doos/README.md

# doos

User-level containers in C++20: overlayfs, user/mount/pid namespaces, slirp4netns or pasta networking, optional images (Alpine, Ubuntu, Debian, or your own rootfs), memory/cpu/pid/size limits, default hardening (capabilities, seccomp). No root at runtime.

```
doos list
doos get_or_create [OPTIONS] NAME
doos attach        [OPTIONS] NAME [-- CMD...]
doos delete [--yes] NAME
doos image list | add [--sha256 HEX] SOURCE [NAME] | delete [--yes] NAME
```

Version: see `VERSION` (currently 1.1.0). Release history: `CHANGELOG.md`.

User-level containers in C++20: overlayfs over the host's `/usr`, `/etc`, `/opt`, user/mount/pid namespaces, networking through slirp4netns. No root at runtime.

```
doos list
doos get_or_create [--net slirp|host|none] NAME
doos attach [--net slirp|host|none] NAME [-- CMD...]
doos delete [--yes] NAME
```

Build: `cmake -S . -B build && cmake --build build`. Test: `tests/run.sh`.

Requirements: Linux >= 5.11, `slirp4netns` (default network mode), `pasta` (only for `--net pasta`), `gnupg` (signature checks; optional), `uidmap` (`newuidmap`/`newgidmap`, for more than one id inside; falls back to one id), `curl`, `tar` and `sha256sum` (images). On Ubuntu 24.04+ install the AppArmor profile once:

```
sudo cp apparmor/doos /etc/apparmor.d/doos && sudo apparmor_parser -r /etc/apparmor.d/doos
```

See `SPEC.md` (behavior, limits) and `docs/RESEARCH.md` (what works, measurements, known gaps).

## Usage examples

```
doos attach dev                         # host mode: your system's /usr as base, shell as root inside
doos attach --image alpine box          # first use downloads and verifies Alpine, then opens a shell
doos attach --image ubuntu web -- apt-get update
doos attach --image ubuntu-24.04 old    # a specific Ubuntu release
doos attach --size 8g big               # raise the writable-layer cap (default 4g)
doos attach --net none sandbox -- sh    # no network at all
doos list
doos image list                         # what is installed, what the catalog offers
doos delete dev                         # dry run: shows what would go
doos delete --yes dev                   # really deletes
doos image delete --yes alpine          # refused while a container uses it
```

Your own image (a root filesystem tarball):

```
doos image add https://example.org/debian-rootfs.tar.xz --sha256 <hex> debian   # URL needs the hash
doos image add ./rootfs.tar.gz mybase                                           # local file
doos attach --image mybase c1
```

## Host mode or image mode

| | Host mode (default) | Image mode (`--image`) |
|---|---|---|
| Base system | your `/usr`, `/opt`, copy of `/etc` | the image, extracted once and shared |
| Needs a download | no | first use only |
| `apt-get install` | works when the host is Debian/Ubuntu based | works for the image's own package manager (apt, apk) |
| Sees host files | `/usr`, `/opt`, readable `/etc` at creation | nothing from the host |
| Best for | quick throwaway shells with your tools | reproducible, separate systems |

Writes always stay in the container (copy-on-write); `/root` and `/var` persist, `/tmp` is memory-backed and lost on exit. A container is `running` while its first `attach` command is alive; more `attach` calls join it. Data persists until `delete --yes`.

## Options

| Option | Meaning | Default | Later |
|---|---|---|---|
| `--image NAME` | alpine, ubuntu, ubuntu-24.04, debian, debian-bookworm, or your own | host system | fixed |
| `--net M` | `slirp` (outbound only, host loopback blocked), `pasta` (host-like, private sockets), `host`, `none` | slirp | fixed |
| `--ids X` | `single` or `subuid` | subuid if possible | fixed |
| `--hardening H` | `default` or `relaxed` | default | fixed |
| `--size S` | writable data cap | 4g | changeable |
| `--memory S` | memory cap | 8g | changeable |
| `--pids N` | process/thread cap | 4096 | changeable |
| `--cpus N` | cpu cap, e.g. `0.5`, `2` | all cores | changeable |

Changeable options apply the next time the container starts. Examples: `doos attach --memory 2g --cpus 1 build`, `doos attach --net pasta --image debian-bookworm web`.

