Configuring Digital Subscriber Line Access

Ubuntu also supports the use of a digital subscriber line (DSL) service. Although it refers to the different types of DSL available as xDSL, that name includes ADSL, IDSL, SDSL, and other flavors of DSL service; they can all be configured using the Internet Connection Wizard. DSL service generally provides 256Kbps to 24Mbps transfer speeds and transmits data over copper telephone lines from a central office to individual subscriber sites (such as your home). Many DSL services provide asymmetric speeds with download speeds greater than upload speeds.

Note

DSL service is an "always-on" type of Internet service, although you can turn off the connection under Ubuntu using the Network Device Control. An always-on connection exposes your computer to malicious abuse from crackers who trawl the Internet attempting to gain access to other computer systems. In addition to the capability to turn off such connections, Ubuntu provides a firewall to keep crackers out; you configured a simple firewall during the original installation. The firewall can also be configured from the Security Level Configuration tool found in the System Settings menu selection as Security Level.

A DSL connection requires that you have an ethernet network interface card (sometimes a USB interface that is not easily supported in Linux) in your computer or notebook. Many users also configure a gateway, firewall, or other computer with at least two network interface cards in order to share a connection with a LAN. We looked at the hardware and protocol issues earlier on in this chapter. Advanced configuration of a firewall or router, other than what was addressed during your initial installation of Ubuntu, is beyond the scope of this book.

Understanding Point-to-Point Protocol over Ethernet

Establishing a DSL connection with an ISP providing a static IP address is easy. Unfortunately, many DSL providers use a type of PPP protocol named Point-to-Point Protocol over Ethernet (PPPoE) that provides dynamic IP address assignment and authentication by encapsulating PPP information inside ethernet frames. Roaring Penguin's rp-pppoe clients are available from the Roaring Penguin site (www.roaringpenguin.com/penguin/pppoe/rp-pppoe-3.8.tar.gz), and these clients make the difficult-to-configure PPPoE connection much easier to deal with. You can download and install newer versions (see the Roaring Penguin link in the "Reference" section at the end of this chapter).

Note

ADSL modems were frequently supplied by ISPs when they originally started to roll out ADSL services. Nowadays however, these modems are optional, which is a good thing as many people choose to purchase a router with an in-built modem to create a dedicated connection. Using a router can save many headaches and will allow you to easily connect more than one computer to an Internet connection. Note that if you are using a cable connection then they usually come with an ethernet cable, in which case you just need a router. Check with your ISP before buying to ensure that whatever router you do end up with can be supported by them.

Configuring a PPPoE Connection Manually

You should only need to use these steps if you are using a modem supplied by your ISP, and not a router. The basic steps involved in manually setting up a DSL connection using Ubuntu involve connecting the proper hardware, and then running a simple configuration script if you use rp-pppoe from Roaring Penguin.

First, connect your DSL modem to your telephone line, and then plug in your ethernet cable from the modem to your computer's network interface card. If you plan to share your DSL connection with the rest of your LAN, you need at least two network cardsdesignated eth0 (for your LAN) and etH1 (for the DSL connection).

The following example assumes that you have more than one computer and will share your DSL connection on a LAN.

First, log in as root, and ensure that your first eth0 device is enabled and up (perhaps using the ifconfig command). Next, bring up the other interface, but assign a null IP address like this:

$ sudo /sbin/ifconfig eth1 0.0.0.0 up

Now use the adsl-setup command to set up your system. Type the command like this:

$ sudo /sbin/adsl-setup

You will be presented with a text script and be asked to enter your username and the Ethernet interface used for the connection (such as eth1). You will then be asked to use "on demand" service or have the connection stay up all the time (until brought down by the root operator). You can also set a timeout in seconds, if desired. You'll then be asked to enter the IP addresses of your ISP's DNS servers if you haven't configured the system's /etc/resolv.conf file.

After that, you will be prompted to enter your password two times, and have to choose the type of firewall and IP masquerading to use. (You learned about IP masquerading in the "Using IP Masquerading in Ubuntu" section, earlier in this chapter.) The actual configuration is done automatically. Using a firewall is essential nowadays, so you should choose this option unless you intend to craft your own set of firewall rulesa discussion of which is beyond the scope of this book. After you have chosen your firewall and IP masquerading setup, you will be asked to confirm, save, and implement your settings. You are also given a choice to allow users to manage the connection, a handy option for home users.

Changes will be made to your system's /etc/sysconfig/network-scripts/ifcfg-ppp0, /etc/resolv.conf, /etc/ppp/pap-secrets, and /etc/ppp/chap-secrets files.

After configuration has finished, use the adsl-start command to start a connection and DSL session, like this:

$ sudo /sbin/adsl-start

The DSL connection should be nearly instantaneous, but if problems occur, check to make sure that your DSL modem is communicating with the phone company's central office by examining the status LEDs on the modem. Because this varies from modem to modem, consult your modem user's manual.

Check to make certain that all cables are properly attached, that your interfaces are properly configured, and that you have entered the correct information to the setup script.

If IP masquerading is enabled, other computers on your LAN on the same subnet address (such as 192.168.1.XXX) can use the Internet, but must have the same /etc/resolv.conf name server entries and a routing entry with the DSL-connected computer as a gateway. For example, if the host computer with the DSL connection has an IP address of 192.168.1.1, and other computers on your LAN use addresses in the 192.168.1.XXX range, use the route command on each computer like this:

# /sbin/route add default gw 192.168.1.1

Note that you can also use a hostname instead if each computer has an /etc/hosts file with hostname and IP address entries for your LAN. To stop your connection, use the adsl-stop command like this:

# /sbin/adsl-stop