Книга: Fedora™ Unleashed, 2008 edition

How DHCP Works

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 the following:

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 DHCP assigns to a host. This means that DNS lookups are not possible on machines configured through DHCP alone; there fore, 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 that make use of registered host- names 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 DynDNS (http://www.dyndns.com/), are already offering Dynamic DNS services and have clients for Linux.

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