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Creating a Custom Ubuntu Server Installation Image

This guide walks through the process of creating a customized Ubuntu Server installation image with automated setup.

Prerequisites

Install the required tools:

sudo apt install xorriso

Step 1: Extract the Original ISO Contents

# Create working directory
mkdir -p ubuntu-custom/{extract,iso}

# Enter the working directory
cd ubuntu-custom

# Download the iso file if we don't have it already
test -e ubuntu-24.04.2-live-server-amd64.iso || curl -O https://releases.ubuntu.com/24.04.2/ubuntu-24.04.2-live-server-amd64.iso

# Mount the original ISO
sudo mount -o loop ubuntu-24.04.2-live-server-amd64.iso iso

# Copy all contents
rsync -av iso/ extract/

# Unmount when done
sudo umount iso/

Step 2: Create Autoinstall Configuration

# Create directory for autoinstall files
chmod u+w extract/casper/
mkdir -p extract/casper/autoinstall/

# Create user-data file with your configuration
cat > extract/casper/autoinstall/user-data << 'EOF'
#cloud-config
autoinstall:
  version: 1
  locale: en_US
  keyboard:
    layout: us
  storage:
    layout:
      name: direct
  identity:
    hostname: myserver
    username: user
    # Generate with: mkpasswd --method=SHA-512
    password: $6$rounds=4096$....
  ssh:
    install-server: yes
    authorized-keys:
      - ssh-rsa AAAAB3N....
  packages:
    - openssh-server
    - fail2ban
    - ufw
  user-data:
    disable_root: true
  late-commands:
    - echo 'user ALL=(ALL) NOPASSWD:ALL' > /target/etc/sudoers.d/user
    - curtin in-target -- ufw allow OpenSSH
    - curtin in-target -- ufw --force enable
EOF

# Create empty meta-data file (required)
touch extract/casper/autoinstall/meta-data

Step 3: Modify Boot Configuration

Edit the GRUB configuration to enable autoinstall:

# Backup original config
chmod u+w extract/boot/grub/
cp extract/boot/grub/grub.cfg extract/boot/grub/grub.cfg.backup

# Edit the grub.cfg file
nano extract/boot/grub/grub.cfg

Replace or modify the first menuentry to:

menuentry "Automated Ubuntu Server Install" {
    set gfxpayload=keep
    linux /casper/vmlinuz autoinstall ds=nocloud\;s=/cdrom/casper/autoinstall/ quiet ---
    initrd /casper/initrd
}

Step 4: Create the Custom ISO

xorriso -as mkisofs -r \
  -V "Ubuntu Custom" \
  -o custom-ubuntu.iso \
  -J -joliet-long \
  -b boot/grub/i386-pc/eltorito.img \
  -no-emul-boot -boot-load-size 4 -boot-info-table \
  -eltorito-alt-boot -e EFI/boot/bootx64.efi \
  -no-emul-boot \
  extract/

Step 5: Test the ISO

Test the ISO using a virtual machine:

# With QEMU
qemu-img create -f qcow2 test-disk.qcow2 20G
qemu-system-x86_64 -cdrom custom-ubuntu.iso \
  -drive file=test-disk.qcow2,format=qcow2,if=virtio \
  -m 2048

Additional Customization Options

You can expand the user-data file to include:

Additional packages
Custom scripts to run during installation
Network configuration
More complex storage layouts with custom partitioning
Time zone and locale settings

Troubleshooting

If the installer doesn't automate:

Check that the paths in the kernel parameters match where you placed the autoinstall files
Verify the syntax of your user-data file
Ensure both user-data and meta-data files exist

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