box/docs/plans/2026-07-18-restricted-tier.md

Restricted incus tier — design and measured results (#74)

Status: implemented and rehearsed. 42/42 rehearsal criteria green on the design host (Debian 13 trixie, Incus 6.0.4, nested KVM), in container mode and VM mode — and green in CI on ubuntu-latest / incus 6.0.0 (whose one version-drift false FAIL is MU-4 in drill/RUNS.md). This doc records the design, what was measured, and why each decision fell where it did. It supersedes the vetoed #72 design (docs/plans/2026-07-17-multiuser-hosts.md on feat/restricted-tier-wip).

What #74 asked

A restricted (incus-group) user can box new/list/shell/snapshot/rm their own boxes, on a network carrying box's full isolation contract, seeing no one else's; the convergence is a documented, idempotent path; the rehearsal passes criteria (a)–(f); the admin tier is unchanged.

The three facts that shaped the design

Task-0 (#72) measured one: incus-user confines users to user-<uid> projects (sound), but pins them to a private auto-created bridge incusbr-<uid> and restricted.networks.access: incusbr-<uid> — they cannot even see boxnet.

This round measured two more:

1. The private bridge is worse than unhardened. incusbr-<uid> is a fully functional NAT bridge — ipv4.nat=true, ipv6.nat=true — with no ACL, no dns.mode=none, no resolver pin, no port isolation, and IPv6 egress box's contract explicitly forbids. Any instance placed on it holds a door to the host's LAN.
2. incus-user projects block snapshots (Project "user-<uid>" doesn't allow for snapshot creation) — box's entire reuse workflow.

And two open questions from #74, answered from incus-user's own source (cmd/incus-user/server.go, stable-6.0) and then confirmed live:

• Daemon-level project template? None exists — the project config is hardcoded in serverSetupUser(). A per-user admin hook is the only path.
• Does widening survive a re-sync? Yes. Setup runs only when the project does not exist (and early-outs when the user's certificate is already trusted); incus-user never rewrites an existing project. Confirmed live: systemctl restart incus-user.socket leaves the convergence intact.

The decision: option 1, tightened

#74 offered (1) converge users onto the shared boxnet or (2) harden each private bridge. Option 2 multiplies every mechanism per user (ACL, resolver pin, dnsmasq, nft rules, firewall coexistence) and turns the shipped static profile into N generated ones. Option 1 keeps one hardened network and one shipped profile — and the existing box↔box mechanisms already make cross-user isolation free: the nft bridge-family drop and dns.mode=none are host/network-owned, so they bind every instance on boxnet no matter whose project it lives in.

One tightening beyond the issue's sketch: the issue proposed restricted.networks.access boxnet,incusbr-<uid> ("must list both" — true as long as the default profile still references the private bridge). Listing both leaves fact 1's unhardened bridge one --network flag away, forever. Instead, box grant:

• removes eth0 from the project's default profile (nothing references the private bridge anymore, so the narrowing validates), and
• sets restricted.networks.access boxnet — only.

After which the hardened network is not the user's default placement but the only placement their certificate can express. Measured: incus launch --network incusbr-<uid> as the user → Network not found; the user cannot widen their own project (Error: Certificate is restricted); they cannot touch boxnet's config or the ACL (no permission for project "default").

A restricted user CAN edit the box-net profile copy in their own project (they own project profiles — features.profiles=true), including stripping security.port_isolation. That is why the host-owned nft bridge drop is the second layer: meta ibrname boxnet obrname boxnet drop fires on every port-to-port frame regardless of per-NIC flags. Cross-user sibling probes are dropped either way — measured from inside the boxes.

What box grant <user> converges (idempotent, re-run to refresh)

1. usermod -aG incus (not incus-admin — that is the tier)
2. first-touch incus-user as the user (runuser/sudo -u, stdin pinned) — the project is created lazily and cannot be pre-created by an admin
3. remove the default profile's private-bridge eth0
4. restricted.networks.access boxnet
5. restricted.snapshots allow
6. install/refresh the shipped box-net profile into the project
7. verify from the user's side of the socket

box revoke <user> is the inverse, two strengths: bare = group removal (the socket closes; boxes keep running; re-grant restores), --purge = boxes, images, project, private bridge, trust-store certificate, incus-user state — then asserts the absence (the wipe.sh discipline).

The tier in the CLI

box_tier() — UID 0 / incus-admin → admin, incus alone → restricted, neither → none — decided from live process credentials (argless id -nG), byte-identical in bin/box and host/setup-host.sh (diffed by a test). Tier-aware surface: new pre-flights the profile and names the right fix per tier; expose refuses before any daemon call (its plumbing is daemon-global; without the guard the failure is a lie — a restricted user cannot read boxnet's redacted config, so box_net_ip would claim their running box has no address); setup-host exits 0 with the honest note; doctor runs a restricted check-set (is the tier granted, does their box resolve/route) instead of judging host state they cannot see.

Found along the way, fixed for every tier: box restore dispatched incus restore, which does not exist in Incus 6 (incus snapshot restore). It had never worked.

Rehearsal and CI

drill/multiuser.sh (root, opt-in via BOX_MULTIUSER_REHEARSAL=1) proves criteria (a)–(f) from #74 plus the measured extensions (g)–(l): the in-box isolation contract (egress, DNS, box→host, RFC1918, cross-user sibling drop, name enumeration, IPv6-off), the closed escape hatches, re-sync survival, and scoped revoke. Two real users, real grants, real mints, probes from inside; --container for CI, VM mode on real hardware; cleanup deletes everything it made.

CI gains a rehearsal job (ubuntu-latest): install incus, stage the checkout at /opt/box (the #71 layout), setup-host, doctor, then the rehearsal in container mode. The tier's semantics are proven on every PR against a live daemon; the VM trust boundary stays a real-hardware ritual, like the drill.

Environment

Debian 13 (trixie), Incus 6.0.4, incus-user.socket shipped in the incus package (Debian 13 and Ubuntu 24.04 both), /dev/kvm present (VM-mode run), btrfs pool. Companion work: rig#24 (box role), rig#12/#25 (host-class).