Implement a DNS Resolver

DNS Message Layout

DNS translates names like example.com into IP addresses like 93.184.216.34. A query fits in a single UDP datagram to port 53 on a resolver such as 8.8.8.8.

The 12-byte header contains:

ID: 16-bit transaction identifier (e.g. 0x1234)
Flags: QR bit, recursion desired, response code
Counts: questions, answers, authority, additional records

Question and Answer Sections

A type-A query (type 1, class 1) asks for an IPv4 address. The name is encoded in length-prefixed labels: 7example3com0 for example.com.

The answer section returns the resolved name, type, TTL, and the 4-byte IPv4 address. Other record types include AAAA for IPv6, MX for mail routing, and NS for delegating authority to another nameserver. Recursive resolvers set the RD (recursion desired) bit and rely on upstream servers to chase referrals through the authority section.

Why This Exercise

This task requires you to build a DNS query packet and parse the response. glibc's getaddrinfo() does this internally against resolvers like 8.8.8.8 or 1.1.1.1, encoding names into DNS's length-prefixed label format before parsing the answer section. Dan Kaminsky's 2008 cache-poisoning attack exploited weak randomization in the very transaction ID field you set here, which is why modern resolvers randomize both the ID and the source port.

Write a C program that builds a DNS query packet, sends it via UDP to 8.8.8.8 (Google Public DNS), and parses the response.

Requirements:

Create a UDP socket and connect to 8.8.8.8 on port 53
Build a DNS query packet with a valid header and question section
Encode the domain name using DNS label format (length-prefixed labels)
Set query type to A (1) and class to IN (1)
Send the packet and receive the response
Parse the response header to extract answer count
Skip the question section and parse at least one answer
Extract and display the IPv4 address from the answer
Print the raw packet bytes in hexadecimal for debugging
Print a clear message showing the resolved IP address

DNS Message Layout

DNS translates names like example.com into IP addresses like 93.184.216.34. A query fits in a single UDP datagram to port 53 on a resolver such as 8.8.8.8.

The 12-byte header contains:

ID: 16-bit transaction identifier (e.g. 0x1234)
Flags: QR bit, recursion desired, response code
Counts: questions, answers, authority, additional records

Question and Answer Sections

A type-A query (type 1, class 1) asks for an IPv4 address. The name is encoded in length-prefixed labels: 7example3com0 for example.com.

Why This Exercise

Write a C program that builds a DNS query packet, sends it via UDP to 8.8.8.8 (Google Public DNS), and parses the response.

Requirements:

Create a UDP socket and connect to 8.8.8.8 on port 53
Build a DNS query packet with a valid header and question section
Encode the domain name using DNS label format (length-prefixed labels)
Set query type to A (1) and class to IN (1)
Send the packet and receive the response
Parse the response header to extract answer count
Skip the question section and parse at least one answer
Extract and display the IPv4 address from the answer
Print the raw packet bytes in hexadecimal for debugging
Print a clear message showing the resolved IP address

Topic Overview

DNS Message Layout

Question and Answer Sections

Why This Exercise

Implementation Task

Progressive Hints

Resources

Open Source

Implement a DNS Resolver

Topic Overview

DNS Message Layout

Question and Answer Sections

Why This Exercise

Implementation Task

Progressive Hints

Resources

Open Source