NS Lookup 1

[Chr1sUK]

two questions:

Firstly, sometimes when i do an nslookup, you get, for example:

Server: ns7.bt.net
Address: 194.74.x.x (me)

Non-authoritative answer:
Name: tek-tips.com
Address: 216.45.19.33
Aliases:

http://www.tek-tips.com

What is the diffrence between authoritative and non-authoritative?


Secondly, sometimes when you do an nslookup for a website, it does as it is supposed to and returns an IP address, and if you then resolve that IP address back to a host, you get the original hostname.

Ie:

C:\Documents and Settings\Chris>nslookup

http://www.tek-tips.com

Server: ns7.bt.net
Address: 194.74.x.x

Non-authoritative answer:
Name: tek-tips.com
Address: 216.45.19.33
Aliases:

http://www.tek-tips.com

C:\Documents and Settings\Chris>nslookup 216.45.19.33
Server: ns7.bt.net
Address: 194.74.x.x

Name: tek-tips.com
Address: 216.45.19.33

Just as normal....

however, when i do this on a message board I post on...this happens:

C:\Documents and Settings\Chris>nslookup

http://www.djsource.co.uk

Non-authoritative answer:
Name: djsource.co.uk
Address: 216.193.202.216
Aliases:

http://www.djsource.co.uk

Note the IP address....when i then try to resolve this back to the hostname.....

C:\Documents and Settings\Chris>nslookup 216.193.202.216

Name: valina.lunarpages.com
Address: 216.193.202.216

Its different. I assume it's the hosting company and that several websites must have the same IP address (as they are on the same server), so how does this work? And what can you do about it?

Thanks!

 

[Serbtastic]

What do you mean "what can you do about it"?

 

[ChrisAC]

valina.lunarpages.com is just the name of the server run by the hosting provider. On that server there will be many web sites hosted so all those site need to resolve to that address. You don't "need" to do anything about it. That's just the way it is!

"What is the diffrence between authoritative and non-authoritative?".

An authoritive answer is one from a DNS server that the domain is designated to, ie. the zone files are located on that server and the name server is authoritive for that domain.

If the DNS query is answered by a non-authoritive server then the answer was cached on the server, eg. If your ISP's DNS server resolved

http://www.microsoft.com

and passed that to your resolver, you would be getting the answer from your ISP's name servers, not Microsofts name servers and so the answer would be non-authoritive.

Chris.


**********************
Chris Andrew, CCNA, CCSA
chris@iproute.co.uk
**********************

 

[Chr1sUK]

Cheers

What I mean't by what can you do is...

...how would you be able to pinpoint that website with just the IP address if it is on a server with other websites.

It's not important was just wondering how it worked, i was under the impression that it would be a bit more unique. Since the only point in text web addresses is for people to remember stuff easier, and the browser then translates that into an IP address....how does it know which to access on the server? The name?

 

[ChrisAC]

Valid question. DNS only resolves a URL to an IP address. The server that the URL resolves to may host hundreds of web sites and so it is the responsibility of the web server to determine which directory to serve from. This is done with the use of name 'based virtual servers'. When your DNS resolver on your local machine determines the IP address to send the HTTP request to your browser sends a 'GET' request for say

http://www.website.com.

The server receives that request and then looks for a virtual server for

http://www.website.com.

If it has that server then it will serve pages from within that directory (and sub-directory).

http://httpd.apache.org/docs-2.0/vhosts/name-based.html

Chris.


**********************
Chris Andrew, CCNA, CCSA
chris@iproute.co.uk
**********************

 

[Chr1sUK]

ah right! I understand that

Thanks!

 

[Perplexer]

iproute, you said:

"If the DNS query is answered by a non-authoritive server then the answer was cached on the server, eg. If your ISP's DNS server resolved

http://www.microsoft.com

and passed that to your resolver, you would be getting the answer from your ISP's name servers, not Microsofts name servers and so the answer would be non-authoritive."

I was first reading how DNS stuff works on this URL:

http://computer.howstuffworks.com/dns5.htm

As written there, the nameserver which is defined in your OS networking settings is the one that always returns the asked-for IP, no matter if it has it cached or retrieves it from another nameserver. Not sure now how cached or not cached plays any role here. Almost everything is 'cached' or gotten through another DNS eventually, isn't it ? That would mean 99% of all domain name recolves would be non-authoritative.

I guess I don't understand the concepts correctly yet so if possible, explain it a bit.

Thanks.

 

[ChrisAC]

Right, the chances are that you will be querying a DNS cache only and so all the answers that you get from that will be non-authoratative, ie. the cache doesn't have that zone delegated to it. However, the server that you query will have to get an answer to your query and so will have to send non-recursive queries to other servers (iterative queries), so it may get back a non-authoratative answer from another cache or it might have to follow referrals and eventually query the authoratative server.

So, to see an authoratative answer, you would need to find the delegated name server for the domain in question and send a query to them. This is useful when troubleshooting DNS problems. If you can query the authoratative servers then you can compare the answer to any info that your DNS cache has.

Chris.


**********************
Chris Andrew, CCNA, CCSA
chris@iproute.co.uk
**********************