Tài liệu Chapter-18-Connecting to the Internet pptx

2Apr il 2003, 17:00:47 The Complete FreeBSD (isp.mm), page 315 18 Connecting to the Internet In this chapter: • The physical connection • Establishing yourself on the Internet • Choosing an Internet Ser vice Provider • Who’sthat ISP? • Making the connection In this chapter: • The physical connection • Establishing yourself on the Internet • Choosing an Internet Ser vice Provider • Who’sthat ISP? • Making the connection To implement the reference network shown in the previous chapter,weneed to do a lot of things that interface with the outside world. Theycan takesome time, so we should look at them first: • What kind of physical connection should we use? We’llconsider that in the next section. • We may want to register a domain.Manypeople don’t, but I strongly recommend it. Find out about that on page 317. • We may also want to register a network.Inour example, we have used the network 223.147.37.0.Inreal life, we can’tchoose our own network: we takewhat is given to us. We’lllook at this on page 318. • We need to find an Internet Service Provider.We’ll look at what that entails on page 319. The physical connection Just twoorthree years ago, the way to connect to the outside world was simple: a phone line. Since then, things have changed quite a bit, and you may have quite a choice: • Analogue telephone line connections are still the most common way of connecting small networks in most countries, but their bandwidth is limited to about 7 kB/s at isp.mm,v v4.10 (2003/04/02 03:09:55) 315 The physical connection 316 2April 2003, 17:00:47 The Complete FreeBSD ( /tools/tmac.Mn), page 316 best. You can run PPP or SLIP overthis kind of line, though nowadays most ISPs support only PPP. • ISDN stands for Integrated Systems Digital Network.It’sthe new, better,washes- whiter telephone system that is replacing POTS (Plain Old Telephone Service)in some countries, notably in Europe. FreeBSD supports ISDN with the isdn4bsd driver. Wewon’tlook at ISDN further in this book. • Leased lines form the backbone of the Internet. They’re invariably more expensive than dialup lines, but theycan provide quite high speeds—in the USA, a T1 line will give you 1,536 kbps, and in the rest of the world an E1 will give you 2,048 kbps. Leased lines are becoming less interesting, and we won’tlook at them in more detail in this book. • Cable modems use existing cable TV networks to deliverahigh speed connection, up to several megabits per second. Theyuse the cable as a broadcast medium, rather like an Ethernet, and suffer from the same load problems: you share the speed with the other users of the cable. There are also some security issues to consider,but if you have a cable service in your area, you’ll probably find it superior to telephones. The cable modem is effectively a bridge between the cable and an Ethernet. From the FreeBSD point of view, the cable modem looks likejust another Ethernet device. • DSL (Digital Subscriber Line)isthe telephone companies’ reaction to cable modems. Until recently,the L stood for Loop,not Line.Aloop is the telco term for the pair of wires between the exchange (or Central Office)and the subscriber premises. There are a number of variants on DSL: ADSL (Asynchronous Digital Subscriber Line)has different speeds for the uplink and the downlink, while SDSL (Symmetric Digital Subscriber Line)and HDSL (High-speed Digital Subscriber Line)hav e the same speed in each direction. Speeds and capabilities differ widely from one location to another.Bymodifying the way theytransmit data overnormal phone wires, including the use of special modems, ADSL can get speeds of up to 6 Mb/s downstream (towards the end user), and about 640 kbps upstream. HDSL has similar speeds, but the speed is the same in each direction. In contrast to cable modems, you don’thav e to share this bandwidth with anybody.Technical considerations limit the loop length to about four miles, so eveninbig cities you may not be able to get it. ManyDSL services are plagued by technical problems. There are a number of different ways to connect to a DSL service, but most of them involveaconversion to Ethernet. • In some parts of the world, satellite connections are a viable alternative.These usually use a telephone line for outgoing data and a satellite receiverfor incoming data. Pricing varies from very cheap to quite expensive,but if you can’tget cable or DSL, this might be your only choice. isp.mm,v v4.10 (2003/04/02 03:09:55) 317 Chapter 18: Connecting to the Internet 2April 2003, 17:00:47 The Complete FreeBSD ( /tools/tmac.Mn), page 317 Establishing yourself on the Internet The first thing you need to decide is the extent of your presence on the Net. There are various possibilities: • Youcould get a dialup service where you use your computer just to connect to the ISP,and perform network functions such as reading mail and news on the ISP’s machine (a shell account). It’salot faster to perform these functions on your own machine, and you have all the software you need to do so, so this option isn’tvery desirable. This option is becoming increasingly uncommon. • Youcould perform all the functions on your machine, but using names and addresses assigned to you by the ISP. • Youcould perform all the functions on your machine, using addresses assigned to you by the ISP,but you would use your own domain name. • Youget your own address space and use your own domain name. Does it matter? That’sfor you to decide. It’scertainly a very good idea to have your own domain name. As time goes on, your email address will become more and more important. If you get a mail address like 4711@flybynight.net,and Flybynight goes broke, or you decide to change to a different ISP,your mail address is gone, and you have to explain that to everybody who might want to contact you. If, on the other hand, your name is Jerry Dunham, and you register a domain dunham.org,you can assign yourself anymail address in that domain. But howdoyou go about it? One way would be to pay your ISP to do it for you. You don’tneed to do that: it’seasy enough to do yourself on the World-Wide Web.You must be connected to the Internet to perform these steps. This implies that you should first connect using your ISP’sdomain name, then establish your domain name, and change to that domain. Whichdomain name? We’llcontinue to assume that your name is Jerry Dunham. If you live in, say,Austin, Te xas, you have a number of domain names you can choose from: dunham.org, dunham.com, dunham.net,oreven dunham.tx.us if you want to use the geographical domain. If you live in, say,Capetown, people will probably suggest that you get the domain dunham.za,the geographical domain for South Africa. The problem with that is that you are limiting yourself to that country.Ifyou move to, say,Holland, you would have to change to dunham.nl—a situation only fractionally better than being bound to an ISP. The same considerations apply to dunham.tx.us,ofcourse. Your choice of domain name also affects the way you apply.Inthe following sections, I assume you takemyadvice and apply for an organizational rather than a geographical domain. isp.mm,v v4.10 (2003/04/02 03:09:55) Establishing yourself on the Internet 318 2April 2003, 17:00:47 The Complete FreeBSD ( /tools/tmac.Mn), page 318 Preparing for registration Once upon a time, registration was handled by InterNIC, a professional body.Since then it has been delegated to commercial companies, and the quality of service has suffered correspondingly: theydon’tevenappear to knowthe technical terms. Forexample, you may find them referring to a domain name as a ‘‘Web Address.’’ Things are still deteriorating at the time of writing: additional companies are being allowed to register domain names, and the field seems to attract a lot of cowboys. Registering a domain name The only prerequisites for registering a domain name are: • The name must be available, though there are some legalimplications that suggest that, though you might be able to register a domain such as microsoft.edu,itmight not be good for you if you do. In fact, microsoft.edu wasonce registered to the BISPL business school in Hyderabad, India, presumably not in agreement with Microsoft. • Youmust be able to specify twoname servers for it—see Chapter 21 for further details about name servers. First, check that the name is available: $ whois dunham.org No match for "DUNHAM.ORG". The InterNIC Registration Services Host contains ONLY Internet Information (Networks, ASN’s, Domains, and POC’s). Please use the whois server at nic.ddn.mil for MILNET Information. Next, try to find a reputable registrar.Immediately after the transfer of registrars from InterNIC, the only companytooffer this service was Network Solutions, but nowthere are many. I do not recommend Network Solutions: they’re expensive and incompetent. If, as I recommend, you set up your mail server to refuse mail from servers without reverse mapping, you will not be able to communicate with them, since theydonot have reverse DNS on their mail servers, and theyuse unregistered names for them. Judge for yourself what this says about their