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Document and harden dos-shell automation

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# DOS Remote Environment Stack
This repository packages a repeatable SvarDOS environment that can be exposed over SSH (with optional X forwarding) and telnet. It wraps the upstream `dosemu2` emulator with automation for bootstrapping media, distributing pre-approved files, and hardening the guest runtime so multiple users can log in without trampling each other.
The project is split into small, testable shell helpers plus a Docker image that glues them together. You can run everything with `docker compose`, or treat the image as a standalone component inside an existing infrastructure.
---
- [Quick Start](#quick-start)
- [Repository Layout](#repository-layout)
- [How the Container Boots](#how-the-container-boots)
- [Customising the DOS Drive](#customising-the-dos-drive)
- [Controlling `dos-shell`](#controlling-dos-shell)
- [Environment Variables (Container / Compose)](#environment-variables-container--compose)
- [Managing the SvarDOS Base Image](#managing-the-svardos-base-image)
- [Running Without Docker Compose](#running-without-docker-compose)
- [Troubleshooting](#troubleshooting)
- [Maintenance & Housekeeping](#maintenance--housekeeping)
- [Security Notes](#security-notes)
---
## Quick Start
Requirements:
- Linux host with Docker 24+ (`docker compose` plugin included)
- X11 server on your workstation if you intend to use `ssh -X`
- Optional: PulseAudio/PipeWire socket forwarded if you want sound
Bring the stack up:
```sh
docker compose up -d --build # rebuilds the image so scripts stay in sync
```
Default access:
- **User:** `dosuser`
- **Password:** `dosuser`
- **SSH:** `ssh -X dosuser@localhost -p 2222`
- **Telnet (optional):** `telnet localhost 2323` (disabled if `ENABLE_TELNET=0`)
Use `exit` from the DOS shell to terminate the session; the container keeps running for the next login.
## Repository Layout
| Path | Purpose |
|-----------------------|--------------------------------------------------------------------------------------------------|
| `Dockerfile` | Builds the Ubuntu 22.04 based image with dosemu2 and helper scripts. |
| `compose.yml` | Reference deployment that exposes SSH/Telnet and mounts custom content. |
| `scripts/` | Automation scripts (`dos-shell`, `prepare-svardos`, `start-services`). |
| `config/` | Baseline configuration for the container (`sshd_config`, `dos_allowed` allow-list). |
| `allowed_repo/` | Host directory whose contents are copied to `C:\` when permitted. |
| `dos_env/` | Optional templates for `AUTOEXEC.BAT` / `CONFIG.SYS`; copied on every login. |
| `docs/` | Room for auxiliary documentation (currently empty). |
All persistent user data inside the guest lives under `/home/dosuser/.dosemu`, which is created on first login.
## How the Container Boots
1. **`prepare-svardos`** runs during build, downloading the latest SvarDOS ZIP (override with `SVARDOS_IMG_URL`) and staging it under `/opt/svardos/base`.
2. **`start-dos-services`** starts BusyBox `telnetd` (if enabled) and then `sshd`.
3. Whenever `dosuser` logs in, **`dos-shell`**:
- Detects terminal mode (X11 window, terminal, or dumb) and composes the corresponding `dosemu` flags.
- Ensures a private C: drive under `/home/dosuser/.dosemu/drive_c`, copying SvarDOS files if the sentinel `.svardos_installed` is missing or if you asked for a reinstall.
- Synchronises approved host files from `/opt/allowed_repo` based on the policy (`DOS_ALLOW_MODE` and `/etc/dos_allowed`).
- Applies optional `AUTOEXEC.BAT` / `CONFIG.SYS` templates from `/etc/dos_env`.
- Automatically amends the user’s `dosemurc` when hardware features are unavailable (e.g. no `/dev/kvm`, no X11/PulseAudio) so dosemu starts quietly.
- Falls back to the `dosuser` account if the login happened as root (useful for forced commands via sshd).
## Customising the DOS Drive
- **`allowed_repo/` volume** – drop files here on the host; they are copied into C:\ during login.
- **`config/dos_allowed`** – list relative paths (from `allowed_repo/`) to permit when `DOS_ALLOW_MODE=list`. With `DOS_ALLOW_MODE=all` every file in the repo is staged.
- **`dos_env/` templates** – place `AUTOEXEC.BAT` and/or `CONFIG.SYS` to control boot scripts.
- **Forcing a reinstall** – set `DOS_FORCE_INSTALL=1` in the environment before logging in; the script re-seeds the drive from `/opt/svardos/base`.
