How do I connect to the Internet?
By putting a network cable into the DSL router, well, most of the time, yes. But sometimes my laptop needs to establish the connection to the Internet using a wireless data modem. Obviously this will change a few things on the machine that are not entirely clear to someone starting on this track for the first time.
In this article we will look behind the curtain to understand the process of dialling into the Internet via a wireless modem.
Let's begin with the basic differences. In using the modem, we have decided that the laptop itself will become part of the Internet, while it was only part of the local area network up to now. First of all, that would require to run a firewall on the laptop if you did not already have set up a firewall before. Protecting your machine while connected to the Internet is indispensable, and you have to make sure, that your firewall is not disabled entirely. Check your firewall rules with the following command:
target prot opt source destination
Chain FORWARD (policy DROP)
target prot opt source destination
Chain OUTPUT (policy ACCEPT)
target prot opt source destination
The output above shows, that there is no protection and you have to solve this problem first. Anyway, firewall configuration will be covered in a separate post.
Where do all these packets go?
As long as your machine had been using the router an IP number belonging to the router was listed in the laptop's routing table. More precisely, the ethernet adaptor would send every data packet destined for the Internet to the machine with this default address, and for the laptop to be able to establish its own connection, the old default route to the DSL router has to be removed from the network setting. Nowadays we simply click through the network dialog and delete the "default gateway address" there. But if you like to know where to check that setting, please examine the file "ifcfg-eth0" in a directory called "/etc/sysconfig/network-scripts", there should be no GATEWAY line.
It is one of the tasks performed by a process called "pppd" to set up a new default route when the connection to the Internet is successfully installed using the modem and a new network interface "ppp0". Getting the Point-To-Point-Protocol-Daemon (pppd) up and running will be the most important thing to get right, but then, fortunately it is the only thing to do. There was no need for a pppd as long as we had the router, but now this process working in the background is responsible for shovelling all packets back and forth into the Internet for us.
Chatting up the PPP Daemon
Given that everything is well prepared, starting the pppd process is as easy as the following
As I am using a THREE data modem there are two files that determine the process of dialling into the Internet, an options file and the chat-script. The last parameter the pppd process depends on is the device file for the hardware used. In most cases running the pppd fails because this special file that represents the wireless modem does not exist. Look around if your Linux system lists the device file, and if not, we cannot proceed unless the hardware is actually accessible through the kernel. The kernel module that is responsible to make the wireless modem visible to the kernel is called "usbserial" and it has to be used with two pieces of information, the vendor-id and the product-id, which are different for every modem you use. For my wireless modem I have to use this command.
You have to check your actual hardware and find out which values are correct for your hardware, using the "lsusb" command for further information.
Now that the hardware is available through the device file "/dev/ttyUSB0" we can define a few options the pppd process should honour while starting up.
noipdefault
defaultroute
persist
noauth
updetach
novj
novjccomp
nopcomp
holdoff 5
Apart from requesting the desired baud rate the process tries to get a remote IP address from the ISP's login server (noipdefault), does not require the ISP's end point to authenticate itself (noauth) and establishes the default route in the routing table after setting up the connection.
During this process the pppd process exchanges some information with the ISP's login server on the Internet to get the required IP address. The details of this conversation are lay down in the chat-script.
ABORT VOICE
ABORT "NO CARRIER"
ABORT "NO DIALTONE"
ABORT "NO DIAL TONE"
"" ATZ
OK ATE0V1&D2&C1S0=0+IFC=2,2
OK AT+CGDCONT=1,"IP","3ireland.ie"
OK ATDT*99#
CONNECT ""
This file lists two corresponding pieces of information each line. While reading the modem input the process waits for the appearance of the first part being sent by the server, responding with the second part which is, in most cases of course, an old-fashioned modem AT command. You can watch this conversation between the modem and the ISP's access machine in detail if you display the tail of the logfile "/var/log/messages".
Finally, when the remote IP address is obtained, the interface "ppp0" can be used.
Get a decent Name Service - And Enjoy!
Of course you should be able to surf the web using the new network connection as the default route should be created by the now running pppd process. If you still don't get your Internet applications running there is a last hurdle to overcome. The domain name service (or DNS for short) is always been used by our applications. No matter what kind of program you use, the first step is always the substitution of a domain name by the correct IP address. Because of the fact that this substitution has to be done many times, one or more name servers must be in easy reach for your laptop. I have found some nameservers that work for me and are reliable and fast, but depending on your location, different ones may be better. Try to find the ones that perform best for you and finally record their IP addresses in the file "/etc/resolv.conf" to make sure, that your internet connection is performing well.
Now, this is how I connect to the Internet! The following excerpt is taken from the usual suspect file "/var/log/messages"
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