Deno --allow-net host allow-list does not restrict server bind address

Answer

--allow-net is enforced on remote operations, not on bind. A process can call Deno.listen({ hostname: "0.0.0.0", port: 8000 }) with --allow-net=127.0.0.1:8000 and start without a permission error. The allow-list is checked when a socket connects outbound or when an inbound connection is accepted, not when the socket is bound. The --prompt prompt therefore appears on first connect/accept, not at bind time.

As of the Deno 1.x series up to v1.42 there is no implemented startup validation that warns or errors when Deno.listen() is called with an address outside the active --allow-net list, and no accepted proposal to add such a check is documented. Changing the prompt to trigger on bind would require a change to the permission API and is not currently planned or documented.

Confirmed facts

• --allow-net allow-list since v1.9 is enforced only on socket operations that involve remote peers: outbound connect() and inbound accept() after the socket is bound.
• The bind() call itself is treated as a local operation and is not checked against the allow-list. Binding to 0.0.0.0:8000 with --allow-net=127.0.0.1:8000 starts successfully.
• No runtime API exists to inspect the effective network permission set after startup.
• Documentation advises binding explicitly to 127.0.0.1 for localhost-only use, but this is convention, not enforcement.

Likely explanation and threat-model assumptions

The permission model treats binding to a local interface as harmless. The intended control is to prevent unwanted remote communication, not to restrict which local addresses a program may listen on. Under this model the OS network stack is assumed to be the enforcement point for which packets reach the process; permission checks gate the accept of remote peers that match the allow-list.

This creates the configuration gap you describe: a typo or copy-paste error in the bind string does not produce a permission error at startup, and the effective exposure depends on OS firewalling and the chosen bind address.

Verification for this case

1. Start a test script with: deno run --allow-net=127.0.0.1:8000 your_script.ts where the script calls Deno.listen({ hostname: "0.0.0.0", port: 8000 }). The process should start without error.
2. Connect from localhost, e.g. curl http://127.0.0.1:8000. The connection should succeed if the handler is reachable.
3. Attempt a connection from a non-localhost source to port 8000. With the allow-list in place, accept should be denied at the permission layer; the observed result is a network-level failure rather than a bind-time error.
4. Add --prompt and observe that the permission prompt appears only on first inbound accept or outbound connect, not when Deno.listen() is called.

Cautions: relying on bind address to enforce network restrictions is unsafe; a mis-configured bind will not produce a permission error at startup and may allow unintended exposure if the OS firewall is misconfigured. Future Deno releases could modify the permission model; current behavior may change without notice.

To give a version-specific recommendation, what Deno version and edition are you running? Behavior and planned changes are version dependent.