How to turn off internet access in a sandbox

Create the sandbox with network: { internet: false }, or call sbx.network.off() at any time; every outbound connection is refused.

On Runtime the rule is enforced on the host, so root inside the sandbox cannot undo it. A sandbox has no network card: everything it sends goes through a proxy outside the microVM, which applies the rule at once, to connections already open too. The same call narrows access to a list of hosts instead. A new sandbox ran its first Python command 351 ms after the request at the median on 24 September 2026 (speed), so a fresh offline machine per task is practical.

Offline from the start

TypeScriptimport { Sandbox } from "withruntime";await using sbx = await Sandbox.create({ network: { internet: false } });const run = await sbx.exec("curl -sS --max-time 5 https://example.com", { timeoutMs: 30_000 });console.log(run.exitCode !== 0); // true: the connection is refused

Pythonfrom withruntime import Sandboxwith Sandbox.create(network={"internet": False}) as sbx:    run = sbx.exec("curl -sS --max-time 5 https://example.com", timeout_ms=30_000)    print(run.exit_code != 0)  # True: the connection is refused

Terminalruntime sandbox create --no-internet

Install first, then cut it off

Most jobs need a package before they run untrusted code. Let the sandbox reach only the registry, install, and turn the internet off:

TypeScriptimport { Sandbox } from "withruntime";await using sbx = await Sandbox.create({  network: { internet: true, allow: ["pypi.org", "*.pythonhosted.org"] },});await sbx.exec("pip install requests", { check: true, timeoutMs: 120_000 });await sbx.network.off(); // same as network.set({ internet: false })console.log(await sbx.network.get());

Pythonfrom withruntime import Sandboxwith Sandbox.create(network={"internet": True, "allow": ["pypi.org", "*.pythonhosted.org"]}) as sbx:    sbx.exec("pip install requests", check=True, timeout_ms=120_000)    sbx.network.set(internet=False)    print(sbx.network.get())

sbx.network.on() gives back the account's normal access, and runtime sandbox network <id> --no-internet does the same from the CLI. An agent connected through MCP uses runtime_sandbox_network_set.

The rules

Set them at create with network, or at any time with sbx.network.set(...), which replaces the rules. They apply at once, to open connections too, and they bind root inside the sandbox (the network).

Some things are refused whatever the rules say:

• private and internal addresses, so a sandbox never reaches your network;
• mail ports 25, 465 and 587, unless support enables mail for your account;
• telnet, Windows RPC, NetBIOS and SMB, and IRC.

TCP leaves a sandbox, and a paid one also sends QUIC and NTP over UDP (outbound UDP); DNS is answered inside it.

What still works offline

• Everything inside the sandbox: commands, files, background processes and the code interpreter.
• Your control of it: exec and file calls come in through Runtime's API, so a sandbox with the internet off still runs your commands and returns their output.
• Ways in that you open: nothing on the internet can connect to a sandbox. A private preview, runtime sandbox ssh and runtime sandbox port-forward are the ways in, and each is yours to open (network access).

When the code needs one API

Turning the internet off is the strongest setting. When code must call one service, allow only that host and store its key as a secret the sandbox never sees: the sandbox holds a placeholder, and the host's proxy adds the real key only on HTTPS requests to the hosts you named.

TypeScriptimport { Runtime } from "withruntime";const runtime = new Runtime();await runtime.secrets.set("OPENAI_API_KEY", {  value: process.env.OPENAI_API_KEY ?? "",  hosts: ["api.openai.com"],});await using sbx = await runtime.sandboxes.create({  network: { internet: true, allow: ["api.openai.com"] },});

Pythonimport osfrom withruntime import Runtimeruntime = Runtime()runtime.secrets.set("OPENAI_API_KEY", value=os.environ.get("OPENAI_API_KEY", ""),                    hosts=["api.openai.com"])with runtime.sandboxes.create(network={"internet": True, "allow": ["api.openai.com"]}) as sbx:    print(sbx.network.get())

Code that leaks the key sends a worthless placeholder, and code that tries to send your data anywhere else is refused.

Where this matters

• Running untrusted LLM code: model output runs with no route out.
• Grading student code: submissions cannot fetch answers or phone home.
• Egress control explains allow and deny lists in general.

Each sandbox also has limits on concurrent connections, bandwidth and bytes a day, so one sandbox cannot crowd out others.

Start

Terminalnpx withruntime sandbox run --trial --no-internet -- python3 -c 'print(6 * 7)'

New accounts get 50 free sandbox hours, no card. The first run prints a link to approve in your browser.

Facts on this page were checked on 25 September 2026.