Dynamic ARP Inspection (DAI) is a security feature that protects ARP (Address Resolution Protocol) which is vulnerable to an attack ike ARP poisoning.

DAI checks all ARP packets on untrusted interfaces, it will compare the information in the ARP packet with the DHCP snooping database and/or an ARP access-list. If the information in the ARP packet doesn’t matter, it will be dropped. In this lesson I’ll show you how to configure DAI. Here’s the topology we will use:

Above we have four devices, the router on the left side called “host” will be a DHCP client, the router on the right side is our DHCP server and on top we have a router that will be used as an attacker. The switch in the middle will be configured for dynamic ARP inspection.

Configuration

We’ll start with the switch, first we need to make sure that all interfaces are in the same VLAN:

SW1(config)#interface range fa0/1 - 3
SW1(config-if-range)#switchport mode access
SW1(config-if-range)#switchport access vlan 123
SW1(config-if-range)#spanning-tree portfast

Now we can configure DHCP snooping:

SW1(config)#ip dhcp snooping 
SW1(config)#ip dhcp snooping vlan 123
SW1(config)#no ip dhcp snooping information option 

The commands above will enable DHCP snooping globally, for VLAN 123 and disables the insertion of option 82 in DHCP packets. Don’t forget to make the interface that connects to the DHCP server trusted:

SW1(config)#interface FastEthernet 0/3
SW1(config-if)#ip dhcp snooping trust 

The switch will now keep track of DHCP messages. Let’s configure a DHCP server on the router on the right side:

DHCP(config)#ip dhcp pool MY_POOL
DHCP(dhcp-config)#network 192.168.1.0 255.255.255.0

That’s all we need, let’s see if the host is able to get an IP address:

HOST(config)#interface FastEthernet 0/0
HOST(config-if)#ip address dhcp 

A few seconds later we see this message:

%DHCP-6-ADDRESS_ASSIGN: Interface FastEthernet0/0 assigned DHCP address 192.168.1.1, mask 255.255.255.0, hostname HOST

Let’s check if our switch has stored something in the DHCP snooping database:

SW1#show ip dhcp snooping binding 
MacAddress          IpAddress        Lease(sec)  Type           VLAN  Interface
------------------  ---------------  ----------  -------------  ----  --------------------
00:1D:A1:8B:36:D0   192.168.1.1      86330       dhcp-snooping   123   FastEthernet0/1
Total number of bindings: 1

There it is, an entry with the MAC address and IP address of our host. Now we can continue with the configuration of DAI. There’s only one command required to activate it:

SW1(config)#ip arp inspection vlan 123

The switch will now check all ARP packets on untrusted interfaces, all interfaces are untrusted by default. Let’s see if this will work or not…I’ll configure the IP address of our host on our attacker:

ATTACK(config)#interface FastEthernet 0/0
ATTACK(config-if)#ip address 192.168.1.1 255.255.255.0

Now let’s see what happens when we try to send a ping from the attacker to our DHCP router:

ATTACK#ping 192.168.1.254
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.1.254, timeout is 2 seconds:
.....
Success rate is 0 percent (0/5)

The ping is failing…what does our switch think of this?

SW1#
%SW_DAI-4-DHCP_SNOOPING_DENY: 1 Invalid ARPs (Req) on Fa0/2, vlan 123.([0017.5aed.7af0/192.168.1.1/0000.0000.0000/192.168.1.254/01:20:08 UTC Tue Mar 2 1993])
%SW_DAI-4-DHCP_SNOOPING_DENY: 1 Invalid ARPs (Req) on Fa0/2, vlan 123.([0017.5aed.7af0/192.168.1.1/0000.0000.0000/192.168.1.254/01:20:10 UTC Tue Mar 2 1993])
%SW_DAI-4-DHCP_SNOOPING_DENY: 1 Invalid ARPs (Req) on Fa0/2, vlan 123.([0017.5aed.7af0/192.168.1.1/0000.0000.0000/192.168.1.254/01:20:10 UTC Tue Mar 2 1993])
%SW_DAI-4-DHCP_SNOOPING_DENY: 1 Invalid ARPs (Req) on Fa0/2, vlan 123.([0017.5aed.7af0/192.168.1.1/0000.0000.0000/192.168.1.254/01:20:10 UTC Tue Mar 2 1993])

Above you can see that all ARP requests from our attacker are dropped. The switch checks the information found in the ARP request and compares it with the information in the DHCP snooping database. Since it doesn’t match, these packets are discarded. You can find the number of dropped ARP packets with the following command:

SW1#show ip arp inspection 

Source Mac Validation      : Disabled
Destination Mac Validation : Disabled
IP Address Validation      : Disabled

 Vlan     Configuration    Operation   ACL Match          Static ACL
 ----     -------------    ---------   ---------          ----------
  123     Enabled          Active                         

 Vlan     ACL Logging      DHCP Logging      Probe Logging
 ----     -----------      ------------      -------------
  123     Deny             Deny              Off          

 Vlan      Forwarded        Dropped     DHCP Drops      ACL Drops
 ----      ---------        -------     ----------      ---------
  123              0              5              5              0

 Vlan   DHCP Permits    ACL Permits  Probe Permits   Source MAC Failures
 ----   ------------    -----------  -------------   -------------------
  123              0              0              0                     0

 Vlan   Dest MAC Failures   IP Validation Failures   Invalid Protocol Data
 ----   -----------------   ----------------------   ---------------------
          
 Vlan   Dest MAC Failures   IP Validation Failures   Invalid Protocol Data
 ----   -----------------   ----------------------   ---------------------
  123                   0                        0                       0

Above you see the number of drops increase. So far so good, our attacker has been stopped. We still have one problem though, let me first shut the interface on our attacker before we continue:

ATTACK(config)#interface FastEthernet 0/0
ATTACK(config-if)#shutdown

Let me show you what happens when we try to send a ping from the host to our DHCP router:

HOST#ping 192.168.1.254
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.1.254, timeout is 2 seconds:
.....
Success rate is 0 percent (0/5)

This ping is failing but why? We are not spoofing anything…here’s what the switch tells us: