什么是PXE
1. PXE，全名Pre-boot Execution Environment，预启动执行环境
2. 通过网络接口启动计算机，不依赖本地存储设备或本地已安装的操作系统
3. 由intel和systemsoft公司于1999年9月20日公布的技术
4. client/server的工作模式
5. PXE客户端会调用网际协议（ip），用户数据报协议（udp），动态主机设定协议（dhcp），小型文件传输协议（TFTP）等网络协议
6.PXE客户端（client）这个术语是指机器在PXE启动过程中的角色。一个PXE客户端可以是一台服务器，笔记本电脑或者其他装有PXE启动代码的机器
PXE的工作流程
，
环境准备
1.安装基础软件，如果是最小化安装的话
yum install -y bash-completion vim lrzsz wget expect net-tools nc nmap tree dos2unix htop iftop iotop unzip telnet sl psmisc nethogs glances bc ntpdate
2.配置yum源
wget -O /etc/yum.repos.d/CentOS-Base.repo http://mirrors.aliyun.com/repo/Centos-7.repo
wget -O /etc/yum.repos.d/epel.repo http://mirrors.aliyun.com/repo/epel-7.repo
yum clean all
yum makecache
3.关闭防火墙
[root@kickstart yum.repos.d]# systemctl stop firewalld
[root@kickstart yum.repos.d]# systemctl disable firewalld
Removed symlink /etc/systemd/system/multi-user.target.wants/firewalld.service.
Removed symlink /etc/systemd/system/dbus-org.fedoraproject.FirewallD1.service.
[root@client01 ~]# sed -i 's/enforcing/disabled/g' /etc/selinux/config
[root@kickstart ~]# getenforce
Disabled
4.关闭vmware的dhcp服务，设置linux静态ip
[root@kickstart ~]# cat /etc/sysconfig/network-scripts/ifcfg-ens33
TYPE="Ethernet"
PROXY_METHOD="none"
BROWSER_ONLY="no"
BOOTPROTO="static"
DEFROUTE="yes"
IPV4_FAILURE_FATAL="no"
IPV6INIT="yes"
IPV6_AUTOCONF="yes"
IPV6_DEFROUTE="yes"
IPV6_FAILURE_FATAL="no"
IPV6_ADDR_GEN_MODE="stable-privacy"
NAME="ens33"
UUID="b4c06569-fc94-4e94-9c63-0391c7a35432"
DEVICE="ens33"
ONBOOT="yes"
IPADDR=172.18.41.133
NETMASK=255.255.255.0
DNS1=1.2.4.8
GATEWAY=172.18.41.2
5.系统检查
[root@kickstart ~]# cat /etc/redhat-release
CentOS Linux release 7.5.1804 (Core)
[root@kickstart ~]# uname -r
3.10.0-862.el7.x86_64
[root@kickstart ~]# ifconfig ens33 |awk 'NR==2 {print $2}'
172.18.41.133
[root@kickstart ~]# iptables -L
Chain INPUT (policy ACCEPT)
target prot opt source destination
Chain FORWARD (policy ACCEPT)
target prot opt source destination
Chain OUTPUT (policy ACCEPT)
target prot opt source destination
6.时间同步更新
[root@kickstart ~]# ntpdate -u ntp.aliyun.com
修改时区
[root@kickstart ~]# ln -sf /usr/share/zoneinfo/Asia/Shanghai /etc/localtime
[root@kickstart ~]# date
Tue Jul 21 19:05:06 CST 2020
DHCP报文
DHCP协议中的报文 DHCP报文共有一下几种： DHCP DISCOVER ：客户端开始DHCP过程发送的包，是DHCP协议的开始 DHCP OFFER ：服务器接收到DHCP DISCOVER之后做出的响应，它包括了给予客户端的IP（yiaddr）、客户端的MAC地址、租约过期时间、服务器的识别符以及其他信息 DHCP REQUEST ：客户端对于服务器发出的DHCP OFFER所做出的响应。在续约租期的时候同样会使用。 DHCP ACK ：服务器在接收到客户端发来的DHCP REQUEST之后发出的成功确认的报文。在建立连接的时候，客户端在接收到这个报文之后才会确认分配给它的IP和其他信息可以被允许使用。 DHCP NAK ：DHCP ACK的相反的报文，表示服务器拒绝了客户端的请求。 DHCP RELEASE ：一般出现在客户端关机、下线等状况。这个报文将会使DHCP服务器释放发出此报文的客户端的IP地址 DHCP INFORM ：客户端发出的向服务器请求一些信息的报文 DHCP DECLINE :当客户端发现服务器分配的IP地址无法使用（如IP地址冲突时），将发出此报文，通知服务器禁止使用该IP地址。
DHCP作用域
作用域是指DHCP服务器可分配租用给DHCP客户机的IP地址范围。DHCP服务器应该至少配置一个作用域，各作用域的IP地址范围（IP地址池）不能出现重叠。
保留IP
保留IP用于有特定服务的客户机，需要将保留的IP与客户机的MAC地址绑定。
IP租约的跟新
当客户机重新启动或租期达50%时，就需要重新更新租约，客户机直接向提供租约的服务器发送请求DHCPREQUEST消息，要求更新现有的地址租约。如果DHCP服务器接收到请求，它将发送DHCP确认信息给客户机，更新客户机租约。如果客户机无法与提供租约的服务器取得联系，则客户机一直等到租期到达87.5%时，进入到一种重新申请的状态，它向网络上所有的服务器广播DHCPDISCOVER请求更新现有的IP租约。
