UNIX Operating System I Module 12 Introduction to UNIX Networking

UNIX Operating System I

Module 12Introduction to UNIX Networking


Objectives

• Identify basic networking concepts, including how a network works.

• Identify the purpose and characteristics of network services configuration files

• Edit basic network services configuration files

• Configure basic network settings.• Run and interpret ifconfig.• Use network diagnostic tools


Basic Functions of Network Hardware

• Network hardware is designed to allow two or more computers to communicate with one another.

• Most networks rely upon wires or cables to transmit data between machines as electrical impulses, but some devices use radio waves or even light to do the job.


Types of Network Hardware

• Ethernet– Ethernet is the most common type of network

hardware on local networks today.– It comes in several varieties ranging from the

old 10Base-2 and 10Base-5 to 10Base-T and 100Base-T to the cuttingedge 1000Base-T and 1000Base-SX.

– In all these cases, the number preceding the “Base” indicates the technology’s speed in megabits per second (Mbps).

– Plans are underway to develop another ten-fold speed increase.



• Token Ring– At one time an important competitor to

Ethernet, IBM’s Token Ring technology is rapidly falling behind.

– The fastest type of Token Ring clocks in at just 16Mbps.

– Just as important, it’s costlier than Ethernet and has less in the way of hardware support.



• FDDI– Fiber Distributed Data Interface (FDDI)

is a networking technology that’s comparable to 100Base-T Ethernet in speed.

– FDDI uses fiber optic cables, but a variant known as CDDI works over copper cables similar to those of 100Base-T.


Network Packets

• Modern networks operate on discrete chunks of data known as packets.– Suppose you want to send a 100KB file from

one computer to another.– Rather than send the file in one burst of data,

you break it down into smaller chunks.– You might send 100 packets of 1KB each.– This way, if there’s an error sending one

packet, you can resend just that one packet, rather than the entire file.


Network Packets

• Typically, each packet includes an envelope, which includes the sender address, the recipient address; and a payload, which is the data intended for transmission.

• When the recipient system receives packets, it must hold onto them and reassemble them in the correct order to re-create the complete data stream.


Network Protocol Stacks

• The packing and unpacking of network data is frequently described in terms of a protocol stack.

• Understanding how the pieces of such a stack fit together can help you understand networking as a whole, including the various network protocols used by Linux.


What Is a Protocol Stack?

• A protocol stack is a set of software that converts and encapsulates data between layers of abstraction.– For instance, the stack can take the commands

of e-mail transfer protocols, and the e-mail messages that are transferred, and package them into packets.

– Another layer of the stack can take these packets and repackage them into Ethernet frames.


The OSI Model

• A common model used for describing protocol stacks generically is the Open System Interconnection (OSI) model.

• This model breaks networking tasks down into seven layers, from the Application layer to the Physical layer.

• Each layer in between these does some task related to the packaging of data for transport or its unpacking.


The OSI Model


TCP/IP vs OSI Model


Basic Network Terminology

• Hub• Switch• Full-duplex• Half-duplex• Bridge• Router• Gateway


Bridge

• Bridge– a network device that connects two or more

networks at the Data Link layer.– Forwards frames from one network to another.


Router

• Router– a network device that connects two or more networks at

the Network layer.– A router forwards packets between networks making

decisions about what network should receive each packet.


Gateway• Gateway

– a network device that does some data conversion at layers higher than the Network layer.

– Gateways are often used to provide connection between different applications or different network layers.

– The picture below shows a gateway that provides for the secure connection of two networks using an insecure network:


Types of Network Addresses

• Hardware Addresses• IP Addresses

– IP address– Broadcast address– network mask (subnet mask or netmask)


Network Ports

• Contacting a specific computer is important, but one additional type of addressing is still left: The sender must have an address for a specific program on the remote system.– For instance, suppose you’re using a Web browser.– The Web server computer may be running more servers

than just a Web server - it might also be running an e-mail server or an FTP server.

• Another number beyond the IP address allows you to address traffic to a specific program.

• This number is a network port number, and every program that accesses a TCP/IP network does so through one or more ports.


Basic Network Configuration

• Clients and Servers– one important distinction is the one

between clients and servers.– A client is a program that initiates a

network connection to exchange data.– A server listens for such connections

and responds to them.


DHCP Configuration

• One of the easiest ways to configure a computer to use a TCP/IP network is to use the Dynamic Host Configuration Protocol (DHCP).

