[PLUG] Mysteries of DNS

[Mike C.]

On 8/24/17 8:40 AM, plug-request at lists.pdxlinux.org wrote:
> Folks,
> I thought I had my Mikrotik router setup right, indeed it was doing just
> fine, I did
> an upgrade on it and all of a sudden my linux boxes no longer keep the DNS
> When I first reboot the router it appears to tell everyone about DNS, I can
> set
> for a while on my Linux boxen and go everywhere, then all of a sudden each
> one of them no longer resolves, interesting thing is the Android devices DO
> continue to resolve just fine.
>
> Anyone seen that one. I am running Mate on Ubuntu 16.04 all machines are up
> to date as far as I know.
Here's how I'd go about troubleshooting this.

When your Linux boxen can't get dns name resolution do the following.
1. Go to cli and run the command "cat /etc/resolv.conf"
e.g.
cat /etc/resolv.conf
# Generated by NetworkManager
nameserver 8.8.8.8
nameserver 8.8.4.4

2. Is the nameserver ip addr the same ip addr of the microtik router? If 
no, there's most likely a config problem.

3. If yes, ping the ip addr of the nameserver

4. If it doesn't respond, there's a network communication issue between 
the two devices.

5. If it does respond, run the command, "nslookup google.com". Does the 
command provide a non-authoritative answer?
e.g.
nslookup google.com
Server:        8.8.8.8
Address:    8.8.8.8#53

Non-authoritative answer:
Name:    google.com
Address: 216.58.193.78

6. If no, login in to the Microtik and run the same command.

7. If the Microtik replies with a non-authoritative answer I'd take a 
look at this thread about dnsmasq.
https://askubuntu.com/questions/131342/dnsmasq-sometimes-resolves-things-sometimes-it-doesnt

That should be a good start in running down this problem.