IP租约的释放
使用ipconfig/release使DHCP客户机向DHCP服务器发送DHCPRELEASE消息并释放其租约。
如果客户机在租约时间内保持关闭并且不更新租约，在租约到期后DHCP服务器可能将客户机的IP地址分配给其他的客户机。如果客户机不发送DHCPRELEASE消息，那么它在重启后，将试图尝试继续使用上一次使用过的IP地址。
部署DHCP服务器
# 机器1 dhcp服务端
[root@kickstart ~]# ifconfig ens33 |awk 'NR==2 {print $2}'
172.18.41.133
[root@kickstart ~]# hostname
kickstart
[root@kickstart ~]# yum install dhcp -y
# 查询软件包信息
[root@kickstart ~]# rpm -qa dhcp
dhcp-4.2.5-79.el7.centos.x86_64
# 查看dhcp配置文件
[root@kickstart ~]# cat /etc/dhcp/dhcpd.conf
#
# DHCP Server Configuration file.
# see /usr/share/doc/dhcp*/dhcpd.conf.example
# see dhcpd.conf(5) man page
#
# 查看DHCP示例配置文件
#
# DHCP Server Configuration file.
# see /usr/share/doc/dhcp*/dhcpd.conf.example
# see dhcpd.conf(5) man page
#
# dhcpd.conf
#
# Sample configuration file for ISC dhcpd
#
# option定义选项
# option definitions common to all supported networks...
option domain-name "example.org";
option domain-name-servers ns1.example.org, ns2.example.org;
default-lease-time 600; #默认租约时间单位秒
max-lease-time 7200; #最大租约时间单位秒
# Use this to enble / disable dynamic dns updates globally.
#ddns-update-style none;
# If this DHCP server is the official DHCP server for the local
# network, the authoritative directive should be uncommented.
#authoritative;
# Use this to send dhcp log messages to a different log file (you also
# have to hack syslog.conf to complete the redirection).
log-facility local7;
# No service will be given on this subnet, but declaring it helps the
# DHCP server to understand the network topology.
subnet 10.152.187.0 netmask 255.255.255.0 { #定义子网
}
# This is a very basic subnet declaration.
subnet 10.254.239.0 netmask 255.255.255.224 {
range 10.254.239.10 10.254.239.20; #指定IP地址池
# # 定义其他参数，如 dns,gateway
option routers rtr-239-0-1.example.org, rtr-239-0-2.example.org;
}
# This declaration allows BOOTP clients to get dynamic addresses,
# which we don't really recommend.
subnet 10.254.239.32 netmask 255.255.255.224 {
range dynamic-bootp 10.254.239.40 10.254.239.60;
option broadcast-address 10.254.239.31;
option routers rtr-239-32-1.example.org;
}
# A slightly different configuration for an internal subnet.
# 选项可以写在子网内，优先级更高，全局优先级低
subnet 10.5.5.0 netmask 255.255.255.224 {
range 10.5.5.26 10.5.5.30;
option domain-name-servers ns1.internal.example.org; #DNS服务器，最多三个
option domain-name "internal.example.org"; #可选 设置默认搜索域
option routers 10.5.5.1; #网关
option broadcast-address 10.5.5.31; #可选 备选广播地址
default-lease-time 600;
max-lease-time 7200;
}
# Hosts which require special configuration options can be listed in
# host statements. If no address is specified, the address will be
# allocated dynamically (if possible), but the host-specific information
# will still come from the host declaration.
# host 用于区别不同的主机名
host passacaglia {
hardware ethernet 0:0:c0:5d:bd:95;
filename "vmunix.passacaglia";
server-name "toccata.fugue.com";
