In this article I will share the steps to create custom ISO using original RHEL/CentOS image. The original DVD of RHEL/CentOS is quiet huge (around 9Gb for RHEL 8.3), in production environment we normally create our own custom ISO with reduced number of packages and deliver the same to our clients and customers.

The steps from this article are verified for CentOS 8 but the same will also work for RHEL 8. But if you want to create custom ISO for RHEL/CentOS 7 then I would recommend to follow How to create customized bootable ISO image in RHEL/CentOS 7

Step 1: Access the original ISO Image

First of all you would need access to the original ISO DVD image which we will further customise and generate a small version of the same image with lesser rpm count. I will be using CentOS 8.2 for the demonstration of this article. I have already added the image on my Oracle VirtualBox VM and have mounted it on /mnt:

~]# mount /dev/sr0 /mnt
mount: /mnt: WARNING: device write-protected, mounted read-only.

Step 2: Generate directory structure

Next we need to have same directory structure as used in the original DVD. But since we don't want to copy all the rpms from the original DVD, so we will exclude the folders which contains packages.

NOTE:

In RHEL/CentOS 7 we had a single Packages directory but with RHEL/CentOS 8 we have BaseOS/Packages and AppStream/Packages. So make sure you update your configuration based on your environment.

I will create a base directory /data/custom_iso under which I will copy the ISO content:

~]# mkdir /data/custom_iso

Since we are using CentOS 8, we will copy everything except BaseOS and AppStream directory from the original DVD.

~]# rsync -av --progress /mnt/ /data/custom_iso/ --exclude BaseOS --exclude AppStream

This will copy all the files and folders from ISO mounted on /mnt into /data/custom. But there are also some hidden files in the root folder of the ISO which also must be copied or else your ISO will never work.

~]# cp -vf /mnt/.??* /data/custom_iso/
'/mnt/.discinfo' -> '/data/custom_iso/.discinfo'
'/mnt/.treeinfo' -> '/data/custom_iso/.treeinfo'

These are the final content under my custom_iso directory at this stage:

~]# ls -al
total 48
drwxr-xr-x. 7 root root 4096 Feb 25 01:32 .
drwxr-xr-x. 4 root root 4096 Feb 25 00:09 ..
-r--r--r--. 1 root root   44 Feb 25 00:36 .discinfo
dr-xr-xr-x. 3 root root 4096 Jun  9  2020 EFI
dr-xr-xr-x. 3 root root 4096 Jun  9  2020 images
dr-xr-xr-x. 2 root root 4096 Feb 24 23:18 isolinux
-r--r--r--. 1 root root   87 Jun  9  2020 media.repo
-r--r--r--. 1 root root  664 Jun  9  2020 TRANS.TBL
-r--r--r--. 1 root root 1520 Feb 25 00:36 .treeinfo

Step 3: Create package list

This is the most tricky part unless you are planning to use all the rpms from the original DVD (in which case you can copy the entire directory into /data/custom_iso in the previous step). I know of two methods which can be used to achieve this, you can add any more which you felt was more easier using the comment section.

Method 1: Getting list of packages using DNF/YUM

In this method we will get the list of packages for an individual group using YUM or DNF and then copy those packages into BaseOS/AppStream directory respectively. First you must decide the list of group(s) which you plan to install as part of your custom ISO. To get the list of available groups you can use:

]# dnf grouplist
Last metadata expiration check: 0:00:06 ago on Thu 25 Feb 2021 11:15:49 AM IST.
Available Environment Groups:
   Server
   Workstation
   KDE Plasma Workspaces
   Virtualization Host
   Custom Operating System
Installed Environment Groups:
   Server with GUI
   Minimal Install
Installed Groups:
   Container Management
   Headless Management
   GNOME Desktop Environment
   Hardware Support
Available Groups:
   .NET Core Development
   RPM Development Tools
   Development Tools
   Graphical Administration Tools
   Legacy UNIX Compatibility
   Network Servers
   Scientific Support
   Security Tools
   Smart Card Support
   System Tools
   Fedora Packager
   Xfce
   Games and Entertainment
   Graphical Internet
   KDE (K Desktop Environment)
   Sound and Video

For example I want to have an ISO with rpms required to install "Minimal Environment" group. So to get the list of rpms part of this group we can use:

