1. Chapter 12: Configuring Networks
The Complete Guide to Linux System Administration

2. Objectives
Configure network interfaces using command-line and graphical utilities
Set up a simple DHCP server
Manage networked printing services
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3. Configuring Linux Networking
Learn more about
Networking protocols
Network configuration
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4. Understanding Network Devices in Linux
Linux networking devices
Not shown in /dev directory
Do not “exist” on system until appropriate device driver installed in kernel
Networking device
Named channel over which network traffic can pass
Device drivers for networking are kernel modules
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5. Understanding Network Devices in Linux (continued)
Kernel modules can be loaded or unloaded while Linux is running
/dev/eth0
First Ethernet card installed on system
Media Access Control (MAC) address
Unique address assigned by Ethernet card manufacturer
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6. Understanding Network Devices in Linux (continued)
To obtain MAC address
Host (switch) broadcasts message to entire network segment using Address Resolution Protocol (ARP)
Host with IP address responds directly to computer that sent ARP request with MAC address
Source host stores MAC address and IP address
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7. Understanding Network Devices in Linux (continued)
arp command
Display ARP cache
Mapping of IP addresses to hardware addresses
Used mainly for troubleshooting network connectivity
Refreshed frequently
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8. Configuring Networking with Command-line Utilities
ifconfig command
Set up network configuration in Linux kernel
Parameters include:
Network interface
IP address assigned to interface
Network mask
Syntax
ifconfig device ip_address netmask address broadcast address
$ ifconfig eth0
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9. Configuring Networking with Command-line Utilities (continued)
Packet
Unit of data that network card transmits
Broadcast address sends packet to all computers on same part of network
Maximum transmission unit (MTU)
Maximum size of packet interface supports
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10. Configuring Networking with Command-line Utilities (continued)
View status of interface: ifconfig eth0
Stop Ethernet interface: ifconfig eth0 down
Start Ethernet interface: ifconfig eth0 up
Routing table tells networking software where to send packets that are not part of local network
A real example of configuring an Ethernet card at the command line might look like this:
# ifconfig eth netmask broadcast
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11. Configuring Networking with Command-line Utilities (continued)
route command
View or configure routing table within kernel
Executed at boot time when networking initialized
Output information for addresses
(eth0 IP address)
Other
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12. The Complete Guide to Linux System Administration

13. Configuring Networking with Command-line Utilities (continued)
Route command output
Destination – Ref
Gateway – Use
Genmask – Iface
Flags
Add route example:
route add -net netmask dev eth0
This command adds a default gateway route,
# route add default gw
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14. Configuring Networking with Command-line Utilities (continued)
service command
Start or stop networking
Relies on script /etc/rc.d/init.d/network
/etc/sysconfig/networking/devices configuration directory
Contains file for each network device
ifcfg-eth0 file
Used by /etc/rc.d/init.d/network script
As it executes ifconfig and route commands
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15. and then execute this command: # service network restart
To change the IP address or other networking parameters for your computer,
you can simply change the information in /etc/sysconfig/network-scripts/ifcfg-eth0
and then execute this command:
# service network restart
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16. Configuring Networking with Command-line Utilities (continued)
ifup and ifdown scripts manage single interface, rather than all network interfaces
(instead of the service command)
Better than using ifconfig command directly
Looks at parameters and associated functionality needed to cleanly manage interface
Example:
# ./ifup eth0
# ./ifdown eth0
Some systems have two or more physical network devices
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17. Configuring Networking with Command-line Utilities (continued)
IP forwarding
Allows packets to be passed between network interfaces
Required for any router
To enable:
# echo 1 > /proc/sys/net/ipv4/ip_forward
Linux includes graphical utilities that set up and manage networking
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18. Configuring Networking Using Graphical Tools
system-config-network program
Included with Red Hat Linux and Fedora
Devices tab
List of each Linux networking device
Name and type of hardware driver
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19. Configuring Networking Using Graphical Tools (continued)
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20. Configuring the DNS Resolver
Used to convert host and domain names into IP addresses
Implemented by server that supports DNS
Reverse DNS converts IP address to domain name
Resolving
Process of converting domain name to IP address or vice versa
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21. Configuring the DNS Resolver (continued)
Client part of DNS
Makes requests to DNS server
Test by pinging another system using host name instead of IP address
Configured by file /etc/resolv.conf
Contains IP address of one or more DNS servers preceded by keyword nameserver
Can include up to three DNS servers
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22. Configuring the DNS Resolver (continued)
Keyword
Word to which program reading configuration file attaches special meaning
Utilities for researching DNS problems:
dnsquery – whois
nslookup – host
dig
/etc/hosts file stores IP addresses and corresponding domain names in text file on host
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23. Configuring the DNS Resolver (continued)
/etc/hosts file
By default contains only host name localhost and own host’s name
On small network:
Create hosts file that contains each host and IP address of host
Avoid need for DNS server
Can have nonlocal domain names resolved by request to DNS server
Configuring DNS resolver graphically uses system-config-network to set up resolver
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24. Configuring the DNS Resolver (continued)
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25. Dynamic Routing with Routing Protocols
Static routing
Uses preconfigured routing table
Not good choice for:
Larger networks
Unreliable connections
Dynamic routing uses specialized routing protocol to build and modify routing tables automatically
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26. Dynamic Routing with Routing Protocols (continued)
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27. Dynamic Routing with Routing Protocols (continued)
Routing protocols divided into two categories
Interior: designed for routing packets among networks under your control
Exterior: designed for routing packets between networks controlled by different organizations
All routing protocols designed to exchange information among routers
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28. Dynamic Routing with Routing Protocols (continued)
Routing Information Protocol (RIP)
Interior routing protocol
Oldest routing protocol still in common use
Implemented in Linux using routed daemon
Easy to configure and run
Choice of most network administrators
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29. Dynamic Routing with Routing Protocols (continued)
Open Shortest Path First (OSPF) protocol
Interior routing protocol
Designed to work effectively even in very large networks
Uses technique called flooding
router running OSPF periodlically floods the network with everything it knows about its
neighboring hosts. Other OSPF routers see information coming from other routers and use
this data to intelligently construct a "chart" inside the router that defines the best way to
reach the various networks.
