Headmesh Agent Sandbox

Headmesh agent sandbox

A persistent Docker container on the ASUS that joins Aaryn's Headscale tailnet as its own tagged node, so cloud agents can work with real files, real dependencies and private services. Agents reach it through several front doors:

• an MCP server, headmesh-sandboxd;
• the Claude self-hosted runner and Claude Remote Control;
• Codex over Tailscale SSH;
• the OpenAI Secure MCP Tunnel;
• codex exec-server, driven from the host through the Agents API.

It implements plan §6 and §7 (docs/plan.md).

This is its own Go module, github.com/headmesh-dev/headmesh/sandbox; the root go.work includes it. Run go test -race ./... here.

What's verified (2026-10-27)

All of this was run locally, without tailnet enrollment:

• go test -race ./... passes. That includes integration tests on real tmux and real git, and the harness against an in-process sandboxd. golangci-lint run with the repo config reports 0 issues.
• docker buildx bake -f sandbox/image/docker-bake.hcl sandbox-local builds headmesh-sandbox:dev (2.39 GB). hadolint is clean, and container-structure-test passes 43/43.
• headmesh-sandbox up starts the container through Compose with the §6.3 hardening. It reports healthy with tailscaled logged out, and the unconfigured front doors idle and say why. status and doctor work.
• Against the running container, tools/list (31 tools), exec, session_* and fs_* pass over the unix socket, and over HTTP with a bearer token. Both protocol versions work: 2026-07-28 stateless and the 2025-11-25 handshake (tests/e2e).
• Acceptance, route mcp and route ssh (run inside the container, phase 1, then phase 2 from a new process):
  • Checks 1–3 pass: uid 1000, Ubuntu 26.04, the file hash matches the host's copy, and the dependency runs.
  • Check 5's mechanics pass: the session is still running after the reconnect, and the marker exists.
  • Checks 4 and 5 still fail because the node isn't enrolled yet.
• acceptance recreate passes 5/5: home and ~/.bash_history persist, the boot identity changes, and old sessions report lost (container restarted at …). The tailnet identity part was skipped (not enrolled).
• YOLO:
  • It is refused without a fresh backup record.
  • With one, it starts with the per-subtree read-only /host. Writes fail even as root, propagation is private, /etc/shadow reads as 0 bytes, ~/.ssh is masked, and the host tailscaled state and Docker data-root are left out.
  • It halts itself at the TTL, with an audit entry, and the next up is Regular.

Spike outcomes are in docs/adr/spike-t0{1,2,3,7,8}-*.md and docs/adr/spike-t16-*.md. Other implementation decisions are in docs/adr/0001-sandbox-implementation-decisions.md.

Security findings to act on

1. Install the egress firewall before enrolling (host/firewall, step 1 below). A Docker bridge container can otherwise reach whatever the host can: its LAN, its own services and, on a host that is itself a tailnet node, that tailnet. Host-specific findings are in the internal ops notes.
2. YOLO shows the host read-only. That includes data directories and certificates, wherever file permissions allow. Agents run as uid 1000; root-only files stay unreadable to them, but tailscaled and the runner run as root in the container. Add anything sensitive to [mounts] extra_denylist: denylisted subtrees are left out of /host.
3. Inside the sandbox, UV_PROJECT_ENVIRONMENT=.venv-sandbox, so uv never rebuilds a project's host .venv against container paths. Make sure each project's .gitignore covers .venv-sandbox (or .venv*).

Quick start (host)

cd /mnt/storage/projects/headmesh/sandboxgo build -o ~/.local/bin/headmesh-sandbox ./cmd/headmesh-sandboxdocker buildx bake -f image/docker-bake.hcl sandbox-local     # headmesh-sandbox:devheadmesh-sandbox init                                          # /mnt/storage/headmesh/sandbox$EDITOR /mnt/storage/headmesh/sandbox/config/sandbox.toml      # image = "headmesh-sandbox:dev" until GHCR has a releaseheadmesh-sandbox scan-secrets                                  # review, then --applyheadmesh-sandbox token create claude-code-mac > ~/.config/headmesh/sandbox-tokenheadmesh-sandbox render --diffheadmesh-sandbox upheadmesh-sandbox statusheadmesh-sandbox doctor

Other commands: down [--yolo|--all], recreate, logs [service|audit] [-f], shell [--root], attach (watch agents in tmux), backup, token list|revoke, acceptance run|verify|report|recreate, and agent run --provider openai.

