util.go
func hostname(hostport string) string {
	colon := strings.IndexByte(hostport, ':')
	if colon == -1 {
		return hostport
	}
	if i := strings.IndexByte(hostport, ']'); i != -1 {
		return strings.TrimPrefix(hostport[:i], "[")
	}
	return hostport[:colon]
}
func hostname(hostport string) string {
	colon := strings.IndexByte(hostport, ':')
	if colon == -1 {
		return hostport
	}
	if i := strings.IndexByte(hostport, ']'); i != -1 {
		return strings.TrimPrefix(hostport[:i], "[")
	}
	return hostport[:colon]
}
// contains code taken from go1.8, for purposes of backwards compatability with
// older go versions.
// hostname returns u.Host, without any port number.
//
// If Host is an IPv6 literal with a port number, Hostname returns the
// IPv6 literal without the square brackets. IPv6 literals may include
// a zone identifier.