Dynamic Host Configuration Protocol

Dynamic Host Configuration Protocol (DHCP) is a protocol used by network computers to obtain unique IP addresses,and other parameters like: default router, subnet mask, and IP addresses for DNS servers from DHCP server. This protocol is used when computers (especially notebooks) are added to a network because these settings are necessary for the host to participiate in the network. This setting is periodically refreshed with typical intervals ranging from one hour to several months. This is determined on the server side.

The DHCP server ensures that all IP addresses are unique, that is, no IP address is assigned to a second client while the first client's assignment is valid (its lease has not expired). Thus ip address pool management is done by the server and not by a human network administrator.

DHCP emerged as a standard protocol in October 1993. As of 2006, RFC 2131 provides the latest ([dated March 1997]) DHCP definition. DHCP functionally became a successor to the older BOOTP protocol, whose leases were given for infinite time and did not support options. Due to the backward-compatibility of DHCP, very few networks continue to use pure BOOTP.

The latest non-standard of the protocol, describing DHCPv6 (DHCP in a IPv6 environment), appeared in July 2003 as RFC 3315.

Overview

The Dynamic Host Configuration Protocol (DHCP) automates the assignment of IP addresses, subnet masks, default routers, and other IP parameters. The assignment usually occurs when the DHCP configured machine boots up or regains connectivity to the network. The DHCP client sends out a query requesting a response from a DHCP server on the locally attached network. The DHCP server then replies to the client with its assigned IP address, subnet mask, DNS server and default gateway information.

The assignment of the IP address usually expires after a predetermined period of time, at which point the DHCP client and server renegotiate a new IP address from the server's predefined pool of addresses. Configuring firewall rules to accommodate access from machines who receive their IP addresses via DHCP is therefore more difficult because the remote IP address will vary from time to time. Administrators must usually allow access to the entire remote DHCP subnet for a particular TCP/UDP port.

Most home routers and firewalls are configured in the factory to be DHCP servers for a home network. An alternative to a home router is to use a Linux computer with a fixed IP address as a DHCP server. ISPs generally use DHCP to assign clients individual IP addresses.

DHCP is a broadcast-based protocol. As with other types of broadcast traffic, it does not cross a router unless specifically configured to do so. Users who desire this capability must configure their routers to pass DHCP traffic across UDP ports 67 and 68.

Technical details

DHCP uses the same two IANA assigned ports as BOOTP: 67/udp for the server side, and 68/udp for the client side.

DHCP operations fall into four basic phases. These phases are IP lease request, IP lease offer, IP lease selection, and IP lease acknowledgement.

DHCP discovery

The client broadcasts on the local physical subnet to find available servers. Network administrators can configure a local router to forward DHCP packets to a DHCP server on a different subnet. This client-implementation creates a UDP packet with the broadcast destination of 255.255.255.255 or subnet broadcast address and also requests its last-known IP address (in the example below, 192.168.1.100) although the server may ignore this optional parameter....

DHCP offers

When a DHCP server receives an IP lease request from a client, it extends an IP lease offer. This is done by reserving an IP address for the client and broadcasting a DHCPOFFER message across the network. This message contains the client's MAC address, followed by the IP address that the server is offering, the subnet mask, the lease duration, and the IP address of the DHCP server making the offer.

The server determines the configuration, based on the client's hardware address as specified in the CHADDR field. Here the server, 192.168.1.1, specifies the IP address in the YIADDR field.

DHCP requests

Whenever a computer comes on line, it checks to see if it currently has an IP address leased. If it does not, it requests a lease from a DHCP server. Because the client computer does not know the address of a DHCP server, it uses 0.0.0.0 as its own IP address and 255.255.255.255 as the destination address. Doing so allows the client to broadcast a DHCPDISCOVER message across the network. Such a message consists of the client computer's Media Access Control (MAC) address (the hardware address built into the network card) and its NetBIOS name.

The client selects a configuration out of the DHCP "Offer" packets it has received and broadcasts it on the local subnet. Again, this client requests the 192.168.1.100 address that the server specified. In case the client has received multiple offers it specifies the server from which it has accepted the offer.

DHCP acknowledgement

When the DHCP server receives the DHCPREQUEST message from the client, it initiates the final phase of the configuration process. This acknowledgement phase involves sending a DHCPACK packet to the client. This packet includes the lease duration and any other configuration information that the client might have requested. At this point, the TCP/IP configuration process is complete.

The server acknowledges the request and sends the acknowledgement to the client. The system as a whole expects the client to configure its network interface with the supplied options.