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docker-bastion/README.md

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[![Blax Software OSS](https://raw.githubusercontent.com/blax-software/laravel-workkit/master/art/oss-initiative-banner.svg)](https://github.com/blax-software)
# 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 one preconfigured command per authenticated session. Authenticate by SSH key or HTTP bearer token, the container runs whatever you point `FORCE_COMMAND` at — `docker exec` into a sibling container, `./deploy.sh`, `nginx -s reload`, anything — and 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.
## 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)* | The command run on every authenticated session. Shell metacharacters OK. |
| `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).
- **busybox httpd** (busybox-extras) — minimal HTTP listener for the URL path; CGI-driven; only starts when `HTTP_TOKEN` 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`. `SSH_ORIGINAL_COMMAND` is dropped. 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
├─ 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 $HTTP_TOKEN is set)
└─ exec sshd -D -e
ssh client
└─ key auth as `agent`
└─ ForceCommand /etc/bastion/force-command
└─ exec sh -c "$FORCE_COMMAND"
http client
└─ Authorization: Bearer <HTTP_TOKEN>
└─ /var/www/cgi-bin/run validates token
└─ exec /etc/bastion/force-command
└─ exec sh -c "$FORCE_COMMAND"
```
## License
MIT.
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