~]# dnf groupinfo "Minimal Install"
Last metadata expiration check: 0:02:31 ago on Thu 25 Feb 2021 11:15:49 AM IST.
Environment Group: Minimal Install
 Description: Basic functionality.
 Mandatory Groups:
   Core
 Optional Groups:
   Guest Agents
   Standard

So looks like this group is composed of different sub groups, so we need to query these individual groups to get the list of packages:

~]# dnf groupinfo "Core" "Guest Agents" "Standard"

This will give you a long output with the list of following types of packages for individual groups:

Optional Packages
Default Packages
Mandatory Packages

You can write a small script to get the list of packages and then copy those packages from the original DVD into respective folder of custom_iso. As some of these packages will be part of BaseOS repo while the others would be part of AppStream repo so you must maintain the hierarchy.

WARNING:

You will also have to get the list of dependency rpms for each of these rpms as it is possible that there are multiple dependency rpms for each one of the package which are not mentioned in the list and will most likely lead to installation failure later. This is the reason why I don't prefer this method as it is very hard to get this dependency list of so many rpms.

Method 2: Bring up a new instance using respective ISO

I personally prefer this method as the success rate is much better to get a working list of packages. In this method we will install CentOS 8 on one of my VMs and then select the respective groups which you plan to include in your custom ISO.

Under the installation option stage:

You can select your preferred group:

I would not recommend selecting any extra add-on packages at this stage unless you know what you are doing. You can always manually install the packages later once the node is UP based on your requirement.

NOTE:

You can not select more than one group at this stage but if your requirement to add multiple groups then you can always use "dnf groupinstall <group_name>" once this node is installed. This would install the packages and all the dependencies from the provided group_name. But it is important that you use offline repository for performing this step as if you use online repositories then you may not have access to those rpms in the DVD. In such case you can just download the respective rpm and dependent rpms from online repositories.

Once this VM is installed with CentOS 8, you can go ahead and install any additional packages or groups based on your requirement:

# dnf install <pkg>
# dnf groupinstall <group>

Next query the list of rpms from this newly created VM and store the output in a file and copy this file to your original node where you were creating the custom ISO:

~]# rpm -qa --queryformat %NAME-%VERSION-%RELEASE.%ARCH'\\n' > /tmp/rpms.txt

This will store the rpm along with the version, release and architecture format into /tmp/rpms.txt. So without much hassle we have our list of rpms with all the dependencies.

Step 4: Copy rpms based on package list

Now that we have our package list with the list of packages we want in our custom ISO, we have to copy these rpms from the original DVD to our custom ISO. It would have been very easy in RHEL/CentOS 7 where we had a single location for all the packages but with RHEL/CentOS 8 now we have BaseOS and AppStream directory for the packages so we must make sure that rpms are copied to their respective directory.

At Step 2 we had skipped the copying of BaseOS and AppStream directory so let us manually create these directories to store the packages:

~]# mkdir -p /data/custom_iso/BaseOS/Packages /data/custom_iso/AppStream/Packages

Now to copy the rpms I have written a small script, this script assumes that the original DVD is mounted on /mnt which contains all the rpms and we are storing the output in /data/custom_iso

~]# cat /tmp/copy_rpms_to_myiso.sh
#!/bin/bash

RPM_FILE="/tmp/rpms.txt"
for pkg in `cat $RPM_FILE`; do
    RPM=`find /mnt/ -name $pkg*.rpm`
    if [[ $RPM =~ .*BaseOS.* ]];then
       cp -ap $RPM /data/custom_iso/BaseOS/Packages/
    elif [[ $RPM =~ .*AppStream.* ]];then
       cp -ap $RPM /data/custom_iso/AppStream/Packages/
    else
       echo "$pkg not found"
    fi
done

This script will check if the individual rpm is from BaseOS or AppStream repository and then will copy the rpm into respective directory. If the script fails to find the rpm in both the repository then it will print the package name which was not found.

Let us execute this script:

~]# /tmp/copy_rpms_to_myiso.sh

If everything is correct then this step should copy all the packages into it's respective folder.

Step 5: Create repodata for Custom ISO

Now that we have all the required rpms in place under respective repo directory, next we must create repodata for this individual repositories. To achieve this we will need createrepo tool which you can install using yum or dnf:

~]# dnf install createrepo

Next we would need Comps.xml file from the original DVD for both the repositories. This comps.xml contains the list of groups and the mapping packages for individual group, this file is normally referred by the installation medium to identify the available groups and add-on packages.

