Linux DHCP 中继

具体到一个公司的网络环境中，不可能只有一个VLAN，更不可能对每个VLAN都架设一个DHCP服务器，这时就要做一个DHCP的中继，使得DHCP的广播可以通过VLAN。

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实验拓扑

三层交换机下面连接一台二层交换机，二层交换机下面连接一台DHCP SERVER、两台客户机Win10和Win7。地址规划如下：

名称	IP地址
DHCP SERVER	192.168.100.100/24
Win10	DHCP（192.168.10.188/24）
Win7	DHCP（192.168.20.100/24）

配置方法

网关上配置ip helper-address。

SW1(config)#int vlan 10
SW1(config-if)#ip add 192.168.10.1 255.255.255.0
SW1(config-if)#ip helper-address 192.168.100.100
SW1(config-if)#no sh
SW1(config-if)#ex

NETWORK 配置过程

SW2

• 添加VLAN10、VLAN20、VLAN100。

SW2#configure terminal 
Enter configuration commands, one per line.  End with CNTL/Z.

SW2(config)#vlan 10,20,100
SW2(config-vlan)#ex
SW2(config)#do show vlan-sw b

VLAN Name                             Status    Ports
---- -------------------------------- --------- -------------------------------
1    default                          active    Fa1/0, Fa1/1, Fa1/2, Fa1/3
                                                Fa1/4, Fa1/5, Fa1/6, Fa1/7
                                                Fa1/8, Fa1/9, Fa1/10, Fa1/11
                                                Fa1/12, Fa1/13, Fa1/14, Fa1/15
10   VLAN0010                         active    
20   VLAN0020                         active    
100  VLAN0100                         active    
1002 fddi-default                     act/unsup 
1003 token-ring-default               act/unsup 
1004 fddinet-default                  act/unsup 
1005 trnet-default                    act/unsup 

• 端口添加到VLAN。

SW2(config)#int f1/1
SW2(config-if)#sw m a
SW2(config-if)#sw a vlan 10
SW2(config-if)#ex
SW2(config)#int f1/2
SW2(config-if)#sw m a
SW2(config-if)#sw a vlan 20
SW2(config-if)#ex
SW2(config)#int f1/3
SW2(config-if)#sw m a
SW2(config-if)#sw a vlan 100
SW2(config-if)#ex
SW2(config)#do show vlan-sw b

VLAN Name                             Status    Ports
---- -------------------------------- --------- -------------------------------
1    default                          active    Fa1/0, Fa1/4, Fa1/5, Fa1/6
                                                Fa1/7, Fa1/8, Fa1/9, Fa1/10
                                                Fa1/11, Fa1/12, Fa1/13, Fa1/14
                                                Fa1/15
10   VLAN0010                         active    Fa1/1
20   VLAN0020                         active    Fa1/2
100  VLAN0100                         active    Fa1/3
1002 fddi-default                     act/unsup 
1003 token-ring-default               act/unsup 
1004 fddinet-default                  act/unsup 
1005 trnet-default                    act/unsup 

• 配置Trunk。

SW2(config)#int f1/0
SW2(config-if)#sw m t
SW2(config-if)#sw t e d
SW2(config-if)#ex
SW2(config)#do show int f1/0 switchport
Name: Fa1/0
Switchport: Enabled
Administrative Mode: trunk
Operational Mode: trunk
Administrative Trunking Encapsulation: dot1q
Operational Trunking Encapsulation: dot1q
Negotiation of Trunking: Disabled
Access Mode VLAN: 0 ((Inactive))
Trunking Native Mode VLAN: 1 (default)
Trunking VLANs Enabled: ALL
Trunking VLANs Active: 1,10,20,100
Protected: false
Priority for untagged frames: 0
Override vlan tag priority: FALSE
Voice VLAN: none 
Appliance trust: none

• 关闭路由功能。

SW2(config)#no ip routing

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SW1

• 配置Trunk。

SW1#configure terminal 
Enter configuration commands, one per line.  End with CNTL/Z.

SW1(config)#int f1/0
SW1(config-if)#sw m t
SW1(config-if)#sw t e d
SW1(config-if)#ex
SW1(config)#do show int f1/0 switchport
Name: Fa1/0
Switchport: Enabled
Administrative Mode: trunk
Operational Mode: trunk
Administrative Trunking Encapsulation: dot1q
Operational Trunking Encapsulation: dot1q
Negotiation of Trunking: Disabled
Access Mode VLAN: 0 ((Inactive))
Trunking Native Mode VLAN: 1 (default)
Trunking VLANs Enabled: ALL
Trunking VLANs Active: 1,10,20,100
Protected: false
Priority for untagged frames: 0
Override vlan tag priority: FALSE
Voice VLAN: none 
Appliance trust: none

• 添加VLAN10、VLAN20、VLAN100。

SW1(config)#vlan 10,20,100
SW1(config-vlan)#ex
SW1(config)#do show vlan-sw b

VLAN Name                             Status    Ports
---- -------------------------------- --------- -------------------------------
1    default                          active    Fa1/0, Fa1/1, Fa1/2, Fa1/3
                                                Fa1/4, Fa1/5, Fa1/6, Fa1/7
                                                Fa1/8, Fa1/9, Fa1/10, Fa1/11
                                                Fa1/12, Fa1/13, Fa1/14, Fa1/15
10   VLAN0010                         active    
20   VLAN0020                         active    
100  VLAN0100                         active    
1002 fddi-default                     act/unsup 
1003 token-ring-default               act/unsup 
1004 fddinet-default                  act/unsup 
1005 trnet-default                    act/unsup 

