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Linux+ Guide to Linux
Certification, Third Edition
Chapter 12
Network Configuration
Objectives
• Describe the purpose and types of networks,
protocols, and media access methods
• Explain the basic configuration of TCP/IP
• Configure a Network interface to use TCP/IP
• Configure a modem, ISDN, and DSL interface
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Objectives (continued)
• Describe the purpose of host names and how they
are resolved to IP addresses
• Configure TCP/IP routing
• Identify common network services
• Use command-line and graphical utilities to perform
remote administration
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Networks
• Network: two or more computers joined via media
and able to exchange information
• Local area networks (LANs): connect computers
within close proximity
– e.g., used to allow connection to shared resources
• Wide area networks (WANs): connect computers
separated by large distances
– e.g., used to connect to Internet Service Provider
• Internet service provider (ISP): company providing
Internet access
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Networks (continued)
• Routers: computers capable of transferring
information between networks
• Protocol: set of rules for communication between
networked computers
• Packets: packages of data formatted by a network
protocol
– Packets can be recognized by routers and other
network devices
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Networks (continued)
• Linux network protocols:
– TCP/IP (Transfer Control Protocol/Internet Protocol)
– UDP/IP (User Datagram Protocol/Internet Protocol)
– IPX/SPX (Internetwork Packet Exchange/Sequence
Packet Exchange)
– AppleTalk
– DLC (Data Link Control)
– DECnet (Digital Equipment Corporation network)
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Networks (continued)
• Media access method: defines how networked
computers share access to the physical medium
– Contained within the hardware on NIC or modem
• Ethernet: most common network media access
method
– Ensures that packets are retransmitted onto the
network if a network error occurs
• Token ring: popular media access method
– Controls which computer has the ability to transmit
information
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The TCP/IP Protocol
• Set of protocols with two core components
– TCP: ensures that packets are assembled in the
correct order, regardless of arrival order
– IP: responsible for labeling each packet with
destination address
• Together, TCP and IP ensure that information
packets travel across the network as quickly as
possible without getting lost
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IPv4 Addresses
• IP address: unique number that identifies a
networked computer
– Octet: series of four 8-bit numbers
• Common format of IPv4 addresses
• Unicast: directed TCP/IP communication from one
computer to another single computer
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IPv4 Addresses (continued)
• IPv4 addresses composed of two parts
– Network ID: network on which a computer is located
– Host ID: single computer on that network
• Two computers with different network IDs can have
the same host ID
• Only computers with same network ID can
communicate without a router
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Subnet Masks
• Define which part of IP address is the network ID
and which part is the host ID
– Series of four octets
– Octet in subnet mask containing 255 is part of
network ID
– Octet in subnet mask containing 0 is part of host ID
• ANDing: calculate network and host IDs from an IP
address and subnet mask
– Compare binary bits
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Subnet Masks (continued)
Figure 12-1: A sample IP address and subnet mask
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Subnet Masks (continued)
• 0.0.0.0 = all networks
• 255.255.255.255 = all computers on all networks
• 255 in an IP address can specify many hosts
– Broadcast addresses
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Default Gateway
• IP address of network interface on a router, to
which you send packets
• Routers can distinguish between different networks
– Move packets between them
– Have assigned IP addresses on each attached
network
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IPv4 Classes and Subnetting
• IP address class defines default subnet mask of
associated device
– All IP address classes can be identified by first octet
– Class A: 8 bits for network ID, 24 bits for host ID
• Assigned to very large companies
– Class B: 16 bits for network ID, 16 bits for host ID
• Assigned to larger organizations with several
thousand users
– Class C: 24 bits for network ID, 16 bits for host ID
• Used for small and home networks
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IPv4 Classes and Subnetting
• Multicast: TCP/IP communication destined for a
certain group of computers
– Class D addresses
• Subnetting: divide a large network into smaller
networks
– Control traffic flow
– Take bits from host ID; give to network ID
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IPv4 Classes and Subnetting
(continued)
Table 12-1: IP address classes
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The IPv6 Protocol
• Number of IP addresses using IPv4 is unsuitable
for Internet growth
• IPv6 protocol: uses 128 bits to identify computers
– Addresses written using eight 16-bit hexadecimal
numbers
• IPv6 address contains two portions
– First half assigned by ISP and identifies network
– Last half is link local portion: used to uniquely
identify computers in a LAN
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The IPv6 Protocol (continued)
• Few networks have adopted IPv6
– Proxy servers and NAT routers: computers or
hardware devices that have an IP address and
access to a network
• Used by other computers to obtain network resources
on their behalf
• Allows computers behind different NAT routers or
proxy servers to have the same IPv4 address
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Configuring a Network Interface
