CCNA 200-125 Exam: DHCP Questions With Answers

The command “show ip dhcp conflict” is used to display address conflicts found by a Cisco IOS DHCP Server when addresses are offered to the client. An example of the output of this command is shown below:

We can use the “lease” command to specify the duration of the lease: lease {days [hours][minutes] | infinite}

The default is a one-day lease.

An address conflict occurs when two hosts use the same IP address. During address assignment, DHCP checks for conflicts using ping and gratuitous ARP. If a conflict is detected, the address is removed from the pool. The address will not be assigned until the administrator resolves the conflict.

(Reference: http://www.cisco.com/en/US/docs/ios/12_1/iproute/configuration/guide/1cddhcp.html)

The following example shows how to configure a DHCP Server on a Cisco router:

Configuration	Description
Router(config)#ip dhcp pool CLIENTS	Create a DHCP Pool named CLIENTS
Router(dhcp-config)#network 10.1.1.0 /24	Specifies the subnet and mask of the DHCP address pool
Router(dhcp-config)#default-router 10.1.1.1	Set the default gateway of the DHCP Clients
Router(dhcp-config)#dns-server 10.1.1.1	Configure a Domain Name Server (DNS)
Router(dhcp-config)#domain-name 9tut.com	Configure a domain-name
Router(dhcp-config)#lease 0 12	Duration of the lease (the time during which a client computer can use an assigned IP address). The syntax is “lease{days[hours] [minutes] | infinite}”. In this case the lease is 12 hours. The default is a one-day lease. Before the lease expires, the client typically needs to renew its address lease assignment with the server
Router(dhcp-config)#exit
Router(config)# ip dhcp excluded-address 10.1.1.1 10.1.1.10	The IP range that a DHCP Server should not assign to DHCP Clients. Notice this command is configured under global configuration mode

Note: We checked with both Cisco IOS v12.4 and v15.4 but found no “ip dhcp-server pool” command:

Therefore the answer “ip dhcp-server pool …” is not correct.

Quick review of DHCP Spoofing and DHCP snooping:

DHCP spoofing is a type of attack in that the attacker listens for DHCP Requests from clients and answers them with fake DHCP Response before the authorized DHCP Response comes to the clients. The fake DHCP Response often gives its IP address as the client default gateway -> all the traffic sent from the client will go through the attacker computer, the attacker becomes a “man-in-the-middle”.

The attacker can have some ways to make sure its fake DHCP Response arrives first. In fact, if the attacker is “closer” than the DHCP Server then he doesn’t need to do anything. Or he can DoS the DHCP Server so that it can’t send the DHCP Response.

DHCP snooping can prevent DHCP spoofing attacks. DHCP snooping is a Cisco Catalyst feature that determines which switch ports can respond to DHCP requests. Ports are identified as trusted and untrusted.

Only ports that connect to an authorized DHCP server are trusted, and allowed to send all types of DHCP messages. All other ports on the switch are untrusted and can send only DHCP requests. If a DHCP response is seen on an untrusted port, the port is shut down -> Answer D is correct.

The fundamental use case for DHCP snooping is to prevent unauthorized (rogue) DHCP servers offering IP addresses to DHCP clients. Rogue DHCP servers are often used in man in the middle or denial of service attacks for malicious purposes -> C is correct.

The DHCP snooping binding database is also referred to as the DHCP snooping binding table. The DHCP snooping feature dynamically builds and maintains the database using information extracted from intercepted DHCP messages. The database contains an entry for each untrusted host with a leased IP address if the host is associated with a VLAN that has DHCP snooping enabled. The database does not contain entries for hosts connected through trusted interfaces.

Reference: http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst6500/ios/12-2SX/configuration/guide/book/snoodhcp.html

The “show ip dhcp binding” command displays the IP address bindings and their associated leases. Below is an example of the output of this command.

R1#show ip dhcp binding
IP address       Hardware address        Lease expiration        Type
172.15.1.5      0100.0103.85e9.87       Apr 03 2017 08:55 PM    Automatic
172.15.1.6      0100.50da.2a5e.a2       Apr 03 2017 09:00 PM    Automatic
172.15.1.7      0100.0103.ea1b.ed       Apr 03 2017 08:58 PM    Automatic

The command “show ip dhcp pool” is used to display information about the DHCP address pools. There are some information we can use to check the failure of address assignment. For example we can see how many IP addresses have been leased for a specific pool. If some IP addresses have been assigned from a pool but a client of that pool has not received the assignment then maybe the issue belongs to the client itself.

R1#show ip dhcp pool
Pool SERVER :
 Utilization mark (high/low)    : 100 / 0
 Subnet size (first/next)       : 0 / 0 
 Total addresses                : 1
 Leased addresses               : 1
 Pending event                  : none
 0 subnet is currently in the pool :
 Current index        IP address range                    Leased addresses
 172.16.200.100       172.16.200.100   - 172.16.200.100    1

An address binding is a mapping between the IP address and MAC address of a client. The IP address of a client can be assigned manually by an administrator or assigned automatically from a pool by a DHCP server. Manual bindings are IP addresses that have been manually mapped to the MAC addresses of hoststhat are found in the DHCP database.

All DHCP clients send a client identifier (DHCP option 61) in the DHCP packet. To configure manual bindings, you must enter the client-identifier DHCP pool configuration command with the appropriate hexadecimal values identifying the DHCP client. For example:

ip dhcp pool SERVER
host 172.16.200.100 255.255.255.0
client-identifier 01aa.bbcc.0003.00
default-router 172.16.200.1 
!

Therefore two requirements for DHCP binding is the IP address and the hardware address (MAC address) of the client. Notice that in the above example “aabb.cc00.0300” is the MAC address of the client while prefix “01” represents the Ethernet media type.

Reference: http://www.cisco.com/c/en/us/td/docs/ios/12_2/ip/configuration/guide/fipr_c/1cfdhcp.html

In fact the “DHCP pool” option is also correct but two above choices are better.