code: func ValidateExtraHost(val string) (string, error) {
  	// allow for IPv6 addresses in extra hosts by only splitting on first ":"
  	arr := strings.SplitN(val, ":", 2)
  	if len(arr) != 2 || len(arr[0]) == 0 {
  		return "", fmt.Errorf("bad format for add-host: %q", val)
  	}
  	if _, err := ValidateIPAddress(arr[1]); err != nil {
  		return "", fmt.Errorf("invalid IP address in add-host: %q", arr[1])
  	}
  	return val, nil
  }
docs: // ValidateExtraHost validates that the specified string is a valid extrahost and returns it.
  // ExtraHost is in the form of name:ip where the ip has to be a valid ip (IPv4 or IPv6).