for source maps - //# sourceMappingURL=<url> - but also just got used by v8 for their new explicit compile hints feature.

Via Jim Nielsen

Tags: javascript, programming-languages, v8, typescript, tom-macwright

Reviving PyMiniRacer

2024-03-24T17:00:55+00:00

Reviving PyMiniRacer

PyMiniRacer is “a V8 bridge in Python”—it’s a library that lets Python code execute JavaScript code in a V8 isolate and pass values back and forth (provided they serialize to JSON) between the two environments.

It was originally released in 2016 by Sqreen, a web app security startup startup. They were acquired by Datadog in 2021 and the project lost its corporate sponsor, but in this post Ben Creech announces that he is revitalizing the project, with the approval of the original maintainers.

I’m always interested in new options for running untrusted code in a safe sandbox. PyMiniRacer has the three features I care most about: code can’t access the filesystem or network by default, you can limit the RAM available to it and you can have it raise an error if code execution exceeds a time limit.

The documentation includes a newly written architecture overview which is well worth a read. Rather than embed V8 directly in Python the authors chose to use ctypes—they build their own V8 with a thin additional C++ layer to expose a ctypes-friendly API, then the Python library code uses ctypes to call that.

I really like this. V8 is a notoriously fast moving and complex dependency, so reducing the interface to just a thin C++ wrapper via ctypes feels very sensible to me.

This blog post is fun too: it’s a good, detailed description of the process to update something like this to use modern Python and modern CI practices. The steps taken to build V8 (6.6 GB of miscellaneous source and assets!) across multiple architectures in order to create binary wheels are particularly impressive—the Linux aarch64 build takes several days to run on GitHub Actions runners (via emulation), so they use Mozilla’s Sccache to cache compilation steps so they can retry until it finally finishes.

On macOS (Apple Silicon) installing the package with “pip install mini-racer” got me a 37MB dylib and a 17KB ctypes wrapper module.

Via Hacker News

Tags: ctypes, javascript, open-source, python, v8

Deno Deploy Beta 3

2021-11-07T01:51:30+00:00

Deno Deploy Beta 3

I missed Deno Deploy when it first came out back in June: it’s a really interesting new hosting environment for scripts written in Deno, Node.js creator Ryan Dahl’s re-imagining of Node.js. Deno Deploy runs your code using v8 isolates running in 28 regions worldwide, with a clever BroadcastChannel mechanism (inspired by the browser API of the same name) that allows instances of the server-side code running in different regions to send each other messages. See the “via” link for my annotated version of a demo by Ondřej Žára that got me excited about what it can do.

Via My TILs

Tags: javascript, ryan-dahl, v8, deno

Subsume JSON a.k.a. JSON ⊂ ECMAScript

2019-08-15T10:30:16+00:00

Subsume JSON a.k.a. JSON ⊂ ECMAScript

TIL that JSON isn’t a subset of ECMAScript after all! “In ES2018, ECMAScript string literals couldn’t contain unescaped U+2028 LINE SEPARATOR and U+2029 PARAGRAPH SEPARATOR characters, because they are considered to be line terminators even in that context.”

Via @mathias

Tags: json, v8

Node.js is genuinely exciting

2009-11-23T12:50:22+00:00

I gave a talk on Friday at Full Frontal, a new one day JavaScript conference in my home town of Brighton. I ended up throwing away my intended topic (JSONP, APIs and cross-domain security) three days before the event in favour of a technology which first crossed my radar less than two weeks ago.

That technology is Ryan Dahl's Node. It's the most exciting new project I've come across in quite a while.

At first glance, Node looks like yet another take on the idea of server-side JavaScript, but it's a lot more interesting than that. It builds on JavaScript's excellent support for event-based programming and uses it to create something that truly plays to the strengths of the language.

Node describes itself as "evented I/O for V8 javascript". It's a toolkit for writing extremely high performance non-blocking event driven network servers in JavaScript. Think similar to Twisted or EventMachine but for JavaScript instead of Python or Ruby.

