docker-bastion/README.md

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# docker-bastion

[![Alpine](https://img.shields.io/badge/alpine-3.21-blue?logo=alpinelinux)](https://alpinelinux.org)
[![OpenSSH](https://img.shields.io/badge/openssh-9.9-green)](https://www.openssh.com)
[![Docker CLI](https://img.shields.io/badge/docker--cli-included-2496ED?logo=docker)](https://docs.docker.com/engine/reference/commandline/cli/)
[![Image Size](https://img.shields.io/badge/image-~105MB-lightgrey)](#whats-inside)
[![License](https://img.shields.io/badge/license-MIT-lightgrey)](#license)

A **minimal SSH + HTTP bastion** for routing commands per authenticated session. Authenticate by SSH key or HTTP bearer token, and the container either runs one preconfigured command (`FORCE_COMMAND` — `docker exec` into a sibling, `./deploy.sh`, `nginx -s reload`) or, in **broker mode**, runs whatever the client asks for *as long as it matches a regex allowlist* (`ALLOWED_COMMANDS`) — then streams the output back.

**Why it exists:** giving an agent or CI bot `docker exec` access usually means handing them the docker socket and trusting their entire toolchain not to misbehave. A bastion with a hard-coded `FORCE_COMMAND` is the inverse: the credential authorizes *one specific thing*, the surface is sshd + busybox httpd, and the same image works for a dozen different roles by varying `FORCE_COMMAND`.

## Available Tags

| Tag                              | Base         | Notes                                |
|----------------------------------|--------------|--------------------------------------|
| `blaxsoftware/bastion:latest`    | alpine 3.21  | Default tag, follows alpine releases |
| `blaxsoftware/bastion:alpine3.21`| alpine 3.21  | Pinned alpine version                |

## Quick Start — drop into a WordPress container

The most common use: give a deploy agent SSH-shaped access *into* a running WordPress container. Every session lands inside the `wordpress-app` container's bash; clients can run WP-CLI commands, edit config, debug — same UX as `ssh user@host` against a VPS, but scoped to one container.

```yaml
services:
  # The bastion. SSH on 2222, HTTP behind traefik on https://deploy-wp.example.com.
  bastion:
    image: blaxsoftware/bastion:latest
    restart: unless-stopped
    environment:
      # REQUIRED — the single command that runs on every authenticated session.
      # Shell metacharacters work: &&, ||, pipes, cd, redirects.
      FORCE_COMMAND: "docker exec -it wordpress-app bash"

      # OPTIONAL — enables the HTTP endpoint at /cgi-bin/run with basic auth.
      # Value is "user:password". Works with `curl https://user:pass@host/…`.
      # Without this (and without HTTP_TOKEN), the bastion is SSH-only.
      HTTP_BASIC_AUTH: "${BASTION_HTTP_BASIC_AUTH}"
      # Alternative: HTTP_TOKEN="$(openssl rand -hex 32)" for Bearer auth.
      # Pick one — they're mutually exclusive (Basic wins if both are set).

    volumes:
      # REQUIRED when FORCE_COMMAND talks to docker (docker exec, docker compose, etc).
      # Mounts the host's daemon socket so docker-cli inside the bastion reaches it.
      - /var/run/docker.sock:/var/run/docker.sock

      # OPTIONAL — host-sourced authorized_keys. Your laptop's keys, or anything
      # outside the repo. Read-only mount.
      - ~/.ssh/authorized_keys:/etc/bastion/authorized_keys.host:ro

      # OPTIONAL — repo-sourced authorized_keys. CI / deploy-bot keys committed
      # alongside the project. At least one of these two must exist or the
      # container refuses to start.
      - ./docker/bastion/authorized_keys:/etc/bastion/authorized_keys.repo:ro

      # REQUIRED — host keys persist across rebuilds. Bind mount, NEVER a named
      # volume; `docker compose down -v` would wipe a named volume and clients
      # would see "REMOTE HOST IDENTIFICATION HAS CHANGED" after every redeploy.
      - ./docker-data/bastion/keys:/etc/ssh/keys

    ports:
      # OPTIONAL — expose SSH on the host directly. Skip this entirely if you
      # only want the HTTP path through traefik.
      - "2222:22"

    labels:
      # OPTIONAL — traefik HTTP route. Visit https://deploy-wp.example.com/cgi-bin/run
      # with `Authorization: Bearer $BASTION_HTTP_TOKEN` to invoke FORCE_COMMAND.
      # Remove these labels if you don't want the HTTP path published.
      traefik.enable: "true"
      traefik.docker.network: "web"
      traefik.http.routers.bastion.rule: "Host(`deploy-wp.example.com`)"
      traefik.http.routers.bastion.entrypoints: "websecure"
      traefik.http.routers.bastion.tls: "true"
      traefik.http.services.bastion.loadbalancer.server.port: "8080"

    networks: [web]

  # Your actual WordPress container — bastion's FORCE_COMMAND targets it by name.
  wordpress-app:
    image: wordpress:latest
    container_name: wordpress-app
    environment:
      WORDPRESS_DB_HOST: db
      WORDPRESS_DB_NAME: wp
      WORDPRESS_DB_USER: wp
      WORDPRESS_DB_PASSWORD: "${DB_PASSWORD}"
    volumes:
      - ./docker-data/wordpress:/var/www/html
    networks: [web]

networks:
  web:
    external: true
```

