If you want to build a private `.iso` file you can upload the `.iso` to a secure file hosting service like [Amazon S3](http://docs.aws.amazon.com/AmazonS3/latest/dev/ShareObjectPreSignedURL.html), [Google Cloud Storage](https://cloud.google.com/storage/docs/gsutil/commands/signurl), or [Azure File Service](https://msdn.microsoft.com/en-us/library/azure/dn194274.aspx) and download it at build time using a signed URL. You should convert `.dmg` files to `.iso` and follow a similar procedure.
or [Google Cloud
Storage](https://cloud.google.com/storage/docs/gsutil/commands/signurl) and
Once you have added [variables in your packer template](/docs/templates/user-variables.html) you can specify credentials or signed URLs using Atlas environment variables, or via the `-var` flag when you run `push`.
download it using a signed URL. You can inject the signed URL into your build by
using a build variable (environment variable) in Atlas. Example:
![Configure your signed URL in the Atlas build variables
![Configure your signed URL in the Atlas build variables
menu](/assets/images/packer-signed-urls.png)
menu](/assets/images/packer-signed-urls.png)
You will also need to [configure your packer
template](http://stormchaser.local:4567/docs/templates/user-variables.html) to
use the variable injected by Atlas (or via `push -var`).