Docker Security Isolation

MCPProxy provides Docker isolation for stdio MCP servers to enhance security by running each server in its own isolated container.

New installs: Docker isolation is turned on automatically when mcpproxy creates its initial mcp_config.json and a Docker daemon is reachable (docker info responds within 2 seconds). If Docker isn't available at first run, isolation stays off so stdio servers still work — you can enable it later from the Security page in the Web UI or by editing the config below.

Existing installs: Your current docker_isolation.enabled value is preserved on upgrade. To turn isolation on manually, set the top-level flag in ~/.mcpproxy/mcp_config.json (or use the Web UI toggle):

jsonCopy

{
  "docker_isolation": {
    "enabled": true
  }
}

Existing connections will re-wrap themselves in containers after the next server restart; new connections pick up isolation immediately.

Overview

Docker isolation automatically wraps stdio-based MCP servers in Docker containers, providing:

• Process Isolation: Each server runs in a separate container
• File System Isolation: Servers cannot access host file system
• Network Isolation: Configurable network modes for security
• Resource Limits: Memory and CPU limits prevent resource exhaustion
• Automatic Runtime Detection: Maps commands to appropriate Docker images

Configuration

Global Docker Isolation

Add to your ~/.mcpproxy/mcp_config.json:

{
  "docker_isolation": {
    "enabled": true,
    "memory_limit": "512m",
    "cpu_limit": "1.0",
    "timeout": "60s",
    "network_mode": "bridge",
    "registry": "docker.io",
    "default_images": {
      "python": "python:3.11",
      "python3": "python:3.11",
      "uvx": "python:3.11",
      "pip": "python:3.11",
      "pipx": "python:3.11",
      "node": "node:20",
      "npm": "node:20",
      "npx": "node:20",
      "yarn": "node:20",
      "go": "golang:1.21-alpine",
      "cargo": "rust:1.75-slim",
      "rustc": "rust:1.75-slim",
      "ruby": "ruby:3.2-alpine",
      "gem": "ruby:3.2-alpine",
      "php": "php:8.2-cli-alpine",
      "composer": "php:8.2-cli-alpine",
      "binary": "alpine:3.18",
      "sh": "alpine:3.18",
      "bash": "alpine:3.18"
    },
    "extra_args": []
  }
}

Configuration Options

Per-Server Configuration

You can override isolation settings per server:

{
  "mcpServers": [
    {
      "name": "custom-python-server",
      "command": "python",
      "args": ["-m", "my_server"],
      "isolation": {
        "enabled": true,
        "image": "my-custom-python:latest",
        "network_mode": "none",
        "working_dir": "/app",
        "extra_args": ["--cap-drop=ALL"]
      },
      "enabled": true
    },
    {
      "name": "no-isolation-server",
      "command": "python",
      "args": ["-m", "trusted_server"],
      "isolation": {
        "enabled": false
      },
      "enabled": true
    }
  ]
}

Gotcha: global flag gates per-server opt-ins

Per-server isolation.enabled: true only takes effect when the global docker_isolation.enabled flag is also true. If the global flag is false, MCPProxy runs the server on the host even if you explicitly opted it into isolation in its per-server config.

Starting in this release, MCPProxy emits a one-time warning in the main log when it detects this configuration (look for per-server docker isolation opt-in ignored in ~/.mcpproxy/logs/main.log). To actually isolate those servers, flip the global flag on.

Telemetry

When anonymous telemetry is enabled, MCPProxy reports two Docker-related counters at daily cadence:

• server_docker_available_bool — whether the host has a reachable Docker daemon. Probed with docker info --format {{.ServerVersion}} and cached for up to 15 minutes (5 minutes when the previous probe failed, so a late Docker-Desktop launch is picked up promptly).
• server_docker_isolated_count — how many of your configured stdio servers are configured for isolation, i.e. servers for which ShouldIsolate() returns true. This is a configuration metric, not a count of running containers; it goes to zero whenever the global flag is off regardless of per-server opt-ins.

Runtime Detection

MCPProxy automatically detects the runtime type based on the command:

Python Environments

• python, python3 → python:3.11
• uvx → python:3.11 (includes uv package manager)
• pip, pipx → python:3.11

Node.js Environments

• node → node:20
• npm, npx → node:20
• yarn → node:20

Other Languages

• go → golang:1.21-alpine
• cargo, rustc → rust:1.75-slim
• ruby, gem → ruby:3.2-alpine
• php, composer → php:8.2-cli-alpine

Shell/Binary

• sh, bash → alpine:3.18
• Unknown commands → alpine:3.18

Why Full Images Instead of Slim/Alpine?

MCPProxy uses full Docker images (python:3.11 instead of python:3.11-slim) because:

1. Git Support: Many MCP servers install packages from Git repositories using git+https:// URLs
2. Build Tools: Some packages require compilation during installation
3. System Dependencies: Full images include common libraries needed by MCP servers

This trade-off prioritizes compatibility over image size.

Environment Variables

Environment variables from server configuration are automatically passed to containers:

{
  "mcpServers": [
    {
      "name": "api-server",
      "command": "uvx",
      "args": ["some-package"],
      "env": {
        "API_KEY": "your-secret-key",
        "DEBUG": "true"
      },
      "enabled": true
    }
  ]
}

These become Docker arguments: -e API_KEY=your-secret-key -e DEBUG=true

Docker-in-Docker Prevention

MCPProxy automatically skips isolation for servers that are already Docker commands:

{
  "mcpServers": [
    {
      "name": "existing-docker-server",
      "command": "docker",
      "args": ["run", "-i", "--rm", "mcp/some-server"],
      "enabled": true
      // Isolation automatically skipped
    }
  ]
}

This prevents Docker-in-Docker complications.

Debugging

Check Docker Isolation Status

# Run with debug logging
mcpproxy serve --log-level=debug --tray=false

# Filter for isolation messages
mcpproxy serve --log-level=debug 2>&1 | grep -i "docker isolation"

Monitor Docker Containers

# List MCPProxy containers
docker ps --format "table {{.Names}}\t{{.Image}}\t{{.Status}}"

# View logs from a specific container
docker logs <container-id>

# Watch container resource usage
docker stats

Common Issues

Container startup timeouts:

• Increase timeout in docker_isolation config
• Check if Docker images need to be pulled
• Verify network connectivity for package installations

Environment variables not working:

• Check that variables are defined in server env section
• Use debug logging to see Docker command arguments
• Verify container has access to required environment

Git/package installation failures:

• Ensure using full images (python:3.11 not python:3.11-slim)
• Check container logs for specific error messages
• Verify network access for package repositories

Security Considerations

Docker isolation provides strong security boundaries but consider:

1. Network Access: Containers can still access the network by default
2. Resource Limits: Set appropriate memory/CPU limits
3. Image Trust: Use trusted base images from official repositories
4. Secrets: Environment variables are visible in container inspect output

For maximum security, consider:

• Using "network_mode": "none" for servers that don't need network access
• Adding --cap-drop=ALL to extra_args to remove Linux capabilities
• Using custom minimal images for specific use cases