Ubuntu – No Internet DNS Problem Ubuntu 16.04.03

Looks like an unusual one this...

Firstly I sincerely doubt that DNS is involved, though if you are using 8.8.8.8 et al for your DNS then DNS will fail because you can't reach those servers.

Firstly it can't be your LAN configuration because you can ping your local gateway, so traffic to and from it works.

The biggest hint is the double listing of your gateway in the trace. Your gateway should only be shown once, and this hints at a loop at your gateway - but all your other systems work fine, and according to what you write, it's only the local system that changed.

The only thing that I can remotely think of is if you had an iptables rule roughly like this:

iptables -t nat -D POSTROUTING ! -d 192.168.0.0/24 -j SNAT --to-source 1.2.3.4

Where 1.2.3.4 is some bogus or possibly previously valid IP address that no longer works.

This would let local traffic work but anything going out beyond the local network would break because replies would go to the wrong place. Your gateway may block traffic like that because it is in essence 'spoofed' traffic.

I think you would get a good hint to the issue with some traffic inspection at your gateway if it is at all possible.

If you can't do that - quite understandable - then maybe set up another Linux box on your network and make it the default gateway for this faulty system, and then you can inspect the traffic it is generating. This is assuming that the fault is on the upgraded system. If you configured that 2nd Linux box to ip_forward and make it's gateway the 192.168.0.1 device you might also see more useful info to help nail down the cause.

Will be interesting to see what it was when you finally get it sorted.

A work around to solve this is to temporarily add the domains you need to the /etc/hosts file.

That provided enough connectivity to sudo aptitude to resolve the broken dependencies on the system.

After the upgrade, Ubuntu rebooted normally and I could remove the static IPs from the hosts file again.

I used the following to capture the domains I needed to update and put them into a text file: sudo apt-get update >> domains.txt

I used the following to grab the IPs I needed:

sh domain-nslookup.sh >> apt-domains.txt

The following is the sh file I created and saved as domain-nslookup.sh:

#!/bin/sh
for DOM in `cat ./apt-domains.txt`;
do
    nslookup $DOM 8.8.8.8 |gawk -F": " '/Address/{print $2}'
    printf "\t%-4s" $DOM
done
printf "\n"

After verifying that it worked as planned, I added the IPs and domains to the hosts file:

sudo sh domain-nslookup.sh >> hosts