Exam question help :(

[Andy]

Ah, thanks Barry! Life saver!


Barry Margolin <barmar at alum.mit.edu> wrote in message news:<c4hntt$1hqs$1 at sf1.isc.org>...
> 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.