• 配置网关地址、ip helper-address。

SW1(config)#int vlan 10
SW1(config-if)#ip add 192.168.10.1 255.255.255.0
SW1(config-if)#ip helper-address 192.168.100.100
SW1(config-if)#no sh
SW1(config-if)#ex
SW1(config)#int vlan 20
SW1(config-if)#ip add 192.168.20.1 255.255.255.0
SW1(config-if)#ip helper-address 192.168.100.100
SW1(config-if)#no sh
SW1(config-if)#ex
SW1(config)#int vlan 100
SW1(config-if)#ip add 192.168.100.1 255.255.255.0
SW1(config-if)#ip helper-address 192.168.100.100
SW1(config-if)#no sh
SW1(config-if)#ex
SW1(config)#do show ip int b
Interface                  IP-Address      OK? Method Status                Protocol
FastEthernet0/0            unassigned      YES unset  administratively down down    
FastEthernet0/1            unassigned      YES unset  administratively down down    
FastEthernet1/0            unassigned      YES unset  up                    up      
FastEthernet1/1            unassigned      YES unset  up                    down    
FastEthernet1/2            unassigned      YES unset  up                    down    
FastEthernet1/3            unassigned      YES unset  up                    down    
FastEthernet1/4            unassigned      YES unset  up                    down    
FastEthernet1/5            unassigned      YES unset  up                    down    
FastEthernet1/6            unassigned      YES unset  up                    down    
FastEthernet1/7            unassigned      YES unset  up                    down    
FastEthernet1/8            unassigned      YES unset  up                    down    
FastEthernet1/9            unassigned      YES unset  up                    down    
FastEthernet1/10           unassigned      YES unset  up                    down    
FastEthernet1/11           unassigned      YES unset  up                    down    
FastEthernet1/12           unassigned      YES unset  up                    down    
FastEthernet1/13           unassigned      YES unset  up                    down    
FastEthernet1/14           unassigned      YES unset  up                    down    
FastEthernet1/15           unassigned      YES unset  up                    down    
Vlan1                      unassigned      YES unset  up                    up      
Vlan10                     192.168.10.1    YES manual up                    up      
Vlan20                     192.168.20.1    YES manual up                    up      
Vlan100                    192.168.100.1   YES manual up                    up      

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DHCP 配置过程

安装 DHCP 服务

[root@localhost ~]# yum install dhcp -y

配置网卡静态 IP

[root@localhost ~]# cat > /etc/sysconfig/network-scripts/ifcfg-ens33 << EOF
> TYPE=Ethernet
> BOOTPROTO=static
> DEVICE=ens33
> ONBOOT=yes
> IPADDR=192.168.100.100
> NETMASK=255.255.255.0
> GATEWAY=192.168.100.1
> EOF

[root@localhost ~]# service network restart
Restarting network (via systemctl):                        [  OK  ]

[root@localhost ~]# ifconfig ens33
ens33: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet 192.168.100.100  netmask 255.255.255.0  broadcast 192.168.100.255
        inet6 fe80::20c:29ff:febc:ab96  prefixlen 64  scopeid 0x20<link>
        ether 00:0c:29:bc:ab:96  txqueuelen 1000  (Ethernet)
        RX packets 2142  bytes 1951874 (1.8 MiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 691  bytes 64359 (62.8 KiB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

编辑 DHCP 配置文件

[root@localhost ~]# cp /usr/share/doc/dhcp-4.2.5/dhcpd.conf.example /etc/dhcp/dhcpd.conf 
cp: overwrite ‘/etc/dhcp/dhcpd.conf’? y

[root@localhost ~]# vim /etc/dhcp/dhcpd.conf 

• 在7-8行可以配置DNS服务器名称、地址。

option domain-name "example.org";
option domain-name-servers 8.8.8.8;

• 在10-11行可以配置DHCP租约时间。

default-lease-time 600;
max-lease-time 7200;

• 在22行可以配置日志信息。

log-facility local7;

• 在27-40行配置网段信息。

subnet 192.168.10.0 netmask 255.255.255.0 {
  range 192.168.10.100 192.168.10.200;
  option routers 192.168.10.1;
}

subnet 192.168.20.0 netmask 255.255.255.0 {
  range 192.168.20.100 192.168.20.200;
  option routers 192.168.20.1;
}

subnet 192.168.100.0 netmask 255.255.255.0 {
  range 192.168.100.100 192.168.100.200;
  option routers 192.168.100.1;
}

• 在87-90配置指定IP地址分配。

host win10 {
  hardware ethernet 00:0C:29:ED:26:F4;
  fixed-address 192.168.10.188;
}

启动 DHCP 服务

[root@localhost ~]# systemctl start dhcpd
[root@localhost ~]# systemctl enable dhcpd
Created symlink from /etc/systemd/system/multi-user.target.wants/dhcpd.service to /usr/lib/systemd/system/dhcpd.service.
[root@localhost ~]# netstat -nuap | grep dhcpd
udp        0      0 0.0.0.0:67              0.0.0.0:*                           42901/dhcpd         

• 关闭防火墙、SELinux。

[root@localhost ~]# systemctl stop firewalld
[root@localhost ~]# setenforce 0

结果测试

Win10

Win7