Programming Assignment #I Solution

Domain Name Service is a service utilized  by application protocols,  such as HTTP, SMTP  and FTP

to  translate host  names  to  IP  addresses.   As shown  in class,  DNS runs  over UDP  and  uses port

53.  In this  assignment; however,  you are asked to simulate  a DNS server  using TCP  and  utilizing sockets.  Your server will respond  to client queries mapping  host names to IP addresses.

 

 

2    Details

 

Following are details  for sending/receiving a DNS query and the steps taken  at both  the client side and the server side.

 

2.1     Client

 

We have written the client program  for you.  Please refer to class web page for client code in Python

(2/3) and Java.

 

• A client wishing to resolve a host name to an IP address  will connect to the server at the port specified in the server program.  (We used port  5001).

 

• For each DNS request,  client opens a socket to server to query its desired host names as needed, providing  host name for the query.

 

• The format  of the query will have the host name “www.yahoo.com”  or “google.com” (of course for stdin  input,  you need to follow that with a carriage  return).

 

• Holding the socket open, the client will wait for the server to respond with a corresponding  IP

address  (or an error message, or “Host  Not Found”).

 

• By receiving a ‘q’ from input,  terminate the clients program

 

• Upon  quitting from  the  program  (by  pressing  ‘q’ or  ‘Q’), all  the  sockets  must  be  closed. Remember,  the server should still be up to service other  potential clients.

 

2.2     Server

 

This is the program  you will need to write.  Please refer to the class web page for suggested  skeleton and partial code.  Note though,  if you choose, you can write your own server code.

 

• The  server  will be listening  to any  incoming  connections  from any  client  (on the  same port, e.g. 5001). Therefore,  threading is utilized  to handle  multiple  concurrent connections.

• Upon  receiving  a  query  from  the  client,  the  server  will first  lookup  its  local  cache  (which could be a file).  If the  server  finds a corresponding  entry  in the  cache,  it will be sent  to the client directly.   If the  server  cannot  find one, it will look up the  DNS system  using API  (e.g. gethostbyname ).

 

• When the host name is resolved by the DNS system, it should be added to the local DNS cache as well. This will avoid constantly contacting the DNS system for previously  resolved queries.

 

• The response message will use the format  of “Source:<hostname:<IP address”. For exam- ple “Local DNS:google.com:  216.58.216.68”.  Since this is a simulation  of DNS, you can imple- ment your local cache using files. Your file should have the format  “<host:<IP Address”. (Note:  for the provided  implementation, the file is named as “DNS Mapping.txt”. Please refer to the class web page for a sample file).

 

• One hostname may have multiple IP addresses in the file. For example, “Local DNS:<hostname:<IP

address  1:<IP address  2”. You should select one to send to the client (randomly).

 

• If the  host  name  is neither  resolved  by the  local cache  nor  by the  DNS system,  a response from the  server should be forwarded  to the  client in the  format  “Host  not found”  and  should also be cached.  The  server should also communicate to the  client any formatting issues with a message, such as “Invalid  format”

 

• If the  server  needs  to  be brought down,  an  “exit”  should  be entered  at  the  server  console, which will close all sockets and terminate.

 

 

3    What to submit

 

You should upload  the project  files to the Moodle site under  Submissions  section.  Your submission

(in a tar  or zip format) should include the followings:

 

• The server source code (in one of the languages:  Python, and Java).

 

• A session (using  “script” or any  other  programs)  showing a sample  of the  queries  and  their resolutions.  (Please  refer to the class web page for a sample).

 

• A brief  readme.txt (less than  200 words)  stating any  compilation  script  you  used,  and  any details  that we should be aware  of in order  to compile and  run  your program.   (Please  try  to run your program  on CSE-Lab  machines  before submitting )

This  should also include  two or more paragraphs explaining  what  each segment of the  server code does.  This  is not  a line-by-line  comments,  but  rather the  logic of the  server  code and what  each segment does, such as at what  point is the query cached on the local dns and why? Why does the server needs multiple  2 sockets and why are they setup  differently?  What  is the point of threading and how we handle  multiple  threads?

 

 

4    Tips

 

• Start early to avoid last-minute issues.

 

• Refer to the google group for any questions  and answers regarding  the assignment.

 

• Do not try  to learn a new language,  utilize knowledge of languages  you are familiar with.

 

• Save and compile your work frequently.

 

• Try  to make your program  simple, commented  and easy to read.

 

• Bonus:  Instead  of selecting the address  randomly,  you could use Ping value to pick up a closer server.