## Limits, size cap and hardening

- **Limits** (`--memory`, `--pids`, `--cpus`): cgroup v2, measured here: a 256m container touching 400 MB is killed whole and doos says so; 80 forks against `--pids 32` fail and are reported; `--cpus 0.5` held 4 busy loops to 2.3 CPU-seconds in 4.3 s of wall time.
- **Size cap**: `--size` limits what a container writes. Checked from inside the id map (so other users' files count), warns at 80%, freezes at 100% (nothing is deleted), thaws below 95%. A second `doos attach NAME -- ...` is never frozen, so you can clean up. No single file can exceed the cap.
- **Hardening** (default): capabilities reduced to Docker's default set, a seccomp filter (no mounts, keyrings, bpf, perf, io_uring, nested user namespaces, kexec, module loading...), and a host `/etc` allowlist instead of a full copy. `--hardening relaxed` switches all three off for tools that need them (container runtimes, Chromium's sandbox).
- All limits need cgroup v2 delegation (standard on systemd desktops). Without it doos prints that nothing is enforced.

## Configuration

| Variable | Meaning |
|---|---|
| `DOOS_HOME` | State directory (default `~/.local/share/doos`). No `,` or `:` allowed. |
| `XDG_DATA_HOME` | Used as `$XDG_DATA_HOME/doos` when `DOOS_HOME` is unset. |

## Troubleshooting

| Message | Cause and fix |
|---|---|
| `write /proc/<pid>/uid_map ... blocked` or `mount ...: Permission denied` | AppArmor restricts user namespaces. Install `apparmor/doos` (command above). |
| `slirp4netns failed to start` | Install `slirp4netns`, or use `--net none` / `--net host`. |
| `subuid mapping unavailable` | Install `uidmap` and make sure `/etc/subuid` and `/etc/subgid` list your user, or use `--ids single`. |
| `sha256 mismatch` | The download is not what the publisher lists; nothing was installed. Retry or check the source. |
| `a URL image needs --sha256` | Unverifiable input is rejected by design; pass the hash from the publisher. |
| `limits are not enforced and the size cap only warns` | No delegated cgroup v2 (no systemd user manager). Limits and cap are advisory. |
| `was killed by its memory limit` | Raise `--memory`. `/tmp` is memory-backed and counts. |
| `hit its process limit` | Raise `--pids`; threads count as processes. |
| `Operation not permitted` from `mount`, `unshare`, `keyctl`, io_uring | Hardening blocked it. Use `--hardening relaxed` on that container. |
| `signature NOT checked (gpg is not installed)` | Install `gnupg` for signature checks; the checksum still applies. |
| `doos: seeded ...: unreadable skipped: N` | Host entries the user cannot read were left out of the container. Informational. |
| `state path must not contain ',' or ':'` | Set `DOOS_HOME` to a plain path. |
| `container 'x' is running` on delete | Exit the attached command first. |
| `container limit reached (64)` | Delete one first. |

## Safety summary

Host files are never written: system dirs are reached only through overlayfs. Host `/home`, `/sys` and host
loopback are not reachable in the default network mode. Images are verified before they are extracted: sha256
from the publisher, and for Alpine and Ubuntu a signature checked against a key pinned by fingerprint. Processes
run with Docker's default capability set and a seccomp filter. Memory, pids, cpus and written data are capped.
`--net host` removes network isolation and is opt-in (`--net pasta` is the safer host-like choice).
What is still not covered is listed at the end of `docs/RESEARCH.md`.

## Project docs

| File | Content |
|---|---|
| `SPEC.md` | Behavior contract and limits |
| `docs/ARCHITECTURE.md` | Code map and flows |
| `docs/RESEARCH.md` | What works unprivileged, measurements, gaps |
| `AGENTS.md` | Rules for AI agents, release workflow |
| `CHANGELOG.md` | Release history |