maf-sandbox-docker

Experimental. This package is early-stage (pre-1.0, Development Status :: 4 - Beta) — its API may change or be removed in a future release without notice. Importing it emits a one-time MafSandboxDockerExperimentalWarning; suppress it with warnings.filterwarnings("ignore", category=maf_sandbox_docker.MafSandboxDockerExperimentalWarning) once you've read the notice.

This package is not affiliated with, endorsed by, or a product of Docker Inc. or Microsoft — it is a third-party sandbox backend for Microsoft Agent Framework.

app  ->  maf_sandbox  ->  maf_sandbox_docker  ->  the container

The sandbox backend for everyone wslc leaves out: plain Docker containers, driven through the docker command-line client, on any machine with a Docker-compatible engine — macOS, Linux, Windows with WSL 2, and every GitHub Actions ubuntu-latest runner. No subscription, no login, and no dependency but maf-sandbox itself. A workload written against the protocol runs here unchanged, which is what makes it a workload rather than an integration.

Quickstart

pip install maf-sandbox-docker

from maf_sandbox import Isolation, SandboxRouter
from maf_sandbox_docker import DockerSandboxBackend, DockerSandboxConfig

router = SandboxRouter([DockerSandboxBackend(DockerSandboxConfig())], min_isolation=Isolation.CONTAINER)

samples/06_docker_codeact runs those two lines end to end: an agent that executes model-written Python in a container and reads the result back out. Its siblings 03_acas_codeact and 04_wslc_codeact are the same program on a microVM-isolated Azure backend and on wslc, and the diff between any two of them is two imports and one constructor.

Requirements

A Docker-compatible engine, reachable through the docker client. Docker Desktop (macOS, Linux, Windows with WSL 2) and Docker Engine (Linux, rootful or rootless) are what this backend supports. The client's own configuration — DOCKER_HOST, the active context, TLS settings — is inherited, because every call is a subprocess that inherits this process's environment; point DockerSandboxConfig.docker_path at a different client binary to use another one. Colima, OrbStack, Rancher Desktop and Podman expose Docker-compatible sockets and may work through the same client (Podman's default outbound network is called podman, so set outbound_network="podman" in allowlist mode), but they are not officially supported and nothing here is verified against them.

Docker Engine 28.0.0 or newer, to serve Egress.ALLOWLIST with hosts on the list. Such a sandbox is attached to an internal network whose bridge holds no host address. That is the bridge driver's gateway_mode_ipv4=isolated and its IPv6 twin gateway_mode_ipv6=isolated, both of which arrived in that engine. Both are set, because a daemon with IPv6 enabled would otherwise keep the v6 half addressed, and the route back with it. An older daemon rejects the options, and the acquire fails saying so rather than serving the workload on an addressed bridge — a bridge address is a route to the host that the allowlist does not cover. The floor is only for that case, because it is only there that a network is built at all: Egress.CLOSED, and an ALLOWLIST spec whose egress_allow is empty, both run on --network none and on any supported engine.

Every call spawns the docker client, so the host's event loop has to be one that can start subprocesses — asyncio's default Proactor loop on Windows does, and a host that installs WindowsSelectorEventLoopPolicy has to undo that first, or every acquire fails with a message saying so.

Hosts this backend does not serve: Windows without WSL (Docker Desktop's Hyper-V backend is documented by Docker but not its default, needs Pro or Enterprise, and is not verified here; Windows Home has no route at all), GitHub Actions' windows-latest (Windows containers only) and macos-latest (no Docker, no nested virtualization). For WSL-less Windows the eventual answer is a separate backend over Docker's "Docker Sandboxes" micro-VM product.

