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SUSE Linux Enterprise Server Administration (Course 3037)
Chapter 7 Connect the SUSE Linux Enterprise Server to the Network

Objectives
? Configure Your Network Connectio

n ? Configure and Manage Routes ? Test the Network Interface

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Configure Your Network Connection
? Objectives
– – – – – TCP/IP Fundamentals Network Interfaces in Linux Network Interface Requirements How to Configure a Network Card with YaST How to Modify a Network Card Manually

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TCP/IP Fundamentals
? TCP/IP network components
– Host
? Receives requests for information from the Internet

– TCP/IP
? Defines how packets should be structured

– IP packet
? Electronic package of data sent over the Internet

– Router
? Forwards an IP packet to the next network point

– Firewall
? Protects the resources of a private intranet or network
SUSE Linux Enterprise Server Administration (Course 3037) 4

TCP/IP Fundamentals (continued)
? IP address structure
– Host is also referred to as a node or station
? Routers locate hosts based on their IP addresses

– IP address consists of four bytes:
? A network address (from 1 to 3 bytes) ? A node or station address (from 1 to 3 bytes)

– Node address 0, and 255 are reserved – IP addresses take the form of a dotted octet
? Each byte is separated by a dot

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TCP/IP Fundamentals (continued)

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TCP/IP Fundamentals (continued)

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TCP/IP Fundamentals (continued)

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TCP/IP Fundamentals (continued)
? Network classes and IP addresses
– There are five address classes defined in IP
? Classes A through C can be assigned ? Classes D and E are reserved

– Class A addresses
? Networks: 126 ? Hosts per network: 16,777,216

– Class B addresses
? Networks: 16,384 ? Hosts per network: 65,534

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TCP/IP Fundamentals (continued)

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TCP/IP Fundamentals (continued)

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TCP/IP Fundamentals (continued)
? Network classes and IP addresses
– Class C addresses
? Networks: 2,097,152 ? Hosts per network: 255

– Class D addresses
? First byte is in the 224 to 239 range ? Used for multicast packets

– Class E addresses
? First byte is in the 240 to 255 range ? Reserved for experimental use
SUSE Linux Enterprise Server Administration (Course 3037) 12

TCP/IP Fundamentals (continued)
? Special IP addresses
– See Table 7-1

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TCP/IP Fundamentals (continued)

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Network Interfaces in Linux
? Network interfaces
– Always referred to with a fixed name – Name depends on type and position of the network card in the computer
? First card is called eth0, second is eth1, and so on

? Drives
– Provide support for network cards and protocols – Kernel modules that can be loaded if required

? SUSE Linux Enterprise Server
– Contains drivers for all common network cards
SUSE Linux Enterprise Server Administration (Course 3037) 15

Network Interface Requirements
? Network card is detected during installation
– And a suitable driver is loaded

? Command ifstatus device
– Verifies network card has been integrated correctly
? With the appropriate driver

? Kernel support implemented as a module
– Name of the module is entered by YaST
? In a file in /etc/sysconfig/hardware/

? Drivers are autodetected for hotplug cards
– No configuration is necessary
SUSE Linux Enterprise Server Administration (Course 3037) 16

How to Configure a Network Card with YaST
? Steps
– Start the YaST Network Card module – Select a network card from the list (conditional) – Modify network card settings (conditional)

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How to Configure a Network Card with YaST (continued)

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How to Configure a Network Card with YaST (continued)

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How to Configure a Network Card with YaST (continued)

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How to Configure a Network Card with YaST (continued)
? Steps
– Do one of the following
? Add a new network card configuration ? Modify an existing configuration ? Delete a listed configuration

– Select Next – Save the configuration by selecting Finish – Check network card activation and settings
? From the command line using ifconfig or ip

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How to Configure a Network Card with YaST (continued)

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How to Configure a Network Card with YaST (continued)

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How to Configure a Network Card with YaST (continued)

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How to Modify a Network Card Manually
? Configure the network interface with ifconfig
– ifconfig (/sbin/ifconfig)
? Used to manually configure a network card ? Display information about status of network interfaces

– Changes done with ifconfig are temporary – Syntax
? ifconfig [interface] [address] [options]

– ifconfig eth0 192.168.0.1
? Assigns 192.168.0.1 to network card eth0

– ifconfig eth0 up
? Activates eth0
SUSE Linux Enterprise Server Administration (Course 3037) 25

How to Modify a Network Card Manually (continued)

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How to Modify a Network Card Manually (continued)
? Configure IP aliases with ifconfig
– IP aliases
? Used to define more than one IP address for a network card