From the client:

```bash
# Interactive shell inside the wp container — feels exactly like ssh-into-vps
ssh -p 2222 agent@your-host

# Or trigger from a URL — basic-auth protected, output streams back
curl https://user:pass@deploy-wp.example.com/cgi-bin/run
```

## Quick Start — reload nginx on a webhook

A scoped bastion that does exactly one thing: test the new nginx config and reload if it passes. The HTTP path lets a CI job (GitHub Action, Forgejo runner, anything that can `curl`) trigger a reload after pushing new configs to disk — no SSH keys to provision in CI.

```yaml
services:
  bastion:
    image: blaxsoftware/bastion:latest
    restart: unless-stopped
    environment:
      # `nginx -t` exits non-zero on a syntax error; `&&` short-circuits so a
      # broken config never gets applied. The exit code propagates back to
      # the HTTP client (which sees the connection close mid-stream on failure).
      FORCE_COMMAND: "docker exec nginx-app nginx -t && docker exec nginx-app nginx -s reload"
      HTTP_TOKEN: "${BASTION_HTTP_TOKEN}"
    volumes:
      - /var/run/docker.sock:/var/run/docker.sock
      # SSH path stays available for human debugging — same key, same scope.
      - ~/.ssh/authorized_keys:/etc/bastion/authorized_keys.host:ro
      - ./docker-data/bastion/keys:/etc/ssh/keys
    labels:
      traefik.enable: "true"
      traefik.docker.network: "web"
      traefik.http.routers.bastion-nginx.rule: "Host(`reload-nginx.example.com`)"
      traefik.http.routers.bastion-nginx.entrypoints: "websecure"
      traefik.http.routers.bastion-nginx.tls: "true"
      traefik.http.services.bastion-nginx.loadbalancer.server.port: "8080"
    networks: [web]

  nginx-app:
    image: nginx:alpine
    container_name: nginx-app
    volumes:
      - ./nginx.conf:/etc/nginx/nginx.conf:ro
      - ./conf.d:/etc/nginx/conf.d:ro
    networks: [web]

networks:
  web:
    external: true
```

CI snippet:

```bash
# After updating nginx.conf on disk:
curl --fail-with-body -H "Authorization: Bearer $BASTION_HTTP_TOKEN" \
     https://reload-nginx.example.com/cgi-bin/run
```

`curl --fail-with-body` makes the CI step fail (non-zero exit) if the bastion returns 4xx/5xx, with the body printed — so a `nginx -t` syntax error in the new config shows up in the CI log without extra wiring.

## Quick Start — manage docker-mailserver (broker mode)

The two examples above hard-code *one* command. **Broker mode** is the inverse: the client supplies the command and the bastion runs it only if it matches a regex allowlist. This is how you give a management UI (a Nuxt "mail manager", a cron job, a bot) a *menu* of allowed operations without handing it a shell.

