User:Beno0020

Chapter 12

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.

1. 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. Network Device Configuration Utilities (ip and ifconfig)*(pg. 301) The ifconfig program is primarily responsible for setting up your network

                  interface

cards (NICs). All of its operations can be performed through command-line options,

                 as

its native format has no menus or graphical interface. Administrators that have

                 used

the Windows ipconfig program may see some similarities, as Microsoft

                 implemented

some command-line interface (CLI) networking tools that mimicked functional

                 of their UNIX counterparts.

TIP Administrators still dealing with Windows may find the %SYSTEMROO%\system32\ netsh.exe program a handy tool for exposing and manipulating the details of Windows

                 networking

via the CLI. NOTE The ifconfig program typically resides in the /sbin directory, which is included in

               root’s

PATH. Some login scripts, such as those in Fedora, do not include /sbin in the PATH for

               nonprivileged

users by default. Thus, you may need to invoke /sbin/ifconfig when calling on it as a regular

               user. If

you expect to be a frequent user of commands under /sbin, you may find it prudent to add

              /sbin toyour PATH.

Setting Up NICs under Fedora and RHEL*(pg.304,305) Fedora and other Red Hat–type systems use a simple setup that makes it easy to configure network cards at boot time. It is done through the creation of files in the /etc/ sysconfig/network-scripts directory that are read at boot time. All of the graphical tools under Fedora create and manage these files for you; for other people who like to get under the hood, the following sections show how to manually manage the configuration files. For each network interface, there is an ifcfg file in /etc/sysconfig/network-scripts. This filename is suffixed by the name of the device; thus, ifcfg-eth0 is for the eth0 device, ifcfg-eth1 is for the eth1 device, and so on. If you choose to use a static IP address at installation time, the format for the interface configuration file for eth0 will be as follows: DEVICE=eth0 ONBOOT=yes BOOTPROTO=none Network Configuration 305 NETMASK=255.255.255.0 IPADDR= 192.168.1.100 GATEWAY=192.168.1.1 TYPE=Ethernet HWADDR=00:0c:29:ac:5b:cd Hostnames *(pg. 23-24,535) There are 2 way to configure the hostname -On DNS -On you local computer Domain Name System (DNS) *(pg. 377-413) - “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. 316) - There are a few command that will help diagnose a problem. - They are ping, traceroute, tracepath, netstat, and tcpdump. Ping command *(pg. 316) - 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. 348-349 - 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 *(pg. 312-313) -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 *(pg. 289-294) -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.