• If NIC detected during installation, Fedora Linux
automatically configures appropriate driver
• insmod and modprobe commands: used to load
kernel objects into the Linux kernel
– Can be used to load NIC drivers
• lsmod command: displays a list of currently loaded
modules
• rmmod command: removes module from kernel
• Most modules loaded from dist.conf file in the
/etc/modprobe.d directory
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Configuring a Network Interface
(continued)
• ifconfig command: assign TCP/IP configuration
to a NIC
– Also used without any arguments to view
configuration of all network interfaces in computer
• dhclient command: receive TCP/IP configuration
from DHCP or Boot Protocol (BOOTP) server
• Automatic private IP addressing (APIPA):
automatic assignment of IP address in the absence
of BHCP and BOOTP
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Configuring a Network Interface
(continued)
• /etc/sysconfig/network-scripts/ifcfg-interface file:
Stores NIC configurations
– Allows the system to activate and configure TCP/IP
information at each boot time
• ifdown command: unconfigures a NIC
• ifup command: configures NIC using
/etc/sysconfig/network-scripts/ifcfg-interface file
• ping (Packet Internet Groper) command:
Check TCP/IP connectivity on a network
-c option: limit the number of ping packets sent
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Configuring a Network Interface
(continued)
Figure 12-2: Configuring network interfaces
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Configuring a Network Interface
(continued)
Figure 12-3: Configuring TCP/IP information for a network interface
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Configuring a Network Interface
(continued)
• Mobile commuters typically connect to many
different NICs, both wired and wireless
• Network Manager daemon: allows users to quickly
connect to wired and wireless networks from
desktop environments
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Configuring a PPP Interface
• Run TCP/IP over serial lines using a WAN protocol
• Three common Point-to-Point Protocol (PPP)
connection technologies:
– Modems
– ISDN
– DSL
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Configuring a PPP Interface
(continued)
• Modems: send TCP/IP information across normal
telephone lines
– Considered slow
– Transmit information on a serial port
• ISDN: set of standards designed to transmit data
over copper telephone lines
• DSL: connects to Ethernet NIC and transmits data
across normal telephone lines
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Configuring a PPP Interface
(continued)
• Normally configured manually after Linux
installation is complete
– Requires:
• Support for PPP compiled into kernel
• PPP daemon
• Supporting utilities such as chat program
• Can use graphical programs to configure files and
utilities to allow PPP communication
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Configuring a PPP Interface
(continued)
Figure 12-5: Adding a network interface
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Configuring a PPP Interface
(continued)
• Information about PPP devices stored in files
named ifcfg-InternetServiceProviderName
– Located in /etc/sysconfig/network-scripts directory
• Other configurations used by PPP daemon stored
in /etc/ppp and /etc/isdn directories
• Incorrect passwords are the most common problem
with PPP connections
• Need to activate PPP device after configuration
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Name Resolution
• Hostnames: user-friendly computer name
• Fully qualified domain name (FQDN): hostname
following DNS convention
• DNS: hierarchical namespace for host names
• whois command: used to obtain registration
information about a domain within a name space
• hostname command: view or set a computer’s
host name
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Name Resolution (continued)
Figure 12-6: The domain name space
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Name Resolution (continued)
• TCP/IP cannot identify computers via hostnames
– Must map hostnames to IP addresses
• Can be done by placing entries in the /etc/hosts file
• ISPs list FQDNs in DNS servers on Internet
– Applications request IP addresses associated with a
specific FQDN
– Configure by specifying the IP address of the DNS
server in /etc/resolv.conf file
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Routing
• Route table: list of TCP/IP networks stored in
system memory
• route command: displays the route table
• Multihomed hosts: computers with multiple network
interfaces
• IP forwarding: forwarding packets from one
interface to another
– Also known as routing
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Routing (continued)
• Enabling routing:
– Place number 1 in:
• /proc/sys/net/ipv4/ip_forward for IPv4
• /proc/sys/net/ipv6/conf/all/forwarding for IPv6
• To enable routing at every boot:
– Edit the /etc/sysctl.conf file to include:
• “net.ipv4.ip_forward = 1” for IPv4
• “net.ipv6.conf.default.forwarding = 1” for IPv6
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Routing (continued)
• Large networks may have several routers
– Packet may travel through several routers
– May require adding entries in the router table
• route add <route> command: add entries to
route table
• route del <route> command: remove entries
from route table
• ip command: can be used to manipulate the route
table
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Routing (continued)
Figure 12-7: A sample routed network
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Routing (continued)
• Contents of route table lost when computer
powered off
– Add to /etc/rc.d/rc.local file
• Most routers configured with a default gateway
– For packets addressed to destinations not in route
table
• traceroute command: troubleshoot routing
– Displays routers between current and remote
computer
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Network Services
• Must identify types and features of network
services before they can be configured
• Network services: processes that provide some