}
# Fixed IP addresses can also be specified for hosts. These addresses
# should not also be listed as being available for dynamic assignment.
# Hosts for which fixed IP addresses have been specified can boot using
# BOOTP or DHCP. Hosts for which no fixed address is specified can only
# be booted with DHCP, unless there is an address range on the subnet
# to which a BOOTP client is connected which has the dynamic-bootp flag
# set.
# host 用于区别不同的主机名，例如针对该MAC地址，设置固定IP，主机名
host fantasia { #主机
hardware ethernet 08:00:07:26:c0:a5; #指定文件服务器MAC地址
fixed-address fantasia.fugue.com; #指定固定IP地址
}
# You can declare a class of clients and then do address allocation
# based on that. The example below shows a case where all clients
# in a certain class get addresses on the 10.17.224/24 subnet, and all
# other clients get addresses on the 10.0.29/24 subnet.
class "foo" {
match if substring (option vendor-class-identifier, 0, 4) = "SUNW";
}
shared-network 224-29 {
subnet 10.17.224.0 netmask 255.255.255.0 {
option routers rtr-224.example.org;
}
subnet 10.0.29.0 netmask 255.255.255.0 {
option routers rtr-29.example.org;
}
pool {
allow members of "foo";
range 10.17.224.10 10.17.224.250;
}
pool {
deny members of "foo";
range 10.0.29.10 10.0.29.230;
}
}
编写DHCP配置文件
[root@kickstart ~]# cat /etc/dhcp/dhcpd.conf
#
# DHCP Server Configuration file.
# see /usr/share/doc/dhcp*/dhcpd.conf.example
# see dhcpd.conf(5) man page
#
subnet 172.18.41.0 netmask 255.255.255.0 {
range 172.18.41.100 172.18.41.199; # range设置起始，结束ip范围
option subnet-mask 255.255.255.0; # 选项，设置掩码
option routers 172.18.41.2; # dhcp服务的网关设置，这里不写，客户端则无法上网
option domain-name-servers 1.2.4.8; # 保证dhcp客户端可以域名解析
default-lease-time 21600; # 默认的IP租用期限
max-lease-time 43200; # 最大的IP租用期限
next-server 172.18.41.133; # 告知客户端tftp服务器的ip
filename "/pxelinux.0"; # 指明引导文件，用于指定PXE的运行程序文件，放在TFTP服务器的目录下
}
【配置文件解释】
filename:指明引导文件名称，用于指定PXE的运行程序文件，一般是在TFTP服务器的工作目录下，这个是关于PXE启动的配置。流程如下：
客户机通过网络启动，一般采用的就是intel的PXE来启动；
PXE首先指定DHCP，获取自身IP地址、TFTP服务器或者NFS服务器的IP地址、PXE程序等内容；
执行获取的PXE程序，获得详细配置内容，再获取linux虚拟系统和intrid等内容；
最后加载整个linux系统到内核。
next-server server-name(一般是IP)：客户端启动后，获得了IP地址，会加载引导文件，这里就定义提供引导文件的服务器IP地址
【启动DHCP服务，检查日志】
[root@kickstart ~]# systemctl start dhcpd [root@kickstart ~]# systemctl is-enabled dhcpd disabled [root@kickstart ~]# systemctl is-active dhcpd active # 部分日志如下 [root@kickstart ~]# tail -f /var/log/messages Jul 21 06:23:06 kickstart systemd: Starting DHCPv4 Server Daemon... Jul 21 06:23:06 kickstart dhcpd: Internet Systems Consortium DHCP Server 4.2.5 Jul 21 06:23:06 kickstart dhcpd: Copyright 2004-2013 Internet Systems Consortium. Jul 21 06:23:06 kickstart dhcpd: All rights reserved. Jul 21 06:23:06 kickstart dhcpd: For info, please visit https://www.isc.org/software/dhcp/ Jul 21 06:23:06 kickstart dhcpd: Not searching LDAP since ldap-server, ldap-port and ldap-base-dn were not specified in the config file Jul 21 06:23:06 kickstart dhcpd: Wrote 0 leases to leases file. Jul 21 06:23:06 kickstart dhcpd: Listening on LPF/ens33/00:0c:29:e9:ed:77/172.18.41.0/24 Jul 21 06:23:06 kickstart dhcpd: Sending on LPF/ens33/00:0c:29:e9:ed:77/172.18.41.0/24 Jul 21 06:23:06 kickstart dhcpd: Sending on Socket/fallback/fallback-net Jul 21 06:23:06 kickstart systemd: Started DHCPv4 Server Daemon.
PXE和TFTP