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Rado Kubiak | Rado x Tech
745cc1813e
- In Alpine Linux containers (commonly used as Docker base images),
standard DNS resolution is provided by **musl**, a lightweight C
standard library (libc). Musl implements DNS lookups via
`getaddrinfo()`, which queries AAAA (IPv6) records first.
- Problem: DNS did not work correctly **inside containers**. Any
system command attempting to resolve hostnames
(e.g., `ping dynamodb-admin`) **failed** when the DNS server
responded NXDOMAIN for AAAA records, even if A (IPv4) records
existed. Explicitly forcing IPv4 (`ping -4dynamodb-admin`) worked
correctly, showing the issue is specific to musl’s IPv6-first behavior.
- Consequence: In IPv4-only environments, Alpine-based containers
cannot resolve hostnames using standard tools or libraries.
Applications relying on `getaddrinfo()` fail with ENOTFOUND,
breaking networking and inter-container communication.
- Root cause: Following RFC 8305 / RFC 6724, musl treats NXDOMAIN
for AAAA as “hostname does not exist” and does not fallback to A
(IPv4) records.
- Fix: The Apple Container DNS engine now behaves as follows:
* If an **A record exists**, AAAA queries return **NOERROR with empty
answer (NODATA)**.
* If neither **A nor AAAA** exist, NXDOMAIN is returned.
This ensures that Alpine-based containers in IPv4-only networks can
correctly resolve hostnames inside containers without modifying
container images or application code.