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smartcat

7 posts

Wannabe Geek


#91408 12-Oct-2011 23:31
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I have a issue with not being able to access some web sites. It appears to be an DNS issue but I can't work out whether it is the ISP or my installation

This installation worked fine till a couple of days ago....

Installation is a Dlink ADSL modem followed by a TP-Link wireless router.

ADSL is working okay from checking the status page - can ping the DNS server at the ISP from inside the Dlink - very low latency and no missing pings.

Have checked this using two diferent PC's ( Win 7 & XP ) with similar results. The PC is given an IP address by the DHCP server in either the Modem or Router. The same result occurs with the PC plugged into the Modem or Router.

Access of the ISP own pages works fine ( 99% of time ). Some other sites apart from the ISP's work if you add a second DNS address into the Router / Modem DNS or PC ( IP address set up page )  from a different ISP  !!

Google.co.nz works but Vodafone.co.nz does not - you get the "Google auto-type results" as expected.   However, accessing the pages from the Google search not all pages are found / accessable - get a time out / not available message. ( suspect the pages that fail are those hosted off shore )

Windows dianostics when the page lookup fails results in an message saying Windows doesn't know the problem - doesn't fail with an DNS error message.

Have tried resetting the DNS cache via IPconfig / fixed IP & DNS address in the PC's ( and Router & Modem ) - no change. Have done the usual power resets of all the bits.

Any ideas of what to try next would be cool

thanks

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tigercorp
643 posts

Ultimate Geek


  #532595 12-Oct-2011 23:39
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Is the adsl router using the dns settings provided by your ISP?

Is the problem consistent or can you provoke it?  Or is it intermittent?

smartcat

7 posts

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  #532625 13-Oct-2011 06:19
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The ADSL Modem is using the ISP's Gateway / DNS address's

These appear to be okay as they are the same as found on the ISP's web site.

The issue cleared late last night after about 8hrs of being a problem - no changes on my part !

 
 
 
 


tigercorp
643 posts

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  #532651 13-Oct-2011 08:21
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Next time it happens, maybe try flushing your dns cache - ipconfig /flushdns - and see if pinging the down site resolves the hostname correctly.

You don't run PeerGuardian (or whatever its called these days) do you? 

sbiddle
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  #532664 13-Oct-2011 08:54
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If your router acting as a DNS proxy? Easiest way to tell is to see whether your DHCP server is giving out the router IP as the DNS server or your ISP's.

ptinson
677 posts

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  #532754 13-Oct-2011 11:18
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If you suspect an ISP DNS fault then your real friend on windows is nslookup, or dig if you have it.

Run nslookup, then set the server to the modem if its doing DNS proxy, test the site you have issues with.
Then set server to your ISP DNS server and test again, and also some other provider like another ISP in NZ.

Compare all the results, if its a DNS fault it should be reasonably evident.

The other tool that helps is http://digwebinterface.com its easy enough to use and can help track this sort of thing down.

Also depending on which ISP you are on you may be going through a HTTP proxy cache for international requests, so while your DNS may work if its doing secondary resolution it could be broken. Just to make it fun.




meat popsicle

smartcat

7 posts

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  #533141 13-Oct-2011 22:59
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Thanks for the ideas - my problem is back but very intermittent so we shall try the suggestions

cheers

webwat
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  #533985 16-Oct-2011 16:54
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ptinson: If you suspect an ISP DNS fault then your real friend on windows is nslookup, or dig if you have it.

Run nslookup, then set the server to the modem if its doing DNS proxy, test the site you have issues with.
Then set server to your ISP DNS server and test again, and also some other provider like another ISP in NZ.

Compare all the results, if its a DNS fault it should be reasonably evident.

The other tool that helps is http://digwebinterface.com its easy enough to use and can help track this sort of thing down.

Also depending on which ISP you are on you may be going through a HTTP proxy cache for international requests, so while your DNS may work if its doing secondary resolution it could be broken. Just to make it fun.

Or test by putting another ISPs DNS — that you know always works — directly into your PCs configuration so that its not resolved by the router. I had a similar problem a while ago, and it came right after I took the manual DNS entries out of the router's WAN setup (might be a bug in the netgear router) and switched the modem to full bridge (so might have also been a bug in the modem's half bridge).




Qualified in business, certified in fibre, stuck in copper, have to keep going  ^_^

 
 
 
 


ptinson
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  #534017 16-Oct-2011 17:42
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webwat:
ptinson: If you suspect an ISP DNS fault then your real friend on windows is nslookup, or dig if you have it.

Run nslookup, then set the server to the modem if its doing DNS proxy, test the site you have issues with.
Then set server to your ISP DNS server and test again, and also some other provider like another ISP in NZ.

Compare all the results, if its a DNS fault it should be reasonably evident.

The other tool that helps is http://digwebinterface.com its easy enough to use and can help track this sort of thing down.

Also depending on which ISP you are on you may be going through a HTTP proxy cache for international requests, so while your DNS may work if its doing secondary resolution it could be broken. Just to make it fun.

Or test by putting another ISPs DNS — that you know always works — directly into your PCs configuration so that its not resolved by the router. I had a similar problem a while ago, and it came right after I took the manual DNS entries out of the router's WAN setup (might be a bug in the netgear router) and switched the modem to full bridge (so might have also been a bug in the modem's half bridge).


Sure, you just need to remember that some ISP's like Telecom run transparent proxy caches that do secondary DNS resolution, so changing to another providers DNS will break things in a whole other way.

If you suspect the modem then use your ISP's DNS servers on your desktop and bypass the modem.
Do a dig/nslookup against your modem and your ISP DNS and compare the answers.

CDN and caches really make using random service provider X DNS a pain and unpredictable results will occur.

Paul




meat popsicle

smartcat

7 posts

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  #534109 16-Oct-2011 23:14
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The problem did come back a day or so back - got sidelined wondering why it only happened in the evenings until I looked at the DHCP "lease times" which were set in the order of 4hrs. Guess things stopped when the renewal happened

I had the situation described above with manual entries set for the DNS server address in the both the Modem & Router - I have since removed them from the Router and since then all has run without problems !

Using "Dig" shows the correct DNS is being "served" to the users

Has run for 2.5 days now with all happy so hopefully.........

Thanks for the suggestions

cheers


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