fix: escape dots and special characters in DNS label text representation #36

New Issue

2026-04-07T02:20:52+08:00

razvandimescu commented

2026-04-07 02:20:52 +08:00

(Migrated from github.com)

Bug

Reported by fanf2 on HN: read_qname pushes raw bytes from wire format labels without escaping special characters. A label containing a dot (byte 0x2E) produces ambiguous text output.

Example: Wire format [8]exa.mple[3]com[0] (two labels) → currently outputs exa.mple.com (looks like three labels). Should output exa\.mple.com.

Root cause

src/buffer.rs line 123-125:

let str_buffer = self.get_range(pos, len as usize)?;
for &b in str_buffer {
    outstr.push(b.to_ascii_lowercase() as char);
}

Raw bytes pushed directly without escaping.

Fix needed

read_qname — escape per RFC 1035 §5.1:

Dots inside labels: \.
Backslashes: \\
Non-printable bytes (< 0x21 or > 0x7E): \DDD (3-digit decimal)

write_qname — parse escaped characters when splitting labels:

Don't split on \. (escaped dot)
Unescape \\ and \DDD sequences

Impact

Low in practice — real-world domains don't contain dots in labels. But it's a correctness issue that could cause wire format round-trip failures with adversarial input.

Credit: fanf2 (HN)

## Bug Reported by [fanf2 on HN](https://news.ycombinator.com/item?id=47612321): `read_qname` pushes raw bytes from wire format labels without escaping special characters. A label containing a dot (byte `0x2E`) produces ambiguous text output. **Example:** Wire format `[8]exa.mple[3]com[0]` (two labels) → currently outputs `exa.mple.com` (looks like three labels). Should output `exa\.mple.com`. ## Root cause `src/buffer.rs` line 123-125: ```rust let str_buffer = self.get_range(pos, len as usize)?; for &b in str_buffer { outstr.push(b.to_ascii_lowercase() as char); } ``` Raw bytes pushed directly without escaping. ## Fix needed **`read_qname`** — escape per RFC 1035 §5.1: - Dots inside labels: `\.` - Backslashes: `\\` - Non-printable bytes (< 0x21 or > 0x7E): `\DDD` (3-digit decimal) **`write_qname`** — parse escaped characters when splitting labels: - Don't split on `\.` (escaped dot) - Unescape `\\` and `\DDD` sequences ## Impact Low in practice — real-world domains don't contain dots in labels. But it's a correctness issue that could cause wire format round-trip failures with adversarial input. Credit: fanf2 (HN)

Sign in to join this conversation.

1 Participants

Notifications

Due Date

No due date set.

Dependencies

No dependencies set.

Reference: dearsky/numa#36