CO285: Introduction to Networks

Assignment 8: Domain Naming System

Due: October 22, 1998

Practice using the DNS by running the program 'nslookup' on an NT machine or academic VAX/Alpha. NSLOOKUP is a console application on NT machines (in %systemroot%\system32). To run it on the VAX, log into your student account, and type MU NSLOOKUP.

The possible commands are shown below (they're a lot of help!):

Commands: (identifiers are shown in uppercase, [] means optional)
NAME - print info about the host/domain NAME using default server
NAME1 NAME2 - as above, but use NAME2 as server
help or ? - print info on common commands
set OPTION - set an option

all - print options, current server and host
[no]debug - print debugging information
[no]d2 - print exhaustive debugging information
[no]defname - append domain name to each query
[no]recurse - ask for recursive answer to query
[no]search - use domain search list
[no]vc - always use a virtual circuit
domain=NAME - set default domain name to NAME
srchlist=N1[/N2/.../N6] - set domain to N1 and search list to N1,N2, etc.
root=NAME - set root server to NAME
retry=X - set number of retries to X
timeout=X - set initial time-out interval to X seconds
querytype=X - set query type, e.g., A,ANY,CNAME,MX,NS,PTR,SOA
type=X - synonym for querytype
class=X - set query class to one of IN (Internet), CHAOS, HESIOD or ANY

server NAME - set default server to NAME, using current default server
lserver NAME - set default server to NAME, using initial server
finger [USER] - finger the optional NAME at the current default host
root - set current default server to the root
ls [opt] DOMAIN [> FILE] - list addresses in DOMAIN (optional: output to FILE)

-a - list canonical names and aliases
-d - list all records
-t TYPE - list records of the given type (e.g. A,CNAME,MX,NS,PTR etc.)

view FILE - sort an 'ls' output file and view it with pg
exit - exit the program

Write down the IP numbers of the following systems (if you can):

vax2.rockhurst.edu, euler.cs.rockhurst.edu, thumper.bellcore.com, cs.arizona.edu, gatekeeper.dec.com www.theonion.com, ftp.wustl.edu, acm.org and ieee.org.

Notice that all the responses show up as non-authoritative. This means that the DNS nameserver that you are using  has looked up this information previously and has cached a copy so that it doesn't have to do all the work again. Most of the time, these cached values are perfectly fine, but if you want to be certain, then you need to get an authoritative response from the nameserver that knows what it is talking about. To do this, go to a root name server by typing

> root

then work your way down the name tree by using the server command to change the name server you are using. For instance, to find the nameserver for theonion.com, try

> theonion.com. [in this case, you must add the trailing dot (.) ]

Name:    theonion.com
Served by:
- NS1.ITIS.COM
          209.83.0.27
          theonion.com

- NS2.ITIS.COM
          209.83.0.131
          theonion.com

then change the server to one of these to lookup the machine address.

This will always give you an authoritative answer (i.e., it's from the horse's mouth). Find the authoritative nameservers for each of the previous host names and write down their DNS names and IP addresses. You may think this redundant and/or overkill, but it is the process that your local nameserver goes through whenever it tries to locate a new name and is the big reason why it keeps a cache of the names it has recently looked up. In theory, any computer on the Internet can use any nameserver—you could set up your system to use a nameserver in Thailand. While it certainly would work, it might be a little slow. This is why you typically set up the nameserver to be a computer that you have fast access to.

Also, in the event the system's primary DNS server can't find the address, it falls back to secondary (and tertiary, etc.) configured name servers until it runs out of servers or finds the address. This is also a handy trick that will give you access to non-standard top-level domains; by adding a non-standard nameserver to your list for DNS resolution, you'll have access to the experimental domains registered with AlterNIC. If you're feeling particularly adventuresome, you can set up either of the following names/IP as tertiary/quaternary nameservers:

MX.ALTERNIC.NET
EEK.HTTPD.COM

Summary of Tasks

Turn in:

1. IP addresses you found from the above exercises (hosts and authoritative nameservers).
2. Answer to why you can send email to j.cigas@ieee.org, but can't find their computer's IP address. If you can find it, show how. (Hint, the answer is in the book).