DHCP服務器只作用於局域網同一網段內，客戶端是通過廣播消息來獲得DHCP服務器響應后才能得到IP地址的，但廣播消息不能跨越子網，那么如何讓客戶端獲取到DHCP服務器提供的IP地址呢？這就是DHCP中繼服務器的功能了。
實驗拓撲圖：
實驗環境：
DHCP服務器：CentOS Linux release 7.3.1611
DHCP中繼代理：CentOS Linux release 7.3.1611
客戶端：CentOS 6.9
1、DHCP服務器配置
（1）、配置DHCP服務器的網絡參數
[root@host02 ~]# vi /etc/sysconfig/network-scripts/ifcfg-ens33 TYPE="Ethernet" BOOTPROTO="static" IPADDR=192.168.8.250 NETMASK=255.255.255.0 GATEWAY=192.168.8.254 DEFROUTE="yes" PEERDNS="yes" PEERROUTES="yes" IPV4_FAILURE_FATAL="no" IPV6INIT="yes" IPV6_AUTOCONF="yes" IPV6_DEFROUTE="yes" IPV6_PEERDNS="yes" IPV6_PEERROUTES="yes" IPV6_FAILURE_FATAL="no" IPV6_ADDR_GEN_MODE="stable-privacy" NAME="ens33" UUID="8225206d-ab64-4101-b343-35316fc76a7c" DEVICE="ens33" ONBOOT="yes"
~
~
~
（2）、重啟網絡服務
[root@host02 ~]# systemctl restart network
查看DHCP服務器IP
[root@host02 ~]# ifconfig ens33: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500 inet 192.168.8.250 netmask 255.255.255.0 broadcast 192.168.8.255 inet6 fe80::66d2:4ff4:82d6:a6e4 prefixlen 64 scopeid 0x20<link> ether 00:0c:29:19:7e:d0 txqueuelen 1000 (Ethernet) RX packets 32913 bytes 19217237 (18.3 MiB) RX errors 0 dropped 0 overruns 0 frame 0 TX packets 23740 bytes 5933321 (5.6 MiB) TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0 lo: flags=73<UP,LOOPBACK,RUNNING> mtu 65536 inet 127.0.0.1 netmask 255.0.0.0 inet6 ::1 prefixlen 128 scopeid 0x10<host> loop txqueuelen 1 (Local Loopback) RX packets 620 bytes 31136 (30.4 KiB) RX errors 0 dropped 0 overruns 0 frame 0 TX packets 620 bytes 31136 (30.4 KiB) TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0 virbr0: flags=4099<UP,BROADCAST,MULTICAST> mtu 1500 inet 192.168.122.1 netmask 255.255.255.0 broadcast 192.168.122.255 ether 52:54:00:f7:80:1e txqueuelen 1000 (Ethernet) RX packets 6 bytes 432 (432.0 B) RX errors 0 dropped 0 overruns 0 frame 0 TX packets 0 bytes 0 (0.0 B) TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
（3） 安裝DHCP服務
[root@host02 ~]# yum install dhcp [root@host02 ~]# rpm -qa dhcp dhcp-4.2.5-47.el7.centos.x86_64
（4）編輯DHCP服務器的配置文件
[root@host02 ~]# vi dhcpd.conf #dhcp configuration #整體環境設置 ddns-update-style none; ignore client-updates; default-lease-time 259200; max-lease-time 518400; option routers 192.168.8.254; option domain-name "centos.me"; option domain-name-servers 202.102.224.68,202.102.227.68; #二個網段IP分配 subnet 192.168.8.0 netmask 255.255.255.0 { range 192.168.8.200 192.168.8.210; option subnet-mask 255.255.255.0; option routers 192.168.8.1; } subnet 192.168.100.0 netmask 255.255.255.0 { range 192.168.100.80 192.168.100.100; option subnet-mask 255.255.255.0; option routers 192.168.100.1; }
將編輯好的DHCP配置文件覆蓋初裝后的空白配置文件;
[root@host02 ~]# cp dhcpd.conf /etc/dhcp/dhcpd.conf cp: overwrite ‘/etc/dhcp/dhcpd.conf’? y
（5）啟動DHCP服務，並檢查狀態；
