DNS servers not always connecting, most webpages don't load

[#1]

Hi there.
I'm having two seemingly related problems and it seems the CSC doesn't understand (ticket no. 29464151).
It started on Wednesday (22/7) evening around 11pm. Just before it started, I noticed I couldn't access the bbc.co.uk sites.  So I restarted the router.  Then I couldn't access any websites or use the internet at all.  My router (a Thomson ST 546) reported that the connection to the Gateway was still up, but there was no connection to the two DNS servers (212.159.6.9 and 10).  I tried turning it off for over half an hour and then back on again, to no avail.
Using my iPhone on a slow Edge connection, I went through the Broadband Fault Checker on Thursday around 1pm.  Connecting with the BT test  settings and viewing the test page worked fine.  An automated ticket shows:

Quote

Broadband Fault Checker - DSL Fault [ Completed ] All questions have been completed by the user. Results confirm DSL fault.

It then says it was escalated to the correct team.
It took until 8am the following morning to get a response from a CSC agent, who said that they couldn't see anything wrong, and to try doing a factory reset. 
Once I'd done that, the router came back up, reporting that there was a connection to both DNS servers.  However I could only browse to a few websites.  Most came back with the error "Safari can’t open the page “http://www.bbc.co.uk/” because the server where this page is located isn’t responding." (with the relevant site mentioned, including the bbc one.)  Of the sites that did load, several only loaded partially (e.g. thehungersite.com - the images didn't load, which I noticed in the Activity Window are downloaded from a different domain) and even the few sites that seemed to load entirely (e.g. Google related sites, and Plusnet sites) I noticed in the Activity Window that one element had not loaded and this element was coming from a different domain.
Now on Friday, I reported this, and got a response later that day (thank goodness) suggesting I change the MTU setting on the router to one of various values.
My responses to this are below, but I feel I'm not getting understood, and my problem(s) aren't being dealt with.  Any explanations for what the CSC agent is getting at, or for what might be going on and what I should do, would be gratefully received.
I responded with:

Quote

I've tried a couple of your different MTU values, and each time rebooted my router. This has made no improvement. The default MTU value was set at 1500, so please would you explain why a lower value would help. Each time I rebooted after making an MTU change, I noticed that my router reported a problem with connecting to the DNS servers. Sometimes it hadn't connected to the first DNS server (212.159.6.9), sometimes it hadn't connected to the second DNS server (212.159.6.10), and sometimes it hadn't connected to both DNS servers. This seems to be the same problem as I originally reported, with the added extra problem that when it does connect to one or both DNS servers, it doesn't manage to load some websites. Please don't simply suggest I check my equipment - please explain why. This problem has arisen without anything changing at my end and I find it very difficult to believe it is something wrong with my equipment. Especially given the reports of DNS problems at your end on the community site forums. Looking at the Activity Window for the webpages that only load partially, the elements that don't load are always from a different domain to the elements that do load. And everything from the different domain does NOT load, and everything from the starting domain DOES load. This includes the Plusnet Community Site forums, where everything loads from community.plus.net and google-analytics.com, but the one element from webstats.plus.net does not load. Having been trying to browse the web some more, it seems that mostly everything from Google related domains and most Plusnet related domains loads, but elements of pages and whole sites from almost all other domains don't load. This seems to support my theory that the problems I am experiencing are something to do with your DNS servers.

[#2]

First check - open a Dos Command Window and type ipconfig /all (note the gap before the /)
And report back this bit of it
  Default Gateway . . . . . . . . . : 192.168.0.1
  DHCP Server . . . . . . . . . . . : 192.168.0.1
  DHCPv6 IAID . . . . . . . . . . . : 184556109
  DHCPv6 Client DUID. . . . . . . . : 00-01-00-01-0F-2A-81-41-00-1A-4D-59-68-29
  DNS Servers . . . . . . . . . . . : 208.67.222.222
                                      208.67.220.220
  NetBIOS over Tcpip. . . . . . . . : Enabled

[#3]

My computer is an Apple MacBook running OS X 10.5, so I can't open a DOS command window!
The equivalent in OS X that I know of (going into System Profiler, and clicking "Network"), only reports the IP address of the router as the DNS servers.
I've tried manually putting in all four Plusnet DNS servers into my MacBook, with no change.
And I've tried adding the extra two DNS servers into my router, with no change.

[#4]

Can you go into Apps > Utilities > Terminal
In the terminal can you type the following command

cat /etc/resolv.conf

and paste the results back here

[#5]

cat /etc/resolv.conf

domain lan
nameserver 192.168.1.254

As I said, the MacBook utilities I know of have only ever reported the IP address of the router as the DNS server(s).  I tried manually adding the Plusnet DNS servers to the MacBook again, and "cat /etc/resolv.conf" then gives those servers too.
BTW, my router still has all four Plusnet DNS servers listed - two that it finds automatically (the ones ending 6.9 and 6.10), and two which I added manually.
In case it's relevant, and so you know, my MacBook is connected wirelessly to an Apple Airport Express which is plugged into my router (a Thomson ST 546).  I've tried turning the Airport Express off and back on again too, and it seems that there's nothing wrong with it - it's doing its job of passing everything through in both directions.

