Linux搭建DHCP服务器实现自动分配IP地址

Posted 2021-06-08 白-胖-子

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DHCP服务

Dynamic Host Configuration Protocol，动态主机配置协议

DHCP服务的协议和端口

UDP协议，C/S模式，
DHCP server：67/udp,
DHCPv4 client :68/udp，
DHCPv6 client：546/udp

DHCP工作原理

类似于投简历找工作到达租期的一半重新续约，达到7/8时再去续约

DHCP通讯过程基于广播实现
DHCP服务器打开67端口并进行监听
客户端监听UDP的68端口，在网卡启动前发送广播请求
DHCP广播机制存在跨路由问题
DHCP使用中继代理机制，硬件路由器和三层交换机接口要配置agent，
可以替客户端将DHCP请求单播到DHCP服务器，从而实现跨网段

DHCP八种报文

DHCP DISCOVER：客户端到服务器
DHCP OFFER ：服务器到客户端
DHCP REQUEST：客户端到服务器
DHCP ACK ：服务器到客户端
DHCP NAK：服务器到客户端,通知用户无法分配合适的IP地址
DHCP DECLINE ：客户端到服务器，指示地址已被使用
DHCP RELEASE：客户端到服务器，放弃网络地址和取消剩余的租约时间
DHCP INFORM：客户端到服务器, 客户端如果需要从DHCP服务器端获取更为详细的配置信息，则
发送Inform报文向服务器进行请求，极少用到

DHCP工作原理

DCHP常用架构

两台服务器做主备
地址池二八分配

关闭或隔离本网中已有DHCP服务器

实现DHCP服务前，先将网络已有DHCP服务，
如：vmware中的DHCP关闭，访止冲突

DHCP服务器实现

dchp可以通过多个软件实现
这里我们使用系统自带的
dhcp（CentOS 7 之前版本）
dhcp-server（CentOS 8 中的包名）
DHCP搭建三步走
- 安装DHCP服务器软件
- 按需修改配置文件
- 启动并设为开机启动

安装DHCP服务器软件

在CentOS 8中进行安装

检查DHCP服务是否已安装

rpm -q dchp-server
未安装软件包 dchp-server

查看DHCP安装包信息

yum info dhcp-server

yum info dhcp-server
BaseOS                                                          1.0 kB/s | 3.9 kB     00:03    
AppStream                                                        52 kB/s | 4.3 kB     00:00    
EPEL                                                             52 kB/s | 4.7 kB     00:00    
extras                                                           19 kB/s | 1.5 kB     00:00    
centosplus                                                       17 kB/s | 1.5 kB     00:00    
可安装的软件包
名称         : dhcp-server
时期         : 12
版本         : 4.3.6
发布         : 41.el8
架构         : x86_64
大小         : 530 k
源           : dhcp-4.3.6-41.el8.src.rpm
仓库         : BaseOS
概况         : Provides the ISC DHCP server
URL          : http://isc.org/products/DHCP/ ## 官方网站
协议         : ISC
描述         : DHCP (Dynamic Host Configuration Protocol) is a protocol which allows
             : individual devices on an IP network to get their own network
             : configuration information (IP address, subnetmask, broadcast address,
             : etc.) from a DHCP server. The overall purpose of DHCP is to make it
             : easier to administer a large network.
             : 
             : This package provides the ISC DHCP server.

安装DHCP服务器

yum -y install dhcp-server

systemctl status dhcpd
● dhcpd.service - DHCPv4 Server Daemon
   Loaded: loaded (/usr/lib/systemd/system/dhcpd.service; disabled; vendor preset: disabled)
   Active: inactive (dead)
     Docs: man:dhcpd(8)
           man:dhcpd.conf(5)

查看安装后的文件列表
关注几个重要的文件，比如主程序文件、配置文件、安装目录等

rpm -ql dhcp-server

rpm -ql dhcp-server
/etc/NetworkManager
/etc/NetworkManager/dispatcher.d
/etc/NetworkManager/dispatcher.d/12-dhcpd
/etc/dhcp
/etc/dhcp/dhcpd.conf ## 主配置文件
/etc/dhcp/dhcpd6.conf ## IPv6配置文件
……
/usr/lib/systemd/system/dhcpd.service ## 主程序服务
/usr/lib/systemd/system/dhcpd6.service ## IPv6主程序
/usr/sbin/dhcpd ## 主程序文件
/usr/share/doc/dhcp-server
……
/usr/share/doc/dhcp-server/dhcpd.conf.example ## 配置模板
/usr/share/doc/dhcp-server/dhcpd6.conf.example## 配置模板
……
/var/lib/dhcpd ##分配地址库文件的目录
/var/lib/dhcpd/dhcpd.leases ## IPv4地址库
/var/lib/dhcpd/dhcpd6.leases ## IPv6地址库

修改DHCP配置文件

DHCP服务刚装好是无法直接启动的
安装后如果起不来，看日志

安装后配置文件是空的

cat /etc/dhcp/dhcpd.conf 
#
# DHCP Server Configuration file.
#   see /usr/share/doc/dhcp-server/dhcpd.conf.example
#   see dhcpd.conf(5) man page
#

备份源文件，并将模板文件copy过来

mv /etc/dhcp/dhcpd.conf /etc/dhcp/dhcpd.conf.bak`date +%F` 
cp /usr/share/doc/dhcp-server/dhcpd.conf.example /etc/dhcp/dhcpd.conf