[root@host02 ~]# systemctl start dhcpd [root@host02 ~]# systemctl status dhcpd ● dhcpd.service - DHCPv4 Server Daemon Loaded: loaded (/usr/lib/systemd/system/dhcpd.service; disabled; vendor preset: disabled) Active: active (running) since Thu 2017-08-31 22:01:20 CST; 6s ago Docs: man:dhcpd(8) man:dhcpd.conf(5) Main PID: 27413 (dhcpd) Status: "Dispatching packets..." CGroup: /system.slice/dhcpd.service └─27413 /usr/sbin/dhcpd -f -cf /etc/dhcp/dhcpd.conf -user dhcpd -group dhcpd --no-pid Aug 31 22:01:20 host02.linux.com dhcpd[27413]: No subnet declaration for virbr0 (192.168.122.1). Aug 31 22:01:20 host02.linux.com dhcpd[27413]: ** Ignoring requests on virbr0. If this is not what Aug 31 22:01:20 host02.linux.com dhcpd[27413]: you want, please write a subnet declaration Aug 31 22:01:20 host02.linux.com dhcpd[27413]: in your dhcpd.conf file for the network segment Aug 31 22:01:20 host02.linux.com dhcpd[27413]: to which interface virbr0 is attached. ** Aug 31 22:01:20 host02.linux.com dhcpd[27413]: Aug 31 22:01:20 host02.linux.com dhcpd[27413]: Listening on LPF/ens33/00:0c:29:19:7e:d0/192.168.8.0/24 Aug 31 22:01:20 host02.linux.com dhcpd[27413]: Sending on LPF/ens33/00:0c:29:19:7e:d0/192.168.8.0/24 Aug 31 22:01:20 host02.linux.com dhcpd[27413]: Sending on Socket/fallback/fallback-net Aug 31 22:01:20 host02.linux.com systemd[1]: Started DHCPv4 Server Daemon. [root@host02 ~]# netstat -tunlp |grep dhcp udp 0 0 0.0.0.0:67 0.0.0.0:* 27413/dhcpd udp 0 0 0.0.0.0:62793 0.0.0.0:* 27413/dhcpd udp6 0 0 :::7468 :::* 27413/dhcpd
DHCP服務器一切正常！至此DHCP服務器的配置已經完成；下一步配置DHCP中繼代理服務器。
2、配置DHCP中繼服務器
（1）、做為中繼服務器首先要確保有兩張網卡，否則沒法中繼；
本實驗我已經在中繼服務器上虛擬出兩個網卡；
[root@host-a ~]# lspci |grep ethernet [root@host-a ~]# lspci |grep Ethernet 02:01.0 Ethernet controller: Intel Corporation 82545EM Gigabit Ethernet Controller (Copper) (rev 01) 02:05.0 Ethernet controller: Intel Corporation 82545EM Gigabit Ethernet Controller (Copper) (rev 01)
（2）查看網卡信息
[root@host-a ~]# ifconfig ens33: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500 ether 00:0c:29:a9:16:dd txqueuelen 1000 (Ethernet) RX packets 691 bytes 574904 (561.4 KiB) RX errors 0 dropped 0 overruns 0 frame 0 TX packets 664 bytes 80407 (78.5 KiB) TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0 ens37: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500 ether 00:0c:29:a9:16:e7 txqueuelen 1000 (Ethernet) RX packets 11 bytes 2916 (2.8 KiB) RX errors 0 dropped 0 overruns 0 frame 0 TX packets 462 bytes 70265 (68.6 KiB) TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0 lo: flags=73<UP,LOOPBACK,RUNNING> mtu 65536 inet 127.0.0.1 netmask 255.0.0.0 inet6 ::1 prefixlen 128 scopeid 0x10<host> loop txqueuelen 1 (Local Loopback) RX packets 164 bytes 12880 (12.5 KiB) RX errors 0 dropped 0 overruns 0 frame 0 TX packets 164 bytes 12880 (12.5 KiB) TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0 virbr0: flags=4099<UP,BROADCAST,MULTICAST> mtu 1500 inet 192.168.122.1 netmask 255.255.255.0 broadcast 192.168.122.255 ether 52:54:00:7a:2c:f4 txqueuelen 1000 (Ethernet) RX packets 0 bytes 0 (0.0 B) RX errors 0 dropped 0 overruns 0 frame 0 TX packets 0 bytes 0 (0.0 B) TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
