(M)  s i s t e m a   o p e r a c i o n a l   m a g n u x   l i n u x ~/ · documentação · suporte · sobre


3. Configuring Linux

After you have connected the modem and it's getting sync, then you're ready to configure Linux and verify your connection to your ISP. Although I will refer to a Linux System, you could conceivably connect any type of 10baseT device to the modem. This includes a router, hub, switch, PC, or any other system that you wish to use. We'll just cover the Linux aspects here.


Before you connect to your ISP, make sure you understand all security issues of having a direct connection to the Internet via DSL. Depending on your ISP, most outside users can access your system, and you should setup any firewalls, deactivate ports/services, and setup any passwords prior to connecting your machine to the world. See the Security section below, and the links section for more on this very important topic. Do not make this an afterthought! Be ready.

3.1. Bridged vs PPPoX Networks

Before we get too far into the final stages of installing and configuring our system, let's look at how various DSL ISPs set up their networks. It will be very important for you to know how your ISP does this, as there is more than one possibility and the steps involved are quite different for each. This may not be the kind of thing the ISP is advertising, and since you are not using Windows, you may not have access to the setup disk that the ISP provides. If you're not sure, ask the ISP's tech support staff, or other users.

To muddy the waters even more, some ISPs may be offering more than one kind of service (over and above the various bit rate plans). Example: Verizon (formerly Bell Atlantic) originally offered static IPs with a Bridged connection. Now all new installs use PPPoE with dynamic IPs. For installation and configuration purposes, this is very different.

The two most common DSL network implementations are Bridged/DHCP and PPPoX. Both have mechanisms for obtaining an IP address and other related networking configuration details so we shouldn't have to worry about this. But there are indeed other, less common, means of connecting. Our job will be finding the right client, and doing what we have to, to get it up and running. The most common ones are discussed below.

Important! You need to know beforehand how your ISP is setup for connecting to his network. To re-iterate, the two main possibilities are Bridged/DHCP and PPPoE. These are mutually exclusive implementations. And there are indeed other possibilities as well. So you will need to know exactly what this is beforehand. And it must be the right one or you will waste a lot of time and effort. You cannot choose which one either. It is a matter of how the ISP is doing his network. Note that PPPoE can run over Bridged networks, so just knowing whether you are Bridged or not, is not necessarily good enough. If your provider is giving you a router, there is a good chance that the router's firmware will handle all of this for you.

If you are subscribing with one of the Baby Bells in the U.S., you can probably count on that being PPPoE, and thus you will need a PPPoE client.

There are a few provider specific FAQs and HOWTOs in the Links section below.

3.1.1. Bridged/DHCP

In the good old days of a year or two ago, purely "Bridged" connections were the norm. This type of network puts you on a local subnet just like a big LAN. You are exposed to much of the local subnet traffic, especially ARP and broadcast traffic. The typical means of authenticating in this set up, is via DHCP.

DHCP is a standard, established networking protocol for obtaining an IP address and other important network parameters (e.g. nameservers). This is a standard, well documented networking scheme and is very easy to set up from the end user's perspective. It is also a very stable connection. You can actually unplug the modem for say 10 minutes, plug it back in, let it re-sync, and the connection is still there -- same IP and everything.

3.1.2. PPPoX

The main alternative now is PPPoX, meaning either PPPoE (PPP over Ethernet) or PPPoA (PPP over ATM). Both of these related protocols are currently being deployed, but at the moment, PPPoE seems to be the more common of the two. PPPoX is a relative newcomer, and, as the name implies, is a variation of Point-to-Point Protocol that has been adapted specifically for DSL networks.

There are several PPPoE clients for Linux (see below). At this moment, Linux PPPoA support is not an option, but could be available soon for some modems. PPPoX simulates a dialup type environment. The user is authenticated by user id and password which is passed to a RADIUS server, just like good ol' dialup PPP. A routable IP address, and other related information, is returned to the client. Of course, no actual dialing takes place. The mechanics of how this is handled, will vary from client to client, so best to RTFM closely. Typically you will set up configuration files like pap-secrets, etc.

It is worth noting that PPPoE will also work non-ethernet devices like USB, provided the correct drivers are installed.

From the ISPs perspective, PPP is much easier to maintain and troubleshoot. From the end user's perspective, it is often more work to set up, often uses more CPU, and the connection is maybe not as stable. So anyway, this seems to be the coming trend. Many of the large telcos, especially the RBOCs (Baby Bells), have committed to PPPoX already. Setting up a PPPoX connection is completely different from setting up a bridged/DHCP connection.

