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DHCP Server Operation
Chapter 8 DHCP Server Operation 8.1 DHCP Server overview
This section describes the general operation of the DHCP server.
The DHCP protocol allows a host which is unknown to the network
administrator to be automatically assigned a new IP address out of a pool
of IP addresses for its network. In order for this to work, the network
administrator allocates address pools for each available subnet and enters
them into the dhcpd.conf file.
On startup, the DHCP server software reads the dhcpd.conf file and stores a
list of available addresses on each subnet. When a client requests an
address using the DHCP protocol, the server allocates an address for it.
Each client is assigned a lease, which expires after an amount of time
chosen by the administrator (by default, 12 hours). Some time before leases
expire, the clients to which leases are assigned are expected to renew them
in order to continue to use the addresses. Once a lease has expired, the
client to which that lease was assigned is no longer permitted to use the
leased IP address and must resort back to the DHCPDISCOVER
mechanism.
In order to keep track of leases across system reboots and server restarts,
the server keeps a list of leases it has assigned in the dhcpd.leases file. This
lease file is stored using ISFS, which is in turn committed to flash memory
(if available) according to user requirement, via issuing of the “flashfs
update” command.
Before a lease is granted to a host, it records the lease in this file. Upon
startup, after reading the dhcpd.conf file, the DHCP server reads the