配置ASA的dhcp中继

1.拓扑图:

 

DHCP服务器在dmz区

2.接口配置：

R1:
R1(config)#int e0/0
R1(config-if)#ip address dhcp
R1(config-if)#shutdown

R2:
R2(config)#int e0/0
R2(config-if)#ip add 20.1.1.10 255.255.255.0
R2(config-if)#no sh

R3：
R3(config)#int e0/0
R3(config-if)#ip add 30.1.1.10 255.255.255.0
R3(config-if)#no sh

ASA:
ASA(config)# int e0
ASA(config-if)# ip add 10.1.1.1 255.255.255.0
ASA(config-if)# nameif inside
INFO: Security level for "inside" set to 100 by default.
ASA(config-if)# no sh
ASA(config-if)#
ASA(config-if)# int e1
ASA(config-if)# ip add 20.1.1.1 255.255.255.0
ASA(config-if)# nameif dmz
INFO: Security level for "dmz" set to 0 by default.
ASA(config-if)# sec
ASA(config-if)# security-level 50
ASA(config-if)# no sh
ASA(config-if)# int e2
ASA(config-if)# ip add 30.1.1.1 255.255.255.0
ASA(config-if)# nameif outside
INFO: Security level for "outside" set to 0 by default.
ASA(config-if)# no sh


3.路由配置：

R2(config)#ip route 0.0.0.0 0.0.0.0 20.1.1.1
------R2需要配置到inside接口网段的路由，否则无法为R1分派IP地址

R3(config)#ip route 0.0.0.0 0.0.0.0 30.1.1.1

4.配置R2为DHCP服务器:

A.先必须配置地址排除
R2(config)#ip dhcp excluded-address 10.1.1.1

----实际测试发现如果没有先配置inside接口地址排除，会把inside接口地址分配给DHCP客户端，因为DHCP服务器分派IP地址前发出的icmp包不能得到回应

-----添加完地址池后再添加地址排除，不会生效

B.再配置DHCP地址池
R2(config)#ip dhcp pool dhcppool
R2(dhcp-config)#network 10.1.1.0 /24

5.配置ASA的DHCP中继：

ASA(config)# dhcprelay server 20.1.1.1

-----设定DHCP服务器的地址
ASA(config)# dhcprelay setroute inside

----设定默认路由有inside接口地址
ASA(config)# dhcprelay enable inside

----在inside接口启用DHCP中继
6.测试:

A.开启R1的e0/0端口，并在R2端口抓包

R1成功获得ip地址，和下条为防火墙接口地址的默认路由:

R1(config-if)#no sh
R1(config-if)#
*Mar  1 01:46:31.595: %LINK-3-UPDOWN: Interface Ethernet0/0, changed state to up
*Mar  1 01:46:32.595: %LINEPROTO-5-UPDOWN: Line protocol on Interface Ethernet0/0, changed state to up
R1(config-if)#
*Mar  1 01:46:35.903: %DHCP-6-ADDRESS_ASSIGN: Interface Ethernet0/0 assigned DHCP address 10.1.1.2, mask 255.255.255.0, hostname R1


R1#show ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route

Gateway of last resort is 10.1.1.1 to network 0.0.0.0

     20.0.0.0/32 is subnetted, 1 subnets
S       20.1.1.10 [254/0] via 10.1.1.1, Ethernet0/0
     10.0.0.0/24 is subnetted, 1 subnets
C       10.1.1.0 is directly connected, Ethernet0/0
S*   0.0.0.0/0 [254/0] via 10.1.1.1

B.分析抓包

1)DHCP的广播包，经过中继后成了单播包
2）DHCP服务器分派ip地址前会尝试ping该地址，但是被防火墙策丢弃，因为超时认为该地址没有被使用