We have been tearing our hair out over an issue where DNS service seems to disappear on our BT fibre connection. It fails several times a day.

We moved office and took with us our trusty Vigor 2820 to the new premises where BT initially installed an ADSL2 connection.

From the outset we had issues with internet access - no DNS. Symptoms are

Ping google.com or bbc.co.uk - would not resolve
Ping 8.8.8.8 (or any numeric internet IP) - gets normal reply with packets sent and received being shown

BT persuaded us to upgrade to a fibre (FTTC) connection saying it would be a lot more reliable. Same issue.
To be fair their Tech Support guy has been very helpful in investigating the issue but now does not believe it to be anything on their side after two months of trying different things.

Here are some details:-

1. We have to use a BT router or modem as the 2820 does not have VDSL support for fibre. We have tried two BT Hub 6's, a BT Hub 5 and now purchased an OpenReach Modem (Huaiwei HG612) at BT suggestion to try and diagnose the issue. All work for a while.
2. We have replaced the Vigor 2820 with a known good Vigor 2820n (same as previous but with WiFi built in), and updated it to latest firmware.
3. We have tried the BT Hub 6 in Bridge mode.
4. We have tried forcing all DNS to use 8.8.8.8 and 8.8.4.4 (Google's DNS) as BT seems to change theirs every few days. We have also tried fixing the DNS to one of BT servers given by their Techie.
5. When it fails we can access the internet only through the BT Router's WiFi (when it is not in Bridge Mode), not through our LAN.
6. Rebooting either the BT Hub 6 or the Draytek works for a while then it drops the DNS again
7. We have a complex LAN with 3 Cisco Catalyst 3750G switches with 4 VLANS set up on them and have static routes set up on the Vigor to map back to the VLANS. However disconnecting the Cisco network completely from the Vigor LAN ports and connecting a PC to the Vigor WiFi still results in eventual DNS failure, which tends to rule out a fault on our LAN. Apart from the static routing the Vigor setup is otherwise fairly standard.
8. We have had 2 network Guru's (with a lot more experience then I, including a CCNA guy) look at the setup and can't see anything wrong.

Frankly I have run out of things to try and would appreciate any help from anyone who has experience of this type of issue.

Happy to provide any further details if required.

Regards
Andrew

You need some more diagnostic info. to track this down.

(Assuming a Windows PC)

ipconfig /all - will show you the DNS servers assigned.

Code:

Ethernet adapter vEthernet (Outside World): Connection-specific DNS Suffix . : Description . . . . . . . . . . . : Hyper-V Virtual Ethernet Adapter #2 Physical Address. . . . . . . . . : AB-CD-EF-8A-DF-4D DHCP Enabled. . . . . . . . . . . : No Autoconfiguration Enabled . . . . : Yes IPv4 Address. . . . . . . . . . . : 192.168.100.8(Preferred) Subnet Mask . . . . . . . . . . . : 255.255.255.0 Default Gateway . . . . . . . . . : 192.168.100.254 DNS Servers . . . . . . . . . . . : 192.168.100.252 192.168.100.253 NetBIOS over Tcpip. . . . . . . . : Enabled

You can use tracert confirm connectivity to them.

You can use nslookup to interrogate these (or any other) DNS servers.

Code:

** Default DNS server ** C:\>nslookup news.bbc.co.uk Server: dc1.home.somedomainorother.co.uk Address: 192.168.100.252 Non-authoritative answer: Name: newswww.bbc.net.uk Addresses: 212.58.244.56 212.58.246.80 Aliases: news.bbc.co.uk C:\>

or specify a particular DNS server, on the command line:

Code:

C:\>nslookup news.bbc.co.uk 9.9.9.9 Server: dns.quad9.net Address: 9.9.9.9 Non-authoritative answer: Name: newswww.bbc.net.uk Addresses: 212.58.246.80 212.58.244.56 Aliases: news.bbc.co.uk C:\>

If these don't reveal anything, it might suggest issues with the actual PC...

Might be time for those 'Network gurus' to earn their keep - and get "Wiresharking" :wink:

6. Rebooting either the BT Hub 6 or the Draytek works for a while then it drops the DNS again

When it 'drops the DNS', can you use nslookup to connect to any other name servers (by IP address.....)

Please check [Firewall] > [DoS Defense] and if UDP Flood Defense is enabled, try disabling that to see if it helps with your issue.

Thanks to all who responded so far. Unfortunately it did not notify me that you had...

Info:
when I tried nslookup it did not get a response but I have just logged in over VPN and tried your suggested syntax with quad9...

C:\Users\user1>nslookup news.bbc.co.uk 9.9.9.9
Server: dns.quad9.net
Address: 9.9.9.9

Non-authoritative answer:
Name: newswww.bbc.net.uk
Addresses: 212.58.246.81
212.58.244.57
Aliases: news.bbc.co.uk.

And it is not a single PC that cannot get DNS - its the whole network of PCs and Linux Clients. And we are using both of Google's DNS IP's which I'm pretty sure don't fail every few hours. Also same result with BT DNS.

I'll also try the suggestion of turning off UDP Flood defense and report back as I do have DoS enabled, and let you know how it goes

andrewc wrote: And it is not a single PC that cannot get DNS - its the whole network of PCs and Linux Clients.

I know, but you've got more diagnostics available to you on a PC.

It's worth configuring Syslog on a PC if you don't already have it - this is where all the Router debug info appears. It will help you spot if it's stuff like the 'DoS defence' getting involved.

Also, can you clarify this statement "when I tried nslookup it did not get a response" ... That sounds for all the world, like you had no working/accessible DNS server available to you at that point. Which one was it trying to use?

Also, how is the DNS server setting conveyed to your PC clients. Via DHCP? or static? I assume the former - is the 2820 the DHCP server?