Here one bastion fronts [docker-mailserver](https://github.com/docker-mailserver/docker-mailserver)'s `setup` CLI. The app sends `email add jane@example.com <pw>` over SSH on the internal network; the bastion validates it and runs `docker exec -i mailserver setup email add jane@example.com <pw>`.

```yaml
services:
  bastion-mail:
    image: blaxsoftware/bastion:latest
    restart: unless-stopped
    environment:
      # Trusted prefix prepended to every validated request, so clients
      # send clean `email add …` and never see the docker plumbing.
      COMMAND_PREFIX: "docker exec -i mailserver setup"

      # The whitelist. A YAML block scalar reads like a string array:
      # one extended-regex (ERE) rule per line. A request is allowed only
      # if it matches a rule WHOLE-LINE (anchored). Matched commands run
      # WITHOUT a shell — ; | & $() are literal args, never operators —
      # so keep argument classes tight ([^ ]+, not .*).
      ALLOWED_COMMANDS: |
        email add [^ ]+@[^ ]+ [^ ]+
        email update [^ ]+@[^ ]+ [^ ]+
        email del [^ ]+@[^ ]+
        email list
        alias add [^ ]+@[^ ]+ [^ ]+
        alias del [^ ]+@[^ ]+ [^ ]+
        alias list
        quota set [^ ]+@[^ ]+ [0-9]+[KMGT]?
        quota del [^ ]+@[^ ]+
    volumes:
      - /var/run/docker.sock:/var/run/docker.sock
      - ./docker-data/bastion/users.d:/etc/bastion/users.d
      - ./docker-data/bastion/keys:/etc/ssh/keys
    networks: [web]   # same network as the `mailserver` container

networks:
  web:
    external: true
```

From the mail-manager (same docker network, no host port needed):

```bash
# allowed → runs `docker exec -i mailserver setup email add …`
ssh agent@bastion-mail "email add jane@example.com $(openssl rand -base64 18 | tr -d /+=)"

ssh agent@bastion-mail "email list"

# refused → exits 126, nothing runs
ssh agent@bastion-mail "email del jane@example.com; rm -rf /"
```

A complete copy-paste setup (compose + a live-editable `allowed-commands.list`) is in [`examples/docker-mailserver/`](examples/docker-mailserver/).

## Quick Start — a "fake VPS" scoped to one directory

Broker mode hands out a *menu* of commands. Sometimes you want the opposite: a
real interactive shell that feels like a VPS, but where the only thing on the
"server" is **one directory** — e.g. hand someone your `docker-mailserver`
directory to manage, and nothing else of the host.

The trick is a **disposable jail container** whose only real mount is that one
directory; the bastion's `FORCE_COMMAND` drops every session into a shell
inside it. The jail's own root filesystem is throwaway image data, *not* your
server — so the only host data reachable over SSH is the directory you mounted.

```yaml
services:
  # Disposable shell box. ONLY real host data inside it = the one directory.
  dms-jail:
    image: docker:27-cli          # or build a richer image — see the example
    container_name: dms-jail
    restart: unless-stopped
    working_dir: /opt/docker-mailserver
    entrypoint: ["tail", "-f", "/dev/null"]   # stay alive for `docker exec`
    volumes:
      # THE one directory. Keep host path == in-jail path so `docker compose`
      # inside the jail resolves the stack's bind mounts to host paths.
      - /opt/docker-mailserver:/opt/docker-mailserver
      # Lets the shell drive `docker compose`/restart. Host-root-equivalent —
      # drop this mount (here and below) for a true no-escape jail.
      - /var/run/docker.sock:/var/run/docker.sock
    networks: [web]

  bastion-vps:
    image: blaxsoftware/bastion:latest
    restart: unless-stopped
    depends_on: [dms-jail]
    environment:
      # Every SSH session becomes an interactive shell in the jail, in the
      # directory. `-it` = TTY → use SSH (not the HTTP path) for this.
      FORCE_COMMAND: "docker exec -it -w /opt/docker-mailserver dms-jail sh"
    volumes:
      - /var/run/docker.sock:/var/run/docker.sock   # only to exec into the jail
      - ./docker-data/bastion/users.d:/etc/bastion/users.d
      - ./docker-data/bastion/keys:/etc/ssh/keys
    ports:
      - "2222:22"
    networks: [web]

networks:
  web:
    external: true
```

```bash
ssh -p 2222 agent@your-host       # lands you in the directory, real shell
```

**The docker-socket tradeoff:** giving the jail the socket (so `docker compose`
works) is host-root-equivalent — from that shell, `docker run -v /:/host …`
reaches the whole host. It's a *practical* one-directory VPS for trusted
operators, not a hard sandbox. For a true no-escape boundary, remove the
`docker.sock` mount from both services (and manage the stack via a separate
broker-mode bastion).