[#6]

In the router replace two of the Plusnet ones with these which are the OpenDNS ones and see if it makes any difference
208.67.222.222
208.67.220.220

[#7]

Odd when I do cat /etc/resolv.com I get

adie$ cat /etc/resolv.conf
nameserver 208.67.222.222
nameserver 212.159.13.49
nameserver 212.159.13.50
nameserver 208.67.220.220

[#8]

I've added the openDNS addresses given to the router, but nothing has changed.
I suppose I'm doing it right?  There isn't a way to manually specify DNS servers in the router's web interface, so I login to it with telnet.
I then issue this command:
dns server route list
to get this list:

DNS Server Entries:
  DNS Server     Source              Metric  Intf     State  Domain            
D 212.159.6.10                         10    Internet  UP      *             
D 212.159.6.9                          10    Internet  UP      *             
S 208.67.222.222                       10    Internet  UP      *             
S 208.67.220.220                       10    Internet  UP      *             

I'd used the following command:
dns server route add
to add the openDNS servers, and specified the Metric 10 and Intf Internet parts, as they were the same as the DNS servers the router had automatically found.

[#9]

Just to make sure, I've plugged my MacBook in to the ST546 router directly, and turned off Airport on my MacBook.  The problems remain the same, so it's not the wireless network causing the problems.
Also I notice that cat /etc/resolv.conf still only lists the router's IP address, even when using a direct wired ethernet connection.
I'm trying to borrow an alternate router.  Any other ideas?  It still seems to me more likely that it's something at Plusnet's end that's wrong.  And it's frustrating that the CSC responses to my ticket have been so slow, and seem not to try to understand my problems nor explain their suggestions, and merely give stock responses. 
Four days without useable internet connection 
It's amazing how much it affects my life.  Being ill, living alone, and at home for long periods means I'm even more dependent on it.

[#10]

By chance I was browsing my router log and I am getting frequent DNS Failures with the message:
Time (since reboot) Facility Severity Message
7th day 12:30:56 user warn dnsprobe[247]: dns query failed 
My router is set up to automatically source the DNS and at the last reboot it got:
Primary DNS Server: 212.159.6.10
Secondary DNS Server: 212.159.6.9
The failures are coming every few seconds - is this the same problem as others are seeing in this thread?

[#11]

@paxy I don't know if yours is the same problem as mine, sorry.  But I would interpret the message you quote as: your router was asked (by a computer on the network) to retrieve a website, so it queried the DNS servers (the ones it finds automatically, which are the Plusnet ones) to find out where to get the website from, and this particular query failed.  So there's probably nothing wrong with the process of your router finding the DNS servers, it's just something about how those servers failed to carry out a request. 
@paxy Alternatively, the message you quote could be a logged case of a computer or server on the internet trying to probe your router, and your router not allowing this and so "failing" it.  Which is behaviour you'd want to encourage!
Now, is this the end of my story?
I borrowed a router, and first of all had it working with the username and password of the person I'd borrowed it from (my parents, also Plusnet customers).  Then I had it working with my username and password.
I then tried my original router with my parents' username and password, and it worked.  Then I tried it with my own username and password again.  And it worked. 
So now my own router is working, with my username and password, and I can access any website successfully.  And I've successfully rebooted AND factory reset it, and it still works with no problems.  (I'd already removed any manually set DNS server details and MTU settings from my MacBook and router.)  And it works with my Airport Express and everything is back as it was before 11pm on Wednesday and it works.
It therefore seems that something must have changed at Plusnet's end... any ideas?

[#12]

Out of interest can you post the results of Adie's suggested test again and see if they have changed

[#13]

cat /etc/resolv.conf

domain lan
nameserver 192.168.1.254

It's still the same.
And my router is still working today, with no problems, as it was on Wednesday.  So I've had 5 days without a useable internet connection, with no explanation, and with Plusnet CSC agents telling me to try changing my equipment and settings.  And now it is working again, with no explanation!

[#14]

Hiya,
We've certainly made no DNS changes at this end in the last 24 hours or so.

[#15]

I've just encountered the same/similar problem again. The router was reporting a connection to the Gateway and to both DNS servers, but I could only browse to a certain few websites.
Firstly, I tried restarting my router and computer, with no change. I could still only browse to the same few websites.
Then, based on what worked on Tuesday, I tried my parents' username and password in my router. This worked and gave me a totally working connection.
Then I put my own username and password back in to my router. The same problems were apparent.
Then I put my parents' username and password in again - working connection.
I used this for a few minutes, before trying again with my own username and password.
This time, with my details, I had a working connection.
Given the reports on the Community forums of similar problems to this, and the related problem I experienced of some reboots not finding one or both DNS servers, I find it difficult to believe it is something at my end.
And given that I've had a working connection since Monday with "normal" settings, it seems unlikely that it is my router that is at fault.
One thought: Is it possible that my request, now cancelled, to change to an "unlimited" account has introduced a fault into my account on the Plusnet servers?