Rebuild an OVA appliance: A free approach to enable legacy hypervisors utilize newer OVA format

Introduction

Recently built OVA packages prefer SHA-256 algorithm instead of SHA1. If you consult VMWare, they will point you to some procedure to download and use a  tool to named ovftool, to recode with another algorithm and generate a converted version of the original appliance. In this article, we will go through an alternative approach using your favorite archive manager and text editor.

Assumption

The following has been assumed for simplicity:

• You are on a 64-bit Windows (But actually not limited to)
• You are using 7Zip as your favorite archive manager.
• We use notepad++ as your favorite text editor. 

Procedure

Here are the steps

• Rename appliance.ova to appliance.ova.tar
• Extract the archive with 7-Zip into a sub-folder
•  Open the file with .mf extension with notepad++, for each file mentioned there do the following steps:

• Right-click on that file and calculate the SHA-1 using the 7-Zip context menu
• Press CTRL-C (yes, do that!) on the pop-up window that displays the SHA-1 checksum

• Paste your clipboard into a new notepad instance. Among those lines, copy the SHA1 checksum into your clipboard

• Paste the checksum overwriting the existing one in .mf file you kept open from previous steps.
• Change the SHA-256 identifier to SHA1 on that line. Repeat the above steps until all lines are updated accordingly.

•  Save the .mf file
• Open the appliance.ova.tar once again with 7-Zip
• Drag the modified .mf files into the tar archive to freshen and overwrite the previous one

• Close 7-zip and rename file back to the original appliance.ova name.
• Deploy your appliance now. 

Final Word

You don't need to add a new tool to your toolbox to resolve import-ability issues of some OVA appliance files. In fact acquiring, installing, learning and invoking the tool to convert your OVA appliance to a new format may sound overkill just to make an import possible. Here we leveraged the fact that an OVA is just a packaged collection of OVF, Virtual disks and some metadata to disassemble, adjust and reassemble an OVA with the fun and joy of doing that in a transparent process and using our everyday tool-set.

Further Discussions

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