Why are request.URL.Host and Scheme blank in the development server?

I'm very new to Go开发者_开发知识库. Tried this first hello, world from the documentation, and wanted to read the Host and Scheme from the request:

package hello

import (
    "fmt"
    "http"
)

func init() {
    http.HandleFunc("/", handler)
}

func handler(w http.ResponseWriter, r *http.Request) {
    fmt.Fprint(w, "Host: " + r.URL.Host + " Scheme: " + r.URL.Scheme)
}

But their values are both blank. Why?

Basically, since you're accessing the HTTP server not from an HTTP proxy, a browser can issue a relative HTTP request, like so:

GET / HTTP/1.1
Host: localhost:8080

(Given that, of course, the server is listening on localhost port 8080).

Now, if you were accessing said server using a proxy, the proxy may use an absolute URL:

GET http://localhost:8080/ HTTP/1.1
Host: localhost:8080

In both cases, what you get from Go's http.Request.URL is the raw URL (as parsed by the library). In the case you're getting, you're accessing the URL from a relative path, hence the lack of a Host or Scheme in the URL object.

If you do want to get the HTTP host, you may want to access the Host attribute of the http.Request struct. See http://golang.org/pkg/http/#Request

You can validate that by using netcat and an appropriately formatted HTTP request (you can copy the above blocks, make sure there's a trailing blank line after in your file). To try it out:

cat my-http-request-file | nc localhost 8080

Additionally, you could check in the server/handler whether you get a relative or absolute URL in the request by calling the IsAbs() method:

isAbsoluteURL := r.URL.IsAbs()