修改copy过来的模板文件

vim /etc/dhcp/dhcpd.conf

需要修改的选项

domain-name

# option definitions common to all supported networks...
option domain-name "example.org";## 自己的域名
## DNS服务器地址
option domain-name-servers ns1.example.org, ns2.example.org;

修改为：

 option domain-name "timonium.co";## 自己的域名
  ## DNS服务器地址
  option domain-name-servers 114.114.114.114, 8.8.8.8;

租期
长租期可以减少服务器压力

default-lease-time 600; ## 默认租期，比较短
max-lease-time 7200;## 最大租期

修改为更长时间

default-lease-time 86400; ## 默认租期，比较短
max-lease-time 106400;## 最大租期

修改默认地址池

# 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 {
}

修改为当前网段
subnet 10.0.0.0 netmask 255.255.255.0 {
range 10.0.0.180 10.0.0.200；
option routers 10.0.0.2；
}

启动服务并设为开机启动

systemctl enable --now dhcpd.service

[07:14:27 root@C8-88[ ~]#systemctl enable --now dhcpd.service 
[07:14:30 root@C8-88[ ~]#systemctl status dhcpd.service 
● dhcpd.service - DHCPv4 Server Daemon
   Loaded: loaded (/usr/lib/systemd/system/dhcpd.service; enabled; vendor preset: disabled)
   Active: active (running) since Wed 2021-05-05 07:14:30 CST; 48s ago
     Docs: man:dhcpd(8)
           man:dhcpd.conf(5)
 Main PID: 3432 (dhcpd)
   Status: "Dispatching packets..."
    Tasks: 1 (limit: 11337)
   Memory: 5.4M
   CGroup: /system.slice/dhcpd.service
           └─3432 /usr/sbin/dhcpd -f -cf /etc/dhcp/dhcpd.conf -user dhcpd -group dhcpd --no-pid

May 05 07:14:30 C8-88 dhcpd[3432]: 
May 05 07:14:30 C8-88 dhcpd[3432]: No subnet declaration for eth0 (no IPv4 addresses).
May 05 07:14:30 C8-88 dhcpd[3432]: ** Ignoring requests on eth0.  If this is not what
May 05 07:14:30 C8-88 dhcpd[3432]:    you want, please write a subnet declaration
May 05 07:14:30 C8-88 dhcpd[3432]:    in your dhcpd.conf file for the network segment
May 05 07:14:30 C8-88 dhcpd[3432]:    to which interface eth0 is attached. **
May 05 07:14:30 C8-88 dhcpd[3432]: 
May 05 07:14:30 C8-88 dhcpd[3432]: Sending on   Socket/fallback/fallback-net
May 05 07:14:30 C8-88 dhcpd[3432]: Server starting service.
May 05 07:14:30 C8-88 systemd[1]: Started DHCPv4 Server Daemon.

客户端测试

手动启动DHCP客户端程序

dhclient -d

可以查看到获取IP的详细情况

[root@c7-53 ~]# ip a
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
    inet6 ::1/128 scope host 
       valid_lft forever preferred_lft forever
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP group default qlen 1000
    link/ether 00:0c:29:50:04:8d brd ff:ff:ff:ff:ff:ff
    inet 10.0.0.53/24 brd 10.0.0.255 scope global noprefixroute eth0
       valid_lft forever preferred_lft forever
    inet6 fe80::20c:29ff:fe50:48d/64 scope link 
       valid_lft forever preferred_lft forever
[root@c7-53 ~]# dhclient -d
Internet Systems Consortium DHCP Client 4.2.5
Copyright 2004-2013 Internet Systems Consortium.
All rights reserved.
For info, please visit https://www.isc.org/software/dhcp/

Listening on LPF/eth0/00:0c:29:50:04:8d
Sending on   LPF/eth0/00:0c:29:50:04:8d
Sending on   Socket/fallback
DHCPDISCOVER on eth0 to 255.255.255.255 port 67 interval 6 (xid=0x51e6bfe1)
DHCPREQUEST on eth0 to 255.255.255.255 port 67 (xid=0x51e6bfe1)
DHCPOFFER from 10.0.0.88
DHCPACK from 10.0.0.88 (xid=0x51e6bfe1)
bound to 10.0.0.180 -- renewal in 693 seconds.

客户端日志位置
/var/lib/dhclient/dhclient.leases

服务器端查看日志确认分配IP

tail -f /var/lib/dhcpd/dhcpd.leases

[07:15:18 root@C8-88[ ~]#tail -f /var/lib/dhcpd/dhcpd.leases

lease 10.0.0.180 {
  starts 2 2021/05/04 23:17:42; ## 租期开始时间
  ends 2 2021/05/04 23:44:22;##租期结束时间
  cltt 2 2021/05/04 23:17:42;
  binding state active;
  next binding state free;
  rewind binding state free;
  hardware ethernet 00:0c:29:50:04:8d;
}

至此，DHCP服务器已经搭建完成

IP地址和MAC地址的静态绑定

固定IP地址可以防止IP冲突
固定地址有利于服务器互相访问

修改DHCP服务器配置文件相关内容实现

# 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 passacaglia {
  hardware ethernet 0:0:c0:5d:bd:95;
  filename "vmunix.passacaglia";
  server-name "toccata.example.com";
}

参考并修改为

	host webServer1 {
	  hardware ethernet 0:0:c0:5d:bd:95;
	  fixed-address 10.0.0.188;
	  filename "web1";
	  server-name "web1.timonium.co";
	}

重启服务后生效

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