So copy the respective comps.xml and place it in your custom ISO

[root@centos8 custom_iso]# cp /mnt/BaseOS/repodata/*comps*.xml BaseOS/comps_base.xml
[root@centos8 custom_iso]# cp /mnt/AppStream/repodata/*comps*.xml AppStream/comps_app.xml

Next create repo using these group xml file for both BaseOS and AppStream:

[root@centos8 custom_iso]# createrepo -g comps_app.xml AppStream/
Directory walk started
Directory walk done - 29 packages
Temporary output repo path: AppStream/.repodata/
Preparing sqlite DBs
Pool started (with 5 workers)
Pool finished


[root@centos8 custom_iso]# createrepo -g comps_base.xml BaseOS/
Directory walk started
Directory walk done - 360 packages
Temporary output repo path: BaseOS/.repodata/
Preparing sqlite DBs
Pool started (with 5 workers)
Pool finished

This step will create repodata directory with the required database files under /data/custom_iso/BaseOS and /data/custom_iso/AppStream directory:

[root@centos8 custom_iso]# ls -l BaseOS/repodata/ AppStream/repodata/
AppStream/repodata/:
total 808
-rw-r--r--. 1 root root  10856 Feb 25 02:02 231deccd5b8e4dc32047ccbe4f6b0b0683a6b0253cc3e3dc89093b6ae9e522da-other.xml.gz
-rw-r--r--. 1 root root  15238 Feb 25 02:02 39e06ef307b975417fcfd5ec9bc23c84495407c136e430837eda3508140e7964-other.sqlite.bz2
-rw-r--r--. 1 root root  19034 Feb 25 02:02 4d1f47a7d096fdbe9bdfccb84ea596be9a09ec6d51cf0b8b67f5c3663775261d-filelists.sqlite.bz2
-rw-r--r--. 1 root root 556980 Feb 25 02:02 636d97191c77227309938cd1eb97fce7139c880dd52a57d950d1094bd170ddfa-comps_app.xml
-rw-r--r--. 1 root root  18477 Feb 25 02:02 8b15e126c078293ffb47e38f32c73ebf5c19878f920b696fac81e37f974aafaf-primary.sqlite.bz2
-rw-r--r--. 1 root root   9714 Feb 25 02:02 9e9d9bfebabb219028fc2fb1b479b1868dc30aecfd07ce7da75935183a9ba644-primary.xml.gz
-rw-r--r--. 1 root root 116050 Feb 25 02:02 e9524923e26fe119880aa4929454fcff9ed8877f7e5ace47928805a5cbeeb920-comps_app.xml.gz
-rw-r--r--. 1 root root  10460 Feb 25 02:02 fbc30afdbd054a4868d5d3fa10f91d647e95d833ebe0452a8fb15167708a643f-filelists.xml.gz
-rw-r--r--. 1 root root   4316 Feb 25 02:02 repomd.xml

BaseOS/repodata/:
total 2680
-rw-r--r--. 1 root root 153383 Feb 25 01:38 38630dc278c0790d7a0ca019208c223dc9cdd384f265272e6b3489effa7ae63c-other.xml.gz
-rw-r--r--. 1 root root 347027 Feb 25 01:38 586a8e96ad3b73414e5c6ae94a5e4f128a087fcb0ca9837674573a8d6cac4a9c-comps_base.xml
-rw-r--r--. 1 root root 373357 Feb 25 01:38 77b091786d530232d4465543457fe73307a61d359dbd3704abb9656b38d3d339-filelists.sqlite.bz2
-rw-r--r--. 1 root root 426066 Feb 25 01:38 7c83828e61d655fa0908d64f69d9f0b88c72972efb880c5e0285d3f9dd7eb3ec-primary.xml.gz
-rw-r--r--. 1 root root 185860 Feb 25 01:38 ae1b59594f9141e982cf97cab724c4f2c454814740edde657791bf1f2f75a7d2-other.sqlite.bz2
-rw-r--r--. 1 root root  78056 Feb 25 01:38 c1437f9c1bd0a3940a7ef97288dbfc6247fa71f9fded4689cc3d16213ad78897-comps_base.xml.gz
-rw-r--r--. 1 root root 298383 Feb 25 01:38 d6b6730e96a51b83763e5e75109e603a5023b7237496fb3ee47567f65d700cfb-filelists.xml.gz
-rw-r--r--. 1 root root 857212 Feb 25 01:38 e1c8b7e4a5d8da239510f494f92cb156db3722dcf4b1bf7b24553ab3c3974cf7-primary.sqlite.bz2
-rw-r--r--. 1 root root   3873 Feb 25 01:38 repomd.xml

