Security · Advanced
Build a DNS Resolver
Write a DNS resolver that builds and parses binary DNS packets over UDP and recursively resolves names starting from the root servers.
You build a DNS resolver that constructs raw DNS query packets, sends them over UDP, and parses the binary responses including the header, question, and answer sections with their compressed name pointers. You then implement full recursive resolution: starting at a root server, you follow referrals down through TLD and authoritative servers until you obtain the final A record, rather than relying on an upstream resolver. It is worth building because DNS underpins essentially all internet traffic, yet most engineers have never seen how a name actually gets resolved. Implementing it teaches binary protocol parsing, UDP networking, and the delegation hierarchy of the internet, and the security angle on spoofing and cache poisoning makes it a standout project for security-focused roles.
What you build
- Encodes DNS queries into the binary wire format
- Sends and receives datagrams over UDP
- Parses headers, questions, answers, and resource records
- Decompresses domain names that use pointer compression
- Follows referrals recursively from the root name servers
- Resolves A, NS, and CNAME record types
- Reports the resolution path and final IP address
What it teaches
- Binary packet construction and bit-level layout
- UDP socket programming
- DNS message format and name compression
- Recursive resolution and the delegation hierarchy
- Reading protocol specifications precisely
- Security concerns like spoofing and cache poisoning
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Take it further
- Add a response cache that respects record TTLs.
- Support more record types like MX, AAAA, and TXT.
- Add DNS-over-TLS or experiment with spoofing detection.
