---
title: Container runtime · Cloudflare Sandbox SDK docs
description: Each sandbox runs in an isolated Linux container with Python,
  Node.js, and common development tools pre-installed. For a complete list of
  pre-installed software and how to customize the container image, see
  Dockerfile reference.
lastUpdated: 2025-10-21T14:02:11.000Z
chatbotDeprioritize: false
source_url:
  html: https://developers.cloudflare.com/sandbox/concepts/containers/
  md: https://developers.cloudflare.com/sandbox/concepts/containers/index.md
---

Each sandbox runs in an isolated Linux container with Python, Node.js, and common development tools pre-installed. For a complete list of pre-installed software and how to customize the container image, see [Dockerfile reference](https://developers.cloudflare.com/sandbox/configuration/dockerfile/).

## Runtime software installation

Install additional software at runtime using standard package managers:

```bash
# Python packages
pip install scikit-learn tensorflow


# Node.js packages
npm install express


# System packages (requires apt-get update first)
apt-get update && apt-get install -y redis-server
```

## Filesystem

The container provides a standard Linux filesystem. You can read and write anywhere you have permissions.

**Standard directories**:

* `/workspace` - Default working directory for user code
* `/tmp` - Temporary files
* `/home` - User home directory
* `/usr/bin`, `/usr/local/bin` - Executable binaries

**Example**:

```typescript
await sandbox.writeFile('/workspace/app.py', 'print("Hello")');
await sandbox.writeFile('/tmp/cache.json', '{}');
await sandbox.exec('ls -la /workspace');
```

## Process management

Processes run as you'd expect in a regular Linux environment.

**Foreground processes** (`exec()`):

```typescript
const result = await sandbox.exec('npm test');
// Waits for completion, returns output
```

**Background processes** (`startProcess()`):

```typescript
const process = await sandbox.startProcess('node server.js');
// Returns immediately, process runs in background
```

## Network capabilities

**Outbound connections** work:

```bash
curl https://api.example.com/data
pip install requests
npm install express
```

**Inbound connections** require port exposure:

```typescript
await sandbox.startProcess('python -m http.server 8000');
const exposed = await sandbox.exposePort(8000);
console.log(exposed.exposedAt); // Public URL
```

**Localhost** works within sandbox:

```bash
redis-server &      # Start server
redis-cli ping      # Connect locally
```

## Security

**Between sandboxes** (isolated):

* Each sandbox is a separate container
* Filesystem, memory and network are all isolated

**Within sandbox** (shared):

* All processes see the same files
* Processes can communicate with each other
* Environment variables are session-scoped

To run untrusted code, use separate sandboxes per user:

```typescript
const sandbox = getSandbox(env.Sandbox, `user-${userId}`);
```

## Limitations

**Cannot**:

* Load kernel modules or access host hardware
* Run nested containers (no Docker-in-Docker)

## Related resources

* [Architecture](https://developers.cloudflare.com/sandbox/concepts/architecture/) - How containers fit in the system
* [Security model](https://developers.cloudflare.com/sandbox/concepts/security/) - Container isolation details
* [Sandbox lifecycle](https://developers.cloudflare.com/sandbox/concepts/sandboxes/) - Container lifecycle management