（2）接下來分別配置這兩個網卡的網絡參數
網卡ens33與DHCP服務器相連，屬於192.168.8.0/24網段，注意網卡ens33的IP地址是DHCP服務器的網關地址，
配置如下：
[root@host-a ~]# vi /etc/sysconfig/network-scripts/ifcfg-ens33 TYPE="Ethernet" BOOTPROTO="static" IPADDR=192.168.8.254 NETMASK=255.255.255.0 DEFROUTE="yes" PEERDNS="yes" PEERROUTES="yes" IPV4_FAILURE_FATAL="no" IPV6INIT="yes" IPV6_AUTOCONF="yes" IPV6_DEFROUTE="yes" IPV6_PEERDNS="yes" IPV6_PEERROUTES="yes" IPV6_FAILURE_FATAL="no" IPV6_ADDR_GEN_MODE="stable-privacy" NAME="ens33" UUID="065ff1ad-9117-4cf4-a716-c61d5fdc0c00" DEVICE="ens33" ONBOOT="yes"
網卡ens37與192.168.100.0/24網段相連，注意網卡ens37的IP地址是192.168.100.0/24網段的網關地址，配置如下：
[root@host-a ~]# cat /etc/sysconfig/network-scripts/ifcfg-ens37 TYPE=Ethernet BOOTPROTO=static IPADDR=192.168.100.1 PREFIX=24 DEFROUTE=yes IPV4_FAILURE_FATAL=no IPV6INIT=yes IPV6_AUTOCONF=yes IPV6_DEFROUTE=yes IPV6_PEERDNS=yes IPV6_PEERROUTES=yes IPV6_FAILURE_FATAL=no IPV6_ADDR_GEN_MODE=stable-privacy NAME="ens37" UUID=e3854409-f832-4d4c-b7ae-271d0aa5ecc5 ONBOOT=yes
（3）配置好后，重起網絡服務使設置生效
[root@host-a ~]# systemctl restart network [root@host-a ~]# ifconfig ens33: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500 inet 192.168.8.254 netmask 255.255.255.0 broadcast 192.168.8.255 inet6 fe80::f31:c380:a398:1df6 prefixlen 64 scopeid 0x20<link> ether 00:0c:29:a9:16:dd txqueuelen 1000 (Ethernet) RX packets 703 bytes 575996 (562.4 KiB) RX errors 0 dropped 0 overruns 0 frame 0 TX packets 759 bytes 93828 (91.6 KiB) TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0 ens37: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500 inet 192.168.100.1 netmask 255.255.255.0 broadcast 192.168.100.255 inet6 fe80::3409:4bde:b57c:957d prefixlen 64 scopeid 0x20<link> ether 00:0c:29:a9:16:e7 txqueuelen 1000 (Ethernet) RX packets 11 bytes 2916 (2.8 KiB) RX errors 0 dropped 0 overruns 0 frame 0 TX packets 522 bytes 78535 (76.6 KiB) TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0 lo: flags=73<UP,LOOPBACK,RUNNING> mtu 65536 inet 127.0.0.1 netmask 255.0.0.0 inet6 ::1 prefixlen 128 scopeid 0x10<host> loop txqueuelen 1 (Local Loopback) RX packets 164 bytes 12880 (12.5 KiB) RX errors 0 dropped 0 overruns 0 frame 0 TX packets 164 bytes 12880 (12.5 KiB) TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0 virbr0: flags=4099<UP,BROADCAST,MULTICAST> mtu 1500 inet 192.168.122.1 netmask 255.255.255.0 broadcast 192.168.122.255 ether 52:54:00:7a:2c:f4 txqueuelen 1000 (Ethernet) RX packets 0 bytes 0 (0.0 B) RX errors 0 dropped 0 overruns 0 frame 0 TX packets 0 bytes 0 (0.0 B) TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
（4）安裝DHCP