– It is always better to use genuine addresses – Configure an IP alias with ifconfig or YaST – View IP aliases by entering ip address show

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How to Modify a Network Card Manually (continued)
? Modify network interface configuration files
– Configuration files directory
? /etc/sysconfig/network/ ? Contains file for each configured network adapter

– Network interfaces are activated at boot
? By script /etc/init.d/network

– /etc/sysconfig/network/config
? Contains general variables for the script

– Directory /etc/sysconfig/network/scripts/
? Contains additional scripts run by /etc/init.d/network
SUSE Linux Enterprise Server Administration (Course 3037) 28

How to Modify a Network Card Manually (continued)
? Modify network interface configuration files
– Activating and deactivating network interfaces
? Use commands /sbin/ifup, /sbin/ifdown, /sbin/ifstatus ? Use script /etc/init.d/network

? Configure host and domain names
– Modify the following files
? /etc/HOSTNAME ? /etc/resolv.conf

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Configure and Manage Routes
? Objectives
– – – – – Route Types How to Manage Routes with route How to Modify Route Configuration Files How to Activate Routing How to Manage the Network Interface and Routes with ip

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Route Types
? Host routes
– Define path data packet can take for exactly one destination host

? Network and gateway routes
– Define path data packet can take for an entire destination network

? Default route
– Special gateway route – Defines route data packet can take if no previous route matched destination of the packet
SUSE Linux Enterprise Server Administration (Course 3037) 31

How to Manage Routes with route
? Use command route (/sbin/route)
– To check and edit the routing table

? Create a route
– Syntax
? route add [-net | -host destination] [netmask mask] [gw gateway] [metric n] [dev interface]

– Static routing is set up by default – Dynamic routing needs a routing daemon
? You also need to add the option metric n

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How to Manage Routes with route (continued)
? Delete a route
– Syntax
? route del [-net | -host destination] [gw gateway] [netmask mask] [dev interface]

– route del default
? Deletes the default route

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How to Modify Route Configuration Files
? An entry is generated in the kernel routing table
– For all active network interfaces

? Additional static routes
– Configured in the files
? /etc/sysconfig/network/routes ? /etc/sysconfig/network/ifroute-Interface

? Configuration files include the following fields:
– – – – Destination network/destination host The gateway to use The network mask The network interface
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SUSE Linux Enterprise Server Administration (Course 3037)

How to Activate Routing
? A Linux host can also serve as a router itself
– This property is deactivated by default

? Activate routing by adding entry in the process file
– echo 1 > /proc/sys/net/ipv4/ip_forward – 0 will deactivate routing

? Activate routing permanently
– Set following variable in /etc/sysconfig/sysctl
? IP_FORWARD=“yes”

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How to Manage the Network Interface and Routes with ip
? SUSE Linux Enterprise Server ip command
– Replaces ifconfig and route commands

? ip general syntax
– ip [options] object [command [parameters]]

? Modifications made with command ip are temporary ? Tasks performed with ip:
– Assign Parameters for Network Interfaces (ip link) – Assign IP Addresses (ip address) – Set Up Routes (ip route)
SUSE Linux Enterprise Server Administration (Course 3037) 36

How to Manage the Network Interface and Routes with ip (continued)

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How to Manage the Network Interface and Routes with ip (continued)
? Assign Parameters for Network Interfaces (ip link)
– Use ip commands
? set and show

? Assign IP Addresses (ip address)
– Most important ip commands
? add, del, and show

? Set Up Routes (ip route)
– Use ip commands
? add, change, delete, and show
SUSE Linux Enterprise Server Administration (Course 3037) 38

How to Manage the Network Interface and Routes with ip (continued)

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How to Manage the Network Interface and Routes with ip (continued)

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How to Manage the Network Interface and Routes with ip (continued)

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Test the Network Interface
? Objectives
– – – – Check a Network Connection between Hosts (ping) Check the Routing (traceroute) Analyze Network Traffic (tcpdump and ethereal) Determine the Status of All Network Connections (netstat) – Check for Service Availability (netcat)

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Check a Network Connection between Hosts (ping)
? Command ping (/sbin/ping)
– Simple tool for checking network connections

? How the command tests a connection
– Enter ping host – Server sends ICMP datagram to the target host
? With the echo request

– Target host answers with ICMP datagram
? Containing the message echo reply

– ICMP protocol does not involve higher level protocols

? Command options
– See Table 7-7
SUSE Linux Enterprise Server Administration (Course 3037) 43

Check a Network Connection between Hosts (ping) (continued)

