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Week 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/ifcfginterface 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/ifcfginterface 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 ifcfgInternetServiceProviderName
 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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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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