Dynamic Host Configuration Protocol (DHCP)

Dynamic Host Configuration Protocol (DHCP) is a client/server protocol that automatically provides an Internet Protocol (IP) host with its IP address and other related configuration information such as the subnet mask and default gateway. RFCs 2131 and 2132 define DHCP as an Internet Engineering Task Force (IETF) standard based on Bootstrap Protocol (BOOTP), a protocol with which DHCP shares many implementation details. DHCP allows hosts to obtain required TCP/IP configuration information from a DHCP server.

Why use DHCP?

Every device on a TCP/IP-based network must have a unique unicast IP address to access the network and its resources. Without DHCP, IP addresses for new computers or computers that are moved from one subnet to another must be configured manually; IP addresses for computers that are removed from the network must be manually reclaimed.

With DHCP, this entire process is automated and managed centrally. The DHCP server maintains a pool of IP addresses and leases an address to any DHCP-enabled client when it starts up on the network. Because the IP addresses are dynamic (leased) rather than static (permanently assigned), addresses no longer in use are automatically returned to the pool for reallocation.

The network administrator establishes DHCP servers that maintain TCP/IP configuration information and provide address configuration to DHCP-enabled clients in the form of a lease offer. The DHCP server stores the configuration information in a database that includes:

  • Valid TCP/IP configuration parameters for all clients on the network.
  • Valid IP addresses, maintained in a pool for assignment to clients, as well as excluded addresses.
  • Reserved IP addresses associated with particular DHCP clients. This allows consistent assignment of a single IP address to a single DHCP client.
  • The lease duration, or the length of time for which the IP address can be used before a lease renewal is required.

A DHCP-enabled client, upon accepting a lease offer, receives:

  • A valid IP address for the subnet to which it is connecting.
  • Requested DHCP options, which are additional parameters that a DHCP server is configured to assign to clients. Some examples of DHCP options are Router (default gateway), DNS Servers, and DNS Domain Name.

Benefits of DHCP

DHCP provides the following benefits.

  1. Reliable IP address configuration. DHCP minimizes configuration errors caused by manual IP address configuration, such as typographical errors, or address conflicts caused by the assignment of an IP address to more than one computer at the same time.
  2. Reduced network administration. DHCP includes the following features to reduce network administration:
    1. Centralized and automated TCP/IP configuration.
    1. The ability to define TCP/IP configurations from a central location.
    1. The ability to assign a full range of additional TCP/IP configuration values by means of DHCP options.
    1. The efficient handling of IP address changes for clients that must be updated frequently, such as those for portable devices that move to different locations on a wireless network.
    1. The forwarding of initial DHCP messages by using a DHCP relay agent, which eliminates the need for a DHCP server on every subnet.

How DHCP works

DHCP runs at the application layer of the Transmission Control Protocol/IP (TCP/IP) stack to dynamically assign IP addresses to DHCP clients and to allocate TCP/IP configuration information to DHCP clients. This includes subnet mask information, default gateway IP addresses, and domain name system (DNS) addresses.

DHCP is a client-server protocol in which servers manage a pool of unique IP addresses, as well as information about client configuration parameters, and assign addresses out of those address pools. DHCP-enabled clients send a request to the DHCP server whenever they connect to a network.

Clients configured with DHCP broadcast a request to the DHCP server and request network configuration information for the local network to which they’re attached. A client typically broadcasts a query for this information immediately after booting up. The DHCP server responds to the client request by providing IP configuration information previously specified by a network administrator. This includes a specific IP address, as well as a time period — also called a lease — for which the allocation is valid. When refreshing an assignment, a DHCP client requests the same parameters, but the DHCP server may assign a new IP address based on policies set by administrators. DHCP clients can also be configured on an Ethernet interface.

A DHCP server manages a record of all the IP addresses it allocates to network nodes. If a node is relocated in the network, the server identifies it using its Media Access Control (MAC) address, which prevents the accidental configuration of multiple devices with the same IP address. Configuring a DHCP server also requires the creation of a configuration file, which stores network information for clients.

DHCP is not a routable protocol, nor is it a secure one. DHCP is limited to a specific local area network, which means a single DHCP server per LAN is adequate or two servers for use in case of a failover. Larger networks may have a wide area network (WAN) containing multiple individual locations. Depending on the connections between these points and the number of clients in each location, multiple DHCP servers can be set up to handle the distribution of addresses. If network administrators want a DHCP server to provide addressing to multiple subnets on a given network, they must configure DHCP relay services located on interconnecting routers that DHCP requests have to cross. These agents relay messages between DHCP clients and servers located on different subnets.

DHCP lacks any built-in mechanism that would enable clients and servers to authenticate each other. Both are vulnerable to deception — one computer pretending to be another — and to attack, where rogue clients can exhaust a DHCP server’s IP address pool.

When managing many DHCP servers or DHCP servers in a WAN, users can make use of a command line. Users should also be aware that starting, stopping and restarting will affect the running of the daemon.

Components

DHCP is made up of numerous components, such as the DHCP server, client, and relay. The DHCP server — typically either a server or router — is a networked device that runs on the DHCP service. The DHCP server holds IP addresses, as well as related information pertaining to configuration. The DHCP client is a device — such as a computer or phone — that can connect to a network and communicate with a DHCP server. The DHCP relay will manage requests between DHCP clients and servers. Typically, relays are used when an organization has to handle large or complex networks. Other components include the IP address pool, subnet, lease and DHCP communications protocol.

DHCP DORA process stands for the following message flows between the client and the server.

  • Discover
  • Offer
  • Request
  • Acknowledge

Below diagram depicts the message flow between the DHCP client and the DHCP Server –

DHCP Relay Agent

DHCP relay agent is any TCP/IP host which is used to forward requests and replies between DHCP server and client when the server is present on a different network. Relay agents receive DHCP messages and then generate a new DHCP message to send out on another INTERFACE. Also, DHCP relay agent adds a giaddr (gateway address of packet) field and also the Relay agent information option 82 if enabled. The options field is removed when the server reply is forwarded to the host.

DHCP clients use local broadcast messages to obtain IP addresses from the DHCP server. By default, routers do not forward local broadcast messages. This means, if the DHCP server is configured on another network or a router is configured between the DHCP server and DHCP clients, the DHCP clients will not receive IP addresses from the DHCP server.

In such a situation, a network administrator has two choices either configure a DHCP server in each subnet or configure the router as a DHCP relay agent that connects the subnet to the DHCP server. A DHCP relay agent sits between a DHCP server and DHCP clients and allows the DHCP clients to obtain IP addresses from the DHCP server that is not configured on the same LAN.

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