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Community Member

Weird DNS issue

Hi. One of our customers has recently started having strange DNS issues when trying to browse the internet (email and AV updates are working fine). They use a Cisco 877 integrated device as their default gateway, service provider is BT (UK). I can ping the internet OK but cannot browse to most sites e.g. www.microsoft.com and www.symantec.com are OK but www.google.co.uk, www.bbc.co.uk are not (in fact, microsoft and symantec are the only ones I've managed to connect to so far).

I've checked with BT that our public DNS server IPs are correct. I've also tried using Firefox to no avail.

I've only encountered this problem once before (on a Netgear router) and this required a firmware upgrade. However, my Cisco logon doesn't give me access to download upgrades so I'm hoping thats not the problem this time. IOS version is 12.4(6)T6

Has anyone encountered this problem before? Any ideas on how I can resolve? Any help/pointers would be greatly appreciated.

Thanks Rex.

4 REPLIES

Re: Weird DNS issue

Do you have an ACL on the public interface? If so, I've seen the acl block dns traffic coming from the provider that you specify. I would try to allow your dns providers ip through your router on udp 53 to see if this helps. You can see this in your logs as well.

HTH,

John

HTH, John *** Please rate all useful posts ***
Bronze

Re: Weird DNS issue

Hi Rex, can you ping www.google.com from a host command shell?

You should see dns resolve the ip then successful pings to the destination.

If that works then it is unlikely to be a dns issue and I would look to the firewall/acl configuration.

If it doesn't, manually configure the host to temporarily use 4.2.2.2 as it's dns server and try again.

If that works it is likely to be a provider dns problem.

Hope this helps

Community Member

Re: Weird DNS issue

Hi, and thanks for the reply.

I could ping www.google.com. I also discovered I could browse to https sites. Eventually resolved by turning on http inspection (not something I'm familiar with but seems to work).

My guess is that whoever did the original config didn't save back to startup config and the router got rebooted. Anyway, all working now and thanks for your help.

Bronze

Re: Weird DNS issue

Glad to hear you got it resolved Rex.

Thanks for the rating.

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