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

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.

Đặng Thanh Bình mm

WHY USE DHCP?

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)

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

DHCP server

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

DHCP clients

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

devices from office to office

CONFIGURING A DHCP SERVER

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

client lease database

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

mark (#) are considered comments

DNS update schemes

implemented

— the ad-hoc DNS update mode

— the interim DHCP-DNS interaction update mode

¢ You must configure the DNS server for compatibility with these schemes

¢ To use the recommended mode, add the

following line to the top of the configuration file

ddns-update-style interim;

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

declarations

Ex 1 Subnet Declaration

¢ There are global options for every DHCP client in the subnet and a range declared Clients are assigned an IP address within the range

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

¢ The name of the shared-network must be a descriptive title for the network, such as using the title ‘test-lab to describe all the subnets ina test lab environment

Ex 2 Shared-network Declaration

shared-network name {

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

option domain-name-servers nsl.redhat.com, ns2.redhat.com;

Ex 3 Group Declaration

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

¢ For example, shared networks, subnets, and

hosts can be grouped

Ex 3 Group Declaration

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;

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

systems

Ex 4 Range Parameter

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

iy , |

Ex 5 Static IP Address using DHCP

host apex {

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

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

fixed-address 192.168.1.4;

Lease Database

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

— The dhcpd.leases file is renamed dAgd:-leases~ and the

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

Starting and Stopping the Server

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

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

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

¢ To start the DHCP Relay Agent, use the command service dhcrelay start

CONFIGURING A DHCP CLIENT

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

/etc/sysconfig/network file

¢ The /etc/sysconfig/network file should contain

the following line:

NETWORKING=-yes

¢ The NETWORKING variable must be set to yes if you want networking to start at boot time

¢ A configuration file is needed for each device to

be configured to use DHCP

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

/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

¢ no — Non-root users are not allowed to control this device