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DHCP configuration
As with many network protocols, the Dynamic Host Configuration Protocol (DHCP) is a client/server interface. A DHCP client is a much simpler program, both internally and to configure, than is a DHCP server. Essentially, the job of a DHCP client is to broadcast a DHCPDISCOVER message on its local physical subnet, then await a response.
The DHCPDISCOVER message MAY include options that suggest values for the network address and lease duration. Servers that receive a DHCPDISCOVER message should respond to the requesting MAC address with a DHCPOFFER message. The client, in turn, responds with a DHCPREQUEST message to one of the offering servers, usually to the first (and only) responding server.
The actual configuration parameters a client uses are received in a DHCPACK message. At that point, the client has received an allocated IP address and its communications will move, so to speak, from the Data Link Layer (Ethernet) to the Network Layer (IP).
A DHCP client typically only needs to be configured with the set of information it wishes to obtain. For example, Debian-based distributions typically use the DHCP client, dhclient, which is configured with the /etc/dhcp3/dhclient.conf file. The sample file that is distributed with the dhcp3-client package has all but one configuration option commented out. The one enabled option might look like:
Listing 1. Option for dhclient.conf
request subnet-mask, broadcast-address, time-offset, routers,
domain-name, domain-name-servers, host-name,
netbios-name-servers, netbios-scope;
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In this example, the default configuration, the client essentially just says "ask for everything possible." In the negotiation messages, the DHCPACK message from the server will contain information for all these requested values which the client will use once they are received. Client IP address is implied in this list since that configuration is always negotiated.
As well as specifying timeout and lease time options (and a few others), a client may, but need not in most cases, put some restrictions on the IP addresses it wishes to use. To exclude a particular one, you can use reject 192.33.137.209;. To specify the explicit address the client wishes to use, then use fixed-address 192.5.5.213;.
A client may reject a lease offer with the DHCPDECLINE message, but servers will try to fulfill requests where possible. A DHCP server may also make a fixed assignment of a particular IP address to a requesting MAC address; configuring a per-machine IP address is more often done with server configuration than with client configuration.
In order to keep track of acquired leases, dhclient keeps a list of leases it has been assigned in the /var/lib/dhcp3/dhclient.leases file (the path may vary across distros); this way a non-expired DHCP lease is not lost if a system disconnects from the physical network and/or reboots.
A DHCP server needs to know a bit more about its options since it provides various information to clients in DHCP leases and also must assure that IP addresses are uniquely assigned per client. The DHCP server usually runs as the daemon, dhcpd, and takes its configuration information from /etc/dhcpd.conf (this path may vary across distros). A single dhcpd daemon may manage multiple subnets, generally if multiple physical networks connect to a server; most frequently however, one server manages one subnet. Listing 2 is a fairly complete example of a server configuration.
Listing 2. dhcpd.conf configuration options
# default/max lease in seconds: day/week
default-lease-time 86400;
max-lease-time 604800;
option subnet-mask 255.255.255.0;
option broadcast-address 192.168.2.255;
option routers 192.168.2.1;
# DNS locally, fallback to outside local domain
option domain-name-servers 192.168.2.1, 151.203.0.84;
option domain-name "example.com";
# Set the subnet in which IP address are pooled
subnet 192.168.2.0 netmask 255.255.255.0 {
range 192.168.2.100 192.168.2.199;
}
# Assign a fixed IP to a couple machines by MAC address
group {
use-host-decl-names true;
host first.example.com {
hardware ethernet 00:00:c1:a2:de:10;
fixed-address 192.168.2.200;
}
host second.example.com {
hardware ethernet 00:dd:cc:a1:78:34;
fixed-address 192.168.2.201;
}
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When a client sends out a broadcast message to a server running with this configuration, it will either receive a lease on 192.168.2.200 or 192.168.2.201 if it has the specified MAC address, or it will receive a lease on an available address in the pool 192.168.2.100 through 192.168.2.199.
A client may also use the DHCPINFORM message to tell a server that it already has an assigned IP address (by manual configuration), but wishes to obtain other configuration information. Notice that informing a server that a client is using a particular IP address is not the same as a requesting a specific IP address; in the latter case, the server may or may not grant the request depending on existing leases. In the former case, the server has no voice in the decision and no lease is granted per se at all (however, servers will try to avoid assigning IP addresses known to be in use to new requesting clients).
When leases expire, clients and servers must negotiate new leases for configuration parameters to remain valid. Shorter leases may be used where configuration information on a server is likely to change (for example, with dynamic DNS via an external WAN). A client may gracefully terminate a lease by sending the DHCPRELEASE message, but this is not required for correct operation (clients sometimes crash, reboot, or become disconnected without the opportunity to send this message, after all).
Absent a release message, a lease is maintained by the server for whatever time terms it was granted on, so a rebooted machine will often continue using its prior lease (which will be stored in dhclient.leases on both server and client).
