Ubuntu 15.10 mini iso preseed not recognized

Here is the result of 30 hours of frustration:

• Ubuntu mini ISO cannot preseed from a file inside an ISO
• Debian netinstall ISO can preseed from a file in the ISO
• Ubuntu and debian Netinstalls cannot retrieve preseed files over SSL

Now i created a public available web server and load the preseed file from there. Why the Ubuntu cannot use a preseed file from in the ISO is a mystery to me, but there must be a valid reason.

Not sure if something has changed in later versions, but for Ubuntu 16.04.2 (using mini.iso for amd64), I had to use the following options for preseeding over HTTP:

auto=true priority=critial url=http://...

Note the addition of auto=true (without it, the preseed file is not even considered) and the correct spelling of priority. More information about the meaning of these keywords can be found in the manual: https://help.ubuntu.com/lts/installation-guide/amd64/apbs02.html

One solution is to extract initrd.gz, add the preseed file to root path and pack it again. In boot params file=/preseed.cfg.

Here is an example of how to do that

First, obtain the compressed initrd.gz file and unpack it:

mkdir preseeded
cd preseeded && gzip -d < path/to/initrd.gz | cpio -id

Add the preseed.cfg file and repack the initrd:

cp mypreseed.cfg preseeded/preseed.cfg
cd preseeded && find . | cpio -o -H newC | gzip) > path/to/initrd.gz

I tested this solution on Ubuntu 18.04-LTS and 20.04-LTS, both minimal ISO.

Options with url=http://xxx/ps.cfg do not require an extract, of course; works correctly on Debian and Ubuntu.

Params that should be added, from file auto=true priority=critical file=/preseed.cfg or from network auto=true priority=critical url=http://xxx/ps.cfg