What does "Supported ISO: no" mean in Fedora installer?

Question

I just installed Fedora Workstation 30. On boot, I selected "Test this media & start Fedora-Workstation-Live". The screen shows:

/dev/disk/by-label/Fedora-WS-Live-30-1-2:   b340a38cf5c1f09fdd769e259e449119
Fragment sums: 6dc04c22c2275... (a bit long to type)
Fragment count: 20
Supported ISO: no
Press [Esc] to abort check.
Checking: 025.5%

And completes the check successfully. I created the USB media from Etcher (IIRC) on Windows 10.

The installation is alright, but I'm a bit puzzled by the "Supported ISO: no" line, even though this ISO comes straight from the getfedora.org website.

What on earth does "Supported ISO: no" mean?

Answer 1

The option "Test this media" actually sets the rd.live.check dracut command line option, which in turn triggers Anaconda, the installer, to test the installation media, by running the systemd checkisomd5 one-shot service. The definition for this service is:

[Unit]
Description=Media check on %f
DefaultDependencies=no
Before=shutdown.target

[Service]
Type=oneshot
RemainAfterExit=no
ExecStart=/bin/checkisomd5 --verbose %f
StandardInput=tty-force
StandardOutput=inherit
StandardError=inherit
TimeoutSec=0

The checkisomd5 command (see here) is provided by the isomd5sum package in Fedora:

[root@fedora30 /]# dnf provides checkisomd5
Last metadata expiration check: 0:02:27 ago on Tue 03 Sep 2019 09:31:46 AM CEST.
isomd5sum-1:1.2.3-5.fc30.x86_64 : Utilities for working with md5sum implanted in ISO images
Repo        : fedora
Matched from:
Filename    : /usr/bin/checkisomd5

So let's check the source:

rpm -ivh https://download.fedoraproject.org/pub/fedora/linux/releases/30/Everything/source/tree/Packages/i/isomd5sum-1.2.3-5.fc30.src.rpm
cd /root/rpmbuild/SOURCES/
tar -xvf 1.2.3.tar.gz
cd isomd5sum-1.2.3

Going through the source, you'll find a few references to the supported flag, which apparently means:

Indicate that the image will be written to a "supported" media, such as pressed CD.  On Red Hat-based Anaconda installers, this bypasses the prompt to check the CD.

which agrees with the man page at https://linux.die.net/man/1/implantisomd5.

So, "Supported ISO: no" means that this image wasn't flagged with the "supported" flag at creation time. This will make Anaconda show the prompt to check the CD.