PXE (Preboot Execution Environment) enables computers to boot from a network server, facilitating automated, large-scale operating system installations without physical media. This method relies on a combination of DHCP, TFTP, and FTP services to deliver boot files and installation images to client machines.

Boot Methods for System Installation

• Local Disk Boot: Initiates from an existing operating system on a hard drive.
• External Media Boot: Uses optical drives or USB sticks to load installation media.
• Network Boot (PXE): Downloads boot images and installation files from a remote server over the network.

System Installation Sequence

1. Boot Loader Initialization: Loads the bootloader to set up hardware and memory.
2. Boot Menu Display: Presents installation options to the user.
3. Kernel and Initrd Loading: Loads the Linux kernel and initial RAM disk for the installation environment.
4. Root Filesystem Mounting: Prepares the root filesystem for the instaler.
5. Anaconda Execution: Launches the graphical or text-based installation wizard.

PXE Overview

PXE is a network boot standard that allows systems to retrieve boot files via TFTP after obtaining an IP address from a DHCP server. Key components include:

• PXE-Enabled NIC: Network interface card with PXE firmware to initiate network booting.
• DHCP Server: Assigns IP addresses and directs clients to the TFTP server.
• TFTP Server: Hosts boot files like the PXE bootloader, kernel, and initrd.

Advantages of PXE:

• Scalability: Supports simultaneous installation on multiple systems.
• Automation: Reduces manual intervention during deployment.
• Remote Capability: Eliminates the need for physical installation media.

Prerequisites:

• Client hardware must support PXE booting.
• A DHCP server configured for PXE opsions.
• A TFTP server to serve boot files.

Kickstart for Unattended Installation

Kickstart automates the installation process by using a configuration file (ks.cfg) that specifies installation parameters, such as partitioning, software selection, and network settings. This file is fetched during the PXE boot to guide the installation without user input.

Setting Up a PXE Server

Environment Setup

• Server IP: 192.168.10.17
• Network: Use a dedicated subnet (e.g., 192.168.100.0/24) for PXE clients.
• Disable SELinux and firewall on the server.

Required Packages

Install the following on the PXE server:

yum install -y dhcp tftp-server syslinux xinetd vsftpd system-config-kickstart

DHCP Configuration

Edit /etc/dhcp/dhcpd.conf:

ddns-update-style none;
subnet 192.168.100.0 netmask 255.255.255.0 {
    range 192.168.100.40 192.168.100.50;
    option routers 192.168.100.100;
    option domain-name-servers 114.114.114.114;
    next-server 192.168.100.100;
    filename "pxelinux.0";
}

Start and enable DHCP:

systemctl start dhcpd
systemctl enable dhcpd

TFTP Configuration

Configure /etc/xinetd.d/tftp:

service tftp
{
    socket_type = dgram
    protocol = udp
    wait = no
    user = root
    server = /usr/sbin/in.tftpd
    server_args = -s /var/lib/tftpboot
    disable = no
    flags = IPv4
}

Start services:

systemctl start xinetd tftp
systemctl enable xinetd tftp

Boot File Preparation

Copy necessary files to the TFTP directory:

cp /usr/share/syslinux/pxelinux.0 /var/lib/tftpboot/
mount /dev/cdrom /mnt
cp /mnt/images/pxeboot/vmlinuz /var/lib/tftpboot/
cp /mnt/images/pxeboot/initrd.img /var/lib/tftpboot/

Boot Menu Configurasion

Create /var/lib/tftpboot/pxelinux.cfg/default:

default auto
prompt 1

label auto
    kernel vmlinuz
    append initrd=initrd.img method=ftp://192.168.100.100/centos7

label text
    kernel vmlinuz
    append text initrd=initrd.img method=ftp://192.168.100.100/centos7

label rescue
    kernel vmlinuz
    append rescue initrd=initrd.img method=ftp://192.168.100.100/centos7

FTP Setup for Installation Media

Start and enable vsftpd:

systemctl start vsftpd
systemctl enable vsftpd

Place the CentOS installation files in /var/ftp/centos7.

Kickstart Configuration

Generate a ks.cfg file using system-config-kickstart or manually create it. Example content:

install
keyboard 'us'
rootpw --iscrypted $1$9Rn8W7vF$npbKh8CeIbTxjwHUlcW4n0
url --url="ftp://192.168.100.100/centos7"
lang en_US
auth --useshadow --passalgo=sha512
text
firstboot --disable
selinux --disabled
firewall --disabled
network --bootproto=dhcp --device=ens33
reboot
timezone Asia/Shanghai
bootloader --location=mbr
zerombr
clearpart --all --initlabel
part / --fstype="xfs" --grow --size=1
part swap --fstype="swap" --size=2048
part /boot --fstype="xfs" --size=300
%packages
@base
@^gnome-desktop-environment
%end

Copy the file to the FTP server:

cp ks.cfg /var/ftp/

Update the PXE boot menu to include the Kickstart file:

default auto
prompt 1

label auto
    kernel vmlinuz
    append initrd=initrd.img method=ftp://192.168.100.100/centos7 ks=ftp://192.168.100.100/ks.cfg

label text
    kernel vmlinuz
    append text initrd=initrd.img method=ftp://192.168.100.100/centos7 ks=ftp://192.168.100.100/ks.cfg

label rescue
    kernel vmlinuz
    append rescue initrd=initrd.img method=ftp://192.168.100.100/centos7 ks=ftp://192.168.100.100/ks.cfg

Post-Installation Scripts

Create a local YUM repository for the installed system:

mkdir -p /etc/yum.repos.d/backup
mv /etc/yum.repos.d/*.repo /etc/yum.repos.d/backup/
echo '[local]
name=Local Repository
baseurl=ftp://192.168.100.100/centos7
enabled=1
gpgcheck=0' > /etc/yum.repos.d/local.repo

NTP Configuration for Time Synchronization

Install and configure NTP to keep server time accurate:

yum install -y ntp ntpdate

Edit /etc/ntp.conf:

restrict 192.168.100.0 mask 255.255.255.0 nomodify notrap
server ntp1.aliyun.com
server ntp2.aliyun.com prefer

Start and enable NTP:

systemctl start ntpd
systemctl enable ntpd
ntpdate ntp1.aliyun.com

Check synchronization status:

ntpq -p
ntpstat