Assignment 1: Network Programming and DNS

ECSE316–Signals and Networks
Assignment 1: Network Programming
and DNS
1 Introduction
In this lab you will implement a domain name system (DNS) client using sockets in Java. If you
prefer, the system can also be implemented in Python.
At the end of this lab you should know how to:
1. Understand an existing network protocol specification;
2. Design a network application that adheres to the given protocol; and
3. Implement and test the network application using sockets.
As always, read the instructions carefully!
2 Background
2.1 The Domain Name System (DNS)
The internet’s domain name system provides the essential service of mapping between domain names
(e.g., mcgill.ca or facebook.com) and IP addresses (e.g., 132.216.177.160 or 31.13.65.1). This makes the
internet more human-friendly, since we only have to remember domain names and not IP
addresses.
When you direct an internet-enabled application to a new site (e.g., you open a new website in your
browser, or you send an email to someone at a different domain name), the application needs to
resolvethedomainname toanIPaddressbeforeitcancontacttheserver.This i s accomplishedusing
the DNS request/response mechanism. For more abouttheDNSsystemandthe protocol it uses, see
Section 2.4 of Kurose and Ross (7
th edition) or Section 2.5 in Kurose and Ross (6th edition) and the
primerdocumentpostedalongwiththeseinstructionsonmyCourses.You may alsolookatRFC1035
(Sections 3and 4)whichgive the official descriptionof the DNS resource record format andmessage
format.
2.2 Socket programming
Sockets are the programming mechanism used to implement network applications. If you have
previouslywrittenanapplicationthatreadsor writes fromafileondisk,thenyou knowyoufirstopen
afilehandle,then read/writeusingthefilehandle,andfinally closethefilehandlewhenyouare done.
Sockets work in a similar way, but instead of reading/writing to a file on disk, they allow you to
read/write to another process (usually running on a differentmachine).
Formoreon socketprogramming readSection2.7inKuroseandRoss (6thEdition-Java or7
th edition
-Python) and the JavaNetworkingTutorialfromOracle.If youchoose thePythonapproach, youcan
read the material available at https://docs.python.orG/2/howto/sockets.html
2.3 Summary of background and pre-requisites
Before proceeding with the rest of this lab document make sure you have accomplished these first
steps:
1. You are familiar with basic Java or Python programming, including how to compile and
executea programfromthecommandline.Wealso recommendthatyouarefamiliarwithan
integrated development environment(IDE), such as Eclipse, for debugging.
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2. You have read the background on the DNS and on java/Python socketprogramming
mentioned above.
3 Lab requirements
Write a program using Java/Python sockets that:
• Is invoked from the command line (STDIN);
• Sends a query to the server for the given domain name using a UDP socket;
• Waits for the response to be returned from the server;
• Interprets the response and outputs the result to terminal display (STDOUT).
Your DNS client should be capable of performing the following actions:
• Send queries for A (IP addresses), MX (mail server), and NS (name server) records;
• Interpret responses that contain Arecords (iPad dresses) andCNAME records (Unaliases);
• Retransmit queries that are lost.
Your client must also handle errors gracefully. In particular, if the response message does not
conform to the DNS specification, if it contains fields or entries that cannot be interpreted, or if
the client receives a response that does not match the query it sent, then an appropriate error
message shouldbe printed to the screen indicating what was unexpected or what wentwrong.
For this lab you must use Java or Python. You must write the code that constructs a DNS request
packet and interprets the DNS response packet. You may not use external DNS libraries or classes
in your project (such as net.URL or net.InetAddress). Specifically, in Java you may only use the
net.InetAddress.getByAddress(byte[] addr) method; you may not use any of the net.InetAddress
methods that take an argument of the form String host. (The same requirements apply if you use
net.Inet4Address instead of net.InetAddress.) Similar instructions apply for a Python
implementation; please ask me or the TAs if you are not sure about whether you can make use of a
library or class.
3.1 Calling syntax
Your application should be named DnsClient, and it should be invoked at the command line using
the following syntax (for Java; the options should be the same for Python):
java DnsClient [-t timeout] [-r max-retries] [-p port] [-mx|-ns] @server name
where the arguments are defined as follows.
• timeout(optional) gives how long to wait, inseconds, before retransmitting an
unanswered query. Default value: 5.
• max-retries(optional) is the maximum number of times to retransmit an
unanswered query before giving up. Default value: 3.
