1. Linux+ Guide to Linux Certification, Third Edition
Chapter 12
Network Configuration

2. 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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3. 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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4. 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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5. 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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6. 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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7. 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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8. 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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9. 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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10. 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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11. 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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12. Subnet Masks (continued)
Figure 12-1: A sample IP address and subnet mask
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13. Subnet Masks (continued)
= all networks
= all computers on all networks
255 in an IP address can specify many hosts
Broadcast addresses
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14. 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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15. 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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16. 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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17. IPv4 Classes and Subnetting (continued)
Table 12-1: IP address classes
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18. 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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19. 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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20. 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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21. 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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22. 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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23. Configuring a Network Interface (continued)
Figure 12-2: Configuring network interfaces
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24. Configuring a Network Interface (continued)
Figure 12-3: Configuring TCP/IP information for a network interface
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25. 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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26. 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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27. 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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28. 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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29. Configuring a PPP Interface (continued)
Figure 12-5: Adding a network interface
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30. 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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31. 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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32. Name Resolution (continued)
Figure 12-6: The domain name space
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33. 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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34. 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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35. Routing (continued) Enabling routing: To enable routing at every boot:
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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36. 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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37. Figure 12-7: A sample routed network
Routing (continued)
Figure 12-7: A sample routed network
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38. 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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39. 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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40. 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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41. Network Services (continued)
Table 12-2: Common well-known ports
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42. 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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43. Network Services (continued)
Figure 12-8: Interacting with network services
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44. Network Services (continued)
Table 12-3: Common network services
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45. Network Services (continued)
Table 12-3 (continued): Common network services
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46. Network Services (continued)
Table 12-3 (continued): Common network services
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47. Network Services (continued)
Table 12-3 (continued): Common network services
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48. 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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49. 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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50. 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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51. 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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52. 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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53. 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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54. 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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55. 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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56. 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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57. 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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