#Networking and Compose

A3S Box treats network behavior as an admitted runtime capability. A request that the selected host or isolation backend cannot enforce is rejected instead of being silently weakened.

#Publish a MicroVM port

Map host TCP port 8080 to guest port 80:

a3s-box run -d \
  --name web \
  -p 8080:80 \
  nginx:alpine

a3s-box port web

Published ports are MicroVM features within the limits of the current host backend. Remove the box when it is no longer needed:

a3s-box stop web
a3s-box rm web

#Create a named network

Named bridge networks provide stable peer connectivity for MicroVM workloads:

a3s-box network create backend --subnet 10.89.20.0/24

a3s-box run -d \
  --name api \
  --network backend \
  local/api:dev

a3s-box network inspect backend

Network membership is part of the persisted box configuration. The network connect and network disconnect commands operate on inactive boxes so the next start has a deterministic topology.

#Access a shared-kernel Sandbox

The explicit Linux Sandbox backend does not use MicroVM bridge publication. Forward one host-loopback TCP port to a running Sandbox instead:

a3s-box run -d \
  --name sandbox-api \
  --isolation sandbox \
  local/api:dev

a3s-box port-forward \
  sandbox-api \
  --host-port 18080 \
  --guest-port 8080

The forwarding worker is tied to the current Sandbox generation. A restart or replacement cannot inherit a stale forwarding session.

#Define a project with ACL

ACL is the canonical A3S Box project format:

service "db" {
  image = "postgres:17"
  environment = {
    POSTGRES_PASSWORD = env("POSTGRES_PASSWORD")
  }
  volumes = ["database:/var/lib/postgresql/data"]
  networks = {
    backend = {}
  }
}

service "api" {
  image = "local/api:dev"
  command = ["serve", "--port", "8080"]
  ports = ["8080:8080/tcp"]
  depends_on = {
    db = { condition = "service_healthy" }
  }
  networks = {
    backend = { aliases = ["api.internal"] }
  }
}

volume "database" {
  driver = "local"
}

network "backend" {
  driver = "bridge"
}

Normalize the project before changing runtime state:

a3s-box compose -f compose.acl config
a3s-box compose -f compose.acl up -d
a3s-box compose -f compose.acl down

Docker Compose YAML is accepted as a bounded compatibility input. ACL remains the source of truth for new A3S Box configuration because it has one deterministic parser and canonical form.

#Platform limits

• Linux/KVM provides the broadest named-network and publication surface.
• The shared-kernel Sandbox uses explicit loopback forwarding, not static bridge publication.
• Native WHPX bridge networking and Compose service networking are not currently supported on Windows.
• strict and custom network admission modes are not currently supported.

See the platform matrix before making network behavior part of a deployment contract.

#Next step

Confirm the network boundary on every target host in the platform matrix. Windows users should continue with Windows WHPX; install the Agent Skill when a coding agent should run these workflows.