A complete copy-paste setup — with a richer jail image (bash, editors, git,
compose) and the read-only / no-socket variants spelled out — is in
[`examples/docker-mailserver-vps/`](examples/docker-mailserver-vps/).

## Command broker mode

Set `ALLOWED_COMMANDS` (or mount `/etc/bastion/allowed-commands.list`) and the bastion switches from "one fixed command" to "any command the client asks for, **if** it passes the allowlist". `FORCE_COMMAND` is then optional and ignored.

**How a request is judged**

1. The client-supplied command (SSH `SSH_ORIGINAL_COMMAND`, or the HTTP `X-Bastion-Command` header) is taken verbatim.
2. Multi-line requests are rejected outright (so a benign first line can't smuggle a second).
3. It is matched **whole-line, anchored** against each rule (`grep -Eqx`). First match wins.
4. On a match: the (optional) `COMMAND_PREFIX` and the request are **word-split with globbing off and `exec`'d directly — there is no `sh -c`.** `;` `|` `&` `` ` `` `$()` `<` `>` become literal arguments, never shell operators. On no match: refused, exit `126`, logged as `[broker] DENY: …`.

**Why shell-free execution matters.** It means the regex is a *containment* boundary, not just a *filter*. Even a careless rule like `email .*` cannot escalate to command injection — the worst case is that `setup` receives a weird argument. (Contrast the `sh -c` path used by `FORCE_COMMAND` mode, where a metacharacter *would* be interpreted.)

**Writing rules**

- One ERE per line; `#` comments and blank lines ignored; surrounding whitespace trimmed (so indented YAML block lines work).
- Anchoring is implicit — write `email list`, not `^email list$` (both work).
- Use tight argument classes: `[^ ]+` ("a run of non-spaces") beats `.*`. Add alternation for fixed verbs: `email (add|update|del|list)`.
- Values that must reach the target intact **cannot contain whitespace** (word-splitting) or be quoted (no shell). Generate passwords/tokens from a space-free alphabet — e.g. `openssl rand -base64 24 | tr -d /+=`.

**Two sources, additive:** `ALLOWED_COMMANDS` (env, snapshot at boot) **+** `/etc/bastion/allowed-commands.list` (bind mount, re-read every request — edit without a restart).

**SSH vs HTTP.** SSH is the recommended transport (clean stdout/stderr separation). The HTTP path works too — `POST`/`GET` with `Authorization` + `X-Bastion-Command: <cmd>` — but merges the broker's stderr into the response body (the CGI uses `2>&1`, same as the deploy examples).

**Where the audit line goes.** The broker prints `[broker] ALLOW: …` / `[broker] DENY: …` on the *command's* stderr — so over SSH it returns to the client on the channel's stderr stream, and over HTTP it is folded into the response body (`2>&1`). It does **not** land in the bastion container's own `docker logs` (sshd does not redirect a ForceCommand's stderr to the daemon log). If you want a container-side audit trail, tee it — e.g. have `COMMAND_PREFIX`/the wrapper append to a logfile, or run a syslog sidecar.

## Two channels, two shapes

| Channel | Best for                       | TTY?     | Streaming?         |
|---------|--------------------------------|----------|--------------------|
| SSH     | Interactive (`docker exec -it`) **or** scripts | yes | yes |
| HTTP    | Scripts only — no TTY          | no       | yes (chunked / close-delimited) |

Interactive commands (`docker exec -it app bash`) over HTTP fail because there's no TTY — use SSH for those. Both channels stream output line-by-line; both close as soon as `FORCE_COMMAND` exits and the exit code propagates (SSH: to the client; HTTP: nonzero closes the response mid-stream).

The client cannot override the command. `SSH_ORIGINAL_COMMAND` and HTTP request bodies are intentionally ignored.

