Listen on specific host using warp

Question

When running a warp application using run, it listens on all IP adresses.

For security reasons, I want to listen on localhost only, handling remote access using a reverse proxy.

How do I need to call run to only listen on a specific host/IP?

Note: This question intentionally shows no research effort as it was answered Q&A-Style.

Answer 1

The currently accepted answer got broken by changes to conduit and/or warp. Warp no longer exports a Host constructor. But you don't need it, because the HostPreference type supports the OverloadedStrings extension, so you can just use a string directly.

This example also eliminates the deprecation warnings by switching to setPort and setHost.

{-# LANGUAGE OverloadedStrings #-}

import Network.Wai (responseLBS)
import Network.Wai.Handler.Warp
import Network.HTTP.Types (status200)
import Network.HTTP.Types.Header (hContentType)

main = do
    let port = 3000
    putStrLn $ "Listening on port " ++ show port
    let settings = setPort port $ setHost "127.0.0.1" defaultSettings
    runSettings settings app

app req = return $
    responseLBS status200 [(hContentType, "text/plain")] "Hello world!"