Air-gapped containers

Air-gapped containers let you restrict container network access by controlling where containers can send and receive data. This feature applies custom proxy rules to container network traffic, helping secure environments where containers shouldn't have unrestricted internet access.

Docker Desktop can configure container network traffic to accept connections, reject connections, or tunnel through HTTP or SOCKS proxies. You control which TCP ports the policy applies to and whether to use a single proxy or per-destination policies via Proxy Auto-Configuration (PAC) files.

Who should use air-gapped containers?

Use air-gapped containers if:

• Your organization requires containers to communicate only with approved internal services
• You need to meet compliance standards that mandate network isolation (such as SOC 2, ISO 27001, or PCI DSS)
• You want to prevent containers from leaking data or reaching unapproved external endpoints during builds or at runtime

How air-gapped containers work

Air-gapped containers operate by intercepting container network traffic and applying proxy rules:

1. Traffic interception: Docker Desktop intercepts all outgoing network connections from containers
2. Port filtering: Only traffic on specified ports (transparentPorts) is subject to proxy rules
3. Rule evaluation: PAC file rules or static proxy settings determine how to handle each connection
4. Connection handling: Traffic is allowed directly, routed through a proxy, or blocked based on the rules

Some important considerations include:

• The existing proxy setting continues to apply to Docker Desktop application traffic on the host
• If PAC file download fails, containers block requests to target URLs
• Hostname is available for ports 80 and 443, but only IP addresses for other ports

Prerequisites

Before configuring air-gapped containers, you must have:

• Enforce sign-in enabled to ensure users authenticate with your organization
• A Docker Business subscription
• Configured Settings Management with the admin-settings.json file to manage organization policies

Configure air-gapped containers

Add the container proxy to your admin-settings.json file. For example:

{
  "configurationFileVersion": 2,
  "containersProxy": {
    "locked": true,
    "mode": "manual",
    "http": "",
    "https": "",
    "exclude": [],
    "pac": "http://192.168.1.16:62039/proxy.pac",
    "transparentPorts": "*"
  }
}

Configuration parameters

The containersProxy setting controls network policies applied to container traffic:

Configuration examples

Block all external access:

"containersProxy": {
  "locked": true,
  "mode": "manual",
  "http": "",
  "https": "",
  "exclude": [],
  "transparentPorts": "*"
}

Allow specific internal services:

"containersProxy": {
  "locked": true,
  "mode": "manual",
  "http": "",
  "https": "",
  "exclude": ["internal.company.com", "10.0.0.0/8"],
  "transparentPorts": "80,443"
}

Route through corporate proxy:

"containersProxy": {
  "locked": true,
  "mode": "manual",
  "http": "http://corporate-proxy.company.com:8080",
  "https": "http://corporate-proxy.company.com:8080",
  "exclude": ["localhost", "*.company.local"],
  "transparentPorts": "*"
}

Proxy Auto-Configuration (PAC) files

PAC files provide fine-grained control over container network access by defining rules for different destinations.

Basic PAC file structure

function FindProxyForURL(url, host) {
	if (localHostOrDomainIs(host, 'internal.corp')) {
		return "PROXY 10.0.0.1:3128";
	}
	if (isInNet(host, "192.168.0.0", "255.255.255.0")) {
	    return "DIRECT";
	}
    return "PROXY reject.docker.internal:1234";
}

General considerations

• FindProxyForURL function URL parameter format is http://host_or_ip:port or https://host_or_ip:port

• If you have an internal container trying to access https://docs.docker.com/enterprise/security/hardened-desktop/air-gapped-containers the Docker proxy service will submit docs.docker.com for the host value and https://docs.docker.com:443 for the url value to FindProxyForURL, if you are using shExpMatch function in your PAC file as follows:

  if(shExpMatch(url, "https://docs.docker.com:443/enterprise/security/*")) return "DIRECT";
  

  shExpMatch function will fail, instead use:

  if (host == docs.docker.com && url.indexOf(":443") > 0) return "DIRECT";
  

PAC file return values

Advanced PAC file example

function FindProxyForURL(url, host) {
  // Allow access to Docker Hub for approved base images
  if (dnsDomainIs(host, ".docker.io") || host === "docker.io") {
    return "PROXY corporate-proxy.company.com:8080";
  }

  // Allow internal package repositories
  if (localHostOrDomainIs(host, 'nexus.company.com') ||
      localHostOrDomainIs(host, 'artifactory.company.com')) {
    return "DIRECT";
  }

  // Allow development tools on specific ports
  if (url.indexOf(":3000") > 0 || url.indexOf(":8080") > 0) {
    if (isInNet(host, "10.0.0.0", "255.0.0.0")) {
      return "DIRECT";
    }
  }

  // Block access to developer's localhost
  if (host === "host.docker.internal" || host === "localhost") {
    return "PROXY reject.docker.internal:1234";
  }

  // Block all other external access
  return "PROXY reject.docker.internal:1234";
}

Verify air-gapped container configuration

After applying the configuration, test that container network restrictions work:

Test blocked access:

$ docker run --rm alpine wget -O- https://www.google.com
# Should fail or timeout based on your proxy rules

Test allowed access:

$ docker run --rm alpine wget -O- https://internal.company.com
# Should succeed if internal.company.com is in your exclude list or PAC rules

Test proxy routing:

$ docker run --rm alpine wget -O- https://docker.io
# Should succeed if routed through approved proxy

Security considerations

• Network policy enforcement: Air-gapped containers work at the Docker Desktop level. Advanced users might bypass restrictions through various means, so consider additional network-level controls for high-security environments.
• Development workflow impact: Overly restrictive policies can break legitimate development workflows. Test thoroughly and provide clear exceptions for necessary services.
• PAC file management: Host PAC files on reliable internal infrastructure. Failed PAC downloads result in blocked container network access.
• Performance considerations: Complex PAC files with many rules may impact container network performance. Keep rules simple and efficient.

Next steps

• Explore Enhanced Container Isolation to further restrict what containers can do at runtime
• Understand how Docker Desktop handles host and container networking