What this backend declares

Isolation.CONTAINER. A container shares the host kernel, below SandboxRouter's default min_isolation=Isolation.MICROVM floor — construct the router with min_isolation=Isolation.CONTAINER and it admits this backend; leave the floor at its default and construction raises SandboxBackendNotPermitted. A Docker Desktop or Colima VM does not lift the rung: one shared VM kernel serves every container, the same shape wslc's WSL 2 utility VM has, and the ladder classifies that at container. The declaration is a constant — no configuration raises it, because a security level the backend cannot verify must not become one the router repeats.

declarations.egress_modes = {closed} by default, {allowlist, closed} with a proxy configured. With no proxy configured every container is created --network none: a network namespace with only loopback, enforced by whichever kernel runs the container. That serves a workload declaring Egress.CLOSED, and refuses one declaring Egress.ALLOWLIST — the router never substitutes a mode, so denying everything is no longer offered as a stricter stand-in for a host list. Never UNRESTRICTED: a container backend always cuts or proxies, so it cannot serve a workload that asked to run open.

Set egress_proxy_image and ALLOWLIST joins the set: each sandbox gets its own internal network and a dual-homed filtering proxy, and the spec's allowlist is enforced by topology — the container has no route out except the proxy, which opens a CONNECT tunnel only to the hosts the spec names. The HTTP_PROXY/HTTPS_PROXY variables set on the workload are how ordinary clients find the proxy, not what enforces the allowlist; the topology is. TLS is not decrypted, and the sandbox never resolves an external name itself. The proxy is shipped as source, not as an image you must trust: build it from the packaged recipe, whose only pinned dependency is its Azure Linux base.

from maf_sandbox_docker import proxy_build_context, DockerSandboxConfig

print(f"docker build -t maf-egress-proxy:local {proxy_build_context()}")  # run this once
config = DockerSandboxConfig(egress_proxy_image="maf-egress-proxy:local")

Capability.FILES_OUT, never Capability.FILES_LIST. This backend reads declared outputs back out — docker cp <container>:<path> - streams a tar whose first 512-byte header carries the size, the entry type and any link target, so a file is statted and read from one stream with no stat command and no shell in the image. It does not enumerate directories: Docker has no engine-level primitive for it, which is exactly why the protocol splits enumeration into FILES_LIST. A kind that cannot name its outputs in advance requires that capability and is refused here — served instead by a backend, like ACAS, that has native listing.

Every path component is checked, not just the last one. A symlink is refused on the tar entry's type bit only when it is the entry being tarred; the engine resolves the path daemon-side, so a guest that points out at /etc gets a stat of out/hostname describing a regular file with the parent link nowhere in it. stat_file and read_file therefore stat every parent component from the filesystem root down — not from the working directory, whose own ancestors the guest can replace just as easily: with /maf-sandbox -> / unchecked, /maf-sandbox/work stats as a real directory and serves /. The check itself is maf_sandbox.paths.refuse_symlinked_ancestors, not a copy living here: this backend passes it the unconfined tar-header stat above. A link is refused as a confinement failure and any other non-directory as an ordinary ENOTDIR — the entry comes back as EntryKind.SYMLINK or EntryKind.OTHER, so a caller can tell an escape from a guest tripping over its own fifo. One residual stays open: the check and the read are separate calls and docker cp has no no-follow form, so a guest that swaps a stat-ed component for a link in between is followed.

Whether that is actually enforced is not this package's own claim either. maf_sandbox.conformance is the shared suite every backend serving FILES_OUT answers, and this is the one backend that answers it against a real engine on every pull request — a container on the runner, a hostile layout planted in it through the public surface, and the probes attacking that.

declarations.os_families = {POSIX} on a Linux daemon, and nothing on any other — asked for with create. A workload states the guest shape its commands are written for in SandboxSpec.requires_os_family, and the router refuses a backend whose os_families does not hold it. This backend reads the answer from its own daemon (docker version --format '{{.Server.Os}}') rather than taking it as configuration: a host would be restating what the engine already knows, and a value it typed could only go stale against the engine that has to back it. The read needs an await, so it lives in a factory — __init__ makes no engine calls, and a blocking read in a constructor would do subprocess I/O on your event loop against a daemon that can hang rather than refuse.

