req.Header.Add("Host", "www.example.com")
req.Header.Add("Content-Type", "application/x-www-form-urlencoded")

map[Host:[www.example.com]]

// 發(fā)送請求并獲取響應(yīng)
resp, err := client.Do(req)
if err != nil {
	fmt.Println("發(fā)送請求失敗:", err)
	return
}

func (c *Client) Do(req *Request) (*Response, error) {
	return c.do(req)
}
func (c *Client) do(req *Request) (retres *Response, reterr error) {
    ... ... // 省略
    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{
		Method:   redirectMethod,
		Response: resp,
		URL:      u,
		Header:   make(Header),
		Host:     host,
		Cancel:   ireq.Cancel,
		ctx:      ireq.ctx,
	}
    ... ... // 省略
}

// If the caller specified a custom Host header and the redirect location is relative, preserve the Host header through the redirect.
// 如果調(diào)用者指定了自定義的 Host 標(biāo)頭并且重定向位置是相對路徑的話，通過重定向保留該 Host 標(biāo)頭。

	// For server requests, Host specifies the host on which the
	// URL is sought. For HTTP/1 (per RFC 7230, section 5.4), this
	// is either the value of the "Host" header or the host name
	// given in the URL itself. For HTTP/2, it is the value of the
	// ":authority" pseudo-header field.
	// It may be of the form "host:port". For international domain
	// names, Host may be in Punycode or Unicode form. Use
	// golang.org/x/net/idna to convert it to either format if
	// needed.
	// To prevent DNS rebinding attacks, server Handlers should
	// validate that the Host header has a value for which the
	// Handler considers itself authoritative. The included
	// ServeMux supports patterns registered to particular host
	// names and thus protects its registered Handlers.
	//
	// 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

req.Host = "www.example.com"

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
    }
}

Parent:     645c661a (cmd/compile/internal/syntax: factor out list parsing)
Author:     Tom Bergan <tombergan@google.com>
AuthorDate: 2017-10-13 15:56:37 -0700
Commit:     Tom Bergan <tombergan@google.com>
CommitDate: 2017-10-16 17:44:26 +0000
net/http: preserve Host header following a relative redirect
If the client sends a request with a custom Host header and receives
a relative redirect in response, the second request should use the
same Host header as the first request. However, if the response is
an abolute redirect, the Host header should not be preserved. See
further discussion on the issue tracker.
Fixes #22233
Change-Id: I8796e2fbc1c89b3445e651f739d5d0c82e727c14
Reviewed-on: https://go-review.googlesource.com/70792
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

    // For server requests, Host specifies the host on which the
    // URL is sought. For HTTP/1 (per RFC 7230, section 5.4), this
    // is either the value of the "Host" header or the host name
    // given in the URL itself. For HTTP/2, it is the value of the
    // ":authority" pseudo-header field.

// example: www.example.org or www.example.org:8080
Host = uri-host [":" port];