• port(optional) is the UDP port number ofthe DNS server. Default value: 53.
• -mx or -ns flags (optional) indicate whether to send a MX (mail server) or NS (name server)
query.At most one ofthese can be given, and if neither is given then the client should send a
type A (IP address) query.
• server (required) is the IPv4 address of the DNS server, in a.b.c.d.format
• name (required) is the domain name to query for.
For example to query for www.mcgill.ca IP address using the McGill DNS server, enter
java DnsClient @132.206.85.18 www.mcgill.ca
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or toqueryforthemcgill.camailserverusingGoogle’spublicDNSserverwithatimeoutof10seconds
and at most 2 retries, enter
java DnsClient –t 10 –r 2 –mx @8.8.8.8 mcgill.ca
3.2 Output behavior
Your program should print output to STDOUT using the following standard format. The first lines
should summarize the query that has been sent:
DnsClient sending request for [name]
Server: [server IP address]
Request type: [A | MX | NS]
Wherethefieldsinsquarebracketsarereplacedbyappropriatevalues.Then,subsequentlinesshould
summarize theperformance andcontentofthe response.Whenavalidresponse is received,the first
line should read:
Response received after [time] seconds ([num-retries] retries)
If the response contains records in the Answer section then print:
***Answer Section ([num-answers] records)***
Then, ifthe response containsA(IPaddress) records, eachshouldbe printed on a line ofthe form:
IP <tab [ip address] <tab [seconds can cache] <tab [auth | nonauth]
Where <tab is replaced by a tab character. Similarly, if it receives CNAME, MX, or NS records,
they should be printed on lines of the form:
CNAME <tab [alias] <tab [seconds can cache] <tab [auth | nonauth]
MX <tab [alias] <tab [pref] <tab [seconds can cache] <tab [auth |
nonauth] NS <tab [alias] <tab [seconds can cache] <tab [auth | nonauth]
If the response contains records in the Additional section then also print:
***Additional Section ([num-additional] records)***
along with appropriate lines for each additional record that matches one of the types A, CNAME,
MX, or NS. You can ignore any records in the Authority section for this lab.
If no record is found then a line should simply be printed saying
NOTFOUND
Ifanyerrorsoccurduringexecutionthenlinesshouldbeprintedsaying
ERROR <tab [description of error]
Be specific with your error messages. Some example of errors your DNS client may output are:
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ERROR <tab Incorrect input syntax: [description of specific problem] ERROR <tab
Maximum number of retries [max-retries] exceeded
ERROR <tab Unexpected response [description of unexpected response content]
4 Important dates, deliverables, and evaluation
4.1 Report
The codeandthereport are due at 23:59onFeb. 22nd
, and it will countfor 7.5%of your final grade.
Your report should include the following items:
• The names and McGill ID numbers of both group members
• Design: Describe the design of your application
• Testing: Describe how you tested your application to make sure it behaves as required
• Experiment:
o What are the IP addresses of McGill’s DNS servers? Use the Google public DNS
server (8.8.8.8) to perform a NS query for mcgill.ca and any subsequent follow-up
queries that may be required. What response do you get? Does this match what you
expected?
o Use your client to run DNS queries for 5 different website addresses, of your choice,
in addition to www.google.com and www.amazon.com, for a total of seven
addresses. Query the seven addresses using the Google public DNS server (8.8.8.8).
o Briefly explain what a DNS server does and how a query is carried out. Modern web
browsers are designed to cache DNS records for a set amount of time. Explain how
caching DNS records can speed up the process of resolving an IP address. You can
draw a diagram to help clarify your answer.
To submit your report, upload a PDF file on myCourses. Only one report needs to be uploaded per
group.
4.2 Code
Whensubmitting your report on myCourses, you should also upload a zip file containing your code.
Make sure to include all source (.java) files required to compile and execute your program. Also
include a readme file giving any special instructions required to compile your code and mentioning
what version of Java or Python you used when writing/testing yourprogram.
5 Additional advice
Java’s supportforunsigned integers is somewhatlimitedcomparedto other languages.For example,
a byte is an 8-bit signed integer (taking values in the range -128 to 127), whereas a char is a 16-bit
(unsigned) Unicode character. See the Java Primitive Data Types tutorial for more.
Rather than using character input/output streams, consider using the class java.nio.ByteBuffer when
preparing a packetto send. Thiswill give you somecontrol, byte-by-byte, ofthe data inthe packet.