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transctrl

A secure service that manages Transmission Docker containers declaratively via gRPC over Unix sockets.

Quick Start

Pull the pre-built image and run:

docker run -d \
  --name transctrl \
  -v /var/run/docker.sock:/var/run/docker.sock \
  -v transctrl-socket:/var/run/transctrl \
  -v /mnt:/mnt:ro \
  -e ALLOWED_MOUNT_BASE=/mnt \
  ghcr.io/redsudo/transctrl:latest

Your core service can then connect via the Unix socket at /var/run/transctrl/transctrl.sock.

For a complete setup, see examples/docker-compose.yml.

Configuration

Env Var Default Description
SOCKET_PATH /var/run/transctrl/transctrl.sock Path to Unix socket
DOCKER_HOST unix:///var/run/docker.sock Docker daemon address
ALLOWED_MOUNT_BASE /mnt Only allow mounts under this path
RATE_LIMIT_REQUESTS 10 Max reconciles per window
RATE_LIMIT_WINDOW 60 Rate limit window in seconds

API Example

from client.transctrl_client import TransmissionControllerClient

client = TransmissionControllerClient('/var/run/transctrl/transctrl.sock')

# Reconcile desired state
result = client.reconcile([
    {
        'id': 'user-1',
        'config_path': '/mnt/configs/user-1',
        'data_path': '/mnt/data/user-1',
        'watch_path': '/mnt/watch/user-1',
        'web_port': 9091,
        'data_port': 51413
    }
])

# Get current status
status = client.get_status()

Deployment

With Docker Socket Proxy (Recommended)

Security Note: Mounting the Docker socket directly gives transctrl full control over the Docker daemon. For production deployments, use a Docker socket proxy to restrict API access to only the operations transctrl needs (container create/delete). This limits the blast radius if transctrl is compromised.

See examples/docker-compose.proxy.yml for a complete example.

services:
  docker-socket-proxy:
    image: tecnativa/docker-socket-proxy:latest
    volumes:
      - /var/run/docker.sock:/var/run/docker.sock:ro
    environment:
      CONTAINERS: 1
      POST: 1
      DELETE: 1

  transctrl:
    image: ghcr.io/redsudo/transctrl:latest
    environment:
      DOCKER_HOST: tcp://docker-socket-proxy:2375
      ALLOWED_MOUNT_BASE: /mnt
    volumes:
      - transctrl-socket:/var/run/transctrl
      - /mnt:/mnt:ro
    # No docker.sock mount needed

volumes:
  transctrl-socket:

Docker Compose (without Docker Socket Proxy)

services:
  transctrl:
    image: ghcr.io/redsudo/transctrl:latest
    volumes:
      - /var/run/docker.sock:/var/run/docker.sock
      - transctrl-socket:/var/run/transctrl
      - /mnt:/mnt:ro
    environment:
      ALLOWED_MOUNT_BASE: /mnt

  core:
    image: your-core-service
    volumes:
      - transctrl-socket:/var/run/transctrl:ro
    environment:
      TRANSCTRL_SOCKET: /var/run/transctrl/transctrl.sock

volumes:
  transctrl-socket:

Development

Prerequisites

  • uv
  • Docker
  • Make

Setup

# Install dependencies
uv sync

# Generate gRPC code
make proto

Running Tests

# Unit tests
make test

# Integration tests (runs in Docker-in-Docker)
make test-integration

Building Docker Image

make build-image

Security Design

  • Stateless: The system relies on Docker labels (transctrl.managed=true) as the source of truth.
  • Isolation: Minimal capabilities (CHOWN, SETGID, SETUID) and no-new-privileges for containers.
  • Socket Communication: Uses gRPC over Unix sockets for local, secure inter-process communication.

Path Restriction (ALLOWED_MOUNT_BASE)

transctrl validates that all paths (config_path, data_path, watch_path) in reconcile requests start with ALLOWED_MOUNT_BASE. This prevents a compromised core service from creating Transmission containers with arbitrary host mounts like /etc or /root/.ssh.

Why mount /mnt:/mnt:ro?

transctrl needs to verify that requested paths actually exist before creating containers (os.path.exists()). Without this mount, transctrl can't see host paths from inside its container. The :ro (read-only) mount is sufficient—transctrl only needs to check existence, not write to these paths. The actual read-write mounts are configured via the Docker API when transctrl creates Transmission containers.

Example: If ALLOWED_MOUNT_BASE=/mnt, a request for config_path: /etc/passwd will be rejected, but config_path: /mnt/user1/config will be allowed (if the path exists).