DNS problem or finger trouble!

[#1]

I have spent the day trying to figure out why pages take so long to load at the moment.  I have traced it to my local DNS server not getting any answers from the upstream servers (which are google servers, 8.8.8.8 and 8.8.4.4). My secondary DNS server is a plusnet one and presumable the pages are loading when my laptop finally gets fed up with waiting for my local DNS server and asks the plusnet one. So as an experiment, I tried using google and opendns directly from my laptop using the following commands:-

dig @208.67.220.220 bbc.co.uk

and

dig @8.8.8.8 bbc.co.uk

Both timed out. So I tried :-

dig @212.159.13.49 bbc.co.uk

Instant answer - thats a plusnet dns server selected.

My question is - Whats going on? Why does plusnet deliver an answer when opendns and google do not. Have I missed an announcement or is it finger trouble?  It has to be something stupid or the forums would be in turmoil!

[#2]

No idea but you'd be best off downloading a dns test client that gives you access to the raw packet data that is sent and received. That way you'll be able to see whats going on.

It's possible (though highly unlikely) that the google servers are deliberately not replying to you. It's also possible that your udp broadcasts are not making it out to google.

Have you changed your network setup / configuration recently? - eg changing the internal lan ip of your router for instance?

[#3]

I have got wireshark but trying to see the data leaving and entering the house is a major task in itself. The logs from the local dns clearly report the requests being sent and do not report any response though previously they would have done so. Pings to the relevant DNS addresses work.

No, nothing on my home system has changed recently to my knowledge. Software updates might be a problem but I don't allow many.

I have downloaded an app to my phone that is DNS lookup. If I use the house wifi (so that the request goes to Plusnet) then it works as described above, that is, it gives a response from Plusnet DNS but not from OpenDNS/GoogleDNS. If I switch it to use the mobile signal, then both work perfectly.

Therefore either Plusnet are censoring port 53 requests and only allowing their own servers to be used - which seems unlikely - or some part of my router/modem are doing the same thing - which seems equally unlikely.

I will try some reboots tomorrow but it's too late tonight to risk the chaos that I could end up with by doing that and if it then turned out to be Plusnet, I will be fairly fed-up (ie livid)

[#4]

Have you checked the Plusnet Firewall settings? (login required)

[#5]

Have you checked /etc/resolv.conf for nameserver entries?

Are you running a DNS cache?

[#6]

Thank you all. The problem has solved itself. By way of explanation and commentary:-

Resolv.conf listed the local dns server first and then a plusnet server as backup -  so said what I would have expected.

I run dnsmasq on a local raspberry pi as a general cache for all on my internal network. Dnsmasq was set to use google servers as its upstream servers so stopped working (limited to old, non-time expired entries)  when the google servers became unavailable. My laptop was accessing that cache first, finding it unhelpful so timing out and then using the plusnet server. This meant that the first page of each new URL took 10-15 seconds to access while the timeout occurred. That was the problem I was initially trying to solve and that led me to the discovery that google dns and opendns were not responding to me.

Plusnet firewall was set to on but low. So it blocks incoming ports on various low numbered ports including 53 - but incoming not outgoing.

As an experiment, I reset Plusnet firewall to off and that forced me to disconnect and reconnect the ppp session. Suddenly everything worked, google and opendns were working. However, I subsequently reset Plusnet firewall back to low and disconnected and reconnected and everything still works.

I conclude that it was a Plusnet special feature, cured by the disconnect/reconnect.