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Intermittent loss of DNS (from more than one location)

harry
Dabbler
Posts: 17
Registered: 17-09-2010

Intermittent loss of DNS (from more than one location)

I've noticed a couple of times that although I still have some some internet connectivity I cannot ping the DNS servers or get any response from them, and therefore cannot connect to sites.
Here is an example, probably taken between 50 and 60 minutes ago.

Microsoft Windows [Version 6.1.7601]
Copyright (c) 2009 Microsoft Corporation.  All rights reserved.
C:\Users\Harry>tracert -d 11.11.11.11
Tracing route to 11.11.11.11 over a maximum of 30 hops
  1    28 ms    99 ms    99 ms  192.168.1.254
  2    13 ms    13 ms    13 ms  195.166.128.182
  3  ^C
C:\Users\Harry>tracert -d bbc.co.uk
Unable to resolve target system name bbc.co.uk.
C:\Users\Harry>tracert -d 212.159.6.9
Tracing route to 212.159.6.9 over a maximum of 30 hops
  1    94 ms    99 ms    99 ms  192.168.1.254
  2    13 ms    14 ms    14 ms  195.166.128.182
  3    *        *    ^C
C:\Users\Harry>nslookup
Default Server:  dsldevice.lan
Address:  192.168.1.254
> server        212.159.6.9
DNS request timed out.
    timeout was 2 seconds.
Default Server:  [212.159.6.9]
Address:  212.159.6.9
> bbc.co.uk
Server:  [212.159.6.9]
Address:  212.159.6.9
DNS request timed out.
    timeout was 2 seconds.
DNS request timed out.
    timeout was 2 seconds.
DNS request timed out.
    timeout was 2 seconds.
DNS request timed out.
    timeout was 2 seconds.
*** Request to [212.159.6.9] timed-out
>
C:\Users\Harry>ping -t  212.159.6.9



Here's what it looks like when working:

Tracing route to cdns01.plus.net [212.159.6.9]
over a maximum of 30 hops:
  1    52 ms    99 ms    99 ms  dsldevice.lan [192.168.1.254]
  2    13 ms    12 ms    13 ms  lo0-central10.pcl-ag07.plus.net [195.166.128.188
]
  3    12 ms    12 ms    12 ms  84.93.249.204.broadband.plus.dyn.plus.net [84.93
.249.204]
  4    13 ms    12 ms    11 ms  xe-2-2-0.pcl-cr01.plus.net [212.159.1.12]
  5    12 ms    12 ms    12 ms  po2.pcl-gw01.plus.net [195.166.129.41]
  6    12 ms    11 ms    12 ms  vl63.pcl-lb01.plus.net [212.159.2.252]
  7    12 ms    12 ms    11 ms  cdns01.plus.net [212.159.6.9]

So, looks like the network dropped out somewhere between lo0-central10.pcl-ag07.plus.net and broadband.plus.dyn.plus.net on this occasion, though I haven't kept logs for the previous occasions.
I'm not alone. I'm on 020 8805 exchange. My parents (on 020 8524 exchange) were getting something very similar when I looked at their machine on Sunday

Why are DNS1 and DNS2 consecutive net addresses?
By my understanding, DNS2 should be in a vastly different subnet from DNS1, so that it can take over if a network disruption makes the first DNS unreachable. And perhaps, as with many other ISPs, there needs to be more than just two of them ?
5 REPLIES
Community Veteran
Posts: 4,862
Thanks: 126
Fixes: 24
Registered: 14-07-2009

Re: Intermittent loss of DNS (from more than one location)

You are not obliged to use Plusnet's own DNS servers.  You could try:
Google: 8.8.8.8 , 8.8.4.4
OpemDNS: 208.67.222.222 , 208.67.220.220
and there are many many more.
Community Gaffer
Community Gaffer
Posts: 13,321
Thanks: 1,106
Fixes: 89
Registered: 04-04-2007

Re: Intermittent loss of DNS (from more than one location)

There's murmurings in the support centre that the odd customer is still calling to report data transfer problems. I'm not convinced it's anything to do with DNS mind you. It's more likely related to a particular edge router, traffic management switch or customer IP range. We're collecting examples at present in order to try and ascertain some sort of pattern.
Quote from: harry
Why are DNS1 and DNS2 consecutive net addresses? By my understanding, DNS2 should be in a vastly different subnet from DNS1, so that it can take over if a network disruption makes the first DNS unreachable. And perhaps, as with many other ISPs, there needs to be more than just two of them ?