## Authorized keys — three sources, two flavors

sshd consults three sources on every authentication attempt:

| Source                                  | Read when            | Mount UX                                                                 |
|-----------------------------------------|----------------------|--------------------------------------------------------------------------|
| `/etc/bastion/users.d/*.pub`            | **live, every login** | Drop one `.pub` file per user — `users.d/alice.pub`, `users.d/bob.pub`. No restart to add/revoke. |
| `/etc/bastion/authorized_keys.host`     | merged at boot       | Single file from the host — `~/.ssh/authorized_keys`.                    |
| `/etc/bastion/authorized_keys.repo`     | merged at boot       | Single file from the repo — `./docker/bastion/authorized_keys`.          |

**Recommended: `users.d/`** — one file per identity, dropped in via a host bind mount, adds and revokes immediately. The two file-based sources stay for backward compatibility and for the "one big committed file" pattern.

Zero authorized keys is now a warning, not a startup failure — the bastion runs but every SSH attempt fails with `publickey denied` until you drop a key in.

## Environment variables

| Variable                | Default                                | Description                                                                |
|-------------------------|----------------------------------------|----------------------------------------------------------------------------|
| `FORCE_COMMAND`         | *(required unless `ALLOWED_COMMANDS` set)* | FORCE_COMMAND mode: the command run on every authenticated session. Shell metacharacters OK. Ignored in broker mode. |
| `ALLOWED_COMMANDS`      | *(unset)*                              | Broker mode: newline-separated ERE allowlist. Set this (or mount the file below) to enable broker mode. Each rule must match a request whole-line. |
| `COMMAND_PREFIX`        | *(unset)*                              | Broker mode, optional: trusted prefix prepended to every validated request (e.g. `docker exec -i mailserver setup`). |
| *(mount)* `/etc/bastion/allowed-commands.list` | *(unset)*               | Broker mode, optional: same format as `ALLOWED_COMMANDS`, re-read every request (live edits, no restart). Additive. |
| `HTTP_BASIC_AUTH`       | *(unset)*                              | Enables HTTP with Basic auth. Value is `user:password`. Works with `curl -u`, `curl https://user:pass@host/…`, and browser URL bars. |
| `HTTP_TOKEN`            | *(unset)*                              | Enables HTTP with Bearer auth. Clients send `Authorization: Bearer <this>`. Mutually exclusive with `HTTP_BASIC_AUTH` (basic takes precedence). |
| `HTTP_PORT`             | `8080`                                 | Port for the HTTP listener (when either auth var is set).                  |
| `SSH_PORT`              | `22`                                   | Port for sshd inside the container.                                        |
| `AUTHORIZED_KEYS_DIR`   | `/etc/bastion/users.d`                 | Directory of `*.pub` files, **live-read** by sshd via `AuthorizedKeysCommand`. Drop a file → next login picks it up. |
| `AUTHORIZED_KEYS_HOST`  | `/etc/bastion/authorized_keys.host`    | Single-file source, merged at boot. Optional / legacy.                     |
| `AUTHORIZED_KEYS_REPO`  | `/etc/bastion/authorized_keys.repo`    | Single-file source, merged at boot. Optional / legacy.                     |

## Build args

| Arg              | Default | Description                       |
|------------------|---------|-----------------------------------|
| `ALPINE_VERSION` | `3.21`  | Alpine base image tag.            |
| `SSH_UID`        | `1000`  | UID of the bastion `agent` user.  |
| `SSH_GID`        | `1000`  | GID of the bastion `agent` group. |

## What's inside

- **openssh-server** — hardened config: key-only auth, no forwarding, no PAM, no user env, global `ForceCommand` directive (clients cannot bypass).
- **bastion-broker** — the allowlist gate for broker mode: whole-line ERE match, then shell-free word-split `exec`. Shared by the SSH and HTTP paths.
- **busybox httpd** (busybox-extras) — minimal HTTP listener for the URL path; CGI-driven; only starts when `HTTP_TOKEN` or `HTTP_BASIC_AUTH` is set.
- **docker-cli + docker-cli-compose** — so `FORCE_COMMAND` can target containers through a mounted docker socket. Group membership is auto-aligned to the host socket's GID at boot.
- **tini** — PID 1, signal handling, zombie reaping.
- **bash, ca-certificates, tzdata.**

Total image: ~105 MB. Most of that is docker-cli (~50 MB) and docker-cli-compose (~25 MB).

