Decode a DNS-wire label sequence (no pointer compression)

Challenge

DNS encodes a domain name as a sequence of length-prefixed labels followed by a 0-length terminator:

[3]'w' 'w' 'w' [7]'e' 'x' 'a' 'm' 'p' 'l' 'e' [3]'c' 'o' 'm' [0]

For this exercise, we IGNORE pointer compression (top 2 bits of length = 11). We just decode raw labels into name.name.name\0.

Implement int decode_dns_labels(const unsigned char *in, int in_len, char *out, int cap).

Return the number of consumed input bytes on success, -1 on parse failure.

Why this matters

DNS wire format uses length-prefixed labels. A parser that does pointer-compression wrong can loop forever — a classic CVE primitive in resolvers.

Input format

DNS-wire bytes + length + output buffer.

Output format

Bytes consumed or -1.

Constraints

Refuse labels >63 bytes (DNS spec). Refuse output overflow.

Starter code

#include <stddef.h>
int decode_dns_labels(const unsigned char *in, int in_len, char *out, int cap) { /* TODO */ (void)in; (void)in_len; (void)out; (void)cap; return -1; }

Common mistakes

Forgetting that each label is <= 63 bytes (top 2 bits clear).

Edge cases to handle

Root label (just 00) → empty output. Truncated input.

Complexity

O(in_len).

Background lessons

• Safe parsing — the defensive parser shape
• Log parsing — line iteration + classification