# `Expresso.Watch.Host`
[🔗](https://github.com/rellen/expresso/blob/main/lib/expresso/watch/host.ex#L1)

Refuses a request for a host other than 127.0.0.1 or localhost

The watch server listens on 127.0.0.1 only, so no other computer can
connect. But a web page that the user opens can try DNS rebinding:

1. The page's host name points to the page's own server.
2. The host name then changes to point to 127.0.0.1.
3. The browser now sends the page's requests to the watch server, and the
   page can read the deck.

Each such request still has the page's host name in its `Host` header. This
module passes a request only when that header is `127.0.0.1` or
`localhost`, with any port. The server answers each other request, and a
request with no `Host` header, with the status 403.

`Expresso.Watch.Server` puts this module first in the `:httpd` module list,
and `:httpd` calls `do/1` for each request.

# `allowed?`

```elixir
@spec allowed?(String.t()) :: boolean()
```

Tell whether the value of a `Host` header names this computer

The port is not part of the check, because a tunnel can use a different
port.

    iex> Expresso.Watch.Host.allowed?("127.0.0.1:4100")
    true

    iex> Expresso.Watch.Host.allowed?("LocalHost:8080")
    true

    iex> Expresso.Watch.Host.allowed?("localhost")
    true

    iex> Expresso.Watch.Host.allowed?("attacker.example:4100")
    false

    iex> Expresso.Watch.Host.allowed?("localhost.attacker.example")
    false

# `do`

```elixir
@spec do(tuple()) :: {:proceed, list()} | {:break, [{:response, {403, charlist()}}]}
```

Pass a request with an allowed `Host` header to the next module

`:httpd` calls this function with its `mod` record. The function returns
`{:proceed, data}` for an allowed host, and a 403 response otherwise.

---

*Consult [api-reference.md](api-reference.md) for complete listing*