Category: Stupid Linux Tricks

There are several existing How-To’s out on the Internet on the subject of creating a custom CentOS installation DVD/USB storage medium. But unfortunately, actually trying to employ them can be frustrating. So here goes with Yet Another How-To that I hope will fill some of the holes.

Why Custom media?

Why even bother with a custom install? Why indeed. The mousetech.com server farm is a fairly typical in-house setup. It has extensive provisioning capabilities, daily backups, filesystem mirroring and failovers for High Availability. If a server dies, it’s relatively easy to reconstruct it.

But what if a meteor hits the server complex or war breaks out and I have to flee to Argentina? How do I minimize the time and effort required to reconstruct the essential frameworks?

One way is to define a master bootstrap server that can be used to rebuild the main provisioning systems. The master bootstrap doesn’t run in normal operations. It’s independent of them. The normal servers distribute their functions among many machines, VM’s and containers, but the master bootstrap compacts their core functions down onto one temporary machine.

I could do the master bootstrap functions via a stock Centos install DVD set and a kickstart file, but by creating a custom install with the essential packages and kickstart (and customization scripts/data), I can make this a completely unattended operation. And when things are in total disaster mode, the less I need to remember to do, the better.

One note. An unfortunate consequence of the curent CentOS and related OS distros is that the old reliable convention of expecting there to be an eth0 device to network through is pretty much shot. I don’t assume any particular physical machine to be the target of this install, and therefore cannot predict what names the installation will assign to the network ports. So rather than find false comfort, I leave actual NIC setup to manually configuring the /etc/sysconfig/network-scripts after the installation has taken place. Similarly, since I install with no known network or gateway, everything is self-contained – no external servers.

And with that, I begin.

Step 1 – Build a workspace

Making a DVD requires a lot of disk space. So find a place on your build system with lots of room and create a workspace directory. We’ll mostly work out of there. For convenience, I’m going to call this directory “buildiso” and so it’s going to have a pathname of something like /home/timh/buildiso.

cd to this directory. All relative paths given in this howto are relative to this directory.

Step 2 – Start with an existing image

Creating an installation CD from scratch is a monumental task and not worth the effort. So rather than do that, let’s do what everyone else does and modify an existing image. Because this is panic recovery and I want it all on a single medium, I’m going to use the CentOS minimal image and build on it.

So to begin, we make a file-by-file copy of the DVD image. If you have a physical DVD mounted you should be able to copy that. If you have an iso image, mount it like so:

cd /home/timh/buildiso
mkdir mnt     # this is where we'll mount the source ISO file (if we use one)

mkdir bootisoks # this is where we build our new ISO image
mount -t iso9660 -o loop /path/to/centos-disc.iso mnt

Copy the source files from your mounted DVD or loop mount into the bootiso directory. You can use the Linux cp command, rysnc, or whatever you like as long as it copies all files and directories, including the hidden ones.

Once you’ve copied the files, you can unmount the ISO (or DVD). You don’t need it anymore unless you have to go back to the beginning.

Now that we have our model files, change them to be writeable so we can play with them:

chmod -R u+w bootisoks

Create a kickstart file and copy it to the iso image isolinux directory:

cp my_ks.cfg bootisoks/isolinux/ks.cfg

This will end up in the root of the actual DVD we’re creating.

Add any additional RPMs we want to the workspace iso package directory. These can be additional stock RPMs, third-party RPMs or your own custom-built RPMs. They all go directly into the bootisoks/Packages directory.

The CD install process for CentoOS 7 uses yum and the yum repository it uses is stored on the disc itself. Part of the repository infrastructure is the repodata directory and as a precaution, you should make a backup copy of it.

Most of the files in repodata have long twisty names designed to help mirrors keep in sync when distributed. We actually only care about one of them, so we’ll steal that one for later use:

cp bootisoks/repodata/*-comps.xml comps.xml

Gotcha #1

This file defines the installation groups, including the most critical one of all, which is base. So you’re going to need to inject that into the process of building the updated repodata. If you do not, the Linux installer will whine and fail.

Here’s how to properly reconstruct the repodata:

cd bootisoks

rm -rf repodata

createrepo -g ../comps.xml -dp .

cd .. # return to our workspace directory

Gotcha #2

It is critical that the newly-created repodata be pointing to the correct location for the RPMs that will be installed, which is to say the Packages directory. To verify that this happened, you can use this command:

less bootisoks/repodata/*-primary.xml.gz

This is a compressed file, but “less” is helpful and will display it uncompressed.

What you need to see in the ‘package type=”rpm”‘ elements are location sub-elements that look like this:

 <location href="Packages/NetworkManager-glib-1.12.0-6.el7.x86_64.rpm"/>

If you don’t see “Packages/” in the location, then yum won’t look in the DVD’s Packages directory, and it won’t find the RPMs. It will whine profusely and the install will fail.

This is why, contrary to some examples, you should run the createrepo program from the bootisoks directory and not the Packages directory. It will presumably scan (and add) any other rpms it finds in the bootisoks tree, but since there shouldn’t be any, that’s OK. If there’s an option to get the proper location without scanning everything, the createrepo documentation is too vague and my experiments haven’t been productive. Although here’s something I don’t think I tried (Note: I tried. It failed.):

createrepo -g ../comps.xml -dp Packages

Check the primary.xml.gz file and if the location is correct, use that.

The other “gotcha” on package installation can come if you omitted a pre-requisite package needed by one of the packages you are installing. The Linux installer packaging log will name any missing packages, in which case add them to the Packages directory, delete and rebuild the repodata and try again.

Step 3 – Activate the Kickstart file

To use your custom kickstart file, you need to define it to the bootloader directives in the isolinux directory. This is basically a grub menu file named isolinux/isolinux.cfg and you can use sed to update the different boot options in a single swoop like this:

sed -i 's/append\ initrd\=initrd.img/append initrd=initrd.img\
  ks\=cdrom:\/ks.cfg/' bootisoks/isolinux/isolinux.cfg

In other works, add “ks=cdrom:/ks.cfg” to the “append” statements in that file.

Special Gotcha: I ran into serious problems when I created my first image because I gave my DVD a custom volume label (using the -V on the mkisofs command). This proved fatal, because the “append” statements refer to the install media by its label and when the installer could not find a volume with that label. Which resulted in the following cryptically useless installation output:

Starting dracut initqueue hook….

At which point the whole thing would hang forever. Changing the volume ID in isolinux.cfg fixed that.

Step 4 Build the image

At this point, we’ve installed and activated our custom kickstart and setup the packages and repodata. If you have any custom scripts or data files to add the the image, do it now. And then build the ISO file, like so:

cd bootisoks
mkisofs -o ../boot.iso -b isolinux.bin -c boot.cat -no-emul-boot -boot-load-size 4 -boot-info-table -V "CentOS 7 x86_64" -R -J -v -T isolinux/. .
cd ..

You can make the ISO bootable from a thumb drive by doing this:

isohybrid boot.iso

And finally, add the checksum so that media testing will work properly.

implantisomd5 boot.iso

At this point, you should be able to burn the iso to DVD or “dd” it to a USB media device. Happy booting!

If you have problems:

This posting has attempted to correct and amplify what I have learned elsewhere. But of course, it’s likely to have introduced a few errors of its own. Because I don’t want to have to deal with spammers and abusers, this website doesn’t allow comments, but if you have questions or comments, I can be contacted through the Linux forum at http://coderanch.com

References:

https://serverfault.com/questions/517908/how-to-create-a-custom-iso-image-in-centos

https://www.frankreimer.de/?p=522