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Mathieu Virbel da3a2ac3a4 rename Fence to Greywall as GreyHaven sandboxing component

Rebrand the project from Fence to Greywall, the sandboxing layer of the
GreyHaven platform. This updates:

- Go module path to gitea.app.monadical.io/monadical/greywall
- Binary name, CLI help text, and all usage examples
- Config paths (~/.config/greywall/greywall.json), env vars (GREYWALL_*)
- Log prefixes ([greywall:*]), temp file prefixes (greywall-*)
- All documentation, scripts, CI workflows, and example files
- README rewritten with GreyHaven branding and Fence attribution

Directory/file renames: cmd/fence → cmd/greywall, pkg/fence → pkg/greywall,
docs/why-fence.md → docs/why-greywall.md, example JSON files, and banner.

2026-02-10 16:00:24 -06:00

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Concepts

Greywall combines two ideas:

An OS sandbox to enforce "no direct network" and restrict filesystem operations.
Local filtering proxies (HTTP + SOCKS5) to selectively allow outbound traffic by domain.

Network model

By default, greywall blocks all outbound network access.

When you allow domains, greywall:

Starts local HTTP and SOCKS5 proxies
Sets proxy environment variables (HTTP_PROXY, HTTPS_PROXY, ALL_PROXY)
Allows the sandboxed process to connect only to the local proxies
Filters outbound connections by destination domain

Localhost controls

allowLocalBinding: lets a sandboxed process listen on local ports (e.g. dev servers).
allowLocalOutbound: lets a sandboxed process connect to localhost services (e.g. Redis/Postgres on your machine).
-p/--port: exposes inbound ports so things outside the sandbox can reach your server.

These are separate on purpose. A typical safe default for dev servers is:

allow binding + expose just the needed port(s)
disallow localhost outbound unless you explicitly need it

Filesystem model

Greywall is designed around "read mostly, write narrowly":

Reads: allowed by default (you can block specific paths via denyRead).
Writes: denied by default (you must opt-in with allowWrite).
denyWrite: overrides allowWrite (useful for protecting secrets and dangerous files).

Greywall also protects some dangerous targets regardless of config (e.g. shell startup files and git hooks). See ARCHITECTURE.md for the full list.

Debug vs Monitor mode

-d/--debug: verbose output (proxy activity, filter decisions, sandbox command details).
-m/--monitor: show blocked requests/violations only (great for auditing and policy tuning).

Workflow tip:

Start restrictive.
Run with -m to see what gets blocked.
Add the minimum domains/paths required.

Platform notes

macOS: uses sandbox-exec with generated Seatbelt profiles.
Linux: uses bubblewrap for namespaces + socat bridges to connect the isolated network namespace to host-side proxies.

If you want the under-the-hood view, see Architecture.

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