Dynamic Host Configuration Protocol

As its name implies, Dynamic Host Configuration Protocol (DHCP) configures hosts for connection to your network. DHCP allows a network administrator to configure all TCP/IP parameters for each host as he connects to the network after activation of a NIC. These parameters include automatically assigning an IP address to a NIC, setting name server entries in /etc/resolv.conf, and configuring default routing and gateway information for a host. This section first describes how to use DHCP to obtain IP address assignment for your NIC, and then how to quickly set up and start a DHCP server using Ubuntu.

Note

You can learn more about DHCP by reading RFC2131 "Dynamic Host Configuration Protocol." Browse to http://www.ietf.org/rfc/rfc2131.txt.

How DHCP Works

DHCP provides persistent storage of network parameters by holding identifying information for each network client that might connect to the network. The three most common pairs of identifying information are

• Network subnet/host address Used by hosts to connect to the network at will

• Subnet/hostname Enables the specified host to connect to the subnet

• Subnet/hardware address Enables a specific client to connect to the network after getting the hostname from DHCP

DHCP also allocates to clients temporary or permanent network (IP) addresses. When a temporary assignment, known as a lease, elapses, the client can request to have the lease extended, or, if the address is no longer needed, the client can relinquish the address. For hosts that will be permanently connected to a network with adequate addresses available, DHCP allocates infinite leases.

DHCP offers your network some advantages. First, it shifts responsibility for assigning IP addresses from the network administrator (who can accidentally assign duplicate IP addresses) to the DHCP server. Second, DHCP makes better use of limited IP addresses. If a user is away from the office for whatever reason, the user's host can release its IP address for use by other hosts.

Like most things in life, DHCP is not perfect. Servers cannot be configured through DHCP alone because DNS does not know what addresses that DHCP assigns to a host. This means that DNS lookups are not possible on machines configured through DHCP alone; therefore, services cannot be provided. However, DHCP can make assignments based on DNS entries when using subnet/hostname or subnet/hardware address identifiers.

Note

The problem of using DHCP to configure servers using registered hostnames is being addressed by Dynamic DNS which, when fully developed, will enable DHCP to register IP addresses with DNS. This will allow you, for example, to register a domain name (such as imalinuxuser.com) and be able to easily access that domain's web server without needing to use static IP addressing of a specific host. The largest hurdle to overcome is the security implication of enabling each host connecting to the system to update DNS. A few companies, such as http://www.dyndns.org/, are already offering Dynamic DNS services and have clients for Linux.

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