#安裝 [root@host-a network-scripts]# yum install dhcp #安裝成功 [root@host-a network-scripts]# rpm -qa dhcp dhcp-4.2.5-47.el7.centos.x86_64 #注意dhcp包含dhcp中繼服務 [root@host-a ~]# rpm -ql dhcp |grep dhcrelay /usr/lib/systemd/system/dhcrelay.service /usr/sbin/dhcrelay /usr/share/man/man8/dhcrelay.8.gz
（5）打開DHCP中繼服務器路由轉發功能
編輯/etc/sysctl.conf 文件在文件添加一行內容為：
“net.ipv4.ip_forward=1”
開啟路由轉發功能
[root@host-a ~]# vi /etc/sysctl.conf # sysctl settings are defined through files in # /usr/lib/sysctl.d/, /run/sysctl.d/, and /etc/sysctl.d/. # # Vendors settings live in /usr/lib/sysctl.d/. # To override a whole file, create a new file with the same in # /etc/sysctl.d/ and put new settings there. To override # only specific settings, add a file with a lexically later # name in /etc/sysctl.d/ and put new settings there. # # For more information, see sysctl.conf(5) and sysctl.d(5). net.ipv4.ip_forward = 1
使設置立即生效，執行以下命令：
[root@host-a ~]# sysctl -p
net.ipv4.ip_forward = 1
查看內核參數設置，確認生效；
[root@host-a ~]# sysctl -a |grep ip_forward
net.ipv4.ip_forward = 1
net.ipv4.ip_forward_use_pmtu = 0
[root@host-a ~]#
（6）開啟DHCP中繼服務
[root@host-a ~]# dhcrelay 192.168.8.250 Dropped all unnecessary capabilities. Internet Systems Consortium DHCP Relay Agent 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/virbr0/52:54:00:7a:2c:f4 Sending on LPF/virbr0/52:54:00:7a:2c:f4 Listening on LPF/ens37/00:0c:29:a9:16:e7 Sending on LPF/ens37/00:0c:29:a9:16:e7 Listening on LPF/ens33/00:0c:29:a9:16:dd Sending on LPF/ens33/00:0c:29:a9:16:dd Sending on Socket/fallback [root@host-a ~]# [root@host-a ~]# netstat -tunlp |grep dhcrelay udp 0 0 0.0.0.0:49976 0.0.0.0:* 10428/dhcrelay udp 0 0 0.0.0.0:67 0.0.0.0:* 10428/dhcrelay udp6 0 0 :::17324 :::* 10428/dhcrelay
3、客戶端測試
（1）配置客戶端網絡為自動獲取IP
（2）重啟網絡服務
（3）查看IP
（4）查看客戶端在重啟網絡時都做了什么
（5）查看DHCP服務器日志文件
[root@host02 ~]# tail -n 20 /var/log/messages Aug 31 23:01:01 host02 systemd: Stopping user-0.slice. Aug 31 23:01:02 host02 dhcpd: DHCPREQUEST for 192.168.6.6 from 00:0c:29:78:4c:ea via 192.168.100.1: ignored (not authoritative). Aug 31 23:01:02 host02 dhcpd: DHCPREQUEST for 192.168.6.6 from 00:0c:29:78:4c:ea via 192.168.100.1: ignored (not authoritative). Aug 31 23:01:15 host02 dhcpd: DHCPDISCOVER from 00:0c:29:78:4c:ea via 192.168.100.1 Aug 31 23:01:16 host02 dhcpd: DHCPOFFER on 192.168.100.80 to 00:0c:29:78:4c:ea (host01) via 192.168.100.1 Aug 31 23:01:16 host02 dhcpd: DHCPREQUEST for 192.168.100.80 (192.168.8.250) from 00:0c:29:78:4c:ea (host01) via 192.168.100.1 Aug 31 23:01:16 host02 dhcpd: DHCPACK on 192.168.100.80 to 00:0c:29:78:4c:ea (host01) via 192.168.100.1 Aug 31 23:01:16 host02 dhcpd: DHCPREQUEST for 192.168.100.80 (192.168.8.250) from 00:0c:29:78:4c:ea (host01) via 192.168.100.1 Aug 31 23:01:16 host02 dhcpd: DHCPACK on 192.168.100.80 to 00:0c:29:78:4c:ea (host01) via 192.168.100.1
(6)檢查客戶端獲取的DNS是否正確
DNS與DHCP服務器設置的一致！
可以看到客戶端已經可以正常獲取DHCP服務器設置的網絡參數了！！