## Security model

The security boundary is **the authorized_keys file (SSH) and the `HTTP_TOKEN` (HTTP), plus the `ForceCommand` wrapper**. Once a key or bearer token authenticates, the session runs exactly one command — there is no fallback shell. The bastion holds the docker socket, which is host-root-equivalent, so the only thing standing between a remote attacker and host root is the auth layer + your key/token hygiene.

Practical checklist:

1. **Key-only SSH, no passwords** — enforced in `sshd_config`.
2. **HTTP requires auth** — either basic auth (via busybox httpd's `-c` conf, with `REMOTE_USER` set on the authenticated CGI) or bearer token (validated by the CGI script). No anonymous path.
3. **No agent / TCP / X11 forwarding, no port tunnels** — enforced in `sshd_config`.
4. **ForceCommand cannot be bypassed.** Clients can request any command (`ssh user@host arbitrary-thing`); sshd ignores it and runs `/etc/bastion/force-command`. In FORCE_COMMAND mode `SSH_ORIGINAL_COMMAND` is dropped entirely. In **broker mode** it is *read* but only ever run if it matches the allowlist whole-line, and even then via shell-free word-split `exec` (no `sh -c`), so the regex is a containment boundary, not just a filter — see [Command broker mode](#command-broker-mode). The bastion user's login shell is `/bin/sh` (not `nologin` — that would break ForceCommand itself, since sshd invokes the user's shell as `shell -c "<forced-command>"`), but it has no path to anything other than the wrapper.
5. **`PermitUserEnvironment no`, `PermitUserRC no`** — clients cannot inject env vars or rc files.
6. **Bind host ports to `127.0.0.1` or hide them behind traefik+TLS unless you genuinely need them publicly open on raw TCP.** The traefik path with `entrypoints: websecure` and `tls: true` is the recommended public exposure.
7. **Rotate `HTTP_TOKEN` regularly.** Generate with `openssl rand -hex 32`, store in `.env`, never commit.
8. **Keep alpine + openssh patched.** An unauth RCE in sshd or httpd here means host root. `apk upgrade` in a rebuild cycle.
9. **Lock down siblings.** Anyone who can `docker exec` into the app via this bastion can also `docker exec` into `mysql`/`redis`/etc through the same socket. `cap_drop: [ALL]` and `no-new-privileges: true` on every sibling caps the blast radius.
10. **One bastion per role.** Don't reuse a single `FORCE_COMMAND` for both interactive shells and deploy automation — separate ports and separate token/key sets make audit trails meaningful.

## Architecture

```
start-container (entrypoint)
  ├─ generate host keys (idempotent, persisted via /etc/ssh/keys bind mount)
  ├─ merge AUTHORIZED_KEYS_HOST + AUTHORIZED_KEYS_REPO into authorized_keys
  ├─ pick mode: broker if $ALLOWED_COMMANDS / allowed-commands.list, else force-command
  │    ├─ broker:        snapshot allowlist + prefix; wrapper → bastion-broker
  │    └─ force-command: write /etc/bastion/force-command wrapper from $FORCE_COMMAND
  ├─ align docker socket group membership to host GID (if socket is mounted)
  ├─ start httpd → /var/www/cgi-bin/run (if $HTTP_BASIC_AUTH or $HTTP_TOKEN is set)
  └─ exec sshd -D -e

FORCE_COMMAND mode                      broker mode
------------------                      -----------
ssh client                              ssh client
  └ key auth as `agent`                   └ key auth as `agent`
     └ ForceCommand wrapper                  └ ForceCommand wrapper
        └ exec sh -c "$FORCE_COMMAND"           └ exec bastion-broker "$SSH_ORIGINAL_COMMAND"
                                                    ├ match vs allowlist (whole-line, anchored)
http client                                         └ exec $COMMAND_PREFIX $request  (no shell)
  └ Authorization header
     └ /cgi-bin/run validates auth       http client
        └ exec force-command wrapper        └ Authorization header
           └ exec sh -c "$FORCE_COMMAND"       └ /cgi-bin/run validates auth
                                                  └ exec bastion-broker "$X-Bastion-Command"
```

## License

MIT.

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