c-ares / c-ares / 24313282876

92%

Build Type

push

github

Committed by web-flow

Commit Message

validate RR counts before preallocation to reject malformed packets early (#1134)

## Summary

Validate DNS RR counts before preallocation to reject malformed packets
early.

## What’s the issue?

The parser currently trusts RR counts from the DNS header (`ANCOUNT`,
`NSCOUNT`, `ARCOUNT`) without verifying that the packet contains enough
data.

A malformed packet can claim a large number of records without providing
sufficient payload, causing the parser to proceed with unnecessary work
and incorrect handling.

## What changed?

- Added validation to ensure RR counts are feasible based on the
remaining buffer size
- Moved RR preallocation from `ares_dns_parse_header` to
`ares_dns_parse_buf`
- Perform validation after question parsing and before preallocation  
- Return `ARES_EBADRESP` early for malformed packets  

## Why this approach?

This keeps the parser aligned with its natural flow:

parse header → parse question → validate → preallocate → parse records

It avoids trusting unverified input and ensures malformed packets are
rejected early and consistently.

## Test

Added a regression test (`ParseMalformedRRCount`) that reproduces the
issue using a malformed packet with inconsistent RR count.

Signed-off-by: @jmestwa-coder

Coverage Stats

20 of 20 new or added lines in 2 files covered. (100.0%)

1 existing line in 1 file now uncovered.

22800 of 24861 relevant lines covered (91.71%)

13010.11 hits per line