CNT-2311-Chapter 8 Notes

Network Hardware Configuration

-The most basic part of network configuration is getting the network hardware up and running.

-Most of the time that is a fairly automatic task since most systems ship with startup scripts that auto-detect the ::network card and loads the correct driver module.

-If the network hardware is not detected subsequent configuration will not work. You will need to load your network ::hardware driver.

-You will use the modprobe command. You must know the name of the networks hardware kernel module.

# modprobe <insert kernel name here>

Configuring with DHCP

-One of the most easy ways to configure a computer to use a TCP/IP network is to use DHCP. This allows one computer on ::a network to manage the settings for many other computers.

-When a computer running a DHCP client boots up it send out a broadcast looking for a DHCP server. The server then ::replies with the configuration information needed by the client to communicate with the computers on the network.

-Linux has three common DHCP clients.

pump

dhclient

dhcpcd

-Some distributions ship with one of these but others ship with two or all three. They will all have a default DHCP ::client, that is installed when you tell the system you want to use DHCP at the installation.

-On systems that ship with more then one DHCP client you can swap one out for another by removing the old client and ::installing the package for the new one.

-The DHCP client runs at system bootup and is handled by its own SysV startup file or as part of the main network ::configuration startup file.

-This SysV file is typically named networking or network.

-The system uses a line in a configuration file to determine whether to run a DHCP client.

-Red Hat and Fedora set this option in a file called /etc/sysconfig/network-scripts/ifcfg-eth0 (the filename will be ::different if something other than a single Ethernet interface is used)

-The line in question looks like this: BOOTPROTO=dhcp

-If the BOOTPROTO variable is set to something different changing to the above line will set the system to use DHCP.

-Ubuntu uses the /etc/network/interfaces file for a similar purpose but the details differ.

-On a system that uses DHCP you will see a line like: iface eth0 inet dhcp

-You may prefer to use the GUI system to adjust these options.

-Once the DHCP client is configured to run when the system boots the configuration task is complete. On rare occasions ::you may need to tweak the DHCP settings to work around client/server incompatibilities.

-If you have to manually run a DHCP client you can do so by typing its name (as root) followed by a network identifier ::such as: dhclient eth0 to have the DHCP client attempt to configure eth0 with the help of any DHCP server it find on the

network.

Configure with a Static IP Address

-When a network lacks a DHCP server you will need to provide basic network configuration options manually.

-These options can be set using interactive commands but to sent them long term you need to adjust a configuration file ::such as:

/etc/sysconfig/ network-scripts/ifcfg-eth0 or /etc/network/interfaces

-IP address: You can set the IP address manually with the ifconfig command or at the IPADDR line in the configuration ::file.

-Network mask: You can set the netmask manually with the ifconfig command or at the NETMASK line in the configuration ::file.

-Gateway address: You can manually set the gateway via the route command. To make it permanent you need to adjust the ::configuration file. This file may be the same that holds other options or another file such as: /etc/sysconfig/

network/routes and is most likely called GATEWAY.

-DNS settings: In order for Linux to use DNS to translate between IP addresses and hostnames you must specify at least ::one DNS server in the /etc/resolv.config file. Adjusting this file is all you need to do to set the name server address.

-You can also set your computers local domain name in this file using the domain option: domain <name of the domain you ::want to use>

-The network configuration script may hold other options such as:

DEVICE=eth0 to specify the interface name

BOOTPROTO=static to assign a static IP address

ONBOOT=yes to bring up an interface when the system boots

NETWORK and BROADCAST are derived from the IPADDR and NETMASK and can be changed if you understand the consequences of ::doing so.

-This command will bring up eth0 using address 192.168.29.39 and netmask 255.255.255.0

1. ifconfig etho up 192.168.29.39 netmask 255.255.255.0

-This command links the specified address to the card so that the computer responds to the address and claims to be that ::address when sending data.

-Both ifconfig and route can display information on the current network configuration.

-For ifconfig, omit up and everything that follows; for route, omit add everything that follows.

-To view the interface you may use this command: # ifconfig eth0

-When configured correctly ifconfig should show a hardware address an IP address and some other statistics.

Configuring Routing

-Routers pass traffic from one network to another. You give the computer a routers address which your system uses as a ::gateway to the internet.

-Any traffic not directed to the network is directed to the router which then sends it on to it’s destination. Every ::router has a list of rules concerning where to send data based on the destination IP address.

-The –net and –host force route to interpret the target as a network or computer address, respectively.

-Netmask lets you set a netmask as you desire and gw lets you specify a router through which packets to the specified ::target should go.