from maf_sandbox_docker import DockerSandboxBackend, DockerSandboxConfig


async def wired() -> DockerSandboxBackend:
    # Asks the daemon once, and declares what it answered.
    return await DockerSandboxBackend.create(DockerSandboxConfig())


# Unchanged, and declares no family — which refuses only a spec that names one.
backend = DockerSandboxBackend(DockerSandboxConfig())

A windows daemon declares nothing, not WINDOWS. Everything this backend runs in a guest is POSIX: sh -c for a string command, rm -rf for a removal, and /-rooted path arithmetic that refuses a backslash outright. Declaring WINDOWS would be a promise no code path here backs, and would move the failure from the guest's first command to the router's certificate. A daemon that will not answer declares nothing for the same reason: silence refuses a spec that asks for a family and serves every spec that does not, which is what this backend did before it asked at all.

The declaration is a snapshot, so a cold acquire re-asks. The client resolves DOCKER_HOST and the active context on every call, so switching Docker Desktop to Windows containers moves the engine under a running backend. The router matched the old answer when your tool was attached and cannot ask again — so an acquire that is about to create or restart a container reads the daemon once more, ahead of the container and its network, and raises SandboxOsFamilyNotSupported if the answer changed. A restart counts because one that fails falls through to a create, and one that succeeds hands out a container from whichever daemon is answering now. Reusing an already-running container does not re-ask — that would cost a round trip on every tool call — and a backend built by the plain constructor never asks at all.

The backend

DockerSandboxBackend implements maf_sandbox.SandboxBackend:

Container names are derived from the key and kind rather than remembered, so acquire and dispose agree on one without a registry to keep in sync. Labels are the durable record dispose_scope selects on, and their values are digested when they are long or carry a separator — the same mapping on both sides, because transforming one and not the other makes a purge quietly select nothing.

No bind mounts, no host paths, and never the Docker socket cross into a sandbox — files go in and out only through docker cp. The hardening flags --security-opt no-new-privileges and --pids-limit go on every container; --cap-drop ALL, --memory and --cpus are opt-in through the config.

stop is never used. A container whose init process ignores SIGTERM takes ten seconds to stop and a fraction of a second to remove, and there is nothing in a sandbox worth waiting for.

Upgrading to 0.10

The four optional declarations moved into one BackendDeclarations. maf-sandbox 0.26 replaced capabilities, limits, egress_modes and os_families as backend attributes with one declarations object holding them as fields, and this backend follows it. A host that read them off the backend gets an AttributeError:

Nothing about what this backend declares changed — the values, and how they are derived from the config, are exactly as they were. maf-sandbox's own README carries the reasoning and what a backend author has to do.

Upgrading to 0.7

0.7.0 requires maf-sandbox 0.19, which made the egress mode a thing a workload declares and a set a backend enforces.

egress is replaced by egress_modes: frozenset[Egress]. A host that read backend.egress gets an AttributeError; read backend.egress_modes instead. Nothing in the wiring changes — the set is still derived from egress_proxy_image exactly as the single value was.

Without a proxy image this backend now refuses an allowlist workload rather than confining it further. That is the upgrade's one behavioural break, and it is most likely to reach you through a kind whose default asks for one — maf-sandbox-bicep 0.9 does:

SandboxEgressNotEnforced: sandbox backend 'docker' cannot enforce the 'allowlist'
egress the 'bicep' workload runs in (it enforces closed).

Configure egress_proxy_image if the workload is meant to reach the hosts it names, or ask the kind for Egress.CLOSED if it is meant to run offline. The old behaviour — serve it anyway, warn, and let the workload fail at the first fetch — is gone deliberately: a fetch failure deep in a tool call is a worse report than a refusal at attach.

Maintained by SOKOLAI BV.