doos/README.md

# doos

User-level containers in C++20: overlayfs, user/mount/pid namespaces, slirp4netns networking, optional images (Alpine, Ubuntu, or your own rootfs), a size cap per container. No root at runtime.

```
doos list
doos get_or_create [--image I] [--size S] [--net M] [--ids X] NAME
doos attach        [--image I] [--size S] [--net M] [--ids X] NAME [-- CMD...]
doos delete [--yes] NAME
doos image list | add [--sha256 HEX] SOURCE [NAME] | delete [--yes] NAME
```

Version: see `VERSION` (currently 1.0.0). 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), `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`.

## Size cap

`--size` limits what a container writes (default 4g, 64m to 1t). The parent checks every 10 s: a warning at 80%, at 100% the container is frozen (nothing is deleted), and it thaws on its own below 95% after you free space with a second `doos attach NAME -- ...`. Needs cgroup v2 delegation (standard on systemd desktops); without it the cap only warns.

## 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. |
| `size cap only warns, cannot freeze` | No delegated cgroup v2 (no systemd user manager). Cap is advisory. |
| `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 mode. Images are
verified against the publisher's checksum before they are extracted. `--net host`
removes network isolation and is opt-in. Known gaps (no cgroups, readable host `/etc`)
are listed with researched solutions (G1 to G6, not implemented) in `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 |