With trusted_proxies covering client networks (e.g. 10.0.0.0/8 over LAN
clients), the right-to-left XFF walk skipped every entry, exhausted, and
fell back to the proxy's RemoteAddr — collapsing all clients into one
rate-limit/lockout bucket, so one user's failed logins locked out
everyone. On exhaustion the walk now returns the leftmost valid entry
(furthest-upstream hop), the best distinct per-client key such a config
allows. An untrusted RemoteAddr still never gets its headers honoured.
Also (W3-3): the CIDR list parses once per request instead of once per
XFF candidate, config load warns about invalid CIDR entries at startup
(a silently skipped entry silently un-trusts the proxy), and the sample
config documents that trusted_proxies must list only proxy hops.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>