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Comment by mdavid626 | Hacker News Reader

I trust sandbox-exec more, or Docker on Linux. Those come from the OS, well tested and known.

MITM proxy is nice idea to avoid leaking secrets. Isn’t it very brittle though? Anthropic changes some URL-s and it’ll break.

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Thanks for sharing that. Zerobox _does_ use the native OS sandboxing mechanisms (e.g. seatbelt) under the hood. I'm not trying to reinvent the wheel when it comes to sandboxing.

Re the URLs, I agree, that's why I added wildcard support, e.g. `*.openai.com` for secret injection as well as network call filtering.

How do you intercept network traffic on mac os? How do you fake certificates?

Zerobox creates a cert in `~/.zerobox/cert` on the first proxy run and reuses that. The MTIM process uses that cert to make the calls, inject certs, etc. This is actually done by the underlying Codex crate.

Yeah, but how does the sandboxed process “know” that it has to go through the proxy? How does it trust your certificate? Is the proxy fully transparent?

Oh I see. It inject HTTP_PROXY/HTTPS_PROXY/etc. env vars into the process so that all sandboxed subprocesses go through the proxy.

What if the program doesn’t respect those env vars? Can Zerobox still block network calls in that case?

Great question! On Linux, yes, network namespaces enforce that and all net traffic goes through the proxy. Direct connections are blocked at the kernel level even if the program ignores proxy env vars, but I will test this case a bit more (unsure how to though, most network calls would respect HTTPS_PROXY and other similar env vars).

That being said, the default behaviour is no network, so nothing will be routed if it's not allowed regardless of whether the sandboxed process respects env vars or not.

How about on macOS?

On macOS, the proxy is best effort. Programs that ignore HTTPS_PROXY/HTTP_PROXY can connect directly. This is a platform limitation (macOS Seatbelt doesn't support forced proxy routing).

BUT, the default behaviour (no net) is fully enforced at the kernel level. Domain filtering relies on the program respecting proxy env vars.

I thought seatbelt-exec had mechanisms for that?

  (allow network-outbound
    (remote tcp "127.0.0.1:8080"))

It does but because I'm inheriting the seatbelt settings from Codex, I'm not resetting it in Zerobox (I thought it's a safer option). Let me look into this, there should be a way to take Codex' profile and safely combine/modify it.

Does this work inside of Podman containers?

You know, the thing is, that it is super easy to create such tools with AI nowadays. …and if you create your own, you can avoid these unnecessary abstractions. You get exactly what you want.

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