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Bài giảng Quản trị Linux: Dynamic host configuration protocol - Đặng Thanh Bình

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Bài giảng Quản trị Linux trình bày những kiến thức cơ bản về dynamic host configuration protocol. Nội dung chính trong chương này gồm có: Why use DHCP? Configuring a DHCP server, configuring a DHCP client. Mời các bạn cùng tham khảo.

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Đặng Thanh Bình mm

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WHY USE DHCP?

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Why Use DHCP?

¢ Dynamic Host Configuration Protocol (DHCP) is a network protocol that automatically assigns

TCP/IP information to client machines

¢ Each DHCP client connects to the centrally

located DHCP server, which returns that client's

network configuration (including the IP address, gateway, and DNS servers)

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Why Use DHCP?

¢ DHCP is useful for automatic configuration of

client network interfaces

¢ When configuring the client system, the administrator chooses DHCP instead of specifying

an IP address, netmask, gateway, or DNS servers

¢ The client retrieves this information from the

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Why Use DHCP?

¢ DHCP is also useful if an administrator wants to change the IP addresses of a large number of systems

¢ Instead of reconfiguring all the systems, he can just edit one DHCP configuration file on the

server for the new set of IP addresses

— If the DNS servers for an organization changes, the changes are made on the DHCP server, not on the

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Why Use DHCP?

¢ If an organization has a functional DHCP server properly connected to a network, laptops and other mobile computer users can move these

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CONFIGURING A DHCP SERVER

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Configuring a DHCP Server

¢ To configure a DHCP server, you must create the dhcpd.conf configuration file in the /etc/dhcp

directory

A sample file can be found at

/usr/Share/doc/dhcp-<version>/dhcpd.conf.sample

¢ DHCP also uses the file

/var/lib/dhcpd/dhcpd.leases to store’ the

client lease database

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Configuration File

¢ The first step in configuring a DHCP server is to create the configuration file that stores the

network information for the clients

¢ Use this file to declare options and global options for client systems

¢ The configuration file can contain extra tabs or blank lines for easier formatting Keywords are case-insensitive and lines beginning with a hash

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DNS update schemes

® IWO DNS update schemes are currently

implemented

— the ad-hoc DNS update mode

— the interim DHCP-DNS interaction update mode

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DNS update schemes

¢ To use adhoc mode, add the following line to the top of the configuration file

ddns-update-style ad-hoc;

¢ To use the recommended mode, add the

following line to the top of the configuration file

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Types Of Statements

¢ 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

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Ex 1 Subnet Declaration

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Ex 2 Shared-network Declaration

¢ All subnets that share the same physical network Should be declared within a shared-network declaration

e Parameters within the shared-network, but outside the enclosed subnet declarations, are

considered to be global parameters

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Ex 2 Shared-network Declaration

shared-network name {

option domain-name "test.redhat.com";

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Ex 3 Group Declaration

¢ The group declaration is used to apply global parameters to a group of declarations

¢ For example, shared networks, subnets, and

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Ex 3 Group Declaration —=— { ian option routers 192.168.1.254; option subnet-mask 2 AO a2 Os option domain-name "example.com"; option domain-name-servers 192.168.1.1; option time-offset -18000; # Eastern Standard Time host apex {

option host-name "apex.example.com"; hardware ethernet 00:A0:78:8E:9E:AA;

fixed-address 192.168.1.4;

host raleigh {

option host-name "raleigh.example.com"; hardware ethernet 00:A1:DD:74:C3:F2;

fixed-address 192.168.1.6;

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Ex 4 Range Parameter

¢ To configure a DHCP server that leases a dynamic IP address to a system within a subnet, modify Example 4, “Range Parameter” with your values

e 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 192.168.1100 to client

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Ex 5 Static IP Address using DHCP

¢ 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 5, “Static IP Address using DHCP”, the host apex declaration specifies that the NIC with the MAC address 00:A0:78:8E:9E:AA always receives the IP address 192.168.1.4

¢ Note that the optional parameter host-name can also be used to assign a host name to the client

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Ex 5 Static IP Address using DHCP

host apex {

option host-name "apex.example.com";

hardware ethernet 00:A0:7/8:8E:9E:AA;

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Lease Database

°Ồ Ơn the DHCP server, the file

/var/lib/dhcpd/dhcpd.leases stores the DHCP client

lease database

e All times in the lease database are in Coordinated Universal Time (UTC), 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 dAgd:-leases~ and the

temporary lease database is written to dhq@pd:- leases:

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Starting and Stopping the Server ¢ To start the DHCP service, use the command /sbin/service dhcpd start ¢ To stop the DHCP server, use the command /sbin/service dhcpd stop

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Starting and Stopping the Server

¢ If more than one network interface is attached to the system, but the DHCP server should only be Started on one of the interfaces, 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

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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

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DHCP Relay Agent

¢ 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

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CONFIGURING A DHCP CLIENT

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Configuring DHCP Clients

° The first step is to make sure the kernel recognizes the network interface card

¢ To configure a DHCP client manually

— Modify the /etc/sysconfig/network file to enable networking

— Modify the configuration file for each network device

in the f/etc/sysconfig/network-scripts directory

¢ In this directory, each device should have a configuration file named ifcfg-ethO, where ethO is the network device

name

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/etc/sysconfig/network file

¢ The /etc/sysconfig/network file should contain

the following line:

NETWORKING=-yes

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etc/sysconfig/network-scripts/ifcfg-ethO

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etc/sysconfig/network-scripts/ifcfg-ethO

¢ Other options for the network script

— DHCP_HOSTNAME — Only use this option if the DHCP server requires the client to specify a hostname before receiving an IP address (The DHCP server daemon in Red Hat Enterprise Linux does not support this feature.)

— PEERDNS=<answer>, where <answer> is one of the

following:

¢ yes — Modify /etc/resolv.conf with information from the server If using DHCP, then yes is the default

¢ no — Do not modify /etc/resolv.cont

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/etc/sysconfig/network-scripts/ifcfg-ethO

¢ Other options for the network script

— SRCADDR=<address>, where <address> Is the

specified source IP address for outgoing packets

— USERCTL=<answer>, where <answer> is one of the

following:

¢ yes — Non-root users are allowed to control this device

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