• There are three DHCP clients in common use on Linux: pump, dhclient, and dhcpcd.

• Red Hat Linux sets this option in a file called /etc/sysconfig/network-scripts/ifcfg-eth0– If the BOOTPROTO variable is set to

BOOTPROTO="dhcp"– the system will use DHCP.


Static IP Address Configuration

• If a network lacks a DHCP server, you must provide basic network configuration options manually.

• There are several specific items that are required:– IP address

• The IPADDR item in /etc/sysconfig/network-scripts/ifcfg-eth0 contains the IP address.

– Network mask• The NETMASK item in /etc/sysconfig/network-

scripts/ifcfg-eth0 contains the network mask.


Static IP Address Configuration

• There are several specific items that are required (continue):– Gateway address

• You can manually set the gateway via the route command, or

• set it in /etc/sysconfig/network-scripts/ifcfg-eth0 (the GATEWAY item) or /etc/sysconfig/network (also the GATEWAY item).

– DNS settings• In order for Linux to use DNS to translate between IP

addresses and hostnames, you need to specify at least one DNS server in the /etc/resolv.conf file.

• Use the keyword nameserver in the /etc/resolv.conf file

nameserver IPADDRESS_OF_DNS


Other Network Configuration Files

• /etc/hosts• /etc/host.conf• /etc/sysconfig/network


the ifconfig program

• To view interface configuration, issue the following command:# ifconfig or $ ifconfig eth0



• Basic use of ifconfig to bring up a network interface:# ifconfig interface up addr netmask mask

– For instance, the following command brings up eth0 (the first Ethernet card) using the address 192.168.100.18 and the netmask 255.255.255.0:

# ifconfig eth0 up 192.168.100.18 netmask 255.255.255.0

– This command links the specified IP address to the card so that the computer will respond to the address and claim to be that address when sending data.



• After executed the ifconfig program, use ifconfig eth0 to view the interface configuration


Routing

• To see your routing table, you might use the route program:# route


Set Default Gateway

• To set up a route for traffic beyond your current network. For that, you need to use the route command:# route add default gw 192.168.100.254


Set Default Gateway

• This shows that data destined for 192.168.100.0 (that is, any computer with an IP address between 192.168.100.1 and 192.168.100.254) goes directly over eth0.

• The 127.0.0.0 network is a special interface that “loops back” to the originating computer. Linux uses this for some internal networking purposes.

• The last line shows the default route - everything that doesn’t match any other entry in the routing table.– This line specifies the default route’s gateway system as

192.168.100.254.– If it’s missing or misconfigured, some or all traffic destined for external

networks, such as the Internet, won’t make it beyond your local network segment.


Network Diagnostic Tools

• Network configuration is a complex topic, and unfortunately, things don’t always work as planned.

• Fortunately, there are a few commands you can use to help diagnose a problem.– ping– netstat– route


ping• ping is the simplest network diagnostic tool.• This command sends a simple packet to the system (via IP address

or hostname) and waits for a reply.• In Linux, ping continues sending packets once every second or so

until you interrupt it with a Ctrl+C keystroke.• Here’s an example of its output:


netstat

• netstat is a multi-network tools because it can be used in place of several others, depending upon the parameters it is passed. It can also return information that’s not easily obtained in other ways.– Interface information– Routing information– Program use– Open ports

• netstat is a very powerful tool, and its options and output aren’t entirely consistent from one distribution to another.

• You may want to peruse its man page and experiment with it to learn what it can do.


netstat – Interface information

• Pass netstat the --interface or -i parameter to obtain information on your network interfaces similar to what ifconfig returns.

[root@redhat9 root]# netstat -iKernel Interface tableIface MTU Met RX-OK RX-ERR RX-DRP RX-OVR TX-OK TX-ERR TX-DRP TX-OVR Flgeth0 1500 0 113 0 0 0 75 0 0 0 BMRUlo 16436 0 18 0 0 0 18 0 0 0 LRU


netstat – Routing information

• You can use the --route or -r parameter to obtain a routing table listing similar to what the route command displays.


netstat – Program use• Some versions of netstat support the --program or –p

parameters, which attempt to provide information on the programs that are using network connections.

• This attempt isn’t always successful, but it often is, so you can see what programs are making outside connections.


netstat – Open ports

• When used with various other parameters, or without any parameters at all, netstat returns information on open ports and the systems to which they connect.

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UNIX Operating System I Module 12 Introduction to UNIX Networking