@yzl Sorry I think I mis-typed. I meant to type something like this:

```
"iso_checksum": null,
"iso_checksum_type": "md5",
"iso_url": null,
```
Basically we should default to null/blank values for the `iso_url` and `iso_checksum` variables. 

According to the [packer docs](http://www.packer.io/docs/templates/user-variables.html) this will make these values required:

> If the default value is null, then the user variable will be required. This means that the user must specify a value for this variable or template validation will fail. 

In our case there are no valid default values for `iso_url` and `iso_checksum` so we want packer to fail during validation if a user forgets to set values via `-var` or `-var-file`.