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

Objectives Configure network interfaces using command-line and graphical utilities Set up a simple DHCP server Manage networked printing services The Complete Guide to Linux System Administration

Configuring Linux Networking Learn more about Networking protocols Network configuration The Complete Guide to Linux System Administration

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 The Complete Guide to Linux System Administration

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 The Complete Guide to Linux System Administration

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 The Complete Guide to Linux System Administration

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 The Complete Guide to Linux System Administration

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 The Complete Guide to Linux System Administration

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 The Complete Guide to Linux System Administration

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 eth0 192.168 . 100.1 netmask 255.255.255.0 broadcast 192. 168.100.255 The Complete Guide to Linux System Administration

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 192.168.100.0 (eth0 IP address) 127.0.0.0 Other The Complete Guide to Linux System Administration

The Complete Guide to Linux System Administration

Configuring Networking with Command-line Utilities (continued) 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 This command adds a default gateway route, # route add default gw 192.168.100.5 The Complete Guide to Linux System Administration

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 The Complete Guide to Linux System Administration

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 The Complete Guide to Linux System Administration

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 The Complete Guide to Linux System Administration

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 The Complete Guide to Linux System Administration

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 The Complete Guide to Linux System Administration

Configuring Networking Using Graphical Tools (continued) The Complete Guide to Linux System Administration

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 The Complete Guide to Linux System Administration

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 The Complete Guide to Linux System Administration

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 The Complete Guide to Linux System Administration

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 The Complete Guide to Linux System Administration

Configuring the DNS Resolver (continued) The Complete Guide to Linux System Administration

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 The Complete Guide to Linux System Administration

Dynamic Routing with Routing Protocols (continued) The Complete Guide to Linux System Administration

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 The Complete Guide to Linux System Administration

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 The Complete Guide to Linux System Administration

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 The Complete Guide to Linux System Administration

Dynamic Routing with Routing Protocols (continued) Border Gateway Protocol (BGP) Designed for routing between major national networks Supported by gated daemon The Complete Guide to Linux System Administration

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 The Complete Guide to Linux System Administration

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 The Complete Guide to Linux System Administration

Networked Printing Services Linux includes network printing capabilities The Complete Guide to Linux System Administration

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 The Complete Guide to Linux System Administration

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 The Complete Guide to Linux System Administration

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 The Complete Guide to Linux System Administration

Using Traditional Linux Printing: LPRng (continued) The Complete Guide to Linux System Administration

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) The Complete Guide to Linux System Administration

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 The Complete Guide to Linux System Administration

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 The Complete Guide to Linux System Administration

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 The Complete Guide to Linux System Administration

Printing Remotely Using LPRng (continued) The Complete Guide to Linux System Administration

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 The Complete Guide to Linux System Administration

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 The Complete Guide to Linux System Administration

Understanding the Common UNIX Printing System (CUPS) (continued) The Complete Guide to Linux System Administration

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 The Complete Guide to Linux System Administration

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 The Complete Guide to Linux System Administration

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 The Complete Guide to Linux System Administration

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 The Complete Guide to Linux System Administration

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 The Complete Guide to Linux System Administration

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 The Complete Guide to Linux System Administration