Typical layout when using `docker compose`:
```
allowed_repo/
games/
doom/
doom.exe
dos_env/
AUTOEXEC.BAT
CONFIG.SYS
```
## Controlling `dos-shell`
You can influence runtime behaviour with environment variables. Set them either in the container environment (e.g. via `compose.yml`, `.env`, or `docker run -e`).
| Variable | Values / Default | Effect |
|-----------------------|------------------------|--------------------------------------------------------------------------------------------------|
| `DOS_TERMINAL_MODE` | `auto` (default), `x`, `sdl`, `terminal`, `dumb` | Forces the video backend used for dosemu. |
| `DOS_AUDIO_MODE` | `auto` (default), `force`, `mute` | Auto-mute when PulseAudio/PipeWire isn’t available over SSH, or force sound on/off. |
| `DOS_LANDLOCK_MODE` | `auto` (default), `force`, `off` | Controls Landlock sandboxing; `auto` disables it if kernel headers don’t expose the ABI we need. |
| `DOS_ALLOW_MODE` | `all` (default) or `list` | Stage everything from `/opt/allowed_repo`, or only the entries listed in `/etc/dos_allowed`. |
| `DOS_FORCE_INSTALL` | `0` (default) or `1` | Rebuilds the C: drive from the SvarDOS base on the next login. |
| `DOS_TERMINAL_MODE` | `auto` | Determines whether `dosemu` launches with X11 (`-X`), terminal (`-td`) or `-dumb`. |
| `DOS_ENV_DIR` | defaults to `/etc/dos_env` | Override if you mount templates somewhere else. |
| `SVARDOS_ROOT`/`SVARDOS_BASE` | default `/opt/svardos` | Changes where the base image lives (mostly useful during debugging). |
| `AO_DRIVER` | auto-set to `null` when sound is muted | You can override to force libao to a specific backend. |
`dos-shell` also writes a managed `~/.dosemu/dosemurc` (marked with `# Managed by dos-shell…`) when it needs to enforce CPU or audio fallbacks. Delete the file to restore defaults; it is regenerated on demand.
## Environment Variables (Container / Compose)
`compose.yml` exposes a few knobs. You can drop a `.env` file next to it to override the defaults.
| Compose Variable | Default | Description |
|--------------------------|---------------------|----------------------------------------------------------|
| `DOS_IMAGE_NAME` | `dos-env` | Tag assigned to the built image. |
| `DOS_CONTAINER_NAME` | `dos-env` | Name of the running container. |
| `DOS_SSH_PORT` | `2222` | Host port forwarded to container port 22. |
| `DOS_TELNET_PORT` | `2323` | Host port forwarded to container port 23. |
| `ENABLE_TELNET` | `1` | Toggle BusyBox telnetd. |
| `TELNET_PORT` | `23` | Port inside the container where telnetd listens. |
| `TELNET_LOGIN` | `/bin/login` | Login command invoked by telnetd. |
| `DOS_ALLOW_MODE` | `all` | Passed straight through to `dos-shell`. |
| `SVARDOS_IMG_URL` | *(empty)* | When set, overrides the SvarDOS ZIP downloaded at build time. |
Example `.env` snippet:
```
DOS_SSH_PORT=2022
ENABLE_TELNET=0
DOS_ALLOW_MODE=list
SVARDOS_IMG_URL=https://example.com/custom-svardos.zip
```
## Managing the SvarDOS Base Image
`prepare-svardos` downloads and unpacks the official SvarDOS `svardos-*-dosemu.zip`. Key environment variables:
| Variable | Purpose |
|----------------------|----------------------------------------------------------------------------|
| `SVARDOS_IMG_URL` | Fetch from a custom URL instead of the default published snapshot. |
| `SVARDOS_ROOT` | Root directory under which files are staged (defaults to `/opt/svardos`). |
| `SVARDOS_BASE_DIR` | Override the final `base` directory location. |
| `SVARDOS_REFRESH` | Set to any value to force a re-download even if `COMMAND.COM` already exists. |
During image build the script also patches `INSTALL.BAT` and `AUTOEXEC.BAT` to work smoothly with contemporary package paths.
If you want to refresh the base files inside a running container:
```sh
docker exec -e SVARDOS_REFRESH=1 dos-env /usr/local/bin/prepare-svardos
```
Subsequent logins should pick up the new contents once you also export `DOS_FORCE_INSTALL=1`.
