DNS Resolver

This collaborative project is a toy DNS resolver written in C and based on Julia Evans' Implement DNS in a Weekend Python guide.

Overview

The program features DNSQuery, DNSHeader, DNSQuestion, and DNSRecord structs that structure the various component parts of a DNS query and the parsed DNS response. At a high level, the program recursively resolves a given domain using the resolve() function. Each iteration of resolve does the following:

1. open a socket file descriptor using socket()
2. instantiate and populate a DNSQuery struct with details about the DNS request using constructor function NewDNSQuery()
3. send above DNSQuery struct bytes using sendto()
4. receive response with recvfrom() and store response bytes in a buffer
5. close socket file descriptor with `close()
6. parse response into a DNSPacket struct comprising of header, questions, answers, authorities, and additionals members. With the exception of the header member (a DNSHeader struct), the other components are linked lists of DNSQuestion and DNSRecord structs.

Based on the response, resolve() does one of three things:

1. If the response packet's answers member contains an DNSRecord struct with a valid answer, we're done!
2. Else if the response packet's additionals member contains a DNSRecord struct with a valid A record, send a new query for the original domain name to that nameserver IP.
3. Else if the response packet's authorities member contains a DNSRecord struct with a valid NS (nameserver) record, recursively resolve that nameserver domain for its IP address and, once obtained, send a new query for the original domain name to that newly-obtained nameserver IP.

Sandbox and Side Projects

The sandbox/ directory contains a number of DNS-related and DNS-adjacent experients and toy projects designed to improve intuition about networking in general and C's networking systems calls.