DNS Resolver

Author: Oleksandr Turytsia
Created at: 11/17/2023

A simple DNS resolver, implemented in C, that performs the task of translating human-readable domain names into IP addresses.

It supports such query types as A, AAAA, PTR. And response type A, AAAA, PTR, CNAME and SOA.

Files

Source code for this project is located at src/ folder. I should contain following files:

• args.c = Source file, that contains functions to parse input arguments
• args.h = Header file for args.c
• dns.c = Source file of a program. Main is located here.
• dns.h = Header file for dns.h
• error.c = Source file, that contains error handling function
• error.h = Header file for error.c
• libs.h = Header file with all the libs
• utils.c = Source file, that contains common functions for multiple source files
• utils.h = Header file for utils.c

Prerequisites

Before using the DNS resolver, make sure you have the following prerequisites installed:

• GCC (Testing was done for the version 10.5.0)
• Unix-based system (Testing was done on FreeBSD)

Installation

All the source code is located at src folder. Run following command in order to compile the project:

make

After the project is compiled, you can run following command to run a quick test:

./dns -r -s dns.google www.github.com

Usage

./dns [−r] [−x] [−h] [−t] [−6] −s server [−p port] address

• -r: Recursion Desired (Recursion Desired = 1), otherwise no recursion.
• -6: Query type AAAA instead of the default A.
• -x: Reverse query instead of direct query. Note, user can specify here ipv6 or ipv4 address without specifying -6 option to make a reverse query.
• -s server: IP address or domain name of the server to which the query should be sent. Note, user can specify server by its domain or ipv6 address.
• -p port: The port number to send the query to, default is set to 53.
• -h: Display help info.
• -t: Enables testing mode (TTL is set to 0).
• address: The address to be queried

Output

The program will construct dns query based on user’s input and send packet over udp to a specified dns server. Dns response may have following format:

Authoritative: Yes/No, Recursive: Yes/No, Truncated: Yes/No
Question section (1)
[ADDRESS], [QTYPE], [QCLASS]
Answer section (N)
[ADDRESS], [TYPE], [CLASS], [TTL], [RDATA]
...
Authority section (N)
[ADDRESS], [TYPE], [CLASS], [TTL], [RDATA]
...
Additional section (N)
[ ADDRESS ], [TYPE], [CLASS], [TTL], [RDATA]
...

First line displays response header flags (RD, TC, AA) followed by request response (RR) sections

Error codes

DNS resolver is also suitable to be used as a part of a script, because it provides distinctive exit error codes, which can help potential programmers validate results

• Arguments parser error codes:
  • 1 - Unknown option
  • 2 - Invalid port
  • 3 - Port is missing
  • 4 - Source address is missing
  • 5 - Target address is missing
  • 6 - Option already specified
• Sending query error codes:
  • 10 - Socket creation error
  • 11 - Sending query error
  • 12 - Timeout
  • 13 - Recieving response error
• DNS header error codes (By subtracting 30 it is possible to get error codes that correspond RFC 1035 documentation):
  • 31 - Format error
  • 32 - Server fail
  • 33 - Name error
  • 34 - Not implemented
  • 35 - Refused
• Other errors:
  • 20 - perror code
  • 21 - other errors
  • 22 - Family is not supported

Bibliography

RFC 1035 RFC 3596