How to remove "Server" header in Servant?

Question

I am using servant library. I tried modifying an approach from some blog post, but it doesn't do anything in case of removing "Server" header. (It only works for adding custom headers.)

removeServerHeaderMiddleware :: Middleware
removeServerHeaderMiddleware baseApp = \req responseFunc -> baseApp req (newResponseFunc responseFunc)
  where
    newResponseFunc :: (Response -> IO ResponseReceived) -> Response -> IO ResponseReceived
    newResponseFunc responseFunc = removeServerHeader >>> responseFunc
    removeServerHeader :: Response -> Response
    removeServerHeader r = trace (show $ responseHeaders r) $ mapResponseHeaders (filter (\(k, _) -> k /= "Server")) r

app1 :: Application
app1 = serve imageAPI1 server1

main :: IO ()
main = do
  let port = 8080
  putStrLn $ "Starting server on port " <> show port <> "."
  run port $ removeServerHeaderMiddleware app1

Response:

$ http :8080/images
HTTP/1.1 200 OK
Content-Type: application/json;charset=utf-8
Date: Sat, 15 Jun 2019 06:12:58 GMT
Server: Warp/3.2.27
Transfer-Encoding: chunked

Looking at trace, maybe the header gets added later?

[("Content-Type","application/json;charset=utf-8")]

I found setServerName in warp, but I have no clue how to incorporate that with servant.

Dan D. Dan D. · Accepted Answer · 2019-06-15T06:33:21

The documentation for setServerName states:

Default server name to be sent as the "Server:" header if an application does not set one. If an empty string is set, the "Server:" header is not sent. This is true even if an application set one.

From http://hackage.haskell.org/package/warp-3.2.27/docs/Network-Wai-Handler-Warp.html

So to disable the Server header one setServerName with an empty string.

To use this one replaces run port with

runSettings (setPort port (setServerName "" defaultSettings))

How to remove "Server" header in Servant?

1 Answers