Evented I/O?

As I discussed in my talk, event driven servers are a powerful alternative to the threading / blocking mechanism used by most popular server-side programming frameworks. Typical frameworks can only handle a small number of requests simultaneously, dictated by the number of server threads or processes available. Long-running operations can tie up one of those threads - enough long running operations at once and the server runs out of available threads and becomes unresponsive. For large amounts of traffic, each request must be handled as quickly as possible to free the thread up to deal with the next in line.

This makes certain functionality extremely difficult to support. Examples include handling large file uploads, combining resources from multiple backend web APIs (which themselves can take an unpredictable amount of time to respond) or providing comet functionality by holding open the connection until a new event becomes available.

Event driven programming takes advantage of the fact that network servers spend most of their time waiting for I/O operations to complete. Operations against in-memory data are incredibly fast, but anything that involves talking to the filesystem or over a network inevitably involves waiting around for a response.

With Twisted, EventMachine and Node, the solution lies in specifying I/O operations in conjunction with callbacks. A single event loop rapidly switches between a list of tasks, firing off I/O operations and then moving on to service the next request. When the I/O returns, execution of that particular request is picked up again.

(In the talk, I attempted to illustrate this with a questionable metaphor involving hamsters, bunnies and a hyperactive squid).

What makes Node exciting?

If systems like this already exist, what's so exciting about Node? Quite a few things:

JavaScript is extremely well suited to programming with callbacks. Its anonymous function syntax and closure support is perfect for defining inline callbacks, and client-side development in general uses event-based programming as a matter of course: run this function when the user clicks here / when the Ajax response returns / when the page loads. JavaScript programmers already understand how to build software in this way.
Node represents a clean slate. Twisted and EventMachine are hampered by the existence of a large number of blocking libraries for their respective languages. Part of the difficulty in learning those technologies is understanding which Python or Ruby libraries you can use and which ones you have to avoid. Node creator Ryan Dahl has a stated aim for Node to never provide a blocking API - even filesystem access and DNS lookups are catered for with non-blocking callback based APIs. This makes it much, much harder to screw things up.
Node is small. I read through the API documentation in around half an hour and felt like I had a pretty comprehensive idea of what Node does and how I would achieve things with it.
Node is fast. V8 is the fast and keeps getting faster. Node's event loop uses Marc Lehmann's highly regarded libev and libeio libraries. Ryan Dahl is himself something of a speed demon - he just replaced Node's HTTP parser implementation (already pretty speedy due to it's Ragel / Mongrel heritage) with a hand-tuned C implementation with some impressive characteristics.
Easy to get started. Node ships with all of its dependencies, and compiles cleanly on Snow Leopard out of the box.

With both my JavaScript and server-side hats on, Node just feels right. The APIs make sense, it fits a clear niche and despite its youth (the project started in February) everything feels solid and well constructed. The rapidly growing community is further indication that Ryan is on to something great here.

What does Node look like?

Here's how to get Hello World running in Node in 7 easy steps:

git clone git://github.com/ry/node.git (or download and extract a tarball)
./configure
make (takes a while, it needs to compile V8 as well)
sudo make install
Save the below code as helloworld.js
node helloworld.js
Visit http://localhost:8080/ in your browser

Here's helloworld.js:

var sys = require('sys'), 
  http = require('http');

http.createServer(function(req, res) {
  res.sendHeader(200, {'Content-Type': 'text/html'});
  res.sendBody('<h1>Hello World</h1>');
  res.finish();
}).listen(8080);

sys.puts('Server running at http://127.0.0.1:8080/');

If you have Apache Bench installed, try running ab -n 1000 -c 100 'http://127.0.0.1:8080/' to test it with 1000 requests using 100 concurrent connections. On my MacBook Pro I get 3374 requests a second.