They're virtual IP addresses which sit in front of eight physical boxes spread across multiple data centres.

Bob Pullen
Plusnet Product Team
If I've been helpful then please give thanks ⤵

harry
Dabbler
Posts: 17
Registered: 17-09-2010

Re: Intermittent loss of DNS (from more than one location)

Quote
eight physical boxes spread across multiple data centres.

That sounds good, but then I do tracert to each DNS ...
Tracing route to cdns01.plus.net [212.159.6.9]
over a maximum of 30 hops:
  1    48 ms    98 ms    98 ms  dsldevice.lan [192.168.1.254]
  2    12 ms    12 ms    13 ms  lo0-central10.pcl-ag02.plus.net [195.166.128.183
]
  3    12 ms    12 ms    12 ms  84.93.249.48.broadband.plus.dyn.plus.net [84.93.
249.48]
  4    12 ms    12 ms    12 ms  xe-0-2-0.pcl-cr01.plus.net [212.159.1.0]
  5    11 ms    12 ms    12 ms  po2.pcl-gw01.plus.net [195.166.129.41]
  6    13 ms    12 ms    13 ms  vl63.pcl-lb01.plus.net [212.159.2.252]
  7    12 ms    12 ms    13 ms  cdns01.plus.net [212.159.6.9]
Tracing route to cdns02.plus.net [212.159.6.10]
over a maximum of 30 hops:
  1    51 ms    99 ms    98 ms  dsldevice.lan [192.168.1.254]
  2    12 ms    12 ms    12 ms  lo0-central10.pcl-ag02.plus.net [195.166.128.183
]
  3    11 ms    12 ms    12 ms  84.93.249.40.broadband.plus.dyn.plus.net [84.93.
249.40]
  4    11 ms    11 ms    12 ms  xe-0-2-0.pcl-cr01.plus.net [212.159.1.0]
  5    99 ms    79 ms    12 ms  po2.pcl-gw01.plus.net [195.166.129.41]
  6    13 ms    12 ms    13 ms  vl63.pcl-lb01.plus.net [212.159.2.252]
  7    13 ms    12 ms    12 ms  cdns02.plus.net [212.159.6.10]

It looks to me as if there are still 6 out of 7 possible common failure points between me and both of the DNS servers.
Of course, it could possibly use a different route when one of the above is down -- but as yesterdays trace shows, it doesn't always use any route at all.
harry
Dabbler
Posts: 17
Registered: 17-09-2010

Re: Intermittent loss of DNS (from more than one location)

Quote from: ReedRichards
You are not obliged to use Plusnet's own DNS servers

Agreed. I used to have OpenDNS appended at one time, but haven't previously needed it with plusnet. Problem there is you have to decide to do it before the DNS goes down (or, at least, have to keep a local copy of the openDNS addresses).
I'm less to add google's. Do we really need google keeping a log of every DNS access we do, especially as google makes it clear these days that privacy isn't one of google's main attributes.
jim:green quote fixed mod:end
Highlighted
Community Gaffer
Community Gaffer
Posts: 13,321
Thanks: 1,106
Fixes: 89
Registered: 04-04-2007

Re: Intermittent loss of DNS (from more than one location)

Quote from: harry
It looks to me as if there are still 6 out of 7 possible common failure points between me and both of the DNS servers.

It's not quite as simple as that. OSPF is used to route your traffic to the closest caching DNS server. This tends to be based on the edge router your connection is terminated on. If one route is broken another would be taken. The two sets of DNS servers are located in London data centres that are miles from each other.
Regarding the data transfer problem itself, we've identified some overlapping IPs that would have led to the symptoms you describe (ref: 72177). We've fixed those conflicts and are currently carrying out an audit to make sure there aren't any more. Let me know if the problem resurfaces ...

Bob Pullen
Plusnet Product Team
If I've been helpful then please give thanks ⤵