How to check implicitly set http headers with golang server?

Question

I created a simple http2 server, If I send a request to it with curl, it responds with some headers, although I did not set them explicity. How can I acces them inside the requesthandling function ( sayhello )? My code ( I've never used golang before)

server.go

package main

import (
  "net/http"
  "strings"
  "fmt"
  "github.com/gorilla/mux"
  "golang.org/x/net/http2"
)

func sayHello(w http.ResponseWriter, r *http.Request) {
  message := r.URL.Path
  message = strings.TrimPrefix(message, "/")
  message = "Hello " + message

  w.Header().Set("myFirst", "golangQuestion")
  w.Write([]byte(message))
  for k, v := range w.Header() {
    fmt.Println("[RESPONSE][Header]", k,":", v)
    }
}

func main() {
    router := mux.NewRouter()
    router.PathPrefix("/").HandlerFunc(sayHello) // catch everything else rule
    var srv = &http.Server{
        Addr: "127.0.0.1:8081",
    }
    http2.ConfigureServer(srv, nil)
    srv.Handler = router
    sslCert := "./ssl.cert"
    sslKey := "./ssl.key"
    if err := srv.ListenAndServeTLS(sslCert, sslKey); err != nil {
        panic(err)
    }
}

Sending request:

curl --head --insecure https://127.0.0.1:8081

HTTP/1.1 200 OK
Myfirst: golangQuestion
Date: Tue, 18 Jun 2019 09:18:29 GMT
Content-Length: 6
Content-Type: text/plain; charset=utf-8

I can see that some headers are sent back, the one which I set explicitly is also recieved, but the output of

go run server.go

[RESPONSE][Header] Myfirst : [golangQuestion]

How can I acces the other headers, which were not explicitly set, but recieved by curl as well? I loopd through w.Headers, but it did not contain the implicitly set headers

  for k, v := range w.Header() {
    fmt.Println("[RESPONSE][Header]", k,":", v)
    }

My expectation that the output of go run server.go shall be something like this:

[RESPONSE][Header] Myfirst : [golangQuestion]
[RESPONSE][Header] Date: [2019.02.12 ]
[RESPONSE][Header] Content-Length: [6]

icza · Accepted Answer · 2019-06-18 09:44:36Z

3

Those headers are sent automatically when you call ResponseWriter.Write(). Quoting from its doc:

// Write writes the data to the connection as part of an HTTP reply.
//
// If WriteHeader has not yet been called, Write calls
// WriteHeader(http.StatusOK) before writing the data. If the Header
// does not contain a Content-Type line, Write adds a Content-Type set
// to the result of passing the initial 512 bytes of written data to
// DetectContentType. Additionally, if the total size of all written
// data is under a few KB and there are no Flush calls, the
// Content-Length header is added automatically.
//
// Depending on the HTTP protocol version and the client, calling
// Write or WriteHeader may prevent future reads on the
// Request.Body. For HTTP/1.x requests, handlers should read any
// needed request body data before writing the response. Once the
// headers have been flushed (due to either an explicit Flusher.Flush
// call or writing enough data to trigger a flush), the request body
// may be unavailable. For HTTP/2 requests, the Go HTTP server permits
// handlers to continue to read the request body while concurrently
// writing the response. However, such behavior may not be supported
// by all HTTP/2 clients. Handlers should read before writing if
// possible to maximize compatibility.
Write([]byte) (int, error)

ResponseWriter.Header() contains only the headers set explicitly. The Content-Type and Content-Length were sent by w.Write().

Note: if you want to suppress such automatic headers, you have to set their values to nil, e.g.:

w.Header()["Date"] = nil

Also note that if you set the values of such headers manually, those values will be sent without being changed.

edited Jun 18, 2019 at 9:44

answered Jun 18, 2019 at 9:37

icza

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6 Comments

user3598726 Over a year ago

Ok I saw that too, I figured the headers are added automatically, becouse I did not set them, but they did appear. My looping through the w.Header object is after calling ResponseWriter.Write(). My problem is not that some go library is adding headers, be it. My problem is how can I acces them inside the sayhello function

icza Over a year ago

@user3598726 You can't access them. You may suppress them if you don't want them sent (see edited answer), or you may set them explicitly to some other value, if you set them, they won't be changed / overwritten.

user3598726 Over a year ago

Thanks, How can I know which one to supress If I don't know which one will be set? I can't find documentation about which headers will be added implicitly by the library

icza Over a year ago

@user3598726 ResponseWriter.Write() documents it may add Content-Type and Content-Length. And another automatic header is Date. As your test shows, there is no other at the moment.

user3598726 Over a year ago

Thanks u helped a lot, I still can't wrap my head around not being able to acces those headers. Shall I use a lower level library?

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