Exam question help :(

[Danny Mayer]

At 09:38 PM 3/31/2004, Andy wrote:
>Hello group!
>=20
>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?

That depends on the browser being used, whether or not it caches
the results of lookups, the number of images, frames, pages, etc in
the starting HTML page, etc. Each file can be at a different location.
Have you looked at the pages and determined how many files need
to be retrieved and from where?

>I believe this to be 2, one for the two http requests and one for the
>ftp request, but i'm not really sure!

No. In addition to the number of files that the browser needs to fetch
you need to examine whether or not the file has been/can be cached,
the DNS responses that the browser has cached, the TTL's on
those responses assuming that the browser obeys the rules on
DNS records.


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

That depends on the URL's that need to be fetched. It doesn't care
where the browser is situated. It also depends on the caching of the
resolver, the caching on nameservers in the chain looking up the
address, the TTL of each record involved including the NS records
of each server in the chain.

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

See above. Possibly none.

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

HTTP uses TCP, binds to the socket and listens for connection requests.

>v) If the session was extended to send an email to
>notab at kiwi.doc.ic.ac.uk, is a new DNS request required.

You need the MX record for email, followed by requests for A records
to get the IP address of the mail server.


>I think that it does not require a new request as its something to do
>with it already being cached...?

Depends on the caching strategy of each client and server and TTL
of each record.

>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! :(

TXT records can be added to the zone.


>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.

Yes. Proxy servers are designed to do this, among other things.


>Any help or pointers in the right direction will be a life saver as my
>exams are soon!!!!

You have a lot of work to do.

Danny

>Thanks a lot people!
>
>Andy