Page 193 - DCAP106_OPERATING_SYSTEM_TOOLS

Operating System Tools

Notes The initial statements you should be concerned with are the allow and deny statements. These
statements control how dhcpd handles client requests. The three allow/deny statements are as
follows:

z z booting: This statement is used within a host statement to allow or deny the exact host
from obtaining any configuration information from the DHCP server. By default, hosts are
allowed booting.

z z bootp: This statement is used to inform the server whether or not it should act as both a
BootP and DHCP server or just a DHCP server. By default, BootP clients are allowed, so
you need only one server to handle both DHCP and BootP requests.

z z unknown clients: This statement controls whether dhcpd will allow or deny clients for
which it does not have detailed host entries. By default, this is allowed, as denying this
feature takes away about 90 percent of the reason why anyone would want to use a DHCP
server.

Example: To deny booting, you would use:

deny booting;
There are also many additional configuration options that control the server and the DHCP
protocol. The more commonly used options are as follows:
z z boot_unknown_clients [true|false]: If the value is false, simply clients that have a host
statement are assigned an address. The default is true.
z z default_lease_time [seconds]: This option describe the length of time, in seconds, for an IP
address lease if the client does not request a specific lease length.

z z dynamic_bootp_lease_cutoff [date]: This option defines a termination date for addresses
assigned to BootP clients. By default, BootP clients are assigned a permanent address.
z z dynamic_bootp_lease_length [seconds]: This option defines the length of time in seconds
for an IP address lease for BootP clients. Note, though, that BootP clients do not renew
address leases, so a client that does not boot and contact the server often enough will lose
its lease.

z z fixed_address [address[,address...]]: This option assigns a permanent IP address to a host
as part of a host statement. More than one address can be assigned for a client that boots
on more than one subnet.

z z get_lease_hostnames [true|false]: If the value is true, dhcpd will execute a reverse lookup
for every dynamically assigned address and send to the client the host name it gets from
DNS. However, this can add a lot of extra overhead for servers on larger networks. By
default, this value is false, and no reverse lookups are done.
z z hardware ethernet [address]: This option defines the client’s Ethernet address within a
host statement. The DHCP server uses the Ethernet address to map host information to
a specific client. For BootP clients, this is the only way dhcpd can map the information;
however, DHCP clients can use other values in addition to the Ethernet address to identify
themselves to the server. To obtain the Ethernet address of a Linux client, run ifconfig and
check the Hwaddr field.
z z max_lease_time [seconds]: This option defines the maximum length, in seconds, of a lease
length. This is the maximum lease length a client may receive regardless of what it requests.
z z range [dynamic_bootp] [low address] [high address]: This option defines the range of IP
addresses available for the server to dynamically assign. The argument [dynamic_bootp]
tells dhcpd to assign dynamic IP addresses to BootP clients as well as DHCP clients. By
default, BootP clients are not assigned dynamic IP addresses because BootP was not
designed for it.

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