【golang】http request正确设置host

问题：直接在Header中设置Host是否可以生效？
问题背景：需要区分Host调用不同环境的接口，但Host设置有问题导致接口调用失败

服务端代码示例

func StartServer(addr string) error {
	http.HandleFunc("/hello", SayHello)
	if err := http.ListenAndServe(addr, nil); err != nil {
		return err
	}
	return nil
}
func SayHello(rsp http.ResponseWriter, req *http.Request) {
	fmt.Printf("get request %s to %s \n", req.RemoteAddr, req.Host)
	// response
	rsp.WriteHeader(200)
	if _, err := rsp.Write([]byte("request received")); err != nil {
		// deal with "write" error
	}
}

客户端代码示例

func DoRequest(url string) (*http.Response, error) {
	client := &http.Client{}
	request, err := http.NewRequest(http.MethodGet, url, nil)
	if err != nil {
		return nil, err
	}
	// request.Header.Set("Host", "specific-host") //错误方式
	request.Host = "specific-host"
	rsp, err := client.Do(request)
	if err != nil {
		return nil, err
	}
	return rsp, nil
}

尝试以下两种方式设置Host字段并发起请求

	request.Header.Set("Host", "specific-host")
	// 响应结果：get request 127.0.0.1:40280 to 127.0.0.1:11010
	// 不符合预期

	request.Host = "specific-host"
	// 响应结果：get request 127.0.0.1:40293 to specific-host
	// 符合预期

结论：直接通过header设置Host无法生效，需要设置Request中的Host成员

扫一下源码：
Request将Host从Header中独立出来

type Request struct {
	// For client requests, certain headers such as Content-Length
	// and Connection are automatically written when needed and
	// values in Header may be ignored. See the documentation
	// for the Request.Write method.
	Header Header
	// For client requests, Host optionally overrides the Host
	// header to send. If empty, the Request.Write method uses
	// the value of URL.Host. Host may contain an international
	// domain name.
	Host string
}

调用do方法时，根据req内容设置Host字段；服务端接收到的request可以读到Host，如上响应内容部分的“127.0.0.1:11010”和“specific-host”

func (c *Client) do(req *Request) (retres *Response, reterr error) {
	...
	host := ""
	if req.Host != "" && req.Host != req.URL.Host {
		// If the caller specified a custom Host header and the
		// redirect location is relative, preserve the Host header
		// through the redirect. See issue #22233.
		if u, _ := url.Parse(loc); u != nil && !u.IsAbs() {
			host = req.Host
		}
	}
	ireq := reqs[0]
	req = &Request{
		Host:     host,
		...
	}
}