Question

Can't reach certain sites. DNS issues?

Asked by: bobinorlando

In one office location, I have a small network setup consisting of Window Server 2008 (handles active directory, DNS, DHCP), a router (DHCP disabled) and a DSL modem. A couple of machines are hanging off this little network.

It all works fine most of the time, however, there are certain sites they can't seem to reach. e.g. bankofengland.co.uk. I'm trying to troubleshoot why this is.

What should I do?




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Asked On
2009-10-23 at 05:58:21ID24837780
Tags

dns

Topics

Windows NT Networking

,

Domain Name Service (DNS)

Participating Experts
2
Points
500
Comments
7

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Answers

 

by: Chris-DentPosted on 2009-10-23 at 06:06:05ID: 25643943


Yes DNS first...

nslookup bankofengland.co.uk

If you get an IP back there, test it on the client with:

ping bankofengland.co.uk

If both of those give you IP addresses, check the browser to see if it's using a Proxy, if it does, troubleshooting needs to happen there. That's the end of DNS's involvement though, it would be time to check network connectivity (telnet bankofengland.co.uk 80), if it fails to connect, see if it's being blocked by your infrastructure.

Chris

 

by: einangenPosted on 2009-10-23 at 06:10:55ID: 25643984

Sounds like an MTU (Max Transfer Unit) problem.  DSL lines require you to use a lower MTU than 1500 (which is standard for ethernet,) since there is some overhead when you encapsulate the ethernet packages to go through the DSL modem.

It is best to have the MTU set as large as possible, otherwise you get fragmentation of packages, effectively making the line appear slower.

With most DSL lines, you should probably use MTU=1452, but to get an accurate value, issue the command

"ping -f -l 1452 <some-internet-ip>"

If it complains about fragmented packages, lower the number, if not, make it bigger.  When you reach the maximum possible, subtract 28 (I think it was), and the result should be the correct MTU value.

How to set this value for the network depends on the hardware/operating system.

 

by: bobinorlandoPosted on 2009-10-23 at 06:27:40ID: 25644152

nslookup from a command line on the server returns server unknown
ping can't find it on the server or the client

max MTU that doesn't return a fragmented error is 1472. i forget where to set this.


 

by: Chris-DentPosted on 2009-10-23 at 06:29:23ID: 25644171


Back to nslookup... can you show me exactly what it returns? I'd expect NXDomain or ServFail, unknown isn't an RCode.

If it returns ServFail, are you using Forwarders? If you're using Root Hints, are you aware of the issues with Windows 2008 DNS and the .co.uk TLD?

Chris

 

by: bobinorlandoPosted on 2009-10-23 at 06:39:57ID: 25644285

From the client I get this:

C:\Documents and Settings\User>nslookup
DNS request timed out.
    timeout was 2 seconds.
*** Can't find server name for address 192.168.1.200: Timed out
*** Default servers are not available
Default Server:  UnKnown
Address:  192.168.1.200

From the server (the .200 machine) all it says:
C:\Users\Username>nslookup bankofengland.co.uk
Server: Unknown
Address: ::1

And no, am not aware of .co.uk issues with win server 2008

 

by: bobinorlandoPosted on 2009-10-23 at 06:48:37ID: 31644990

Searched for the tld issue. Found this code at http://www.tinyint.com/index.php/2009/06/09/windows-2008-dns-query-issues-with-root-hints/

and it fixed it.

NET STOP DNS
REG ADD HKLM\SYSTEM\CurrentControlSet\Services\DNS\Parameters /v MaxCacheTTL /t REG_DWORD /d 0x0002a300
NET START DNS

 

by: Chris-DentPosted on 2009-10-23 at 06:57:48ID: 25644455



The Unknown you're getting there is the result of a failed reverse lookup for your servers IP address. Essentially, it's this:

nslookup -q=ptr 192.168.1.200

NsLookup just likes to check those resolve when you start it up, it isn't a response to a specific query.

Chris

20120131-EE-VQP-002

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