# box design `box` is a CLI that mints and manages **trust-less, network-isolated VMs with a coding agent installed** (`claude`, `codex`, `grok`, or `blank` for none). It is infrastructure, not a project provisioner. See issue #3 for the full reframe and rationale. This doc captures the durable design decisions. ## Principle: separate the tool from the agent - **The tool** mints isolated boxes with the agent installed but **unauthenticated**. It knows nothing about projects, secrets, recipes, or memory. - **The agent** (Claude Code, Codex, Grok — whichever template, inside the box) reads an optional `.box/` runbook in a cloned repo and acts on it. The recipe's consumer is the reasoning agent, not host machinery. ## Boxes are strictly creds-free `box new --name ` launches a blank box: everything installed, **no** git credentials and **no** agent credentials. The operator authenticates interactively *inside* the box: - **The coding agent** — e.g. `claude` → `/login` (paste-a-code OAuth: copy the URL, open it in your own browser, paste the code back); `codex` and `grok` have their own login step. Works because the box is outbound-only; the tool never handles a token. - **Git** — the operator adds their own PAT / `gh auth login` inside the box. The tool stores and injects **no** credentials, ever. This dissolves the multi-user problem: nothing shared, nothing committed. ## Snapshots are the reuse mechanism Re-authing every fresh box would be toil, so authenticated state is reused via snapshots, not a secrets store: - `box snapshot [label]` — checkpoint after login + clone. - `box new --name --from [/]` — clone an existing box or snapshot (authed state and all). Isolation is preserved: the clone keeps the `box-net` profile + `boxnet` + ACL. - `box restore ` — roll a box back to a checkpoint. Log in once → snapshot → spin up authed boxes from it. ## The box announces itself to the agent cloud-init installs a global agent-context file in every coding-agent box (`~/.claude/CLAUDE.md`, `~/.codex/AGENTS.md`, `~/.grok/AGENTS.md`) telling the agent it is running in a box (trust-less, ephemeral, creds-free) and to treat a repo's `.box/` folder as its bootstrap runbook. No "tell it" step, no host execution. ## `.box/` is optional, agent-facing documentation Not host-executed shell. A repo that wants to be easy to stand up in a sandbox ships a runbook (prose + optional scripts the agent may run). A repo that does not, you set up by hand. The tool enforces no contract; there is no `install`. ## What box owns, and what it doesn't Boxes are ordinary Incus instances, tagged `user.box=1`. That makes every Incus verb a candidate feature request — `rename`, `info`, `file push`, on forever — and wrapping them one at a time grows a worse `incus`. The rule: > **box owns a command when it must enforce an invariant Incus cannot see:** > the `user.box=1` boundary (never touch an instance we didn't mint), the > isolation stack (`box-net` profile + `boxnet` + ACL), or the creds-free > snapshot→clone workflow. Everything else is Incus's job. The rule cuts both ways, and that's the point: - `rename` **is** ours — not because it adds logic to `incus rename`, but because resolving the name *is* the logic: check the tag, apply `--remote`, and notice the box is running (Incus won't rename a running instance) so we can say "stop it first" rather than leak an Incus error. - `incus config set security.nesting=false` is **not** ours. It dismantles the trust boundary; wrapping it would imply we bless it. Two mechanisms keep this honest. **The command table** (`CMDS` in `bin/box`) is the single source of truth for what exists, its synopsis, its help line, its preconditions and what runs. Dispatch and help are both rendered from it, so the help cannot describe a command that doesn't exist — the failure that produced #8. A thin verb is one row; a verb that can't be expressed as a row and enforces no invariant of ours doesn't belong in the tool. **The escape hatch** — `box incus -- ` — resolves and tag-checks the box, then hands the rest to Incus verbatim. It means "no" to a proxy request is not "you can't do that", and it keeps the one rail that matters: you cannot aim it at an instance box didn't mint. If the command can move the box off the isolation stack (profile, network, device, `security.*`), it warns and proceeds — from there the trust boundary is yours to keep. ## Isolation Dedicated NAT bridge `boxnet` + Incus `box-isolate` ACL dropping all RFC1918/CGNAT/link-local egress, plus host-firewall rules blocking instance → host. Entry is `incus exec` over the local socket — no inbound path. The VM is the trust boundary. **A box reaches the public internet and nothing else — including no other box.