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&long y = 365**24;
System.out.println(y);
结果： --错误
long y = 365**24L;
System.out.println(y);
结果： --正确
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声明：本站为我们域名爱好者共同分享知识的平台,非域名注册机构.Configuring a DHCP ServerRed Hat Enterprise Linux 3: System Administration GuidePrevChapter 25. Dynamic Host Configuration Protocol (DHCP)Next25.2. Configuring a DHCP Server To configure a DHCP server, modify the configuration file
/etc/dhcpd.conf.
DHCP also uses the file /var/lib/dhcp/dhcpd.leases
to store the client lease database. Refer to Section 25.2.2 Lease Database for more information.
25.2.1. Configuration File
The first step in configuring a DHCP server is to create the
configuration file that stores the network information for the
clients. Global options can be declared for all clients, and options
can be declared for each client system.
The configuration file can contain any extra tabs or blank lines for
easier formatting. The keywords are case-insensitive, and lines
beginning with a hash mark (#) are considered comments.
Two DNS update schemes are currently implemented — the ad-hoc
DNS update mode and the interim DHCP-DNS interaction draft update
If and when these two are accepted as part of the IETF
standards process, there will be a third mode — the standard DNS
update method.
The DHCP server must be configured to use one of the
two current schemes. Version 3.0b2pl11 and previous version used the
ad- however, it has been deprecated. To keep the same
behavior, add the following line to the top of the configuration file:
ddns-update-style ad-
To use the recommended mode, add the following line to the top of the
configuration file:
ddns-update-
Refer to the dhcpd.conf man page for details about
the different modes.
There are two types of statements in the configuration file:
Parameters — state how to perform a task, whether to
perform a task, or what network configuration options to send to
the client.Declarations — describe the topology of the network,
describe the clients, provide addresses for the clients, or
apply a group of parameters to a group of declarations.
Some parameters must start with the option
keyword and are referred to as options. Options configure DHCP
whereas, parameters configure values that are not optional or
control how the DHCP server behaves.
Parameters (including options) declared before a section enclosed in
curly brackets ({ }) are considered global parameters. Global
parameters apply to all the sections below it.
Important&
If the configuration file is changed, the changes do not take effect
until the DHCP daemon is restarted with the command service
dhcpd restart.
In Example 25-1, the routers,
subnet-mask, domain-name,
domain-name-servers, and
time-offset options are used for any
host statements declared below it.
As shown in Example 25-1, a subnet can
be declared. A subnet declaration must be
included for every subnet in the network. If it is not, the DHCP
server fails to start.
In this example, there are global options for every DHCP
client in the subnet and a range
declared. Clients are assigned an IP address within the
subnet 192.168.1.0 netmask 255.255.255.0 {
option routers
192.168.1.254;
option subnet-mask
255.255.255.0;
option domain-name
option domain-name-servers
192.168.1.1;
option time-offset
# Eastern Standard Time
range 192.168.1.10 192.168.1.100;
}Example 25-1. Subnet Declaration
All subnets that share the same physical network should be declared
within a shared-network declaration as shown in
Example 25-2. Parameters within the
shared-network but outside the enclosed
subnet declarations are considered global
parameters. The name of the shared-network
should be a descriptive title for the network such as test-lab to
describe all the subnets in a test lab environment.
shared-network name {
option domain-name
option domain-name-servers
option routers
192.168.1.254;
more parameters for EXAMPLE shared-network
subnet 192.168.1.0 netmask 255.255.255.0 {
parameters for subnet
range 192.168.1.1 192.168.1.31;
subnet 192.168.1.32 netmask 255.255.255.0 {
parameters for subnet
range 192.168.1.33 192.168.1.63;
}Example 25-2. Shared-network Declaration
As demonstrated in Example 25-3, the
group declaration can be used to apply global
parameters to a group of declarations. For example, shared networks,
subnets, hosts, or other groups can be grouped.
option routers
192.168.1.254;
option subnet-mask
255.255.255.0;
option domain-name
option domain-name-servers
192.168.1.1;
option time-offset
# Eastern Standard Time
host apex {
option host-name "";
hardware ethernet 00:A0:78:8E:9E:AA;
fixed-address 192.168.1.4;
host raleigh {
option host-name "";
hardware ethernet 00:A1:DD:74:C3:F2;
fixed-address 192.168.1.6;
}Example 25-3. Group Declaration
To configure a DHCP server that leases a dynamic IP address to a system
within a subnet, modify Example 25-4 with your
values. It declares a default lease time, maximum lease time, and
network configuration values for the clients. This example assigns IP
addresses in the range 192.168.1.10 and
192.168.1.100 to client systems.
default-lease-time 600;
max-lease-time 7200;
option subnet-mask 255.255.255.0;
option broadcast-address 192.168.1.255;
option routers 192.168.1.254;
option domain-name-servers 192.168.1.1, 192.168.1.2;
option domain-name "";
subnet 192.168.1.0 netmask 255.255.255.0 {
range 192.168.1.10 192.168.1.100;
}Example 25-4. Range Parameter
To assign an IP address to a client based on the MAC address of the
network interface card, use the hardware ethernet
parameter within a host declaration. As
demonstrated in Example 25-5, the
host apex declaration specifies that the network
interface card with the MAC address 00:A0:78:8E:9E:AA always
receives the IP address 192.168.1.4.
