Access by IP address but not URL?

Question

Access by IP address but not URL?

One laptop on our network exhibits strange behaviour occasionally. It will be working fine, then will suddenly lose the ability to access a website or any other server by its domain name.

Using Network Utility to lookup the address, the DNS responds with all the correct information. And in Terminal, nslookup and dig both correctly resolve the domain name to the IP address.

However no other application seems able to use this information. Safari, Mail, Software Update all fail to connect to the requested servers.

Mac OS X 10.6.8, MacBook Pro 2.66 Ghz Intel Core i7

Hope someone can assist

Cheers

James

Posted on Nov 14, 2011 4:26 AM

Reply

Answer 1

X423424X

Level 6

14,271 points

Nov 14, 2011 11:24 AM in response to James Knight3

Try using different DNSs. Two are OpenDNS (208.67.222.222, 208.67.220.220) and Google DNS (8.8.8.8, 8.8.4.4).

Reply

Answer 2

Nov 14, 2011 1:08 PM in response to X423424X

It's nice of you to reply. But the existing DNS server (which by the way is on our LAN) is giving the proper answers.

This only happens with *one* computer on the network, and then only sometimes. But when the computer is exhibiting this problem, dig and nslookup both resolve IP addresses correctly - but apps like Safari and Mail seem unable to use this information.

The only way I have found of sorting this is to restart the client computer, at which point it works fine.

Logging out and back in as a different user has no effect.

Any ideas?

Cheers

James

Reply

Answer 3

Nov 28, 2011 9:40 AM in response to James Knight3

can anyone help? this happens every time we bring this laptop on to our network. I've deleted the ethernet interface and added it again, tried with different users, but nothing works apart from a restart.

The DNS resolves IP addresses perfectly, and the IP addresses can be pinged but no URL can be pinged or used in any way.

Help!

Reply

Answer 4

Camelot

Level 9

59,807 points

Nov 28, 2011 10:29 AM in response to James Knight3

Check /etc/hosts

You can override normal lookups by setting a hostname in /etc/hosts to give it a different IP address.

Safari (and just about every other application) will use the system resolver to resolve the hostname, and that first checks /etc/hosts to see if an entry exists. If it does, it uses that address rather than whatever DNS would say.

However, dig specifically queries the name server, so will return whatever DNS says, completely ignoring /etc/hosts and any caches.

Reply

Answer 5

Nov 28, 2011 11:44 AM in response to Camelot

Thanks Camelot - that would be fine if this was one IP address I was having problems with. But that's not it. No URLs at all can be accessed - whether hosted locally or on the internet. Both dig and nslookup resolve IP addresses correctly but the URLs do not work.

From your post, it sounds as thought the system resolver may be at the root of the problem. But I have no idea how I would go about unclogging it?

Cheers

James

Reply

Answer 6

varjak paw

Level 10

182,778 points

Nov 28, 2011 11:46 AM in response to James Knight3

Try:

dscacheutil -flushcache

Hope that helps.

Reply

Answer 7

Nov 28, 2011 11:47 AM in response to varjak paw

I have already tried that, sorry I did not mention it in original post

Reply

Answer 8

Mar 7, 2012 5:27 AM in response to James Knight3

does anyone have any more ideas on this? The laptop only appears to have this problem on our LAN - it's fine at another office or at home. But no other computers exhibit this behaviour at all. Every time the user plugs on to our LAN, it works at first and then after a period it suddenly gives up resolving domain names. Help please!

Reply

Answer 9

fane_j

Level 4

3,704 points

Mar 7, 2012 5:38 AM in response to James Knight3

James Knight3 wrote:

does anyone have any more ideas on this?

Create a new user account on the same machine and test it. At least it will give you an idea on where to look for the trouble. If Safari and Mail don't display the same issue in the new user account, then the problem must be somewhere in the other account's Home folder.

Reply