How to check implicitly set http headers with golang server? - http

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]

Answer 1

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.