Notice that the optional parameter host-name can
be used to assign a host name to the client.
host apex {
option host-name "";
hardware ethernet 00:A0:78:8E:9E:AA;
fixed-address 192.168.1.4;
}Example 25-5. Static IP Address using DHCPTip&
The sample configuration file provided can be used as a starting
point. Custom configuration options can be added to it. To copy it
to the proper location, use the following command:
cp /usr/share/doc/dhcp-<version-number>/dhcpd.conf.sample /etc/dhcpd.conf
(where <version-number> is the DHCP
version number).
For a complete list of option statements and what they do, refer to
the dhcp-options man page.
25.2.2. Lease Database
On the DHCP server, the file
/var/lib/dhcp/dhcpd.leases stores the DHCP client
lease database. This file should not be modified by hand. DHCP lease
information for each recently assigned IP address is automatically
stored in the lease database. The information includes the length of
the lease, to whom the IP address has been assigned, the start and end
dates for the lease, and the MAC address of the network interface card
that was used to retrieve the lease.
All times in the lease database are in Greenwich Mean Time (GMT), not
local time.
The lease database is recreated from time to time so that it is not
too large. First, all known leases are saved in a temporary lease
database. The dhcpd.leases file is renamed
dhcpd.leases~, and the temporary lease database
is written to dhcpd.leases.
The DHCP daemon could be killed or the system could crash after the
lease database has been renamed to the backup file but before the new
file has been written. If this happens, the
dhcpd.leases file does not exist, but it is
required to start the service. Do not create a new lease file.
do, all the old leases will be lost and cause many problems. The
correct solution is to rename the dhcpd.leases~
backup file to dhcpd.leases and then start the
25.2.3. Starting and Stopping the ServerImportant&
When the DHCP server is started for the first time, it will fail
unless the dhcpd.leases
file exists. Use the command touch
/var/lib/dhcp/dhcpd.leases to create the file if it does
not exist.
To start the DHCP service, use the command /sbin/service
dhcpd start.
To stop the DHCP server, use the command
/sbin/service dhcpd stop. To configure the daemon to
start automatically at boot time, refer to Chapter 21 Controlling Access to Services
for information on how to manage services.
If more than one network interface is attached to the system,
but the DHCP server should only be started on one of the interface,
configure the DHCP server to start only on that device. In
/etc/sysconfig/dhcpd, add the name of the
interface to the list of DHCPDARGS:
# Command line options here
DHCPDARGS=eth0
This is useful for a firewall machine with two network cards. One
network card can be configured as a DHCP client to retrieve an IP
address to the Internet. The other network card can be used as a DHCP
server for the internal network behind the firewall. Specifying only
the network card connected to the internal network makes the system
more secure because users can not connect to the daemon via the
Other command line options that can be specified in
/etc/sysconfig/dhcpd include:
<portnum> —
Specify the udp port number on which dhcpd
should listen. The default is port 67. The DHCP server transmits
responses to the DHCP clients at a port number one greater than
the udp port specified. For example, if the default port 67 is
used, the server listens on port 67 for requests and responses to
the client on port 68. If a port is specified here and the DHCP
relay agent is used, the same port on which the DHCP relay agent
should listen must be specified. Refer to Section 25.2.4 DHCP Relay Agent for details.
-f — Run the daemon as a foreground
process. This is mostly used for debugging.
-d — Log the DHCP server daemon to the
standard error descriptor. This is mostly used for debugging. If
this is not specified, the log is written to
/var/log/messages.
-cf <filename> —
Specify the location of the configuration file. The default
location is /etc/dhcpd.conf.
-lf <filename>
— Specify the location of the lease database file. If a
lease database file already exists, it is very important that the
same file be used every time the DHCP server is started. It is
strongly recommended that this option only be used for debugging
purposes on non-production machines. The default location is
/var/lib/dhcp/dhcpd.leases.
-q — Do not print the entire copyright
message when starting the daemon.
25.2.4. DHCP Relay Agent
The DHCP Relay Agent (dhcrelay) allows for the
relay of DHCP and BOOTP requests from a subnet with no DHCP server on
it to one or more DHCP servers on other subnets.
When a DHCP client requests information, the DHCP Relay Agent forwards
the request to the list of DHCP servers specified when the DHCP Relay
Agent is started. When a DHCP server returns a reply, the reply is
broadcast or unicast on the network that sent the original request.
The DHCP Relay Agent listens for DHCP requests on all interfaces
unless the interfaces are specified in
/etc/sysconfig/dhcrelay with the
INTERFACES directive.
To start the DHCP Relay Agent, use the command service
dhcrelay start.
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