type of valuable service for client computers on
network
– Often presented by daemon processes that listen to
certain requests
• Daemons identify packets to which they should
respond using a port number
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Network Services (continued)
• Port: number uniquely identifying a network service
– Ensure that packets delivered to proper service
– Range from 0 to 65534
• /etc/services file: lists ports and associated protocol
• Well-known port: ports from 0 to 1023
– Represent commonly used services
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Network Services (continued)
Table 12-2: Common well-known ports
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Network Services (continued)
• Internet super daemon (xinetd): initializes
appropriate daemon to provide needed network
service
• Stand-alone daemons: daemons that provide
network services directly
– Log information themselves to subdirectories under
/var/log
– chkconfig command or ntsysv utility can be used
to configure most stand-alone daemons to start in
various runlevels
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Network Services (continued)
Figure 12-8: Interacting with network services
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Network Services (continued)
Table 12-3: Common network services
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Network Services (continued)
Table 12-3 (continued): Common network services
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Network Services (continued)
Table 12-3 (continued): Common network services
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Network Services (continued)
Table 12-3 (continued): Common network services
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Remote Administration: Telnet
• telnet command: traditionally used to obtain a
command-line shell on remote server
– Receives host name or IP address of remote
computer as argument
– Easiest way to perform remote administration
• Need to install telnet daemon using yum command
• Use regular commands and exit to kill remote
BASH shell
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Remote Commands
• Remote commands: set of commands that can be
used to execute commands on remote systems
– yum install rsh-server
• rlogin command: obtains a shell from remote
computer on network
• rcp command: copies files between computers
• rsh command: used to execute a command on a
remote computer
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Remote Commands (continued)
• r commands allow access to remote computers
without a password, if remote computer has trusted
access
• Trusted access: computers allowed to access a
computer without providing a password
– Does not apply to root user
– Methods of setting up:
• Add host names of computers to /etc/hosts.equiv
• Create and .rhosts file in the home directory of each
user who should get trusted access
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Secure Shell (SSH)
• Secure Shell (SSH): encrypts information passing
between computers
– Secure replacement for r commands
• ssh command: connects to a remote computer
running ssh daemon
– Receives host name or IP address of target
computer as argument
– Accept RSA encryption fingerprint for target
computer
– Can be used to transfer files between computers
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Secure Shell (SSH) (continued)
• Main types of encryption supported by ssh
daemon:
–
–
–
–
–
Triple Data Encryption Standard (3DES)
Advanced Encryption Standard (AES)
Blowfish
Carlisle Adams Stafford Tavares (CAST)
ARCfour
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Remote X Windows
• X Windows is a network application
– Can be used to obtain graphical utilities from remote
Linux computer
• To obtain graphical utilities using X Windows:
– Edit /etc/gdm/custom.conf
– Use xhost + command within a graphical terminal
to allow other computers to write X Windows clients
on your computer
– Connect to remote computer with telnet or ssh,
and export DISPLAY variable of the local computer
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Virtual Network Computing (VNC)
• Graphical option for administering Linux remotely
• Other computers run VNC client that connects to
VNC server daemon installed on local computer to
obtain a desktop environment
• Remote FrameBuffer (RFB): platform-independent
protocol used to transfer graphics, mouse
movements and keystrokes across network
• vncpasswd command: used to configure
password for VNC connection
• vncviewer command: connects to VNC server
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Summary
• A network is a collection of connected computers
that share information
• A protocol is a set of rules that defines the format
of information that is transmitted across a network
• Each computer on a TCP/IP network must have a
valid IPv4 or IPv6 address
• The IPv4 configuration of a network interface can
be specified manually, obtained automatically from
a DHCP or BOOTP server, or autoconfigured by
the system
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Summary (continued)
• The IPv6 configuration of a network interface can
be obtained from a router using ICMPv6, from a
DHCP server, or autoconfigured by the system
• The /etc/sysconfig/network-scripts directory
contains the configuration for NIC and PPP
interfaces
• Host names are computer names that are easy for
humans to remember; host names that follow the
DNS are FQDNs
• Host names must be resolved to an IP address
before network communication can take place
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Summary (continued)
• Routers are devices that forward TCP/IP packets
from one network to another; each computer and
router has a route table used to determine how
TCP/IP packets are forwarded
• Network services are started by the Internet Super
Daemon or by stand-alone daemons
• There are many ways to remotely administer a
Linux system, including the telnet, rsh, rcp,
rlogin, and ssh commands, X Windows and
VNC
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