So Node is fast - but where it really shines is concurrency with long running requests. Alter the helloworld.js server definition to look like this:

http.createServer(function(req, res) {
  setTimeout(function() {
    res.sendHeader(200, {'Content-Type': 'text/html'});
    res.sendBody('<h1>Hello World</h1>');
    res.finish();
  }, 2000);
}).listen(8080);

We're using setTimeout to introduce an artificial two second delay to each request. Run the benchmark again - I get 49.68 requests a second, with every single request taking between 2012 and 2022 ms. With a two second delay, the best possible performance for 1000 requests 100 at a time is 1000 requests / (1000 / 100) * 2 seconds = 50 requests a second. Node hits it pretty much bang on the nose.

The most important line in the above examples is res.finish(). This is the mechanism Node provides for explicitly signalling that a request has been fully processed and should be returned to the browser. By making it explicit, Node makes it easy to implement comet patterns like long polling and streaming responses - stuff that is decidedly non trivial in most server-side frameworks.

djangode

Node's core APIs are pretty low level - it has HTTP client and server libraries, DNS handling, asynchronous file I/O etc, but it doesn't give you much in the way of high level web framework APIs. Unsurprisingly, this has lead to a cambrian explosion of lightweight web frameworks based on top of Node - the projects using node page lists a bunch of them. Rolling a framework is a great way of learning a low-level API, so I've thrown together my own - djangode - which brings Django's regex-based URL handling to Node along with a few handy utility functions. Here's a simple djangode application:

var dj = require('./djangode');

var app = dj.makeApp([
  ['^/$', function(req, res) {
    dj.respond(res, 'Homepage');
  }],
  ['^/other$', function(req, res) {
    dj.respond(res, 'Other page');
  }],
  ['^/page/(\\d+)$', function(req, res, page) {
    dj.respond(res, 'Page ' + page);
  }]
]);
dj.serve(app, 8008);

djangode is currently a throwaway prototype, but I'll probably be extending it with extra functionality as I explore more Node related ideas.

nodecast

My main demo in the Full Frontal talk was nodecast, an extremely simple broadcast-oriented comet application. Broadcast is my favourite "hello world" example for comet because it's both simpler than chat and more realistic - I've been involved in plenty of projects that could benefit from being able to broadcast events to their audience, but few that needed an interactive chat room.

The source code for the version I demoed can be found on GitHub in the no-redis branch. It's a very simple application - the client-side JavaScript simply uses jQuery's getJSON method to perform long-polling against a simple URL endpoint:

function fetchLatest() {
  $.getJSON('/wait?id=' + last_seen, function(d) {
    $.each(d, function() {
      last_seen = parseInt(this.id, 10) + 1;
      ul.prepend($('<li></li>').text(this.text));
    });
    fetchLatest();
  });
}

Doing this recursively is probably a bad idea since it will eventually blow the browser's JavaScript stack, but it works OK for the demo.

The more interesting part is the server-side /wait URL which is being polled. Here's the relevant Node/djangode code:

var message_queue = new process.EventEmitter();

var app = dj.makeApp([
  // ...
  ['^/wait$', function(req, res) {
    var id = req.uri.params.id || 0;
    var messages = getMessagesSince(id);
    if (messages.length) {
      dj.respond(res, JSON.stringify(messages), 'text/plain');
    } else {
      // Wait for the next message
      var listener = message_queue.addListener('message', function() {
        dj.respond(res, 
          JSON.stringify(getMessagesSince(id)), 'text/plain'
        );
        message_queue.removeListener('message', listener);
        clearTimeout(timeout);
      });
      var timeout = setTimeout(function() {
        message_queue.removeListener('message', listener);
        dj.respond(res, JSON.stringify([]), 'text/plain');
      }, 10000);
    }
  }]
  // ...
]);

The wait endpoint checks for new messages and, if any exist, returns immediately. If there are no new messages it does two things: it hooks up a listener on the message_queue EventEmitter (Node's equivalent of jQuery/YUI/Prototype's custom events) which will respond and end the request when a new message becomes available, and also sets a timeout that will cancel the listener and end the request after 10 seconds. This ensures that long polls don't go on too long and potentially cause problems - as far as the browser is concerned it's just talking to a JSON resource which takes up to ten seconds to load.