3.1.3. ATM

Since the traffic on the wire from the DSLAM to the modem is typically ATM, a raw ATM connection would seem to make sense. While possible, this is rare, if it exists at all in the U.S, and would require a modem in addition to a PCI ATM card, such as the Efficient Networks 3010. There is an ATM project for Linux, that is being incorporated into the 2.4 kernel. (See the Links section for more information.)

This may be a viable solution at some point, but it is just not "there" yet.

3.2. Configuring the WAN Interface

The most common configuration is a DSL modem in "bridging" mode. Both PPPoX and DHCP can use this setup. In this scenario, the WAN interface typically means your NIC. This is where your system meets the outside world. (If you have a router see below for router specific instructions.) So essentially we will be configuring the NIC, typically "eth0" since it is an ethernet interface.

With PPPoX, once the connection comes up, there will be a "ppp0", or similar, interface, just like dialup. This will become the WAN interface once the connection to the PPP server is up, but for configuration purposes we will we be concerned with "eth0" initially.

There are various ways an ISP may set up your IP connection:

  • Static IP.

  • Dynamic IP on Bridged Network via DHCP.

  • Dynamic IP via PPPoX.

  • Static IP via PPPoX.

Let's look at these individually.

3.2.1. Static IP Configuration

A "static" IP address is an IP that is guaranteed not to change. This is the preferred way to go for those wanting to host a domain or run some type of public server, but is not available from all ISPs. Note that while there are some noteworthy benefits to having a static IP, the disadvantage is that is more difficult to remain "invisible". It is harder to hide from those with malicious intentions. Skip this section if you do not have a static IP, or if you have a router, and the router will be assigned the static IP.

Configure the IP address, subnet mask, default gateway, and DNS server information as provided by the ISP. Each Linux Distribution (Redhat, Debian, Slackware, SuSE, etc.) has a different way of doing this, so check on your distro's docs on this. Each may have their own tools for this. Redhat has netcfg for example. You can also do this manually using the ifconfig and route commands. See the man pages on these or the Net HOWTO for more information and specifics. A quick command line example with bogus IPs:

 # ifconfig eth0 111.222.333.444 up netmask
 # route add default gw 111.222.333.1 dev eth0

Be sure to add the correct nameservers in /etc/resolv.conf.

3.2.2. Bridged/DHCP Configuration

ISPs that have Bridged networks typically use DHCP to assign an IP addresses, and authenticate the user. All distributions come with one or more DHCP clients. dhcpcd seems to be the most common. pump comes with Redhat based distributions as of Redhat 6.0. The DHCP client will obtain an IP "lease" from the ISP's server as well as other related information: gateway address, DNS servers, and network mask. The lease will be "renewed" at regular intervals according to the ISP's configuration.

You will want the DHCP client started on boot, so use your distribution's means of doing this. There generally is little to configure with DHCP as it is fairly straightforward and easy to use. You may need to tell it which interface to listen on if the NIC is something other than "eth0". You can also start it from the command line to get started. See the respective man pages for more.

Unless you have a static IP, the ISP will need some way to know who you are when you connect. There are two ways this authentication process is accomplished with DHCP. The first and most common method is via the MAC (or hardware) address of the network device. Typically this would be the NIC. The MAC address is a unique identifier and can be found among the boot messages, or with ifconfig, and looks something like 00:50:04:C2:19:BC. You will need to give the ISP the MAC address before your first connection.

The other DHCP authentication method is via an assigned hostname. In this case, the ISP will have provided you with this information. Your DHCP client will need to pass this information to the server in order for you to connect. Both dhcpcd and pump accept the "-h" command line option for this purpose. See the client's man page, or your distribution's documentation, for specifics.

Note: If your ISP uses MAC address authentication, and you change your network device (e.g. NIC), you will need to register the new address with the ISP or you won't be able to connect.

3.2.3. PPPoE Configuration

PPPoE (PPP over Ethernet) is an alternate way for ISPs to control your connection, and is becoming increasingly popular with ISPs. Setting this up is quite different, and may be a little more work than with static IPs or DHCP above. Recent distro releases are now shipping PPPoE clients. If this is not the case for you, then you will have to download one. Check any Linux archive site like http://freshmeat.net, etc. or look below.

Some of the current GPL PPPoE clients available:

  • The Roaring Penguin (rp-pppoe): http://www.roaringpenguin.com/pppoe/, by David F. Skoll. Reportedly very easy to set up, and get started with. This is a popular Linux PPPoE clients due to it's reputation for ease of installation, and is now being bundled with some distributions.

  • PPPoEd: http://www.davin.ottawa.on.ca/pppoe/ by Jamal Hadi Salim is another popular Linux client and is also bundled with some distros. This is a kernel based implementation for 2.2 kernels. A setup script is now included so no patching is required, making installation quick and easy. Also, less CPU intensive than user space alternatives like rp-pppoe.