## Running Without Docker Compose
You can launch the container directly:
```sh
docker build -t dos-env .
docker run -d --name dos-env \
-p 2222:22 -p 2323:23 \
-v "$(pwd)/allowed_repo:/opt/allowed_repo" \
-v "$(pwd)/config/dos_allowed:/etc/dos_allowed:ro" \
-v "$(pwd)/dos_env:/etc/dos_env:ro" \
dos-env
```
To override behaviour, append `-e` flags:
```sh
docker run … -e ENABLE_TELNET=0 -e DOS_ALLOW_MODE=list -e DOS_AUDIO_MODE=force …
```
## Troubleshooting
| Symptom / Log line | Cause & Fix |
|----------------------------------------------------------------------|---------------------------------------------------------------------------------------------------------------------|
| `dos-shell: disabling KVM acceleration (no usable /dev/kvm)` | The kernel lacks `/dev/kvm` (common inside nested VMs). CPU emulation is already enabled; no action required. |
| `dos-shell: muting DOS audio for this session` | No PulseAudio/PipeWire endpoint detected when logging in over SSH. Export `DOS_AUDIO_MODE=force` if you need sound.|
| `ERROR: ladspa: failed to load filter.so / libao: unable to open` | Happens when audio is muted; harmless once the override is applied. |
| `Landlock ABI … not defined / landlock_init() failed` | Older kernels don’t expose `LANDLOCK_ACCESS_FS_REFER`. `dos-shell` disables Landlock automatically. |
| `ERROR: using outdated config file ~/.dosemurc` | Remove the legacy file (`rm ~/.dosemurc`). `dos-shell` now writes to `~/.dosemu/dosemurc`. |
| `ssh: connect … port 2222: Connection refused` | Container not running. `docker compose ps` or `docker compose up -d` to start it. |
Enemy separation: `dos-shell` prints the config overrides it applies; review those messages first when diagnosing odd behaviour.
## Maintenance & Housekeeping
- **Backing up user data:** everything lives under `/home/dosuser`; mount a volume if you need persistence beyond container lifetimes.
- **Upgrading dosemu2/SvarDOS:** rebuild the image (`docker compose build`) after adjusting `Dockerfile` or `SVARDOS_IMG_URL`.
- **Resetting to stock files:** remove `/home/dosuser/.dosemu` or log in with `DOS_FORCE_INSTALL=1`.
- **Extending the allow-list:** add entries to `config/dos_allowed` (one per line) or switch the mode to `all`.
- **Audit logs:** `sshd` is configured to run in debug mode (`-e`); use `docker logs dos-env` for a quick view.
## Security Notes
- SSH is preferred; telnet is available only if explicitly enabled and should be disabled on untrusted networks.
- When audio is muted the script points libao to the `null` backend to avoid opening `/dev/dsp` or Pulse pipes.
- Landlock sandboxing provides extra filesystem isolation when available. The script downgrades gracefully when the kernel is too old.
- The default credentials are intentionally simple for local development. Change them (or add public keys) before exposing the service.
---
For questions or contributions, open an issue or send a patch – the shell scripts are deliberately compact so it is easy to audit every change.

133
scripts/dos-shell
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@ -137,6 +137,138 @@ apply_templates() {
detect_dosemu_args
FORCE_CPU_EMULATION=0
detect_kvm_support() {
if [ ! -c /dev/kvm ]; then
return 1
fi
if [ "$(id -u)" -eq 0 ]; then
if command -v runuser >/dev/null 2>&1; then
runuser -u "${DOS_USER}" -- test -r /dev/kvm 2>/dev/null || return 1
return 0
fi
return 0
fi
[ -r /dev/kvm ]
}
if ! detect_kvm_support; then
echo "dos-shell: disabling KVM acceleration (no usable /dev/kvm)" >&2
FORCE_CPU_EMULATION=1
fi
should_disable_audio=0
audio_mode="${DOS_AUDIO_MODE:-auto}"
