The DNS auth endpoint was vulnerable to resource exhaustion if an
attacker controlled a domain with a slow/non-responding authoritative
DNS server. Without a timeout, handler goroutines would block on DNS
resolution indefinitely, allowing an attacker to pile up goroutines
until OOM.
Production is protected by NGINX ingress's default 60s proxy timeout,
but this adds defense-in-depth with a tighter 5s bound.
dcb5a6f696