Secrets live in /mnt/storage/headmesh/sandbox/secrets/<name>, mode 0600; an empty file means that front door is off. They are never mounted. The CLI passes them to docker compose through the environment, and Compose copies each one into the container as root:root 0400 (spike T-1).

Steps that need Aaryn

Each step needs sudo, a production change or an account. None of them has been done.

1. Host (sudo)

• Install the egress firewall and the home backup units, then run a first backup. The exact commands are in host/README.md. Check the result with headmesh-sandbox doctor; it should show egress firewall ✓ and home backup ✓.
• Install restic (plan §5.3). Sleep targets are already masked (doctor checks this).

2. Headscale policy (production ACL change; maintenance window)

This builds on plan §5.2 and §6.6. Add the tags and grants to api/infra/headscale/policy.hujson in the Xcelsior repo. Run headscale policy check first; the policy tests must pass.

"tagOwners": { "tag:sandbox": ["group:admin"], "tag:sandbox-yolo": ["group:admin"] },"grants": [  // Regular: only the private services you list (keep sandbox.toml [mode.regular] reach in sync).  { "src": ["tag:sandbox"], "dst": ["tower-server"], "ip": ["tcp:11434"] },  { "src": ["tag:sandbox-yolo"], "dst": ["*"], "ip": ["*"] },],"ssh": [  { "action": "accept", "src": ["xcelsior@"], "dst": ["tag:sandbox", "tag:sandbox-yolo"], "users": ["agent"] },],

xcelsior@ must mean Aaryn's own devices only. That's true once the provider hosts are tagged (§5.2).

3. Enroll the node

# On prod-hetzner: a single-use pre-auth key that carries the tag (Headscale 0.29.6).sudo headscale preauthkeys create --user <owner> --tags tag:sandbox --expiration 1h# On the ASUS: store it and recreate.install -m 600 /dev/stdin /mnt/storage/headmesh/sandbox/secrets/ts_authkey   # paste, then Ctrl-Dheadmesh-sandbox recreateheadmesh-sandbox status        # tailnet Running, node asus-sandbox, dns-check ok

• The key alone sets the tags. --advertise-tags is never passed, because Headscale rejects it with a tagged key (spike T-8). tailscale-up checks the enrolled tags against TS_TAGS instead.
• tailscale up runs with the key only while no state exists, so later restarts never re-authenticate. Tagged nodes don't expire. Clear the key file after enrollment.
• YOLO is a separate node (asus-sandbox-yolo, tag:sandbox-yolo) with its own state directory. Enroll it the same way, with a tag:sandbox-yolo key, during the first up --yolo.

4. Front doors

MCP over the tailnet (Claude Code, any MCP client):

headmesh-sandbox token create claude-code-mac          # prints hms_… onceclaude mcp add --transport http sandbox http://asus-sandbox.mesh.xcelsior.ca:8780/mcp \  --header "Authorization: Bearer hms_…"

Tokens are scoped (sandbox:read|exec|write|admin) and can be revoked: headmesh-sandbox token revoke claude-code-mac takes effect within seconds. OAuth (RFC 9728 metadata is already served at /.well-known/oauth-protected-resource/mcp) arrives with the Headmesh authorization server in M3.

Claude self-hosted runner (plan §7.1):

1. Turn on "Allow self-hosted environments" in claude.ai admin settings.
2. Put the environment secret in secrets/claude_env_secret, and fill secrets/github_token with a fine-grained PAT.
3. Set [entrypoints.claude_runner] enabled = true, then run headmesh-sandbox up.

The runner runs as root. Every session goes through claude-session, which drops to agent with setpriv, so sessions can't read the secret (spike T-1). The checkout hook links repos mapped in [repos] to the shared working copy, on main. One live check is still pending: what the runner puts in the sessions' environment.

Claude Remote Control (plan §7.2):

1. Set [entrypoints.claude_rc] enabled = true, then run headmesh-sandbox up.
2. Run headmesh-sandbox shell, then claude auth login (a full claude.ai login), trust /home/agent, and run claude remote-control once to answer "Enable Remote Control? y".
3. Run headmesh-sandbox recreate.

