Troubleshooting: Broadband Linux Connectivity
by Mark Rais, from reallylinux.com
In this new beginner guide, Rais shares some quick tips for overcoming some common broadband connectivity hurdles. This article should be read in conjunction with our Linux DSL and Internet connectivity articles.
article is written for beginners who are having difficulty with their
Here are a few tips that have reconciled nearly all problems I have personally experienced with broadband connectivity.
Almost every flavour of Linux now comes with a preinstalled and an automatically enabled firewall. In most cases, the interfaces to the firewall configurations may vary, but the underlying configuration for IPTables is the same on most major flavours.
There should not ordinarily be any issue running basic IPTables (firewall) configurations on Linux systems hooked to broadband modems. In fact, although many times tech support staff or even documents on the internet say to “check that your firewall is not blocking your broadband connectivity” this is a rare issue for Linux. Of the thirteen Linux versions I’ve recently used, I only encountered two related issues.
Security Enabled Linux may be the culprit of some connectivity issues. Now it is important to note that SELinux is a more robust but often temperamental security parameter found automatically enabled from installation on certain flavours including Fedora, Centos and a few others. The main reason SELinux causes issues is because it's default setting blocks pretty much every primary socket connection. In other words, trying to perform any http related requests hang.
Although I would never recommend disabling your primary firewall, it may be okay to temporarily disable SELinux (if your flavour uses it) and see whether that solves your connectivity issue. If it does, then you can fine tune the configuration.
On very few flavours of Linux, the IPTables basically block all Ethernet connectivity or close all available ports. Without ensuring that some ports are enabled, you are obviously going to get connectivity failures through your Ethernet connection.
This issue is rare, since most recent Linux flavours properly preset the firewall to allow basic Internet connectivity. But it is wise to verify that the firewall settings on your flavour are not set to block all. Please do not make the other extreme error I’ve seen some people make, as they react by enabling or setting as “trusted” every Ethernet (eth0) connection. In some flavours, such as those based on RedHat, there is an option to set the entire Ethernet (every port/connection) as trusted, which is only useful for situations when you are not connecting to the Internet.
In almost every instance where my Linux broadband connectivity failed, the problem was traced down to the local telco exchange or the configuration of the cable box for the neighbourhood.
The problem is not so much how to configure or test this with Linux, as it is in dealing with staff from the related company and helping them see that it is likely an exchange issue.
Here are a few tips when dealing with them.
Do not speak about your system configurations or that you are using Linux. They tend to be following a “cheat sheet” for their support and you need to make sure you quickly move past the idiotic questions like “have you tried to turn your modem off and then back on to reset?” I suppose there may be plenty of people that do such things, but rarely are they Linux users!
Ensure that they understand that you had proper connectivity until just recently. Ask if they have had any recent outages or network anomalies in your area recently or if they are doing any repair work currently?
They will try to check the line. This is a simple “line noise” test where they send a signal down the pipe to your house/office. This does very little to actually diagnose a problem. Obviously the line noise test will show if you are not properly using the filter on your phone connections (those little boxes that MUST be connected to all phones and connections to all of your wall jacks for DSL use). The noise can also come if you have a bad cable or a problem with cabling outside your location. But in most cases, the line noise test shows nothing. They will then get very confused since the problem is obviously more complex than their set of quick notes.
Configuring the modem tips
One of the key items is to ensure that your modem has properly been configured. For one reason or another the DSL modems tend to “lose” their authentication details after some use with Linux systems. This can come from a variety of reasons that are not necessary to describe right now. Please try to check authentication (that the password and username you were given by the provider) are still enabled on the mode. You can check this by actually going to the broadband modem’s internal configuration usually with something like 10.1.1.1, then typing the administration name and password. In most cases this is just admin and admin. You would usually see a basic menu for configuring and fixing mode errors. Under one of the options, usually WAN (wide area network) you will find the User name and password. If it has the proper information filled in to the box then your modem’s authentication is still in tact and not an issue. If not, then you may need to run the WIZARD or manually fill in and Apply the username and password. Notice that in most cases the username is more than just your login, but includes the full email address. In some cases it may also require a specification. For instance for Telecom ADSL mode use, the username is: firstname.lastname@example.org.
There is also a Status OPTION. This will show whether the mode itself is making a proper connection to the substation (telco exchange or cable switch). You should see the status as Connected.
If it is not, then you may manually need to press a Connect button or reset your modem and see if status changes.
An issue I’ve seen that almost always verifies that a problem is at the exchange has to do with the modem resetting itself regularly. You can watch this happen sometimes. You start Linux, begin browsing and then all of a sudden everything slows down to a halt.
Observe the modem lights. If the ADSL light or connect light goes off, then after a while starts to blink it means your connection is being dropped. If you monitor the length of time it takes from when that light comes back on (not blinking) until it goes off you can identify the total in seconds that connections are being reset. If the timing is roughly consistent (I’ve noted that DSL exchange issues can actually occur at precise line fails such as every 72 seconds) you know it’s not a modem problem but an exchange problem and need to contact your ISP and tell them the exchange is dropping your connection. They may try to send you another modem (because there is a tendency not to admit switching devices are at fault), but it is likely the problem gets resolved once escalated to their technicians.
If however, the connectivity varies greatly (more than several minutes of variance between each restart) it is more likely to be a problem with your modem than an exchange. One easy method for checking this is to test another modem on the same line connection. Borrowing one from a friend and testing whether the modem is faulty is a quick method to check dropped connection issues.
Hopefully these tips enable you to get past any initial hurdles for setting up and enjoying your Linux broadband connectivity.