Testing host preferences in Warp

Main.hs

-- app/Main.hs
{-# LANGUAGE OverloadedStrings #-}

module Main where

import Network.Wai (responseLBS, Application)
import Network.Wai.Handler.Warp (defaultSettings, setHost, setPort,
                                 runSettings, Settings)
import Network.HTTP.Types (status200)
import Network.HTTP.Types.Header (hContentType)
import Data.String

main :: IO ()
main = do
  let port = 3000
  putStrLn $ "Listening on port " <> show port
  -- run port app
  runSettings (warpSettings port) app

app :: Application
app _req f =
  f $ responseLBS status200 [(hContentType, "text/plain")] ("Hello world!")

warpSettings :: Int -> Settings
warpSettings port =
    setHost (fromString "!6") $ setPort port defaultSettings

warp-test.cabal

cabal-version:      3.0
-- The cabal-version field refers to the version of the .cabal specification,
-- and can be different from the cabal-install (the tool) version and the
-- Cabal (the library) version you are using. As such, the Cabal (the library)
-- version used must be equal or greater than the version stated in this field.
-- Starting from the specification version 2.2, the cabal-version field must be
-- the first thing in the cabal file.

-- Initial package description 'warp-test' generated by
-- 'cabal init'. For further documentation, see:
--   http://haskell.org/cabal/users-guide/
--
-- The name of the package.
name:               warp-test

-- The package version.
-- See the Haskell package versioning policy (PVP) for standards
-- guiding when and how versions should be incremented.
-- https://pvp.haskell.org
-- PVP summary:     +-+------- breaking API changes
--                  | | +----- non-breaking API additions
--                  | | | +--- code changes with no API change
version:            0.1.0.0

-- A short (one-line) description of the package.
-- synopsis:

-- A longer description of the package.
-- description:

-- The license under which the package is released.
license:            MIT

-- The file containing the license text.
license-file:       LICENSE

-- The package author(s).
author:             Laurence Isla

-- An email address to which users can send suggestions, bug reports, and patches.
maintainer:         lau.isla.c@gmail.com

-- A copyright notice.
-- copyright:
build-type:         Simple

-- Extra doc files to be distributed with the package, such as a CHANGELOG or a README.
extra-doc-files:    CHANGELOG.md

-- Extra source files to be distributed with the package, such as examples, or a tutorial module.
-- extra-source-files:

common warnings
    ghc-options: -Wall

executable warp-test
    -- Import common warning flags.
    import:           warnings

    -- .hs or .lhs file containing the Main module.
    main-is:          Main.hs

    -- Modules included in this executable, other than Main.
    -- other-modules:

    -- LANGUAGE extensions used by modules in this package.
    -- other-extensions:

    -- Other library packages from which modules are imported.
    build-depends:    base ^>=4.16.4.0,
                      -- warp >= 3.3.19 && < 3.4,
                      warp >= 3.4.0,
                      wai  >= 3.2.1 && < 3.3,
                      http-types >= 0.12.3 && < 0.13

    -- Directories containing source files.
    hs-source-dirs:   app

    -- Base language which the package is written in.
    default-language: Haskell2010

Run with:

cabal install & cabal run

Unexpected behavior:

It allows both ipv4 and ipv6 requests when setHost !6

$ curl "http://localhost:3000" --ipv4 -I
HTTP/1.1 200 OK
Transfer-Encoding: chunked
Date: Thu, 08 Feb 2024 01:19:54 GMT
Server: Warp/3.4.0
Content-Type: text/plain

It does not allow ipv6 requests when setHost *

$ curl "http://localhost:3000" --ipv6 -I
curl: (7) Failed to connect to localhost port 3000 after 0 ms: Couldn't connect to server

setHost !4 working as expected

$ curl "http://localhost:3000" --ipv6 -I
curl: (7) Failed to connect to localhost port 3000 after 0 ms: Couldn't connect to server