technical competence. One registrar that manyFreeBSD people use is Gandi (http://www.gandi.net/ ), which is slightly associated with the FreeBSD project. So far nobody has found anything negative to say about them. UnlikeNetwork Solutions, their web pages are also relatively simple to understand. Getting IP addresses Once upon a time, it was possible to get IP addresses from InterNIC, but this practice is nowrestricted to large allocations for ISPs. Instead, get the addresses from your ISP. Routing considerations makeitimpractical to move IPaddresses from one place to another.Ifyou move a long distance, you should expect to change your IP addresses in the same way as you would change your telephone number. isp.mm,v v4.10 (2003/04/02 03:09:55) 319 Chapter 18: Connecting to the Internet 2April 2003, 17:00:47 The Complete FreeBSD ( /tools/tmac.Mn), page 319 Choosing an Internet Service Provider In most cases, you will get your connection to the Internet from an Internet Service Provider,orISP.Asthe name suggests, an ISP will supply the means for you to connect your system or your local network to the Internet. Theywill probably also supply other services: most ISPs can’tliv e on Internet connections alone. In this chapter we’ll look at the things you need to knowabout ISPs, and howtoget the best deal. We’llconcentrate on what is still the most common setup, PPP overadialup line with a V.90 modem (56 kbps), which will give you a peak data transfer rate of about 7kB/s. Who’sthat ISP? As the Internet, and in particular the number of dialup connections, explodes, a large number of people have had the idea to become involved. In the early days of public Internet access, manyISPs were small companies run by very technical people who have seen a market opportunity and have grabbed it. Other ISPs were small companies run by not-so technical people who have jumped on the bandwagon. Still other ISPs are run by large companies, in particular the cable TV companies and the telephone companies. Which is for you? Howcan you tell to which category an ISP belongs? Do you care? You should care, of course. Let’sconsider what you want from an ISP,and what the ISP wants. You want a low-cost, high-reliability,high speed connection to the Internet. You may also want technical advice and value-added services such as DNS (see Chapter 21) and web pages. The main priority of a small ISP (or anyother ISP,for that matter) is to get a good night’s sleep. Next, he wants to ensure the minimum number of nuisance customers. After that, he wants to ensure that he doesn’tgoout of business. Only then is he interested in the same things that you are. In the last fewyears, a large number of ISPs have gone out of business, and manymore have merged with other companies. In particular,large companies frequently bought out small techie ISPs and then ran them into the ground with their incompetence. Fora humorous viewofthis phenomenon, see the ‘‘User Friendly’’cartoon series starting at http://ars.userfriendly.org/cartoons/?id=19980824. Questions to ask an ISP So howdoyou choose an ISP? Don’tforget the value of word-of-mouth—it’sthe most common way to find an ISP.Ifyou knowsomebody very technical, preferably a FreeBSD user,who is already connected, ask him—he’ll certainly be able to tell you about his ISP.Otherwise, a lot depends on your leveloftechnical understanding. It’s easy to knowmore about the technical aspects of the Internet than your ISP,but it doesn’t often help getting good service. Here are a fewquestions to ask anyprospective ISP: isp.mm,v v4.10 (2003/04/02 03:09:55) Who’sthat ISP? 320 2April 2003, 17:00:47 The Complete FreeBSD ( /tools/tmac.Mn), page 320 What kind of connections do you provide? See the discussion on page 315. Howdoyou charge? By volume, by connect time, or flat rate? Once most ISPs charged by connect time: you paid whether you transfer data or not. This made it unattractive toanISP to provide good performance, since that would have meant that you could finish your session more quickly.Now adays, flat rates are becoming more popular: you pay the same no matter howmuch you use the service. The disadvantage of the flat rate is that there is no incentive todisconnect, so you might find it difficult to establish connections. When comparing connect time and volume rates, expect an average data transfer rate of about 600 bytes per second for most connections via a 56 kbps modem. You’ll get up to 7 kB per second with traffic-intensive operations likefile downloading, but normally,you’ll be doing other things as well, and your data rate overthe session is more likely to be 600 bytes per second if you’re reasonably active,and significantly less if not. Faster lines typically don’tcharge by connect time: in particular,DSL lines are permanently connected and thus charge by data volume or at a flat rate. Another alternative that is again becoming more popular is a ‘‘download limit.’’ Your flat monthly fee allows you to download up to a certain amount of data, after which additional data costs money. This may seem worse than a flat rate, but it does tend to keep people from abusing the service. Do you have a cheaper charge for data from your own network? ManyISPs maintain web proxy caches, ftp archivesand network news. If theycharge by volume, some will give you free access to their own net. Don’toverestimate the value of this free data. What speed connections do you offer? ADSL connections have two different rates, a faster one for downloads and a slower one for the uplink. That’sfine if you’re planning to use the system as a client. If you intend to run servers on your system, things can look very different. If you are using a modem connection, theyshould be the fastest, of course, which are currently 56 kbps. What uplink connections do you have? The purpose of this question is twofold: first, see if he understands the question. An uplink connection is the connection that the ISP has to the rest of the Internet. If it’sinadequate, your connection to the Internet will also be inadequate. To judge whether the link is fast enough, you also need to knowhow manypeople are connected at anyone time. See the question about dialup modems below. Howmanyhops are there to the backbone? Some ISPs are a long way from the Internet backbone. This can be a disadvantage, but it doesn’thav e to be. If you’re connected to an ISP with T3 all the way to the backbone, you’re better offthan somebody connected directly to the backbone by an ISDN Basic Rate connection. All other things being equal, though, the smaller the number of hops, the better. isp.mm,v v4.10 (2003/04/02 03:09:55) 321 Chapter 18: Connecting to the Internet 2April 2003, 17:00:47 The Complete FreeBSD ( /tools/tmac.Mn), page 321 Howmanydialup modems do you have? This question has twopoints to makeaswell. On the one hand, the total bandwidth of these modems should not exceed the uplink bandwidth by too much—let’ssay it shouldn’tbe more than double the uplink bandwidth. On the other hand, you want to be able to get a free line when you dial in. Nothing is more frustrating than having to try dozens of times before you can get a connection. This phenomenon also causes people not to disconnect when they’re finished, especially if there is no hourly rate. This makes the problem evenworse. Of course, the problem depends on the number of subscribers, so ask the next question too. Howmanysubscribers do you have?What is the average time theyconnect per week? Apart from the obvious information, check whether theykeep this kind of statistics. They’re important for growth. What’syour up-time record? Do you keep availability statistics? What are they? ISPs are always nervous to publish their statistics. They’re neverasgood as I would like. But if theypublish them, you can assume that that fact alone makes them better than their competitors. What kind of hardware and software are you running? This question will sort out the good techie ISPs from the wannabes. The real answers aren’t quite as important as the way theyexplain it. Nevertheless, consider that you’ll be better off with an ISP who also runs FreeBSD or BSD/OS. 1 Only small ISPs can afford to use UNIX machines (including FreeBSD) as routers; the larger ones will use dedicated routers. Next, in my personal opinion, come other UNIX systems (in decreasing order of preference, Solaris 2.X, Linux and IRIX), and finally,along way behind, Windows NT.Ifyou’re looking for technical support as well, you’ll be a lot better offwith an ISP who uses FreeBSD or BSD/OS. You’ll also be something special to them: most ISPs hate trying to solveproblems for typical Windows users. Howmanyname servers do you run? The answer should be at least 2. You’ll probably be accessing them for your non-local name server information, because that will be faster than sending requests throughout the Internet. Can you supply primary or secondary DNS for me? Howmuch does it cost? Istrongly recommend using your own domain name for mail. That way,ifyour ISP folds, or you have some other reason for wanting to change, you don’tneed to change your mail ID. Todothis, you need to have the information