chapter 12: configuring networks the complete guide to linux system administration

Chapter 12:Configuring Networks

The Complete Guide to Linux System Administration

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


Configuring Linux Networking

• Learn more about – Networking protocols – Network configuration


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


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



• To obtain MAC address– Host 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



• arp command– Display ARP cache

• Mapping of IP addresses to hardware addresses

– Used mainly for troubleshooting network connectivity

– Refreshed frequently


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


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



• 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



• route command– View or configure routing table within kernel– Executed at boot time when networking initialized– Output information for addresses

• 192.168.100.0 (eth0 IP address)

• 127.0.0.0

• Other



• Route command output– Destination – Ref– Gateway – Use– Genmask – Iface– Flags

• Add route example:– route add -net 192.168.100.0 netmask

255.255.255.0 dev eth0



• 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



• ifup and ifdown scripts manage single interface, rather than all network interfaces– Better than using ifconfig command directly

• Looks at parameters and associated functionality needed to cleanly manage interface

– Example: ./ifdown eth0

• Some systems have two or more physical network devices



• 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


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


Configuring Networking Using Graphical Tools (continued)


Configuring the DNS Resolver

• DNS– 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


Configuring the DNS Resolver (continued)

• Resolver – 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



• 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



• /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


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


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



• 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



• Open Shortest Path First (OSPF) protocol– Interior routing protocol– Designed to work effectively even in very large

networks– Uses technique called flooding– Few Linux network administrators use it– Implemented using gated daemon



• Border Gateway Protocol (BGP)– Designed for routing between major national

networks– Supported by gated daemon


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


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


Networked Printing Services

• Linux includes network printing capabilities


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


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



• 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



• 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)



• 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



• 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


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


Printing Remotely Using LPRng (continued)


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


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



• 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


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


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


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


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


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

chapter 12: configuring networks the complete guide to linux system administration

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