Tài liệu Learning DebianGNU Linux-Chapter 11. Getting Connected to the Internet docx

Getting Connected to the Internet This chapter explains how to use Linux to connect to remote servers.. Then, the chapter explains basic TCP/IP network concepts that you must know in ord

11 Getting Connected to the Internet

This chapter explains how to use Linux to connect to remote servers First, it explains how to use wvdial, a program that makes it easy to connect to the Internet via a PPP connection provided by an ISP Then, the chapter explains basic TCP/IP network concepts that you must know in order to administer a Linux system connected to the Internet or a local area network So that you can use your knowledge of TCP/IP effectively, the chapter explains how to

use pppconfig to configure and administer a system that connects to a

local area network and to a remote server via PPP Next, the chapter

describes several popular network client applications available under Linux, including a web browser and an FTP client The chapter then describes the use of minicom and seyon, which provide dial-out capabilities like those

of Window's hyperterminal Finally, the chapter shows how to make a

PPP connection manually, by using minicom

11.1 Connecting to the Internet

Most Internet service providers (ISPs) offer two primary types of service: Shell accounts and PPP (point-to-point protocol) accounts Shell accounts were more popular before the advent of the Web A shell account lets you use your computer much as if it were a virtual console associated with a remote computer You can type commands, which are interpreted by the remote computer, and view the resulting output on your computer Although

a few web browsers, such as Lynx, can operate via a shell account, they

don't generally support the highly graphical, multimedia pages which web surfers have come to expect

In contrast, a PPP account connects your computer directly to the Internet While your computer is connected to the Internet, you can use it to surf the Web with your favorite browser If your ISP allows, you can even run a web server, providing pages that can be viewed by others around the world

You can compare the two types of Internet accounts - shell and PPP - with two kinds of postal service Imagine that no mail carrier actually comes to your home to pick and deliver mail Instead, every time you want to conduct postal business, you go to the post office This resembles a shell account: The computer that connects you to the Internet is remote, and every time you want to do something on the Internet you must open a terminal, or telnet, session to that computer PPP, on the other hand, is like home delivery: The Internet comes right to your doorstep, and your computer is literally placed

on the Internet by the machine at your ISP that you connect to

Under Microsoft Windows, you use hyperterminal to access a shell

account and Dial-Up Networking to access a PPP account Under Linux, you can choose from among several programs that let you access a shell account The most commonly used programs are minicom and seyon To access a

PPP account under Linux, you use the PPP daemon, pppd The next section

describes how to use wvdial to make the process of establishing a PPP connection simple

11.2 Configuring Your Modem

Your modem is a peripheral device to Linux, like a CD-ROM drive, hard drive, or a mouse Your modem will be connected to a serial port, which means that one of the serial devices in the /dev directory actually points to your modem You may notice that there's also a file, /dev/modem, which serves as a placeholder for your modem Initially this file doesn't point to anything To simplify your operations, so that applications like

minicomand seyon need only refer to /dev/modem, you can create a symbolic link from the serial device connected to your modem to

Table 11.1: Command to Create /dev/modem

Table 11.1: Command to Create /dev/modem

For example, most ISPs use one of three PPP login procedures: PAP

(Password Authentication Protocol), CHAP (Challenge-Handshake

Authentication Protocol), or ordinary user/password authentication PAP is currently the most popular of the three procedures If you ask the tech

Support person at your ISP what login procedure your ISP uses, the tech Support person may have no idea what you're talking about, because most users run Microsoft Windows, and Windows Dial-Up Networking handles the login procedure transparently

The wvdial program transparently establishes a PPP connection in much the same way as Windows Dial-Up Networking It understands a variety of possible dialogs used by ISPs In most cases, it will analyze data sent by your ISP and respond with the proper data in the format required by the ISP