** That last clause is the one that was assumed and turned out to be false, so it is spelled out here with the mechanism, and `drill/` tests it on every run. - **Box → host, LAN, RFC1918, CGNAT, link-local:** the `box-isolate` ACL. - **Box → box: an nftables *bridge-family* rule** (`host/box-firewall.sh`). It cannot be an ACL rule. Two boxes on one bridge share an L2 segment, so their frames are *switched* between bridge ports and never traverse the netfilter path an L3 ACL lives on — the ACL looked airtight (it drops `10.0.0.0/8`, which contains `boxnet`) while box→box was in fact wide open. A live probe found box A's SYN arriving at box B. The bridge family's forward hook fires exactly on port-to-port frames, which on this bridge means box→box and nothing else: gateway traffic and routed egress are delivered locally, not forwarded. Dropping every forwarded frame therefore isolates the boxes and costs them nothing. - **Box → box by NAME:** `dns.mode=none`. dnsmasq on the gateway held a record for every instance, so a box could enumerate its siblings even where it could not reach them. Blocked connections with open reconnaissance is not isolation. - **IPv6:** off (`ipv6.address=none`), and that is a *contract*, not a default — every rule above is IPv4-only, so IPv6 would be an uncovered path. - **`security.ipv4_filtering`: deliberately NOT used.** It breaks the box's networking (in-box Docker cannot pull or run a container). Tested, vetoed. The rule that keeps this honest: **isolation claims are tested, never reasoned about.** The box→box hole existed because a plausible code reading said it could not. See `drill/RUNS.md`. ## Multi-user hosts: access tiers The daemon socket is binary — `incus-admin` holds everything on the machine — so a shared host needs a second tier, and Incus ships one: **incus-user** confines an `incus`-group member to an auto-created project `user-`, behind a restricted certificate that cannot name any other project. The tier is decided once, from the process's live credentials (`box_tier()`: UID 0 or `incus-admin` → admin; `incus` alone → restricted; neither → none), and every tier-aware verb reads that one function. What incus-user does *not* do is honor box's contract — measured on Debian 13 / Incus 6.0.4 (#74), after the design that assumed it (#72) was vetoed by its own Task-0 rehearsal: - it pins each user's project to a private auto-created bridge (`incusbr-`) — a stock NAT bridge with **none** of the hardening: no ACL, no `dns.mode=none`, no resolver pin, IPv6 on; - it blocks snapshots — box's entire reuse workflow; - the `box-net` profile lives in the default project, invisible to theirs. So the tier is an **admin-run convergence** (`box grant `), not a group membership: put them in `incus`, touch incus-user once as them (the project is created lazily; nothing exists to converge until it does), then rewire the project — network access narrowed to `boxnet` **and only `boxnet`**, snapshots allowed, the shipped profile installed. Narrowing is the load-bearing decision: granting `boxnet,incusbr-` (the obvious fix) would leave an unhardened NAT bridge one `--network` flag away from any box they mint. With the private bridge unreferenced (its `eth0` is removed from their default profile) and outside `restricted.networks.access`, the hardened network is not their default placement — it is the only placement their certificate can express. The grant survives incus-user restarts by that tool's own design (it configures a project only at creation), and a restricted certificate cannot widen its own project — both measured, not read. Cross-USER isolation is the same mechanism as cross-box isolation, on purpose: their instances share `boxnet` with everyone's, and the bridge-family drop + port isolation + `dns.mode=none` already make any two boxes strangers. A restricted user CAN strip `security.port_isolation` from the profile copy in their own project — which is why the nft bridge drop, which they cannot touch, exists as the second, host-owned layer. Defense in depth, both layers measured (`drill/multiuser.sh`, criteria a–l). `box revoke` is two strengths: bare, it removes the group — the socket closes, their boxes keep *running* (revoking a person does not kill their workloads) and `grant` restores everything; `--purge` deletes their world (boxes, images, project, private bridge, trust-store certificate) and asserts the absence afterwards. ## Non-goals - Interactive-first: install and setup prompt by default (`BOX_YES=1` and the CI rehearsal job are the sanctioned unattended paths). - No credential storage or injection by the tool. - No per-user resource quotas on the restricted tier (Incus's `limits.*`/`restricted.*` project keys exist when someone needs them).