dnsresolver's Introduction

Welcome to DNS resolver!

Start the command-line program
- make run
Look up a domain name
- lookup x.x.x.x (ex: lookup www.cs.ubc.ca)
Set the root name server (optional, default is 199.7.83.42 which is close to UBC!)
- server x.x.x.x (ex: server 8.8.8.9)
Set verbose tracing on (optional, to print a trace of all queries and responses)
- trace on | off
Print the contents of the DNS cache (optional)
- dump
Quit
- quit

How to understand what is happening

This program is an iterative DNS resolver, meaning that the client repeatedly makes requests to DNS servers to resolve a domain name to an IP address (The code contains a lot of comments). Here are two example lookups, which provide the corresponding dig commands for a live demo:

We can simulate lookup www.cs.ubc.ca as follows:

dig +norecurse @199.7.83.42 www.cs.ubc.ca
dig +norecurse @185.159.196.2 www.cs.ubc.ca
dig +norecurse @137.82.1.1 www.cs.ubc.ca
dig +norecurse @137.82.61.120 www.cs.ubc.ca → we got the answer 142.103.6.5!

We can simulate lookup finance.google.ca (which contains aliases) as follows:

dig +norecurse @199.7.83.42 finance.google.ca
dig +norecurse @185.159.196.2 finance.google.ca
dig +norecurse @199.7.83.42 ns3.google.com → we needed to lookup the IP of an authoritative nameserver
dig +norecurse @192.33.14.30 ns3.google.com → we got the answer 216.239.36.10 for the authoritative nameserver
dig +norecurse @216.239.36.10 finance.google.ca → we got the answer 142.251.33.110!

Reference: https://www.ietf.org/rfc/rfc1035.txt

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