Next we need to create modules file for AppStream repository because if you check the repodata in the original DVD, it will have one additional file:

[root@centos8 custom_iso]# ls -l /mnt/AppStream/repodata/*modules*
-r--r--r--. 1 root root 46270 Aug 16  2019 /mnt/AppStream/repodata/f8cbb89b908e10e2e01b6b5555a116ba365f96cd4b5aa8414aa5465f82a021fd-modules.yaml.gz

So we must add the modular metadata (modulemd) using modifyrepo_c inside the same repodata of our custom ISO. To achieve this we must copy the original modules.xml from the source ISO:

[root@centos8 custom_iso]# cp /mnt/AppStream/repodata/*modules.yaml* AppStream/

Unzip the content of this file as it is in .gz format:

[root@centos8 custom_iso]# cd AppStream

[root@centos8 AppStream]# gunzip f8cbb89b908e10e2e01b6b5555a116ba365f96cd4b5aa8414aa5465f82a021fd-modules.yaml.gz

[root@centos8 AppStream]# ls -l *modules*.yaml
total 876
-r--r--r--. 1 root root 281029 Feb 25 02:01 f8cbb89b908e10e2e01b6b5555a116ba365f96cd4b5aa8414aa5465f82a021fd-modules.yaml

We will rename the file for easier usage:

[root@centos8 AppStream]# cp f8cbb89b908e10e2e01b6b5555a116ba365f96cd4b5aa8414aa5465f82a021fd-modules.yaml modules.yaml

Next to add the modular metadata (modulemd), run:

[root@centos8 custom_iso]# modifyrepo_c --mdtype=modules AppStream/modules.yaml AppStream/repodata/

This will create the respective modules.yaml file inside AppStream/repodata directory:

[root@centos8 custom_iso]# ls -l AppStream/repodata/
total 808
-rw-r--r--. 1 root root  10856 Feb 25 02:02 231deccd5b8e4dc32047ccbe4f6b0b0683a6b0253cc3e3dc89093b6ae9e522da-other.xml.gz
-rw-r--r--. 1 root root  15238 Feb 25 02:02 39e06ef307b975417fcfd5ec9bc23c84495407c136e430837eda3508140e7964-other.sqlite.bz2
-rw-r--r--. 1 root root  19034 Feb 25 02:02 4d1f47a7d096fdbe9bdfccb84ea596be9a09ec6d51cf0b8b67f5c3663775261d-filelists.sqlite.bz2
-rw-r--r--. 1 root root 556980 Feb 25 02:02 636d97191c77227309938cd1eb97fce7139c880dd52a57d950d1094bd170ddfa-comps_app.xml
-rw-r--r--. 1 root root  18477 Feb 25 02:02 8b15e126c078293ffb47e38f32c73ebf5c19878f920b696fac81e37f974aafaf-primary.sqlite.bz2
-rw-r--r--. 1 root root   9714 Feb 25 02:02 9e9d9bfebabb219028fc2fb1b479b1868dc30aecfd07ce7da75935183a9ba644-primary.xml.gz
-rw-r--r--. 1 root root 116050 Feb 25 02:02 e9524923e26fe119880aa4929454fcff9ed8877f7e5ace47928805a5cbeeb920-comps_app.xml.gz
-rw-r--r--. 1 root root  46270 Feb 25 02:02 f8cbb89b908e10e2e01b6b5555a116ba365f96cd4b5aa8414aa5465f82a021fd-modules.yaml.gz
-rw-r--r--. 1 root root  10460 Feb 25 02:02 fbc30afdbd054a4868d5d3fa10f91d647e95d833ebe0452a8fb15167708a643f-filelists.xml.gz
-rw-r--r--. 1 root root   4316 Feb 25 02:02 repomd.xml

So finally our repository configuration is completed, so let's move on to the next step.

