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CMSC 417 Computer Networks Fall 2007
Programming Assignment 2 — Simple DNS Client
Assigned: Sep. 28 Due: Oct. 7, 11:59PM (Phase 0) and Oct. 17, 11:59PM (Phase 1)
1 Introduction
This assignment has two primary purposes:
- Teach you communication using UDP
- Teach you how to use select(2) for IO multiplexing
You will write a simple, partially functioning DNS client. DNS is the Domain Name System, and is the system used to (among other things) map hostnames (bowman.cs.umd.edu) to IP addresses (128.8.128.147). DNS is used to maintain many other types of mappings as well, but we will only query DNS for A (address) records that map hostnames to IP addresses. The host to IP addresses are kept at DNS servers. The input to your code will be a set of pairs of the following form: (IP address of DNS server, query), e.g.:
(128.8.128.7, poole.cs.umd.edu) (128.8.128.13, bowman.cs.umd.edu)
Here, we’re asking 128.8.128.7 to give us the IP address for poole, and 128.8.128.13 for bowman. The server and the queries need not be restricted to the cs.umd.edu domain and your code will not need any change for querying any DNS server on the Internet Even though DNS servers respond to TCP queries, we will rely EXCLUSIVELY on UDP for this assignment. In class, I will give you a handout that describes the DNS protocol (and all the fields etc. in detail). Please read this, but this is NOT necessary for the solution. If you have any questions about any field, ask NOW. You will develop your code in two phases.
1.1 Phase 0
Your code accepts only one (DNS server, target) pair, and sends this single QUESTION to the server, waits for the response, and parses and prints the answer. You will need to be able to create a query packet, and then parse the RRs in the answer. The server may return “compressed” data, and you should be able to handle this. The phase 0 solution will be due on 7th of October, 2007.
1.1.1 Executable
The command line syntax and output for the phase 0 executable is as follows:
./dns_client < query.txt
where ./dns_client is your executable file, query.txt is a plain text file containing a set of pairs of the form: (IP address of DNS server, query) When a response is received, print it in the following form:
Header: ID=, flags=, QDCOUNT=, ANCOUNT=, NSCOUNT=, ARCOUNT= Question: QNAME=<double-quoted string>, QTYPE=, QCLASS= Answer: Authority: ... Additional: ... bytes received
Format of :
NAME=<double-quoted string>, TYPE=, CLASS=, TTL=, RDLENGTH=, RDATA=
1.1.2 Sample output
Header: ID=0x0, flags=0x8480, QDCOUNT=1, ANCOUNT=1, NSCOUNT=3, ARCOUNT= Question: QNAME="poole.cs.umd.edu", QTYPE=1, QCLASS= Answer: NAME="poole.cs.umd.edu", TYPE=1, CLASS=1, TTL=3600, RDLENGTH=4, RDATA=128.8.126. Authority: NAME="cs.umd.edu", TYPE=2, CLASS=1, TTL=72000, RDLENGTH=7, RDATA="dns2.cs.umd.edu" NAME="cs.umd.edu", TYPE=2, CLASS=1, TTL=72000, RDLENGTH=7, RDATA="bozo.cs.umd.edu" NAME="cs.umd.edu", TYPE=2, CLASS=1, TTL=72000, RDLENGTH=7, RDATA="dns1.cs.umd.edu" Additional: NAME="bozo.cs.umd.edu", TYPE=1, CLASS=1, TTL=3600, RDLENGTH=4, RDATA=128.8.128. NAME="dns1.cs.umd.edu", TYPE=1, CLASS=1, TTL=3600, RDLENGTH=4, RDATA=128.8.128. NAME="dns2.cs.umd.edu", TYPE=1, CLASS=1, TTL=3600, RDLENGTH=4, RDATA=128.8.128. 155 bytes received
1.2 Phase 1
Once you can talk to a server and get responses, you will send multiple queries out to a set of servers. In this assignment, please DO NOT use threads. Instead, we will use select(2) and timeouts to
NAME="dns1.cs.umd.edu", TYPE=1, CLASS=1, TTL=3600, RDLENGTH=4, RDATA=128.8.128. NAME="dns2.cs.umd.edu", TYPE=1, CLASS=1, TTL=3600, RDLENGTH=4, RDATA=128.8.128. 155 bytes received
Query #1: responded Header: ID=0x0001, flags=0x8480, QDCOUNT=1, ANCOUNT=1, NSCOUNT=3, ARCOUNT= Question: QNAME="bowman.cs.umd.edu", QTYPE=1, QCLASS= Answer: NAME="bowman.cs.umd.edu", TYPE=1, CLASS=1, TTL=3600, RDLENGTH=4, RDATA=128.8.128. Authority: NAME="cs.umd.edu", TYPE=2, CLASS=1, TTL=72000, RDLENGTH=7, RDATA="dns2.cs.umd.edu" NAME="cs.umd.edu", TYPE=2, CLASS=1, TTL=72000, RDLENGTH=7, RDATA="bozo.cs.umd.edu" NAME="cs.umd.edu", TYPE=2, CLASS=1, TTL=72000, RDLENGTH=7, RDATA="dns1.cs.umd.edu" Additional: NAME="bozo.cs.umd.edu", TYPE=1, CLASS=1, TTL=3600, RDLENGTH=4, RDATA=128.8.128. NAME="dns1.cs.umd.edu", TYPE=1, CLASS=1, TTL=3600, RDLENGTH=4, RDATA=128.8.128. NAME="dns2.cs.umd.edu", TYPE=1, CLASS=1, TTL=3600, RDLENGTH=4, RDATA=128.8.128. 156 bytes received
1.2.4 Sample output (all timeout)
$ ./dns_client -t 1 -r 2 < query.txt Query #0: timeout Query #0: retransmit Query #1: timeout Query #1: retransmit Query #0: timeout Query #0: retransmit Query #1: timeout Query #1: retransmit Query #0: timeout Query #1: timeout
2 Reference
RFC 1035: http://www.faqs.org/rfcs/rfc1035.html Read Section 4. Obviously, you’re encouraged to read the entire RFC, but you only NEED to read Section 4.
3 Bonus
Resolve arbitrary names without using recursion. Get the list of root server names, and “walk” the DNS tree as necessary to resolve any name. Of course, you’re welcome to include other types of queries (MX, HINFO, etc.) once you’ve mastered the basics.
