Networking debuging [closed]

Question

How to systematically find culprit behind loss of internet in linux?

I do know

that to to download files/access webpage/use ssh/etc , there are several layers of communication made: application,transport,internet,link ( wikipedia )

I don't know

• How each of these layers (excluding application layer) map to programs/components in linux?
• How to check each of these components efficiently - if they are working correctly?
• In which order to check them

My goal is

to have (reasonably) simple system for tracking down the problem. In pseudo code it would look something like

 for(component in A..Z) // from hardwarefailure,missing drivers to configuration in /etc/network/interfaces
 {
     if  cmd_to_check(component).output == _OK_
     then continue // problem is 100% not here, continue with next item
     else break    // problem is 100% with current component
 }
 do_adhoc_reseach_on(item)
 fix(item)

I want to focus here on problems which happens on MY machine, not problems with remote or intermediary servers

Notes

If there are differences

• between distributions, debian is preferred
• between wire vs wireless then wire is preferred
• between any other aspect, then most standard is preferred

If available free resources would be preferred

Thanks in advance for any answers and/or references

Answer 1

Steps for testing the Internet connectivity:

• test if the Ethernet links is up and has the expected negotiated speed and duplex: ethtool eth0
• test if the next hop (neighbor router) is available: ping -c 3 next_hop_ip
• test if a well known remote site is available (e.g. ping the google DNS): ping -c 3 8.8.8.8
• test if the DNS is working: host www.google.com
• test if a website is answering: wget -O - http://www.google.com
• if the workstations are directly connected you should be able to see them with arp even if a firewall is configured on them: sudo arping workstation_ip
• test if workstations are reachable from the server: ping workstation_ip

When you fail one step, try to find what is happening before going to next one.