alpine-server-setup/services/basic/global.policy.json

{
    "description": "Restrict all internet access",
    "variable": { "internet_if": "eth0" },
    "zone": {
        "internet": { "iface": "$internet_if" }
    },
    "policy": [
        { "in": "internet", "action": "drop" },
        { "action": "reject" }
    ]
}