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 Fedora.
You can learn more about DHCP by reading RFC 2131, "Dynamic Host Configuration Protocol." Browse tohttp://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 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.
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.
DHCP Software Installation
Installation of the DHCP client and server might be easiest during the initial install of Fedora, but you can also use yum later. This section describes configuring the dhclient and setting up and running the dhpcd daemon.
dhclientAs previously mentioned, using DHCP for an installed NIC is easily accomplished when installing Fedora on your host (read more about installation in Chapter 1, "Installing Fedora"), and during the network step of installation, you can choose to have DHCP initiated at boot time. If you choose to do this (and choose to install the DHCP client package), the DHCP client, dhclient, sends a broadcast message to which the DHCP server replies with networking information for your host. That's it; you're finished.
If you choose to install from source, you will have to (as root) download and install the server packages that include dhclient. Unpack the source file, run ./configure from the root of the source directory, run make, and then run make install. This should put the DHCP client binaries where they will start at the correct time in the boot process.
You can however, fine-tune how dhclient works, and where and how it obtains or looks for DHCP information. You probably will not need to take this additional effort; but if you do, you can create and edit a file named dhclient.conf, and save it in the /etc directory with your settings. A few of the dhclient.conf options include the following:
► timeout time ; — How long to wait before giving up trying (60 seconds is the default)
► retry time ; — How long to wait before retrying (5 minutes is the default)
► select-timeout time ; — How long to wait before selecting a DHCP offer (0 seconds is the default)
► reboot time ; — How long to wait before trying to get a previously set IP (10 seconds is the default)
► renew date ; — When to renew an IP lease, where date is in the form of <weekday><year>/<month>/<day><hour>:<minute>:<second>, such as 4 2004/1/1 22:01:01 for Thursday, January 4, 2004 at 10:01 p.m.
See the dhclient.conf man page for more information on additional settings.
Again, the easiest way to install the DHCP server on your computer is to include the RPMs at install time or to use yum if you have installed your machine without installing the DHCP server. If you are so inclined, you can go to the Internet Software Consortium (ISC) website and download and build the source code yourself (http://www.isc.org/).
If you decide to install from a source downloaded from the ISC website, the installation is straightforward. Just unpack your tar file, run ./configure from the root of the source directory, run make, and finally, if there are no errors, run make install. This puts all the files used by the DHCP daemon in the correct places. It's best to leave the source files in place until you are sure that DHCP is running correctly; otherwise, you can delete the source tree.
For whichever installation method you choose, be sure that a file called /etc/dhcpd.leases is created. The file can be empty, but it does need to exist for dhcpd to start properly.