How to troubleshoot networking
 

I wasted about 24 hours trying to implement a wireless network. Actually, it wasn't really wasted as I learned more about Linux than I ever knew.

Turned out that to enable wlan0, I first had to turn eth0 off.

That being said, in hindsight, what would have been the best way to troubleshoot why a Linux box isn't communicating on a network? What questions should I have asked on this forum to allow others to provide help?

• should-i-use-networkmanager-and-if-so-what-are-the-implications-for-wireless/
• where-is-ifcfg-wlan0/
• desparately-trying-to-set-up-wifi-network/
• super-simple-unencrypted-wireless-network

Would have been more specific if all simple debugging information was gathered, analyzed, and from the finding make a specified post title. Using 'dmesg' or 'message.log' or everything available at /var/log folder.

Well anyway, post title is a matter of taste. This is merely taken from my simple experience.

Good luck.

Thanks malemustaq,

It appears that I don't have a message.log. With Centos, would /var/message/messages be the equivalent? I looked through it, and it did not appear to indicate any issues.

I also ran "dmesg" as well as looked at /var/log/dmesg, and did not see any clues, but only when I connected a mouse or USB or the like.

As for as "everything available at /var/log", any specific logs that would be most telling?

Its a huge subject. As a standard practice, you should troubleshoot according to OSI model. Most of the problem will occur in Application-Layer, and this layer is the hardest to troubleshoot. It all depends on box-to-box and the packages that are installed on it. You should have the knowledge on how to setup network in the chosen distro first.

But before starting to hunt for problem, you should first make sure rest of the OSI layers are trouble-free. I will just give you quick rundown of step. Google is your buddy.

1 Physical Layer

Check Network card is detected:

Ensure physical address in config file is what this says:

2 Data-Link Layer

Check for arp broadcast. The IP address of at least one system should be listed. If there are no systems listed, there is a problem at the physical layer (above):

Check for dropped packets. ERR, DRP and OVR should be zero:

The default Message Transmit Unit (MTU) setting is 1500. If the MTU is set to something other than 1500, the network may run slowly. To set the MTU to a default of 1500:

3 Network Layer

Determine if the default gateway is correct:

Use ping to determine communication:

4 Transport Layer

Track the route packets taken from your machine on their way to a given host and find the miscreant gateway/hops/routers that are discarding your packets (if there are any):

5 Session Layer & 6 Presentation Layer
Issues rarely occur at the session and presentation layers.

7 Application Layer
The application layer is where the client-server issues fall. This includes SMTP, POP3, HTTP, FTP, etc. As i have already told this topic is huge and start from how to setup network. Google for everything... ;).

Thank you mddesai,

This is going to take me a bit to digest before I have any intelligent followup questions to ask :)