/cc @agl @bradfitz

@cavaliercoder, the https://golang.org/pkg/net/http/#Request.URL docs say:

        // For client requests, the URL's Host specifies the server to
        // connect to, while the Request's Host field optionally
        // specifies the Host header value to send in the HTTP
        // request.
        URL *url.URL
...
        // 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

Have you tried setting URL.Host instead of Host?

Yes. The request connects to the correct server, but the SNI and Host header are incorrect and the server does not know which certificate and virtual host to serve.

Setting Request.Host solves half the issue: telling the remove server which virtual host to serve. There's a second half to the problem: the SNI also needs to be set during TLS handshake so the remote server knows which certificate to serve.

What are you trying to do?

I know how SNI works and how net/http works, but I don't know what the use case is here. Nobody else has complained in 8 years or so about the behavior. The closest we've ever had is #14404.

I'm writing a tool to stress test and unit test web servers. I can edit /etc/hosts to direct the tests where they belong, but this quickly breaks down when we want to run the same tests against multiple servers from a CI server. It also sucks when testing our CDN staging network whose IPs need rediscovery for every test run.

The typical solution to that is setting one of the various http.Transport Dial fields and overriding how DNS resolution works.

It's fine to have multiple Transports active, as long as you remember to close their idle connections when you're done with them.

Awesome, I'll head down that path and report back. Thanks for your advice!

This would break usage of net/http for domain fronting, an anti-censorship technique specifically relying on sending different hosts in SNI and HTTP headers. Such a use of net/http is not rare: censorship-resistant tunneling software such as Lantern and my own project Geph both use net/http this way.

The following snippet got the job done for me:

dialer = &net.Dialer{
	Timeout:   30 * time.Second,
	KeepAlive: 30 * time.Second,
	DualStack: true,
}

client := http.Client{
	Transport: &http.Transport{
		DialContext: func(ctx context.Context, network, addr string) (net.Conn, error) {
			// redirect all connections to 127.0.0.1
			addr = "127.0.0.1" + addr[strings.LastIndex(addr, ":"):]
			return dialer.DialContext(ctx, network, addr)
		},
	},
}

The above http.Client will send all requests to 127.0.0.1, no matter what hostname is specified in a http.Request's URL. The hostname in the URL is used to set the Host request header and SNI name as desired.

Thanks for taking a look at this @bradfitz. Love your work!