Datadog Docker socket filter

Filtering proxy for a read-only access to the Docker socket, based on HAProxy.

Quick start

See sample docker-compose file.

Why?

The Docker engine does not currently offer any monitoring interface (to list and inspect containers / images / volumes) other than the management socket. This API is pretty stable and allows our agents to get reliable data, but it comes with a severe security risk: all commands sent to it are executed with root privileges. An attacker could use that as a privilege escalation mechanism.

The authorization subsystem tries to solve this issue by enabling a rule-based authorization workflow, but it has several shortcomings:

• it requires SSL client certificate authentication, which is currently not supported on the unix socket, potentially breaking orchestrators and third party software assuming access to /var/run/docker.sock
• it is not self-contained in the Docker engine, but requires the sysadmin to install and configure a third-party software
• if no certificate chain is already setup, creating one for that use case is a big hurdle

This is why this container aims at providing a simpler solution.

How?

The Docker management API is a standard HTTP REST API, which can be filtered through a filtering HTTP proxy. This container uses HAProxy to provide a read-only access to the API, via a socket created in the shared volume mounted in /safe-docker. Our provided configuration:

• only allows GET/HEAD requests, as all requests that modify the state of an object are either POST, PUT or DELETE
• passes URLs through a whitelist to forbid access to endpoints that might be exploited to escalade access (attach via a websocket) or enable DoS attacks (disk/network intensive read-only operations)

The remaining endpoints are deemed safe for use and are accessible on the safe socket. This socket can then be exposed to monitoring software.

Troubleshooting

Inside the docker-filter container you should be able to access haproxy.sock, see https://www.datadoghq.com/blog/how-to-collect-haproxy-metrics/#socket-communication to know more about how to use it to troubleshoot haproxy.