docker-bastion

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.

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:

# 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.

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:

# 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.

Star History