Exam question help :(

[Barry Margolin]

In article <c4hk4v$1fnk$1 at sf1.isc.org>,
 andyli1234567 at hotmail.com (Andy) wrote:

> Hello group!
>  
> I'm guessing this where all the experts with DNS are gathered...
> 
> I'm currently stuck on this question we have in our Networks and
> Communication course in Imperial, UK! If anyone can help to any parts
> of the question, it will be very much appreciated! Taken from the 2003
> exam...
> 
> 
> A web browsing session visits the following URLs in order:
> 
> http://www.doc.ic.ac.uk/index.html
> ftp://kiwi.doc.ic.ac.uk/README
> http://www.doc.ic.ac.uk:8080/index.html
> 
> Briefly explain your answer to each of the following:
> 
> 
> i) How many DNS lookup operations will the browser need to perform in
> the session?
> 
> I believe this to be 2, one for the two http requests and one for the
> ftp request, but i'm not really sure!

It could be 2 or 3, depending on how the browser caches recent results.  
If the resolution of "www.doc.ic.ac.uk" is no longer in its cache after 
it finishes the FTP, it will need to look it up again.

> 
> ii) If the browser was situated in the java.sun.com domain, and made a
> request for an iterative lookup of address from its local DNS server,
> what are the domains that the DNS server contacts for the lookup of
> the first URL in the list above. (it must be assummed that each level
> in the DNS heirarchy is in a different zone, and that all servers have
> no cached information).
> 
> I came up with this list,but i'm not too sure once again!
> 
> java.sun.com
> sun.com
> .com

The DNS server doesn't need to look up anything in the java.sun.com 
domain to resolve a name in the doc.ic.ac.uk domain.

> <root>
> uk
> ac.uk
> ic.ac.uk
> doc.ic.ac.uk

Without examining the details of how these subdomains are delegated, 
it's not possible to answer this, because it's possible to delegate 
multiple levels at a time.  In this case, the ac.uk zone delegates the 
doc.ic.ac.uk subdomain directly, so there's no need to go through the 
ic.ac.uk step.

> iii) How many server processes will be contacted by the browser during
> the session?

One DNS server, one HTTP server (unless there's server load balancing 
going on), and one FTP server.

> 
> iv) For the first and last URL, how is the server socket constructed?

The first one is a TCP connection from an ephemeral port to port 80 on 
www.doc.ic.ac.uk, the second is a TCP connection from an ephemeral port 
to port 8080 on that server.

> 
> v) If the session was extended to send an email to
> notab at kiwi.doc.ic.ac.uk, is a new DNS request required.
> 
> I think that it does not require a new request as its something to do
> with it already being cached...?

Except that mail makes use of MX records, and only the A record was 
cached earlier.

> 
> vi) DNS does not provide user defined record types, but you are
> required to add details of the owner and telephone number of each
> machine in the doc.ic.ac.uk domain, which can be retrieved by outside
> clients using DNS. Can you suggest a method to achieve this?
> 
> I can't! :(

See the TXT record type, which allows arbitrary character strings to be 
associated with a name.

> 
> vii) Explain if it would be possible or not to use a firewall to block
> access to http://www.doc.ic.ac.uk/c212_exam.ps but allow access to all
> URLs listed in the web browsing session.

A proxy-based firewall can examine the URL and filter on the pathname.

-- 
Barry Margolin, barmar at alum.mit.edu
Arlington, MA
*** PLEASE post questions in newsgroups, not directly to me ***