Self Running demo of http.Request Host behavior

main.go

package main

import (
	"fmt"
	"net/http"
	"net/http/httptest"
)

func main() {

	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		for k, v := range r.Header {
			fmt.Println(k, v)
		}
		fmt.Println("Host", r.Host)
		fmt.Println("--------------")
	}))

	defer srv.Close()
	client := srv.Client()

	header := http.Header{
		"test": []string{"bar"},
		"Host": []string{"foo.com"},
	}

	req, _ := http.NewRequest("GET", srv.URL, nil)
	req.Header = header
	fmt.Println("Request.Host", req.Host)
	fmt.Println("Url.Host", req.URL.Host)
	fmt.Println("Header.Host", req.Header.Get("Host"))
	fmt.Println("--------------")
	client.Do(req)

	req.Host = "bar.net"
	client.Do(req)
}

output.txt

go run .
Request.Host 127.0.0.1:42387
Url.Host 127.0.0.1:42387
Header.Host foo.com
--------------
User-Agent [Go-http-client/1.1]
Test [bar]
Accept-Encoding [gzip]
Host 127.0.0.1:42387
--------------
Test [bar]
Accept-Encoding [gzip]
User-Agent [Go-http-client/1.1]
Host bar.net
--------------