Step 6: Create kickstart configuration file

I hope you are aware that in RHEL/CentOS we use kickstart to automate the installation procedure. I have already written a detailed article on kickstart configuration along with a sample kickstart file. Here you can use the kickstart template from the VM which you might have created at Step 3 (Method-2) under /root/anaconda-ks.cfg

I will not be able to explain all the individual section of the kickstart as that is out of scope for this article, I have just placed a sample kickstart from my working node where I verified these steps:

#platform=x86, AMD64, or Intel EM64T
#version=RHEL8
repo --name="AppStream" --baseurl=file:///run/install/repo/AppStream

# Only use sda disk
ignoredisk --only-use=sda

# System bootloader configuration
bootloader --append="rhgb novga console=ttyS0,115200 console=tty0 panic=1" --location=mbr --driveorder="sda" --boot-drive=sda

# Clear the Master Boot Record
zerombr

# Partition clearing information
clearpart --all

# Reboot after installation
reboot

# Use text mode install
text

# Use CDROM
cdrom

# Keyboard layouts
keyboard --vckeymap=us --xlayouts=''

# System language
lang en_US.UTF-8

# Installation logging level
logging --level=info

# Network information
network --bootproto=dhcp --device=enp0s3 --noipv6 --activate --hostname server1.example.com

# Root password
rootpw --iscrypted $1$oXhMRzpse6FeGBc1uF2JmG2xTeSWPL9

# System authorization information
authselect --enableshadow --passalgo=sha512

# SELinux configuration
selinux --disabled

firstboot --disable

# Do not configure the X Window System
skipx

# System services
services --disabled="kdump,rpcbind,sendmail,postfix,chronyd"

# System timezone
timezone Asia/Kolkata --isUtc --ntpservers=10.43.12.34

# Disk partitioning information
autopart --type=lvm

%packages
@^minimal-environment
smartmontools
sysstat
tmux
tuned
zip
%end

%addon com_redhat_kdump --disable --reserve-mb='auto'
%end

You can also add %pre and %post section in the kickstart to add your custom scripts and configurations.

Place this kickstart file in the home folder of the custom ISO i.e. /data/custom_iso in our example:

[root@centos8 custom_iso]# ls -l ks.cfg
-rw-------. 1 root root 1845 Feb 25 01:32 ks.cfg

Step 7: Create/Modify the GRUB2 Menu

Next we will modify the GRUB2 splash screen which gives us the option to perform the installation, you could also modify the splash screen of the GRUB2 menu. We will use the default configuration file with little modification to support our customization.

Locate isolinux.cfg under isolinux directory of the custom ISO and modify the highlighted section:

FROM:

label linux
  menu label ^Install CentOS Linux 8.0.1905
  kernel vmlinuz
  append initrd=initrd.img inst.stage2=hd:LABEL=CentOS-8-BaseOS-x86_64 quiet

TO:

label linux
  menu label ^Install CentOS Linux 8
  kernel vmlinuz
  append initrd=initrd.img inst.repo=cdrom ks=cdrom:/ks.cfg quiet

If you have placed you kickstart file at a different location then you can modify the above entry accordingly.

Step 8: Create custom ISO

So finally we are at the final stages of this article. You are just one step away from having your own custom ISO. We require mkisofs tool to create our custom ISO which is part of genisoimage in RHEL/CentOS 8 so to install this you can use yum/dnf:

~]# dnf install genisoimage

Next use the following command to generate your ISO (assuming you are under /data/custom_iso):

[root@centos8 custom_iso]# mkisofs -o /tmp/new.iso -b isolinux/isolinux.bin -c isolinux/boot.cat --no-emul-boot --boot-load-size 4 --boot-info-table -J -R -V "CentOS-8-2-2004-x86_64-dvd" .

Here this command will generate /tmp/new.iso with the LABEL CentOS-8-2-2004-x86_64-dvd.

Step 9: Validating the ISO

Now that we have our custom ISO ready, it is time to create a new VM using this image. Once the VM boots using the custom image, select the option to "Install CentOS Linux 8" and press TAB which will show you the isolinux.cfg content which we modified in Step 7

Hit Enter to start the installation:

So as you see the installation has started successfully.

Summary

In this tutorial we learned how to create custom ISO with minimzed package list in RHEL/CentOS 8. You may feel some parts of this article complicate, specially the step where we generated our repodata. In such case you can always choose to take the default repodata from the original DVD for both BaseOS and AppStream and that should also work but the repodata database will contain details of rpms which are actually not present in the DVD. Although this will not impact the functioning of the ISO. You can further enhance the steps by adding %pre scripts to prompt of user password or network details which can then be parsed into the kickstart.

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