To use wvdial, be sure that you've established your nameserver

configuration, as described in Chapter 10, Setting Up a Linux-Based LAN Then, issue the following commands

route del default

wvdial &

The command generates quite a bit of output, which makes further use of this virtual terminal distracting The simplest solution is to switch to another virtual terminal, by pressing Alt- 7, where n stands for the virtual terminal (1-7) Alternatively, you can direct the output of the command to a file, by typing this command in place of the one given earlier:

wvdial 2>/tmp/wvdial.messages &

Of course, you'll need to consult the file if something goes wrong with

wvdial Do so by using the more command:

more /tmp/wvdial.messages

Once your connection is up, you can browse the Web and access other

Internet services, as described later in this chapter For now, simply verify that your connection is working by issuing the command:

ping www.oreilly.com

The ping command should report that echo packets were successfully

received from the server If not, check your nameserver configuration

When you want to log off your ISP, issue the following command:

killall wvdial

11.4 PPP Client

The Linux PPP client lets your Linux system connect to the Internet via a PPP server, much the same way Dial-Up Networking lets your Microsoft Windows system do so Behind the scenes, wvdial calls on the services of the PPP client to connect to a PPP server However, it's possible to use the Linux PPP client directly, without the aid of wvdial Doing so gives you access to the many options provided by PPP, which can prove helpful if your networking needs are too sophisticated to be met by wvdial's simple fire- and-forget mechanism Even if you're content with wvdial, you'll benefit from understanding the PPP client For example, you'll find 1t much easier to

troubleshoot PPP problems if you understand the functions performed by the PPP client on behalf of wvdial

11.4.1 Specifying PPP Options

The Linux PPP client requires several configuration files Rather than build

these files manually, you can use pppconfig, a program that leads you

through a dialog and then creates the proper configuration files based on your responses Login as root, and issue the command:

ppepconfig

Figure 11.1 shows pppconfig's main menu, which the program displays

whenever you start it Notice that pppconfig lets you create a

connection - the term by which it refers to a complete set of configuration parameters - but it does not let you edit an existing connection To change a connection, you must edit the configuration files manually It's usually

simpler to forego editing a connection, creating a new connection instead

Figure 11.1: The pppconfig main menu

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This is the PP? cocfigzatiœn uLiÌitu, [It dose not sake 4 correction to your ISP: it just configures your system so that wou can connect with 5 utility such as ‘pon’, You will be asked for the usernane, pasmord avi Phone number that wour ISP gave you, If your ISP uses PAP or CHAP, that is sll you reed, If you need to use a chat script to cormmect, you will need

to know hoe wou isp prurets for wour usernees and pavaAxYd, lÝ wou don't know what your ISP uses, try PH

Use the up and doen erroy keus to move around the menus Use the TAB keu

to wove from the menu to (DD to <CANTEL> and back, When you ore ready to

move 0ì tọ the next morn Go to <LIO and hit enter To go back to the main

menu go to <DAWCEL> and hit enter

Crud : 9 9f:E 1ΠCreate Create a commection Oust Exit this utility

To create a new connection, use the arrow keys to highlight Next or Create, and then press Enter

If pppconfig determines that the file /etc/resolv.conf does not exist or does not specify one or more nameservers (DNS servers), it will display the screen shown in Figure 11.2, which asks you to specify nameservers You should do so, therefore highlight Yes and press Enter

Figure 11.2: Request to specify nameservers

Figure 11.3: Specifying the primary nameserver

Figure 11.4: Specifying the secondary nameserver

Figure 11.5 shows the next screen displayed by pppconfig, which asks

for a name by which to refer to this connection If your /etc/resolv.conf file already specified one or more nameservers, pppconfig displays this screen after its initial screen Choose a connection name you'll be able to recall, backspace over the default connection name, type your connection name, and press Enter You may find it convenient to use the domain name

of your ISP as the connection name

pppconfig equivocates by sometimes referring to a connection,

sometimes to an /SP (Internet Service Provider), and sometimes to a

provider The term connection is more accurate in this context, because you may have several ways of connecting to a particular ISP - for example, the ISP may have several dial-up numbers Each way of connecting to an ISP

can be configured as a distinct pppconfig connection

Figure 11.5: Specifying the connection (provider) name

you can dial it by just otving the command ‘pon’, Give each additional isp

5 unique name, For exaeple, you might call your employer “theofflce” and

your university “theschool”, Then you oan comect to your IS with “pon”,

your office with “pon UrmotFice’ and war university vith “oor theecool’,

` cv

Next, pppconfig asks what method your ISP uses to authenticate your access to its network, as shown in Figure 11.6 PAP (Password

Authentication Protocol) is currently the most popular method; however, some ISPs continue to use other methods The technical support staff of many ISPs cannot tell what authentication method their system uses So, you may have to guess; if so, initially guess PAP If you find you cannot

establish a connection, you can try the alternative values Select the desired authentication method and press Enter

Figure 11.6: Specifying the authentication method

Pleese select the euthentication method for this connection PHP is the rethod mast often used in Windows SH a0 LÝ ga [SP supports the NT œ- MinSS disi-up client try “PAP”