Few Linux network administrators use it
Implemented using gated daemon
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30. Dynamic Routing with Routing Protocols (continued)
Border Gateway Protocol (BGP)
Designed for routing between major national networks
Supported by gated daemon
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31. Configuring a DHCP Server
DHCP server installed by default on many Linux systems
/etc/dhcpd.conf file
Configuration for DHCP
Instructs DHCP server which IP address ranges are available for DHCP clients
Once you have the DHCP server configured, you can start it using the standard
script in /etc/rc.d/init.d
or run this command:
# service dhcpd start
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32. Configuring a DHCP Server (continued)
When client requests IP address :
DHCP server leases address to client for specified time
At end of lease client must request new IP address
Whole arrangement transparent to user on client host
DHCP client can run on any operating system
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33. Networked Printing Services
Linux includes network printing capabilities
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34. Using Traditional Linux Printing: LPRng
Traditional Linux printing system
Based on BSD version of UNIX
Allows multiple users to print files at same time to:
Local printer
Networked printers
Print queues
System administrator must define printers
Printer definitions describe type of printer and features to be used
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35. Using Traditional Linux Printing: LPRng (continued)
Printing file in LPRng system
Application submits file to be printed (print job)
Print job processed by print filter converts information from Linux application into formatting codes to produce desired output
Printing utility stores print job in print spool directory
Default directory /var/spool/lpd
Lpd print server program keeps track of all print jobs in all print queues on system
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36. Using Traditional Linux Printing: LPRng (continued)
Correlation between print queue and physical printer not always one to one
Linux print filter
Same as printer driver in other operating systems
Converts documents or images into format that printer can use
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37. Using Traditional Linux Printing: LPRng (continued)
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38. Using Traditional Linux Printing: LPRng (continued)
Page description language
Special set of codes
Determine graphic elements, text font, and everything else about what appears on printed page
Most widely used:
PostScript
Printer Control Language (PCL)
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39. Using Traditional Linux Printing: LPRng (continued)
Magic filter
Can convert documents for many different printers
Convenient to support hundreds of printers
Doesn’t allow Linux to use specialized features of each printer
Script /usr/share/printconf/util/mf_wrapper
Main print filter in Red Hat Linux and Fedora
Starts magicfilter-t program
Uses several other programs in conversion process
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40. Using Traditional Linux Printing: LPRng (continued)
Configuring local printer definitions
Each LPRng printer definition created as print queue entry in /etc/printcap configuration file
Uses complex format
Must provide appropriate Linux device name
Parallel ports use device name lp followed by device number
Serial ports use name ttyS followed by device number
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41. Printing Remotely Using LPRng
Define printer on system that refers to remote computer and print queue on remote system
Key options in configuration file
rm remote system specified
rp remote print queue specified
Once print job has been sent to remote system using lpd, user has no direct control over it
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42. Printing Remotely Using LPRng (continued)
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43. Understanding the Common UNIX Printing System (CUPS)
Provides new architecture for Linux printing
Users browse network to find and print to networked printers and other devices
System administrators manage printer definitions and print jobs across network
Installed by default in Red Hat Linux and Fedora
cupsd
Main print server daemon
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44. Understanding the Common UNIX Printing System (CUPS) (continued)
cupsd print server daemon
Uses HTTP protocol
Manages printers using web browser interface
CUPS architecture uses network port 631
Start CUPS user interface
http//localhost:631/
/etc/cups/cupsd.conf file specifies setting for each active server option
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45. Understanding the Common UNIX Printing System (CUPS) (continued)
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46. Understanding the Common UNIX Printing System (CUPS) (continued)
Many printers are configured using PostScript Printer Description (PPD) file
Manufacturers can easily create text-based configuration file
Permits printer to be graphically configured using standard utilities
CUPS class
Group of printers to which user can submit print job
First available printer within used to print job
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47. Managing Printing Printing policy
Brief statement of rules describing:
How printing resources can be used
How printers should be managed
lpc utility controls LPRng or CUPS printing
Alternatives printing system
Maps print command to different programs
Based on which printing system in use
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48. Managing Printing (continued)
Alternatives command
lpr command and lpc command
Symbolic links
Using lpc
Prevent new print jobs from being accepted by print queue
Prevent print jobs from being sent to printer
Cancel print job currently being printed
See status of any printer
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49. Managing Printing (continued)
lpq utility lists each print job in print queue with status information
lprm command deletes print job from queue
Graphical print management utilities
Print Manager
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50. Summary Ethernet card includes unique MAC address
Used by address resolution protocol
ifconfig command used to configure networking interface in kernel
route command modifies internal routing table
system-config-network utility graphically configures network devices
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51. Summary (continued)
DHCP server on Linux requires configuring /etc/dhcpd.conf file
Linux printing relies on printer definitions
Red Hat Linux and Fedora support both LPRng and CUPS printing using alternatives printing system
Print jobs can be managed using command line utilities or graphical interface
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