When a message does become available, calling message_queue.emit('message') will cause all waiting requests to respond with the latest set of messages.

Talking to databases

nodecast keeps track of messages using an in-memory JavaScript array, which works fine until you restart the server and lose everything. How do you implement persistent storage?

For the moment, the easiest answer lies with the NoSQL ecosystem. Node's focus on non-blocking I/O makes it hard (but not impossible) to hook it up to regular database client libraries. Instead, it strongly favours databases that speak simple protocols over a TCP/IP socket - or even better, databases that communicate over HTTP. So far I've tried using CouchDB (with node-couch) and redis (with redis-node-client), and both worked extremely well. nodecast trunk now uses redis to store the message queue, and provides a nice example of working with a callback-based non-blocking database interface:

var db = redis.create_client();
var REDIS_KEY = 'nodecast-queue';

function addMessage(msg, callback) {
  db.llen(REDIS_KEY, function(i) {
    msg.id = i; // ID is set to the queue length
    db.rpush(REDIS_KEY, JSON.stringify(msg), function() {
      message_queue.emit('message', msg);
      callback(msg);
    });
  });
}

Relational databases are coming to Node. Ryan has a PostgreSQL adapter in the works, thanks to that database already featuring a mature non-blocking client library. MySQL will be a bit tougher - Node will need to grow a separate thread pool to integrate with the official client libs - but you can talk to MySQL right now by dropping in DBSlayer from the NY Times which provides an HTTP interface to a pool of MySQL servers.

Mixed environments

I don't see myself switching all of my server-side development over to JavaScript, but Node has definitely earned a place in my toolbox. It shouldn't be at all hard to mix Node in to an existing server-side environment - either by running both behind a single HTTP proxy (being event-based itself, nginx would be an obvious fit) or by putting Node applications on a separate subdomain. Node is a tempting option for anything involving comet, file uploads or even just mashing together potentially slow loading web APIs. Expect to hear a lot more about it in the future.

Going evented with Node.js

2009-11-17T13:09:38+00:00

Going evented with Node.js

Comprehensive Node.js tutorial—from basic principles to installation and writing a simple Twitter search command-line client application.

Tags: javascript, nodejs, twitter, v8

node.js

2009-11-09T23:25:47+00:00

node.js

“Evented I/O for V8 JavaScript”—a JavaScript environment built on top of the super-fast V8 engine which provides event-based IO functionality for building highly concurrent TCP and HTTP servers. The API design is superb—everything is achieved using JavaScript events and callbacks (even regular file IO) and the small standard library ships with comprehensive support for HTTP and DNS. Overall it’s very similar to Twisted and friends, but JavaScript’s anonymous function syntax feels more natural than the Python equivalent. It compiles cleanly on Snow Leopard. Definitely a project to watch.

Tags: dns, eventbasedio, http, io, javascript, nodejs, twisted, v8

And Now For Something Entire... Oooh! Shiny!

2009-04-22T12:19:23+00:00

And Now For Something Entire... Oooh! Shiny!

Alex Russell on O3D, the new 3D browser plugin from Google that makes OpenGL accessible to JavaScript (and embeds V8 so performance won’t suck even on slower browsers).

Tags: 3d, alex-russell, google, javascript, o3d, opengl, v8

V8 Design Elements

2008-09-02T23:58:46+00:00

V8 Design Elements

High level design details of Google’s V8 JavaScript engine, including how it uses “hidden classes” to optimise object property lookups and a bit of information on the machine code generation and garbage collection.

Tags: chrome, google, javascript, v8

Chromium

2008-09-02T21:06:19+00:00

Chromium

Google Chrome is out! Here’s the open source project, including the code for the new V8 JavaScript virtual machine.

Tags: browsers, chrome, chromium, google, javascript, open-source, v8