You can install Linux on a server without booting from a locally attached ISO by using PXE/network boot, starting installer files from an existing disk and bootloader, or provisioning a virtual machine from a network installation source. The best option depends on whether the server is physical or virtual, what it can boot, and whether you control the network services needed to deliver installer files.
What “without booting from an ISO” means
It usually means the server does not start by reading an ISO from a USB drive, optical disc, or virtual CD-ROM. Instead, it starts a small installer environment from the network or from files on an existing disk. The installer can then download packages—or, in Ubuntu’s documented network-boot flow, download and mount an ISO during installation. So avoiding an ISO as the boot device does not always mean avoiding an ISO file altogether.
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This is different from installing an already-running operating system that boots from a network disk or depends on a server at every startup. Here, the network or local boot files start an installer that writes Linux to the target server’s storage.
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| Method | Best fit | What it needs | Important limitation |
|---|---|---|---|
| PXE/network boot | Physical servers, especially when installing several systems | Firmware network-boot support and network boot services, commonly DHCP/BOOTP and TFTP | Coordinate DHCP changes with the network administrator; boot files must match firmware mode and architecture. |
| Installer kernel and initrd from disk | A physical server with an existing usable bootloader, such as GRUB | Access to place compatible installer files on disk and network access for a netboot installation | Repartitioning the disk that contains the boot files can disrupt the installation. |
| Disk-based installer plus local ISO | A server that should not boot from an ISO and may not have internet access | Installer kernel/initrd and an ISO stored on disk | The ISO is still part of the installation, even though it is not the boot device. |
| Hypervisor provisioning | A virtual machine | A hypervisor and a distribution tree, network boot source, ISO, or local installer files | This configures a guest’s virtual boot process, not the firmware of a physical server. |
Use PXE when you can arrange the network services and want a repeatable bare-metal process. Use disk-based boot when the machine already has a reliable boot path and you can safely place files on it. For a guest VM, use the hypervisor’s installer options rather than configuring physical-server firmware.
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Option 1: PXE boot a physical server
PXE (Preboot Execution Environment) lets compatible firmware start an installer over the network. In Canonical’s Ubuntu Server guide for netbooting the amd64 installer, the sequence is: the server requests network configuration; DHCP/BOOTP supplies configuration and a bootloader location; the bootloader retrieves its configuration over TFTP; that configuration identifies a kernel and RAM disk (initrd); and the initrd brings up networking and fetches the server ISO. The installer then mounts the ISO and proceeds with installation.
What you need to arrange
- Firmware support: Enable network boot in firmware or the server’s boot menu. Ubuntu documents PXE for amd64 in both UEFI and legacy BIOS modes; the bootloader files differ by mode.
- Boot services: Provide the network configuration and initial boot files. Ubuntu’s example uses
dnsmasqfor DHCP and TFTP, but its sample configuration assumes a particular network setup. - An approved network plan: Do not start another DHCP service on a production LAN without coordinating with the network administrator. A competing DHCP server can hand out incorrect settings to other devices.
- A reachable installer source: The target needs access to the ISO or installation source specified by the boot configuration. The Ubuntu example can use a release-specific ISO URL or an ISO hosted locally.
Ubuntu’s guide includes a kernel parameter, cloud-config-url=/dev/null, to prevent cloud-init from downloading the ISO a second time in that documented flow. Follow the guide’s current release-specific configuration rather than copying a boot filename, ISO path, or address into a different environment unchanged.
Match boot files to firmware and architecture
Confirm the server’s CPU architecture and whether it is starting in UEFI or legacy BIOS mode. The EFI boot components and legacy PXELINUX route are not interchangeable. Secure Boot requirements also matter: use boot components that the target firmware accepts. Mismatched files can leave a server at a firmware prompt or prevent the installer from starting.
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Option 2: Start installer files from disk or GRUB
If the server already boots an operating system and has a usable GRUB installation, you may be able to put the distribution’s installer kernel and initrd on disk and select them from the bootloader. The Debian stable amd64 Installation Guide describes starting its installer from files on a hard drive. With the netboot kernel and initrd, the installer can repartition that drive, but the guide warns that this must be done carefully. A netboot install still needs network access to fetch the remaining installation components.
Ubuntu’s Community Help Wiki guide to netboot installation from the internet describes a related approach using boot files on disk and an existing bootable GRUB or GRUB2 system. It avoids removable media and a locally configured PXE server, but it depends on a working Linux installation and bootloader. Because the page is community documentation and may not reflect current releases, verify the installer file locations and instructions for the exact release you intend to install.
Protect the boot path before changing partitions
- Keep the installer kernel and initrd somewhere the bootloader can read before the installer starts.
- Check that the installer files belong to a compatible release and architecture.
- Plan how the installer will reach its network source after startup.
- Review the partitioning plan carefully, especially if the boot files are on the disk being repartitioned.
Option 3: Install from disk using a local ISO
If the goal is specifically to avoid booting from an ISO, Debian’s hd-media method is another option: boot the installer’s kernel and initrd from disk and keep the ISO file on the disk for the installer to find. Debian documents this as a way to install without network access. It still requires a local ISO, so it is not the right interpretation of “without an ISO” if you want to avoid the image file entirely.
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Use this route when network access is unavailable or unsuitable and you can safely stage both the installer files and image. Confirm that the installer can locate the ISO and that the image matches the release and architecture you plan to install.
Option 4: Provision a Linux virtual machine
For a virtual machine, the hypervisor can provide an installation source without requiring a physical server to PXE boot. The Ubuntu virt-install manpage for Noble documents --location for a distribution tree served over HTTP or FTP, an ISO, or a local directory; --pxe to network boot on first start; and explicit kernel and initrd paths with installer arguments.
These options control a guest’s virtual devices and installation process. They do not configure network boot in a physical server’s firmware. Check the manpage for the installed virt-install version and use an installation source compatible with the guest architecture.
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Check these details before starting
Firmware mode and architecture
Identify whether the target starts in UEFI or legacy BIOS mode, confirm its CPU architecture, and account for Secure Boot. Select matching bootloader, kernel, and initrd files; Ubuntu’s netboot instructions distinguish EFI components from legacy boot files.
Network ownership and access
PXE commonly relies on DHCP/BOOTP for boot information and TFTP for initial files. Establish who administers those services before changing a network. Also confirm that the target can reach the distribution’s installer source; a boot environment can load successfully and still fail later if the source is unreachable.
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Use boot files, installer sources, and architecture paths that belong together. Ubuntu’s guide includes Noble-specific examples, and release URLs can change. Debian’s stable guide is also release-specific; check the current instructions and paths when implementing the method.
Disk and data risk
Network or disk-based installation does not make partitioning safer. Decide in advance whether the target disk should be erased, repartitioned, or preserved, and verify the installer’s storage choices before committing changes. With the GRUB method, avoid removing or overwriting files the installer still needs to boot.
Quick Recap
Common problems and what to check
- The server skips network boot: Check that network boot is enabled, the correct network interface is selected, and the firmware mode matches the boot files being served.
- The bootloader starts but cannot load its configuration or kernel: Check the DHCP/BOOTP response, TFTP root and filenames, and whether the requested files exist for that client’s firmware mode.
- The installer starts but cannot fetch its source: Check network connectivity from the installer environment, the configured source address, and whether the source is reachable from that network.
- The disk-based installer will not start: Confirm GRUB can read the kernel and initrd, the files match the machine and release, and the disk layout changes will not remove them too soon.
- The installation offers unexpected disks or partitions: Stop before confirming changes and recheck the target disk and partitioning plan. A mistaken selection can overwrite data.
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