Disable SNI in Go HTTPS requests

sni.go

url := "https://perdu.com"
hostname := "perdu.com"

sni := false

var serverName string
if sni {
  serverName = hostname
} else {
  // disable sending the ServerName by using an IP
  // see tls.Config.ServerName
  serverName = "127.0.0.1"
}

client := &http.Client{
  // adapted from http.DefaultTransport
  Transport: &http.Transport{
    Proxy: http.ProxyFromEnvironment,
    DialContext: (&net.Dialer{
      Timeout:   30 * time.Second,
      KeepAlive: 30 * time.Second,
      DualStack: true,
    }).DialContext,
    ForceAttemptHTTP2:     true,
    MaxIdleConns:          100,
    IdleConnTimeout:       90 * time.Second,
    TLSHandshakeTimeout:   10 * time.Second,
    ExpectContinueTimeout: 1 * time.Second,
    TLSClientConfig: &tls.Config{
      ServerName: serverName,
      // bypass Go standard verification to use our own below (VerifyConnection)
      InsecureSkipVerify: true,
      VerifyConnection: func(cs tls.ConnectionState) error {
        // adapted from the tls.Config.VerifyConnection example
        // behaves just like the default verification,
        // except for the overriden DNSName below
        opts := x509.VerifyOptions{
          // override the requested hostname with the server hostname
          DNSName:       hostname,
          Intermediates: x509.NewCertPool(),
        }
        for _, cert := range cs.PeerCertificates[1:] {
          opts.Intermediates.AddCert(cert)
        }
        _, err := cs.PeerCertificates[0].Verify(opts)
        return err
      },
    },
  },
}

res, err := client.Get(url)