Control HTTP headers from outer Go middleware

Question

Let\'s say I have a middleware in Go where I want to override any existing Server headers with my own value.

// Server attaches a Server header          


        

Answer 1

It's not possible to change headers (add/remove/modify) after you use WriteHeader(Status).

For this to work, change to:

// Server attaches a Server header to the response.
func Server(h http.Handler, serverName string) http.Handler {
  return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
    w.Header().Set("Server", serverName)
    h.ServeHTTP(w, r)
  })
}

Answer 2

You can define a custom type which wraps a ResponseWriter and inserts the Server header just before all headers are written, at the cost of an additional layer of indirection. Here is an example:

type serverWriter struct {
    w           http.ResponseWriter
    name        string
    wroteHeader bool
}

func (s serverWriter) WriteHeader(code int) {
    if s.wroteHeader == false {
        s.w.Header().Set("Server", s.name)
        s.wroteHeader = true
    }
    s.w.WriteHeader(code)
}

func (s serverWriter) Write(b []byte) (int, error) {
    return s.w.Write(b)
}

func (s serverWriter) Header() http.Header {
    return s.w.Header()
}

// Server attaches a Server header to the response.
func Server(h http.Handler, serverName string) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        sw := serverWriter{
            w:           w,
            name:        serverName,
            wroteHeader: false,
        }
        h.ServeHTTP(sw, r)
    })
}

I wrote more about this here. https://kev.inburke.com/kevin/how-to-write-go-middleware/