PXE Network Boot Lab

A PXE network boot server that boots a legacy computer over the network using DHCP and TFTP, no local OS required.

LinuxPXE BootDHCPTFTPNetworking

Overview

PXE (Preboot Execution Environment) enables computers to boot from the network instead of local storage. The client downloads the bootloader, kernel, and root filesystem over the network, no local OS required. This is the backbone of centralized operating system deployment in enterprise environments: data centers, computer labs, and server farms.

Boot sequence: Power on → PXE firmware initializes NIC → DHCP Discover → DHCP Offer (IP + TFTP server + bootloader filename) → Download bootloader via TFTP → Download kernel → Download initramfs → Linux boots from RAM.

Architecture

The lab runs both DHCP and TFTP services on a single Linux server. The client is a legacy desktop with a BIOS/UEFI NIC that supports PXE boot.

DHCP Server (dhcpd / isc-dhcp-server): Hands out an IP address and the critical PXE options:

  • Option 66: TFTP server name (IP or hostname)
  • Option 67: Bootloader filename (pxelinux.0 for BIOS, grubx64.efi for UEFI)
  • Standard options: subnet mask, gateway, DNS, lease time

TFTP Server (tftpd-hpa): Serves the boot files from /srv/tftp:

  • pxelinux.0, SYSLINUX/PXELINUX bootloader (BIOS)
  • grubx64.efi, GRUB2 bootloader (UEFI)
  • vmlinuz, Linux kernel
  • initrd.img, Initial RAM filesystem
  • pxelinux.cfg/default, Boot menu configuration

Client Firmware Flow:

  1. NIC initializes and broadcasts DHCP Discover
  2. Server replies with Offer containing Options 66/67
  3. Client TFTP-downloads the bootloader specified in Option 67
  4. Bootloader loads kernel and initramfs via TFTP
  5. Kernel executes, system boots entirely from RAM

Network Topology: Simple flat network, Client ↔ Switch/Router ↔ Linux PXE Server (runs both DHCP and TFTP). No VLANs, no proxy DHCP, no separate boot server.

Challenges Faced

| Problem | Cause | Solution | |---------|-------|----------| | DHCP gives IP but no boot info | Missing options 66/67 in dhcpd.conf | Added option tftp-server-name "192.168.1.10"; filename "pxelinux.0"; | | Kernel "File not found" | TFTP root structure mismatch, typo, restrictive perms | Fixed naming, chmod 644 on kernel/initrd, verified TFTP root config | | "No boot file received" | Firewall, wrong filename, connectivity | Allowed UDP 67/68/69, confirmed same subnet, verified pxelinux.0 exists, used tcpdump | | Boots from HDD not PXE | Network boot not prioritized in BIOS | Enabled PXE, moved Network to top of boot order, disabled fast boot | | UEFI client fails with pxelinux.0 | Client UEFI-only, needs grubx64.efi | Identified firmware type, used GRUB2, placed in /srv/tftp/grub2/, updated DHCP filename |

Solutions Implemented

DHCP Configuration (/etc/dhcp/dhcpd.conf):

subnet 192.168.1.0 netmask 255.255.255.0 {
  range 192.168.1.100 192.168.1.200;
  option routers 192.168.1.1;
  option domain-name-servers 192.168.1.1;
  option tftp-server-name "192.168.1.10";
  filename "pxelinux.0";
}

TFTP Setup: Installed tftpd-hpa, set TFTP_DIRECTORY="/srv/tftp", TFTP_OPTIONS="--secure --create", ensured world-readable permissions on all boot files.

Firewall: Opened UDP 67 (DHCP server), 68 (DHCP client), 69 (TFTP), critical since both DHCP and TFTP run over UDP.

BIOS/UEFI Handling: Detected client firmware type upfront. BIOS gets PXELINUX; UEFI gets GRUB2. DHCP filename option must match the client's firmware.

Troubleshooting Toolkit: tcpdump -i eth0 port 67 or port 68 or port 69 to verify DHCP/TFTP exchange; journalctl -u tftpd-hpa -u isc-dhcp-server for service logs; dhcping to test DHCP responses.

Lessons Learned

  • DHCP options 66 and 67 are the linchpin: without them, the client gets an IP but has no idea where to fetch the bootloader. Most "PXE not working" issues trace back here.
  • TFTP is deliberately primitive: UDP port 69, no auth, 512-byte blocks, no directory listing. That simplicity is why it fits in NIC firmware. But it means permissions and exact paths matter.
  • UDP unreliability is a feature, not a bug: The client firmware handles retransmission. You don't need TCP's overhead in 16KB of boot ROM.
  • Bootloader mismatch is the silent killer: Serving pxelinux.0 to a UEFI-only machine fails with zero useful error. Always verify client firmware type first.
  • Firewall eats PXE silently: UDP 67/68/69 must be open on the server. tcpdump on the server side confirms whether packets even arrive.
  • Diskless computing trades local storage for network dependency: The root filesystem lives in RAM (initramfs) or over NFS/iSCSI/HTTP. Centralized management and security improve, but the network becomes a single point of failure.

Future Improvements

  • iPXE Integration: HTTP boot, scripting, SAN boot, chainloading, better UEFI support. iPXE replaces PXELINUX/GRUB with a scriptable, feature-rich bootloader.
  • HTTP Boot: More reliable than TFTP, supports larger files, modern UEFI native support.
  • Automated OS Deployment: Cobbler/Foreman for provisioning, Kickstart/preseed for unattended installs, Ansible/Puppet for post-install configuration.
  • Multi-OS Support: Boot menu offering multiple distributions, live ISO boot via PXE.
  • Secure Boot: Signed bootloaders/kernels, shim.efi, Microsoft key enrollment for production environments.
  • NFS Root: Persistent storage, shared read-only root with overlayfs for true diskless operation.
  • Centralized Logging: syslog-ng/rsyslog over network for audit trails.
  • HA/Failover: Multiple DHCP/TFTP servers, dhcp-failover, synced TFTP roots for production resilience.