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Check a Network Connection between Hosts (ping) (continued)
? Troubleshooting suggestions
– Start by checking the host-internal network
? ping localhost

– Then check network interface connected to your host
? ping interface_addr

– Check network segment to the next closest network element
? In the direction of the target host ? Until the check ends at the target host

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Check the Routing (traceroute)
? Command traceroute (/usr/sbin/traceroute)
– Helps you follow route taken by an IP datagram

? How a route is traced
– traceroute sends three UDP datagrams with TTL = 1 – Then increases value for the next three datagrams – TTL is reduced by 1 when UDP datagram passes through a router
? Datagram is discarded at TTL=0

– Gateway at TTL=0 sends an ICMP datagram
? With message “TTL exceeded” back to the sender
SUSE Linux Enterprise Server Administration (Course 3037) 46

Check the Routing (traceroute) (continued)
? How a route is traced
– If UDP datagram reaches the target host
? Host replies with ICMP datagram Port Unreachable

– From this message
? Sender recognizes that target host has now been reached

– Syntax
? traceroute host

? Command options
– See Table 7-8
SUSE Linux Enterprise Server Administration (Course 3037) 47

Check the Routing (traceroute) (continued)

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Check the Routing (traceroute) (continued)
? Troubleshooting suggestions
– Common reason for defective communication
? Transport path between these networks contains errors

– traceroute
? Excellent tool for checking routers located on this transport path ? Shows at which hop the transport path is faulty

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Analyze Network Traffic (tcpdump and ethereal)
? How to use tcpdump
– Command tcpdump (/usr/sbin/tcpdump)
? Lets you analyze data packets

– Syntax
? tcpdump –i interface – Puts interface in promiscuous mode

? How to use ethereal
– Graphical tool – Provides the same functionality as tcpdump – Must be first installed using YaST
SUSE Linux Enterprise Server Administration (Course 3037) 50

Analyze Network Traffic (tcpdump and ethereal) (continued)

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Analyze Network Traffic (tcpdump and ethereal) (continued)

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Analyze Network Traffic (tcpdump and ethereal) (continued)
? Command options (tcpdump)
– See Table 7-9

? Troubleshooting suggestions
– Use tcpdump or ethereal to analyze network traffic – Helps you find which packets are exchanged – Recommended for security-critical environments

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Determine the Status of All Network Connections (netstat)
? Command netstat (/bin/netstat)
– Helps you determine the status of all network connections on a host

? How to use netstat
– Displays the status of all open sockets – Information is displayed in two blocks
? First block – Each socket is listed on a separate line ? Second block ? Contains information on the UNIX domain sockets active on the host
SUSE Linux Enterprise Server Administration (Course 3037) 54

Determine the Status of All Network Connections (netstat) (continued)
? Command options
– See Table 7-10

? Troubleshooting suggestions
– netstat
? Ideal for monitoring resources for network sockets on a host ? Provides detailed information on existing and available network sockets or resources

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Determine the Status of All Network Connections (netstat) (continued)

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Check for Service Availability (netcat)
? Command netcat (/usr/bin/netcat)
– Uses TCP and UDP protocols
? To read and write data through network connections

? How to use netcat
– netcat host port

? Advanced use of netcat
– Query ports and display the responses of the services – nmap command
? More suitable for a simple port scan

– Transmit files between hosts

? Command options (see Table 7-11)
SUSE Linux Enterprise Server Administration (Course 3037) 57

Check for Service Availability (netcat) (continued)

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Exercise 7-1: Configure and Test Your Network Connection
? In this exercise, you do the following:
– Part I: View and Record Network Configuration – Part II: Configure a Static Network with YaST – Part III: Test the Network Card Configuration

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Summary
? Each host has an IP address and subnet mask ? IP classes normally used for IP addressing
– Class A, B, and C

? Commands ifconfig and ip
– View and change your TCP/IP configuration

? Network interface and TCP/IP information
– Stored in the /etc/sysconfig/network – Started by the /etc/init.d/network script

? Each host contains a hostname
– Stored in the /etc/HOSTNAME file
SUSE Linux Enterprise Server Administration (Course 3037) 60

Summary (continued)
? /etc/resolv.conf file
– Contains the addresses of up to three DNS servers

? TCP/IP networks are connected via routers
– Host has a routing table
? That typically contains a default gateway route

? Commands route and ip
– View and change the routing table

? Commands ping and traceroute
– Used to test network communication and routing
SUSE Linux Enterprise Server Administration (Course 3037) 61

Summary (continued)
? Command netcat
– Used to test TCP and UDP connections

? Utilities tcpdump and ethereal
– Used to capture and analyze network traffic

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