DHCP Server - Basics

DHCP (Dynamic Host Configuration Protocol) is a client-server technology that allows DHCP servers to assign or lease IP addresses to DHCP clients. DHCP server offers its clients, IPv4/IPv6 addresses to a specified period of time. A DHCP server provides IP addresses to a DHCP client through the exchange of a series of messages, which the process is termed DORA (Discover Offer Request Acknowledgement).

What DORA process is?

As said earlier, it is an exchange of series of messages between DHCP client and DHCP server as shown below:

DHCP Client: Sends a Hello message to the network to discover any DHCP servers available.

DHCP Server: Sends a message again offering an IP to the client.

DHCP Client: After choosing an IP offered by DHCP server, it sends a request to DHCP server for that IP.

DHCP Server: In response to the client request, DHCP server assigns that IP to the DHCP client and both server and client sends an acknowledgement to each other.

Other messages sent between DHCP server and DHCP client:

DHCPNack: Message sent for denying the client’s DHCP Request message. Occurred for two reasons, if the requested address is incorrect (may be due to the client moved to a new subnet) or because the DHCP client’s lease period has expired and cannot be renewed.

DHCPDecline: Message from DHCP Client to DHCP Server, informing that the IP offered by server is being used by another client computer.

DHCPRelease: Message from DHCP Client to DHCP Server, releasing the IP address assigned and cancelling the remaining lease.

DHCPInform: Message from DHCP Client to DHCP Server, asking for local configuration parameters, often used to detect unauthorized DHCP servers.

Responsibilities of DHCP Server include maintenance of scopes, reservations and options.

Scopes: Scope defines the range of IP addresses for a network. Without defining scopes, DHCP server cannot distribute IP addresses to clients. It has got the following properties:

Scope name — name of the scope.

A range of possible IP addresses to be used.

Unique subnet mask.

Lease duration values.

Reservations: As the name specifies this determines that a specified hardware device on a subnet is assigned a specific IP address, all the time.

Options: Additional configuration parameters that a DHCP server assigns to clients. These are also used for DHCP communication between the sever computer and client computers. Typical options are server and scope options.

Server options: These apply globally for all scopes and classes defined at each DHCP server and clients.

Scope options: Apply to any clients that obtain a lease from that particular scope.

Reserved client options: Apply only to a specific client corresponding to the reservation.

Default DHCP options:

Option Code          Option Name

1                              Subnet mask

3                              Router

6                              DNS servers

15                           DNS domain name

44                           WINS/NBNS servers

46                           WINS/NetBT node type

47                           NetBIOS scope ID

51                           Lease time

58                           Renewal (T1) time value

59                           Rebinding (T2) time value

31                           Perform router discovery

33                           Static route

43                           Vendor-specific information

249                         Classless static routes