Fedora™ Unleashed, 2008 edition

applications are provided ports to use to make 'direct' connections for specific software services. These ports help TCP/IP distinguish services so that data can get to the correct application. If you check the file /etc/services, you will see the common ports and their usage. For example, for FTP, HTTP, and Post Office Protocol (email retrieval server), you will see the following:

ftp   21/tcp
http  80/tcp www www-http # WorldWideWeb HTTP
pop3 110/tcp pop-3        # POP version 3

The ports defined in /etc/services in this example are 21 for FTP, 80 for HTTP, and 110 for POP3. Other common port assignments are 25 for Simple Mail Transport Protocol (SMTP) and 22 for Secure Shell (SSH) remote login. Note that these ports are not set in stone, and you can set up your server to respond to different ports. For example, although port 22 is listed in /etc/services as a common default for SSH, the sshd server can be configured to listen on a different port if you edit its configuration file /etc/ssh/sshd_config. The default setting (commented out with a pound sign) looks like this:

#Port 22

Edit the entry to use a different port, making sure to select an unused port number, such as this:

Port 2224

Save your changes, and then restart the sshd server. (Refer to Chapter 11, 'Automating Tasks,' to see how to restart a service.) Remote users must now access the host through port 2224, which can be done using ssh's -p (port) option like so:

$ ssh -p 2224 remote_host_name_or_IP

Beyond the Network and Onto the Internet

Fedora supports Internet connections and the use of Internet resources in many different ways. You will find a wealth of Internet-related software included with this book's version of Fedora, and you can download hundreds of additional free utilities from a variety of sources. To use them, you must have a working Internet connection.

In this section, you learn how to set up an Internet connection in Fedora, using a modem and Point-to-Point Protocol (PPP) as well as other connection methods, including Digital Subscriber Line (DSL) and cable modem services. Just a few years ago, getting a dialup connection working was difficult — hence, an entire chapter of this book was devoted to it. Nowadays, as long as you have a hardware modem, dialup configuration is simple. The Fedora developers and the wider Linux community have made great progress in making connectivity easier.

Although many experienced Linux users continue to use manual scripts to establish their Internet connectivity, new users and experienced system administrators alike will find Fedora's graphical network configuration interface, the Internet Connection Wizard, much easier to use. You learn how to use the Internet Connection Wizard in this chapter, as well as how to configure Fedora to provide dial-in PPP support. The chapter also describes how to use Roaring Penguin's DSL utilities for managing connectivity through a cable modem connection.

Common Configuration Information

Although Fedora enables great flexibility in configuring Internet connections, that flexibility comes at the price of an increase in complexity. To configure Internet connectivity in Fedora, you must know more about the details of the connection process than you can learn from the information typically provided by your ISP. In this section, you learn what to ask about and how to use the information.

Some ISPs are unaware of Linux or unwilling to support its use with their service. Fortunately, that attitude is rapidly changing, and the majority of ISPs offer services using standard protocols that are compatible with Linux, even if they (or their technical support people) aren't aware that their own ISPs are Linux-friendly. You just need to press a little for the information you require.

If you are using a dialup modem account (referred to in Linux as PPP for the Point-to-Point Protocol it uses), your ISP provides your computer with a static or dynamic IP (Internet Protocol) address. A dynamic IP address changes each time you dial in, whereas a static IP address remains the same. The ISP also might automatically provide your computer with the names of the Domain Name Service (DNS) servers. You need to know the telephone number that your computer dials in to for making the connection; your ISP supplies that number, too. You also need a working modem and need to know the device name of the modem (usually /dev/modem).

NOTE

Most IP addresses are dynamically assigned by ISPs; ISPs have a pool of addresses, and you get whatever address is available. From the ISP's viewpoint, a small number of addresses can serve a large number of people because not everyone will be online at the same time. For most Internet services, a dynamic IP works well because it is the ISP's job to route that information to you, and it sits in the middle —between you and the service you want to use. But a dynamic IP address changes, and if someone needs to find you at the same address (if you run a website or a file transfer site, for example), an IP that changes every time you log on will not work well. For that, you need a static IP. Because your ISP cannot reuse that IP with its other customers, it will likely charge you more for a static IP than a dynamic If? The average consumer doesn't need the benefit of a static IP so he is happy paying less for a dynamically assigned IP Also, the DNS information can be provided automatically by the ISP by the DHCP

If you are using DSL access or a cable modem, you might have a dynamic IP provided through DHCP, or you might be assigned a static IP. You might automatically be provided with the names of the DNS servers if you use DHCP, or you might have to set up DNS manually (in which case, you have to know the IP addresses of the DNS servers).

In all cases, you have to know your username, your password, and for the configuration of other services, the names of the mail servers and the news server. This information can be obtained from your ISP if you specifically ask for it.

NOTE

The information in this book will help you understand and avoid many connection issues, but you might experience connection problems. Keep the telephone number of the technical help service for your ISP on hand in case you are not able to establish a connection. But be aware that few ISPs offer Linux support, and you might need to seek help from a Linux-savvy friend or a Linux user's group if your special circumstances cannot be handled from the knowledge you gain from this book. Of course, the best place to look is on the Internet. Use Google's Linux page (http://www.google.com/linux) to research the problem and see whether any other users have found fixes or workarounds.