available from a name server 24 hours per day.DNS can generate a lot of traffic, and unless you’re connected to the network 100% of the time, mail to you can get lost if a system can’tfind your DNS information. Even if you are connected 100% of the time, it’sagood idea to have a backup DNS on the other side of the link. Remember,though, that it doesn’thav e to be your ISP.Some ISPs supply free secondaries to anybody who asks for them, and you might have friends who will also do it for you. The ISP may also offer to perform the domain registration formalities for you—for a fee. 1. BSD/OS is a commercial operating system closely related to FreeBSD. If you have a few thousand dollars to spare, you may evenfind it better than FreeBSD. Check out http://www.wrs.com/ for further details. isp.mm,v v4.10 (2003/04/02 03:09:55) Who’sthat ISP? 322 2April 2003, 17:00:47 The Complete FreeBSD ( /tools/tmac.Mn), page 322 Youcan just as easily do this yourself: see page 318 for more details. Check the fee, though: in some countries, the ISP may get a discount for the domain registration fees. If it’sbig enough, registering via the ISP may possibly be cheaper than doing it yourself. Can you route a class C network for me? What does it cost? If you’re connecting a local area network to the Internet, routing information must be propagated to the Net. ISPs frequently consider this usage to be ‘‘commercial,’’ and may jack up the prices considerably as a result. Alternativestoafull class C network are a group of static addresses (say,8or16) out of the ISP’sown assigned network addresses. There’snoparticular problem with taking this route. If you change ISPs, you’ll have tochange addresses, but as long as you have your own domain name, that shouldn’tbeaproblem. Another possibility might be to use IP aliasing.See page 393 for more details. Can you supply me with a static address? Howmuch does it cost? It’shighly desirable to have static addresses. See page 346 for more details. Unfortunately, manyISPs use static IPs to distinguish links for commercial use from those for home use, and may charge significantly more for a static address. Do you give complete access to the Internet, or do you block some ports? This is a complicated question. ManyISPs block services likesmtp (mail) or http (web servers). If theydo, you can’trun a mail or web server on your own machines. In the case of mail, this is seldom a problem: theywill provide you with their own mail server through which you must relay your mail. This also allows the ISP to limit spam, which might otherwise come from anysystem within the network. Forhttp, the situation is different. Usually ISPs charge moneyfor supplying access to their ownweb servers. On the other hand, this arrangement can provide much faster web access, especially if you are connected by a slowlink, and you may also save volume charges. Ultimately it’sachoice you need to make. Do you have complete reverse DNS? In previous editions of this book, I didn’task this question: it seemed impossible that any ISP would answer ‘‘no.’’ Unfortunately,times have changed, and a number of ISPs not only don’tsupply DNS, theyseem to think it unnecessary.Don’thav e anything to do with them: firstly,itshows complete incompetence, and secondly it will cause trouble for you accessing anumber of sites, including sending mail to the FreeBSD mailing lists. isp.mm,v v4.10 (2003/04/02 03:09:55) 323 Chapter 18: Connecting to the Internet 2April 2003, 17:00:47 The Complete FreeBSD ( /tools/tmac.Mn), page 323 Making the connection After calling a fewISPs, you should be able to makeadecision based on their replies to these questions. The next step is to gather the information needed to connect. Use Table 18-1 to collect the information you need. See Chapter 20 for information about authentication, user name and password. Table 18-1: Information for ISP setup Information Fill in specific value IP address of your end of the link IP address of the other end of the link Kind of authentication (CHAP,PAP, login) User or system name Password or key Primary Name Server name Primary Name Server IP address Secondary Name Server name Secondary Name Server IP address Pop (Mail) Server Name News Server Name isp.mm,v v4.10 (2003/04/02 03:09:55) . understands the question. An uplink connection is the connection that the ISP has to the rest of the Internet. If it’sinadequate, your connection to the Internet. to the Internet 2April 2003, 17:00:47 The Complete FreeBSD ( /tools/tmac.Mn), page 317 Establishing yourself on the Internet The first thing you need to