Never set DISABLE_TELEMETRY, DO_NOT_TRACK, CLAUDE_CODE_DISABLE_NONESSENTIAL_TRAFFIC or DISABLE_GROWTHBOOK: Remote Control refuses to start with any of them (spike T-2). Whether it restarts on its own without a TTY still needs checking after this login.

Codex over Tailscale SSH (plan §7.3):

• This needs the ssh rule from step 2.
• In Codex desktop, add the remote host [email protected].
• Smoke test: ssh [email protected] codex --version.
• Tailscale SSH works inside the container (spike T-3). Codex desktop itself is untested.

OpenAI Secure MCP Tunnel (plan §7.4):

1. In the Platform, create a Tunnel and a runtime key with Read+Use only. Put the key in secrets/openai_tunnel_key and the id in [entrypoints.openai_tunnel] tunnel_id. Set enabled = true, then run up.
2. Create a token for the tunnel route: headmesh-sandbox token create chatgpt-tunnel --route tunnel (exec ≤ 110 s, responses ≤ 8 MiB).
3. In ChatGPT: Plugins → Add custom MCP server → Connection: Tunnel, with bearer auth using that token.

Codex exec-server via the Agents API (plan §7.6):

1. Store the application key outside the sandbox. The CLI runs as you, so use secrets/openai_app_key (0600; never mounted, and denylisted from YOLO's /host). /etc/headmesh/openai-app.key is also accepted if you can read it.

2. Put the environment key (Agents tab, every other permission None) in secrets/openai_env_key.

3. Run:

   bash

   headmesh-sandbox agent run --provider openai --repo xcelsior \  --prompt-file sandbox/tests/acceptance/prompt.md

The executor runs as agent in the repo with --linux-sandbox-pid-namespace inherit and full access inside the container. Codex's bubblewrap sandbox can't run under the container's seccomp and AppArmor profiles, so the container is the boundary (spike T-16). The environment key reaches the executor through stdin, and the transcript goes to logs/agent-runs/<session>.jsonl.

5. Acceptance (plan §6.15)

headmesh-sandbox acceptance run --route mcp \  --url http://asus-sandbox.mesh.xcelsior.ca:8780/mcp --token-file ~/.config/headmesh/sandbox-tokenssh [email protected] headmesh-sandbox acceptance run --route ssh --phase 1ssh [email protected] headmesh-sandbox acceptance run --route ssh --phase 2# runner, exec-server, tunnel: the agent follows tests/acceptance/prompt.mdheadmesh-sandbox acceptance recreateheadmesh-sandbox acceptance verifyheadmesh-sandbox acceptance report --out ../docs/acceptance/$(date +%F).md

Set the targets in sandbox.toml [acceptance]:

• allowed_target must be granted to tag:sandbox.
• denied_target must be blocked. Run that probe only after the firewall is installed.

Check 4 accepts only a tailnet address for the MagicDNS name; a public DNS record for the same name doesn't count.

6. YOLO

1. Make a dedicated deploy key, add it to GitHub as "headmesh-sandbox-yolo (asus)", and pin known_hosts:

   bash

   ssh-keygen -t ed25519 -f /mnt/storage/headmesh/sandbox/secrets/github_ssh_key \  -C "headmesh-sandbox-yolo (asus)"

   It is passed only to the YOLO container.

2. Run headmesh-sandbox backup, then headmesh-sandbox up --yolo --ttl 4h.

The CLI refuses without a backup younger than backup_max_age and asks you to confirm you're watching.

7. Publish the image

Push a sandbox-v0.1.0 tag. .github/workflows/sandbox-image.yml builds it, runs hadolint, container-structure-test and trivy, pushes to ghcr.io/headmesh-dev/sandbox, and signs it with cosign. Then set image = "ghcr.io/headmesh-dev/sandbox:0.1.0@sha256:…".

Troubleshooting

• Run headmesh-sandbox doctor first; every failure comes with a fix.
• headmesh-sandbox logs sandboxd|tailscaled|tailscale-up|claude-runner|claude-rc|openai-tunnel|audit.
• To watch agents live: headmesh-sandbox attach.
• "IP works, names fail": dns-check explains the fix (pitfall 3).