-The reject keyword installs a blocking route , which refuses all traffic destined for the specific network.

-Incorrect routing tables can cause serious problems causing some or all computers on the network not to respond. You ::can use route alone to compare the results to what your routing table should be.

Ifup and ifdown command *(pg. 408)

-The ifup command bring the interface up

-The ifdown command bring the interface down

Example of ifup/ifdown *(pag.408)

“ifup eth0”

–bring up the interface eth0“ifdown eth0”.

–bring down the interface eth0- useful to quickly to take down a interface or

bring it up, because you do not need to remember all the detail of the IP

address, route and so on.

Hostnames *(pg. 409)

There are 2 way to configure the hostname

-On DNS

-On you local computer

Domain Name System (DNS) *(pg. 409)

- “is a distributed database computers that converts between IP addresses and

hostnames”.

- The DNS server just sit and listen for other computers on the network to send

request.

- The server then send the request out to other DNS server if it cannot find it

in it cache.

- If the server found what the request is asking for then it will send a request

back to the computer telling it where to go.

Diagnosing Network Connections *(pg. 412)

- There are a few command that will help diagnose a problem.

- They are ping, traceroute, tracepath, netstat, and tcpdump.

Ping command *(pg. 412)

- The ping command is a very basic network test, which send ICMP packet to the

system you name, hostnames, or ip address and wait for a reply

- In Linux it send the packet every second until you press Ctrl+C key to stop

the stroke.

Traceroute command *(pg. 412-413)

- This command will sends a series of three test packets to each computer

between your system and a specified target system

Tracepath command *(pg. 413)

- This command is a lot similar to traceroute

Checking Network Status

-A useful diagnostic tool is Netstat

-Netstat can be used in place of many other tools.

-It also shows information that is not easily found in other ways.

Netstat options

-Interface information: use netstat –interface or -i parameter to see information about

network interfaces. This is similar to what ifconfig command displays

-Routing information: use netstat –route or -r parameter to see the routing table. This

output is similar to what the route command shows

-Masquerade information: use netstat –masquerade or –M to see information about

connections mediated by Linux’s NAT features . This is a good way to stretch limited

IPv4 addresses.

-Program use: use netstat –program or –p parameter to show information about programs that

are using network connections.

-All connections: use netstat –all or –a parameter to display information about the ports

that server programs open to listen for network connections.

tcpdump

-Advanced network troubleshooting tool.

-tcpdump is a packet sniffer that intercepts network packets and log them on the screen.

-Useful tool to verify that computer is receiving data from other computers

-This is also useful to examine the data in its raw form, which can be helpful if you

-understand protocols implementation details allowing you to spot problems.

-Be aware that packet sniffers can be used by individuals to capture passwords sent over

the network.

-You must use tcpdump in root mode.

-Once you enter tcpdump it summarizes what it is doing and prints lines, one for each packet

it monitors.

-Lines consist of time stamps, stack identifiers, origin system name, IP addresses and

port, destination system name and packet-specific information.

-Once tcpdump starts it dose not stop so you must press Ctrl+C to terminate it.

tcpdump options

- A: displays packet contents in ASCII

- D: Displays a list of interfaces tcpdump can listen to.

- n: Shows all addresses numerically.

- v: Shows additional packet information.

- w file: Used to write captured packets to the specified file.

-You could consult tcpdump’s man page for details and for additional options.

Additional Networking Tools

-Other than networking diagnostic programs you could use some common programs as debugging tools.

-One of the most helpful tools may be Telnet a program and protocol tool that is mainly used as

a remote login tool.

-Telnet is considered a poor choice as a remote login protocol due to the fact that it

is entirely unencrypted.

-A good rule to follow is that you should remove Telnet server from your system and never use

the telnet client program. SSH is a safer alternative to Telnet described in chapter 10.

Using Telnet to Debug Network Protocols

-If you include a port number after the remote hostname, the program connects to the

specified port allowing you to interact with the server.

-To use telnet this way you must know a lot of information about the protocol. Without

the knowledge of the protocol you can still use Telnet to test if a server is running or not.

-If you try to connect and get a connection refused error message you know that a remote

server is not running or is inaccessible. (This may be because a firewall is blocking

the connection)

-If the result is Escape character this means that the server is running, even though it

could not be working correctly.

-This test only works for protocols that us TCP. UDP tools will not connect.

Using Telnet to Debug Network Protocols continued

-Sometimes File Transfer Protocols (FTP) can be a helpful tool.

-This program allows you to transfer files between systems, as its name suggests.

-To use this tool type the program name followed by the FTP servers name, which will then

show you a login prompt, allowing you to issue FTP commands.