case "${audio_mode,,}" in
mute|off|disabled)
should_disable_audio=1
;;
force|on|enabled)
should_disable_audio=0
;;
auto)
if [ -n "${SSH_CONNECTION:-}" ]; then
if [ -z "${PULSE_SERVER:-}" ] && [ -z "${PIPEWIRE_REMOTE:-}" ]; then
should_disable_audio=1
fi
fi
;;
*)
echo "Unknown DOS_AUDIO_MODE '${audio_mode}', defaulting to auto." >&2
if [ -n "${SSH_CONNECTION:-}" ] && [ -z "${PULSE_SERVER:-}" ] && [ -z "${PIPEWIRE_REMOTE:-}" ]; then
should_disable_audio=1
fi
;;
esac
if [ "${should_disable_audio}" -eq 1 ]; then
echo "dos-shell: muting DOS audio for this session" >&2
export AO_DRIVER="${AO_DRIVER:-null}"
fi
detect_landlock_support() {
local header
for header in /usr/include/linux/landlock.h \
/usr/include/uapi/linux/landlock.h \
/usr/lib*/gcc/*/*/include/uapi/linux/landlock.h; do
if [ -f "$header" ] && grep -q 'LANDLOCK_ACCESS_FS_REFER' "$header" 2>/dev/null; then
return 0
fi
done
return 1
}
landlock_mode="${DOS_LANDLOCK_MODE:-auto}"
case "${landlock_mode,,}" in
auto)
if detect_landlock_support; then
:
else
echo "dos-shell: disabling Landlock sandbox (ABI mismatch detected)" >&2
DOSEMU_ARGS+=(-p)
fi
;;
off|disable|disabled)
echo "dos-shell: Landlock sandbox disabled by DOS_LANDLOCK_MODE" >&2
DOSEMU_ARGS+=(-p)
;;
force|on|enabled)
;;
*)
echo "Unknown DOS_LANDLOCK_MODE '${landlock_mode}', defaulting to auto." >&2
if detect_landlock_support; then
:
else
echo "dos-shell: disabling Landlock sandbox (ABI mismatch detected)" >&2
DOSEMU_ARGS+=(-p)
fi
;;
esac
write_autoconfig() {
local need_config=0
local conf_path="${DOSEMU_DIR}/dosemurc"
local legacy_conf="${DOS_HOME}/.dosemurc"
local managed_marker="# Managed by dos-shell for compatibility"
if [ "${FORCE_CPU_EMULATION}" -eq 1 ] || [ "${should_disable_audio}" -eq 1 ]; then
need_config=1
fi
if [ "${need_config}" -eq 1 ]; then
if [ -f "${legacy_conf}" ] && grep -qF "${managed_marker}" "${legacy_conf}" 2>/dev/null; then
rm -f "${legacy_conf}"
fi
local tmp_conf="${conf_path}.tmp"
{
echo "${managed_marker}"
echo "define parser_version_3"
if [ "${FORCE_CPU_EMULATION}" -eq 1 ]; then
echo '$_cpu_vm = "emulated"'
echo '$_cpu_vm_dpmi = "emulated"'
fi
if [ "${should_disable_audio}" -eq 1 ]; then
echo '$_sound = (off)'
echo '$_pcm_hpf = (off)'
fi
} > "${tmp_conf}"
mv "${tmp_conf}" "${conf_path}"
if [ "$(id -u)" -eq 0 ]; then
chown "${DOS_USER}:${DOS_USER}" "${conf_path}"
fi
echo "dos-shell: wrote ${conf_path} overrides" >&2
else
for candidate in "${conf_path}" "${legacy_conf}"; do
if [ -f "${candidate}" ] && grep -qF "${managed_marker}" "${candidate}" 2>/dev/null; then
rm -f "${candidate}"
echo "dos-shell: removed autogenerated ${candidate}" >&2
fi
done
fi
}
if [ "${DOS_FORCE_INSTALL:-0}" = "1" ] || [ ! -f "${INSTALL_SENTINEL_PATH}" ]; then
reset_drive_c
touch "${INSTALL_SENTINEL_PATH}"
@ -149,6 +281,7 @@ fi
sync_allowed_content
apply_templates
write_autoconfig
if [ "$(id -u)" -eq 0 ]; then
chown -R "${DOS_USER}:${DOS_USER}" "${DOSEMU_DIR}"

5
scripts/prepare-svardos.sh
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@ -1,10 +1,11 @@
#!/bin/bash
set -euo pipefail
SVARDOS_BUILD=20250427
SVARDOS_ROOT="${SVARDOS_ROOT:-/opt/svardos}"
SVARDOS_BASE_DIR="${SVARDOS_BASE_DIR:-${SVARDOS_ROOT}/base}"
DEFAULT_URL="http://svardos.org/download/20250427/svardos-20250427-dosemu.zip"
ARCHIVE_URL="${SVARDOS_IMG_URL:-$DEFAULT_URL}"
SVARDOS_URL="http://svardos.org/download/${SVARDOS_BUILD}/svardos-${SVARDOS_BUILD}-dosemu.zip"
ARCHIVE_URL="${SVARDOS_IMG_URL:-$SVARDOS_URL}"
# Skip work if the base tree already exists (unless SVARDOS_REFRESH is set)
if [ -z "${SVARDOS_REFRESH:-}" ] && [ -e "${SVARDOS_BASE_DIR}/COMMAND.COM" ]; then