  • PPPoE Redirector: http://www.ecf.toronto.edu/~stras/pppoe.html. This is a redirector which allows the use of PPPoE with pppd-2.3.7 or later. No recompiling of other system components are required. It is meant as an interim solution until the 2.4.x series, which will include kernel support of PPPoE/A. (Does not seem to be under active development at this time.)

  • 2.4.x kernels include native PPPoE support. The PPPoE for 2.4 page is http://www.math.uwaterloo.ca/~mostrows/ and is by Michal Ostrowski.

  • EnterNet is a non-GPL'd PPPoE client from NTS, http://www.nts.com, that is being distributed by some ISPs as the Linux client. It does come with source code but the it is not available for free download.

Depending on which client you have chosen, just follow the INSTALL instructions and other documentation included with that package (README, FAQ, etc.).

Once a PPPoE client connects, your connection should look something like the below example from Roaring Penguin, where "eth0" is connected to the modem:

$ route -n

Kernel IP routing table
Destination    Gateway      Genmask         Flags Metric Ref Use Iface  *   UH    0      0     0 eth1   *   UH    0      0     0 ppp0    *     U     0      0     0 eth1      *         U     0      0     0 lo
default         UG    0      0     0 ppp0

$ ifconfig
eth0    Link encap:Ethernet  HWaddr 00:A0:CC:33:74:EB
        RX packets:297581 errors:0 dropped:0 overruns:0 frame:0
        TX packets:266104 errors:1 dropped:0 overruns:0 carrier:2
        collisions:79 txqueuelen:100
        Interrupt:10 Base address:0x1300

eth1    Link encap:Ethernet  HWaddr 00:A0:CC:33:8E:84
        inet addr:  Bcast:  Mask:
        RX packets:608075 errors:0 dropped:0 overruns:0 frame:0
        TX packets:578065 errors:0 dropped:0 overruns:0 carrier:0
        collisions:105408 txqueuelen:100
        Interrupt:9 Base address:0x1200

lo      Link encap:Local Loopback
        inet addr:  Mask:
        UP LOOPBACK RUNNING  MTU:3924  Metric:1
        RX packets:1855 errors:0 dropped:0 overruns:0 frame:0
        TX packets:1855 errors:0 dropped:0 overruns:0 carrier:0
        collisions:0 txqueuelen:0

ppp0    Link encap:Point-to-Point Protocol
        inet addr:  P-t-P:  Mask:
        RX packets:297579 errors:0 dropped:0 overruns:0 frame:0
        TX packets:266102 errors:0 dropped:0 overruns:0 carrier:0
        collisions:0 txqueuelen:10

Note: For PPPoX, the correct setting for the ppp0 interface MTU is 1492. If the MTU is set too high, it may cause failure of some web pages to load properly, and possibly other annoying problems. You may need to also set the MTU for interfaces on any masqueraded LAN connections MTU to 1452.

Actually, for PPPoE the real setting should be at least 8 bytes less (the extra PPPoE protocol overhead) than any interface between you and the ultimate destination. All routers normally would be set to 1500, thus 1492 is correct from your end. But, it may happen that somewhere a router is misconfigured at a lower setting, and this can cause problems, especially with web pages loading. The way to test this is to keep dropping the MTU until things work.

3.2.4. PPPoA

PPPoA (PPP over ATM) is a cleaner solution than PPPoE since most of the work is done in hardware, and since the raw DSL traffic is ATM. There is no user space client necessary to manage the connection as with PPPoE, and the additional ethernet protocol layer is not required. Authentication is still the same: user id and password to connect, but the mechanics are different since no ethernet encapsulation takes place.

As of this moment, there is not really a viable, working implementation of PPPoA for Linux. There is an ATM patch for 2.2 kernels, support for ATM in the 2.4.x kernel, and a project based on the Efficient Networks 3010, as well as other ATM cards. The ATM on Linux homepage is here: http://lrcwww.epfl.ch/linux-atm/. And even more info is at http://www.sfgoth.com/~mitch/linux/atm/pppoatm/ from the kernel developer of this project. Existing PPPoA implementations are hardware/driver based, and Linux PPPoA modem drivers are scarce as hen's teeth at this time. The above modem does not seem to be available through normal retail channels. This may be a problem, if this is the only protocol an ISP delivers, and an external modem that supports PPPoA is not available.

If PPPoA is your ISP's only option, you might consider one of the router/modems that can handle PPPoA, and let the hardware handle everything.

