dns, top level domain and isp's name server

[sarahr202]

Hi everybody.

I have a question.

Does our ISp' name server know  ip addresses of root servers only (which contain the ip addresses of all top level domain dns servers) ?

or isp's name server  know ip addresses of all top level domains dns server  plus the ip addresses of root servers?

thanks and happy holidays.

[Edwin Summers · Accepted solution]

The ISP typically runs a caching server that will cache responses of queries up to the TTL value in the query record.  So they'll have the root server locations and a large list of recent queries.

More to your question and what I'm not certain of, does this caching include the TLD responses that would be used in a recursive query by the ISP DNS server.  Standing by for someone to answer that one.

Merry Christmas!

-Ed

[sarahr202]

Hi Edwin.

An ISP's dns does not know the ip addresses of top level domain but knows the ip address of root servers. Just want to confirm that.

This is my understanding.

Let suppose dns query is received by isp 's dns for www.cisco.com  but isp's dns does not have entry in its cache.

Next dns's server contacts one of the root server, which provides the list of ip addresses of top level domain " .com".

Next , isp's dns contacts one of the top level dns for " com". which returns the ip address of " cisco.com".

Next isp' dns contacts dns server at cisco.com asking for ip address for. www.cisco.com"  . Dns server at cisco.com returns the ip address for www.cisco.com.

Here is my question,  we can cut down the traffic if our isp' dns  knows the ip addresses of top level domains as well.

Rather than contacting root server for ip addresses of top level domain server for " com", our isp's dns simply contacts the top level dns for " com" and resolve the query a little faster.

I appreciate your feedback.

[Edwin Summers · Accepted solution]

Sarah,

Curious, I went looking a little further.  My understanding was that the caching DNS servers would also cache the records for the TLD servers exactly for the efficiency reasons you mentioned.  However, I wasn't totally certain so I had to do a bit of searching.  The following is from ISOC, and I think answers the question:

(from: http://www.isoc.org/briefings/020/)

Q: Are the root name servers queried every time I browse the web or send mail?

A: No, information is cached in the DNS. Your computer will query a  caching DNS server to resolve domain names. A well behaved DNS server  needs to query the root name servers only once every 48 hours for each  particular TLD. In the meantime it can resolve names for that TLD  without involvement of the root name servers. Because of this caching  almost all DNS queries are answered without involvement of the root name  servers.

So theoretically*, the ISP's caching DNS server also caches the entries for the TLD DNS servers, and only needs to query the root servers every "48 hours" (likely the TTL for that record).  Thus the efficiency that you discussed is currently in place.

(* - I say theoretically because, inevitably somewhere an ISP will have some odd policy or mis-configured server that operates differently.  The Internet takes all kinds...) 

[sarahr202]

Thanks Edwin