Chat Use “chat” for logint/psssword: suthenticeation

CHAP Challenge Handshake Authentication Protccol

Quit Exit thie utility

Configuration screen, shown in Figure 11.8, appears Type the login

username assigned to you by your ISP and press Enter Your login

username may differ from your actual username, so be sure to use your login

usemame

Figure 11.8: Specifying the username

ĐT

Bạckspaca over the placeholder string acd enter the username given bo you

by vour ISP, If your username includes any space or purctuation merks

Next, you must specify the device that corresponds to your modem, as

shown in Figure 11.9 Type the name of the device file, paying careful

attention to capitalization, and press Enter

Figure 11.9: Specifying the modem device

Enter the poet your modes i2 on,

/de./ttu9%) ss COM in DOS

fdee/ttyS2 is ONS in BOS

/de./ttu53 is CON4 tn DOS

/dev/ttụuŠ1 ¡+ the mọt comwn, Mote that this aust be taped exactly as

thom Capitalization is important: ttySl is not the same as ttysl

The program next asks whether you want it to establish a default route when

it makes a connection, as shown in Figure 11.10 You almost certainly do, so accept the default choice - Enable Default Route - by using the arrow keys to highlight Ok and pressing Enter

Figure 11.10: Enabling a default route

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Enabling default routing tells your system that the way to reach hosts to

which it is not directly connected is vie your ISP, This is almost cartsinly

vet you want, Use the up and down arrow keys to nove among the selections

ard press the spacebe to select one When you are finished, use TAB to

select (OK and ENTER to move on to the next item

MỌI: cổ lu `) Ernable default roœwte ( >) *-defaultroute* Disable default route

Figure 11.11 shows the next pppconfig screen, which requests

information on your computer's network address (IP number) Most ISPs assign your computer a different network address each time you establish a connection Some ISPs give you the option of having a static network

address, meaning that your computer's network address is always the same

Figure 11.11: Specifying your computer's network address

at the end, like this; 192,169,1.2: If wou have been siven both a local

ad a remote ip enter the local ip a colon and the renote ip like this:

Unless your ISP has assigned your computer a static network address,

simply press Enter If your computer has a single static network address, type the address followed by a colon and press Enter If it has separate local

and remote network addresses, type the local address, a colon, and the

remote address; then press Enter

Next, you must specify your modem's speed, as shown in Figure 11.12 This refers to the speed of the connection between your modem and your system, not to the speed of the connection between your modem and your ISP, which

is normally a smaller value Unless you have a particular reason for doing otherwise, you should accept the default speed of 115200 by pressing Enter

Figure 11.12: Specifying the modem speed

Figure 11.13: Specifying a modem initialization string

Erter modem intialization string The default value is ATZ, ehich tells

the woden to use its default settings Re most modems 4re shipped fron the factory with default settings that are appropriate for pop, I suggest you not change this

Figure 11.14: Specifying the phone number

1 ]

1 r

Reckspace over the placeholder string and enter the number to diel Don\'t

insert ary dashes or spaces, See your modern aarual if you need to do

aretthing urweual Like dialing through 4 PRX,

Figure 11.15: Specifying tone or pulse dialing

pulse, Use the up end down arrov keys to move among the selections, and

press the spacebar to select one khen you are finished use TAB to select “OK> and ENTER to move on to the mext item,

Figure 11.16: Reviewing and changing entries

¬———ẺỄễềễ

Pleese select the property you wish to modify, select “Cancel” to go

back to start over or select “Finished® to write out the changed

files,

Humber Telestore nusber to call

User User name for authentication Pasewerd The oaysword fœ- thi cocnecLiœn

Spesd Port speed for the nwoden

Com Serial Port the sodes is on

Defsultroute Enable or disable default route

Ipdefault Set IP addresses Metrod futhentication Method (e.9 PRP?

Finished Write files and return to main menu,

Quit Exit this utility

pppconfig writes the PPP configuration files according to your

specifications Press Enter to exit the Finished screen Unless you want to specify another connection, highlight Quit and press Enter to exit the main

This command continually displays messages as they're posted to the system log file Because PPP writes log entries that describe its progress, the

command lets you see what PPP is doing When you're done viewing log entries, type Ctrl-C to exit the tail command

You can see even more detail by using the command:

e Did you correctly specify your ISP's phone number?

« Did you correctly specify your userid and password?