DHCP

DYNAMIC HOST CONFIGURATION PROTOCOL

HISTORY

• DHCP was created by the Dynamic Host Configuration Working Group of the Internet Engineering Task Force

• ctober 1993: RFC 1531 initially defined DHCP as a standard-track protocol succeeding the Bootstrap Protocol (BOOTP), which is a network protocol used by a network client to obtain an IP address from a configuration server

• October 1997: RFC 2131 released is the current DHCP definition for Internet Protocol version 4 (IPv4) networks

• The extensions of DHCP for IPv6 (DHCPv6) were published as RFC 3315

What is DHCP?

• It is a method for assigning Internet Protocol (IP) addresses permanently or to individual computers in an organization’s network

• DHCP lets a network administrator supervise and distribute IP addresses from a central point and automatically sends a new IP address when a computer is plugged into a different place in the network

DHCP

• DHCP a network service that enables clients to obtain network settings (IP Address, Subnet Mask, Default Gateway, DNS Server, Hostname and Domain) automatically from a central server
• The DHCP client sends a broadcast request to find the DHCP server and the DHCP server in the subnet responds with an IP address (and other common network parameters) from a pool of IP addresses
• The IP address can be bound to the MAC address of the client

                 Dynamically update DNS servers

                 Divide hosts into classes, based on many criteria

• When you configure Windows to “Obtain an address automatically,” DHCP is the service providing that address

• Server offers IP address and network parameters for a limited time (called a lease)
• In practice, leases may vary from 30 minutes to a week or so
• Short lease:

                 Clients get updated parameters quickly

                 Essential if you have more clients than addresses

                 Requires more processing power on the server

• Long Lease:

                  More reliable (clients may continue to operate for a week after DHCP server fails)

                  But takes longer for all clients to get new settings if they change.

Motivation for DHCP

• Configuration parameters for network hosts

1. IP address
2. Router
3. Subnet Mask
4. Others.

Why is DHCP Important?

• Important when it comes to adding a machine to a network
• When computer requests an address, the administrator would have to manually configure the machine

                     Mistakes are easily made

                     Causes difficulty for both administrator as well as neighbors on the network

• DHCP solves all the hassle of manually adding a machine to a network

How does DHCP work?

• When a client needs to start up TCP/IP operations, it broadcasts a request for address information
• The DHCP server will not reallocate the address during the lease period and will attempt to return the same address every time the client requests an address

• The client can extend its lease or send a message to the server before the lease expires it that it no longer needs the address so it can be released and assigned to another client on the network 

Advantages of DHCP

• DHCP minimizes the administrative burden

•  By using DHCP there is no chance to conflict IP address

• By using DHCP relay agent you provide IP address to another network

Disadvantages of DHCP

• When DHCP server is unavailable, client is unable to access enterprises network

• Your machine name does not change when you get a new IP address 

Security problem

• DHCP is an unauthenticated protocol
• When connecting to a network, the user is not required to provide credentials in order to obtain a lease
• Malicious users with physical access to the DHCP-enabled network can instigate a denial-of-service attack on DHCP servers by requesting many leases from the server, thereby depleting the number of leases that are available to other DHCP clients

Limitations 

• Some machines on your network need to be at fixed addresses, for example servers and routers
• You need to be able to assign a machine to run the DHCP server continually as it must be available at all times when clients need IP access 

DHCP operations

DHCPDISCOVER — from client

client has no address, asking for a new one

DHCPOFFER — from server

Offer of address and other parameters

DHCPREQUEST — from client

Client asks if it can use the offered address and parameters

DHCPACK — from server

Server says “yes, go ahead, this address and these parameters are yours; the lease starts now.”

DHCPNAK — from server

client has no address, asking for a new one

DHCPDECLINE — from client

Client has detected another machine is using the offered address, and tells the server about this problem

DHCPRELEASE — from client

Server expires the lease immediately

DHCPINFORM — from client

Client already has an IP address, but wants other network settings from the server

By : mogtaba altyib 
Modification by : Mohammed Bakry PhD