3.2.5. PPTP/PPPoA with Alcatel Ethernet Modems

Alcatel SpeedTouch Home ethernet modems (supercedes the Alcatel 1000) support both bridged and PPPoA connections. The modem itself handles the PPPoA protocol internally. When in PPPoA mode (as opposed to RFC1483 bridging mode), Linux will connect to the modem via PPTP (MS VPN). The Linux PPTP homepage is http://cag.lcs.mit.edu/~cananian/Projects/PPTP/, and works well with this modem. In addition to installing pptp, your kernel must also have support for PPP.

The modem has internal configuration pages than can be reached by pointing a browser to the default IP address of (You will of course have to have your NIC set up for a network with similar IP such as, in order to reach the modem's configuration pages.) For PPPoA, the connection type is 'PPTP'. You will have to get the other settings from your provider if the defaults do not work. Settings such as 'VPI/VCI' and 'encapsulation' can vary from provider to provider. Of course, if the modem is coming from your provider, all this should be already configured.

The next step is to configure pptp, which is done by configuring the pppd files /etc/ppp/pap-secrets (or chap-secrets) and /etc/ppp/options. This is where the username and password is entered. For example:


# client secret server IP address 
login@isp.com  *    my_password_here    *


and /etc/ppp/options:

name "login@isp.com"


Once everything is configured properly, it should be just a matter of starting pptp, pointing it to the modem's address:


Note: Alcatel supplies many sub-models of these modems. These features may not be available on all models, or may be altered from the defaults. Something to be aware of, if buying a used modem.

This modem only supports one concurrent PPTP connection.

3.2.6. Modem/Router Configuration

Some ISPs are providing "routers" as the connection device. Essentially these are mini routers with built in modems. These are all ethernet based devices too, so Linux should be good to go here as well. Again, a compatible, working NIC should be all that is required to make this work.

A "router" has many advantages. The better ones can handle the connection management, IP encapsulation, and authentication, as well as providing a means of segregating your LAN from outside traffic, and possibly other features too. In short they can do it all. One big advantage is that they can handle whatever protocols your ISP requires in order to connect.

If the ISP is requiring PPPoX, then this makes life a little easier since you will not have to install or configure any additional software just to use their network. The modem's firmware will handle this. The downside is that most of these do not have the flexibility of a Linux router, or other software solution. Of course, you could set up a Linux router behind the router, and have the best of both worlds. The ones with more and better features are also going to cost significantly more.

While the physical installation of a router is very similar to the modem installation (see above), the router configuration itself is different since your first "hop" will be the router's interface and not the ISP's gateway. Routers will actually have two interfaces -- one that you connect to from the LAN side, and one that connects to your ISP on the WAN side. Your point of exposure here is the WAN interface of the router.

The router will also have a pre-configured, private IP address that you will connect to from the LAN side. This will be your gateway. The public IP address will be assigned to the WAN side interface. Typically these devices also act as DHCP servers for the LAN side as well. So possibly all you have to do is to start a DHCP client such as dhcpcd or pump (Redhat based distros) to get up and running. Just make sure the modem/router is syncing first. The appropriate steps and configuration should be in the owner's manual, or available from your provider.

If you are a PPPoX customer, and the router is handling this part of the connection, then you will have to configure at least your user id and password before connecting. If a Bridged/DHCP customer, you should just have to activate DHCP on the router, and possibly register the MAC (hardware address) of the router with your provider. Some routers have "MAC cloning" which means that they will report the MAC address of the attached NIC. If static IP, then you will have to configure this as well.

If you need to access the router directly, you will need to know the manufacturer's default setting for its IP address. See the owner's manual, or ask your provider. You will then have to set your NIC's interface to the same network as the router. For instance, if the router has an IP of, set your interface's address to (typically eth0), and netmask to

 # ifconfig eth0 up netmask
 # route add -net
 $ ping

If everything is in working order, the router should respond to pings. How to configure this permanently will vary from distro to distro. So check your distribution's documentation. Now you should be able to ping the modem/router, and, if all is well, beyond. Then use telnet or a web browser to do any further configuration of the router.

Even if the ISP is not offering any router options, there are quite a few available from third party manufacturers such as Netgear, Linksys, Cisco, Zyxel, Cayman, Alcatel and others. These will have all the features already mentioned and maybe more. Just make sure it matches your provider's DSL. This is one good way around the PPPoX bugaboo.


Some manufacturers may be marketing these as having "firewall" capabilities. In some cases, this amounts to nothing more than basic NAT (Network Address Translation or masquerading). Not a full, true firewall by any means. Be sure to read the fine print before buying and make sure you know how much real firewalling is included.

3.3. Connect

Everything should be in place now. You probably have already tested your connection. You should be seeing ping roundtrip times of 10-75 ms to the ISP's gateway. If something has gone wrong, and you cannot connect, either retrace the above steps, or see the Troubleshooting Section below.