Chapter 15DHCP

Dynamic Host Configuration Protocol

• An Application Layer Protocol• A client server protocol that automatically

provides an IP host with its IP address and other items such as the subnet mask and default gateway

• All necessary TCP/IP configuration information for the host is obtained from a DHCP server

When to Use DHCP

• DHCP can help when you are running out of IP addresses.

• In a Class C IP network, there are only 254 addresses to assign to hosts. What to do if you get more hosts than this?

• Use DHCP to assign hosts an IP address for a short period of time.

• This works if you never have more than 254 simultaneous users.

History of DHCP

• Originally called BootP protocol

• Designed to give IP addresses to diskless workstations

• Today DHCP is built onto BootP protocol

How DHCP Works

• When TCP/IP starts up on a DHCP-enabled host, the host sends out a special message to the DHCP server requesting an IP address and a subnet mask

• The DHCP server checks its internal database to find an available IP address

• The DHCP offers the IP address to the client for a specified period of time, called a lease

The Pros of DHCP

• It’s inexpensive

• Human error factors are eliminated

• Configuration problems are minimized

• New or moved hosts can be plugged into a network segment immediately without any configuration or setup

The Cons of DHCP

• If DHCP server is down, then no clients will be operational

• Certain applications require the use of a specific IP address.

DHCP in Action

• Stage One: IP Lease Request

• Stage Two: IP Lease Offer

• Stage Three: IP Lease Selection

• Stage Four: IP Lease Acknowledgement

• DHCP Lease Renewal

• DHCP Lease Release

IP Lease Request

• Begins when a host is started that has TCP/IP configures with DHCP

• Client does not have an IP address initially• It uses 0.0.0.0• The client sends out a DHCP discover message• The message contains the hardware address of the client• The client waits one second for an offer• If no DHCP server responds, then the request is repeated

at 9, 13, and 16 second intervals• If no response still, then a broadcast message is sent every

five minutes

IP Lease Offer

• Information is sent back to the client• The offer consists of an IP address, subnet mask,

lease period (in hours), and the IP address of the proposing DHCP server

• The offer is sent to the hardware address of the client

• The pending IP address is reserved temporarily

IP Lease Selection

• The client machine selects the first IP addressing offer it receives

• Client replies by broadcasting an acceptance message

• The message includes the IP address of the DHCP server whose offer was accepted

• All other DHCP servers revoke their offer

IP Lease Acknowledgement

• The accepted DHCP server assigns the IP address to the client

• The DHCP server then sends an acknowledgement back to the client

• This acknowledgement is called DHCPACK

• Occasionalyy a negative acknowledgement (DHCPNACK) is sent back

• This may happen if the client is trying to lease its old IP address that has already been assigned elsewhere

DHCP Lease Renewal

• IP addresses are leased for a period of time

• The leasing client will send a DHCPREQUEST to the DHCP server when its lease period has elapsed by 50%

• The DHCP server may send a acknowledge message to the client, updating the configuration and resetting the least time

• If the server can’t renew the lease, the client will receive an eviction notice

• The rejected client responds by sending out an additional lease renewal attempt when 87.5% of its lease time is expired

• Any available DHCP server can respond and renew the lease

• If the client receives a DHCPNACK, the client must stop using the IP address immediately and start the leasing process from the beginning

DHCP Lease Release

• The client can elect to cancel the lease by using the IPCONFIG /RELEASE utility

• DHCP leases are not automatically released at system shutdown

DHCP Client Reservation

• Certain machines such as servers must always have the same IP address

• To fulfill this need, a special arrangement called a client reservation must be made with the DHCP server

DHCP Relay

• Used in networks with older routers that don’t support DHCP

• The DHCP Relay Agent forwards an IP lease request from a client on the same subnet directly to the computer running the DHCP service

• The DHCP server then returns an IP address directly to the requesting client