# Hostile-input and overload protection

Tracking: #15

Rendezvous treats every public HTTP request and UDP datagram as hostile. The
server applies bounded fixed-window request budgets and concurrency ceilings in
two stages so malformed input is discarded before expensive work while valid
traffic is also isolated by its authenticated scope.

## Enforcement order

1. Kestrel and the HTTP abuse middleware cap request bodies at 16 KiB. A known
   oversized body receives a typed `413` response before endpoint dispatch.
2. Every HTTP request consumes global, source-prefix, and operation budgets and
   acquires the corresponding concurrency leases. IPv4 sources share a `/24`
   budget and IPv6 sources share a `/56` budget; raw addresses are not retained.
   Non-lease operations also consume a smaller optional-work budget, leaving a
   configured global and source-prefix reserve for renew, update, and delete
   operations during shedding.
   Health probes use their own source-prefix budget so public API overload cannot
   make a healthy instance fail its orchestrator probes, while health traffic is
   still bounded.
   Operator endpoints likewise use a separate bounded rate/concurrency partition
   backed by the critical tracker reserve. They first require an exact source IP
   from the default-deny `OperatorAllowedAddresses` policy, so public traffic
   cannot spend the incident-response budget.
3. Once an endpoint has safely derived identities, it also acquires applicable
   tenant, principal or capability, and listing/attempt budgets. Secret
   capabilities are represented only by bounded SHA-256 fingerprints.
4. Every UDP envelope consumes global, source-prefix, and wire-operation
   budgets before decoding. A structurally and cryptographically valid request
   then consumes capability, role, and mediation-handle budgets before state
   mutation or introduction.
5. HTTP overload returns the stable `RateLimited` error, status `429`, and a
   bounded `Retry-After` value in both the header and response contract. UDP
   overload and every invalid UDP input are silently dropped.

The same HTTP identity budget is computed whether or not a listing or attempt
exists. Rejection therefore does not disclose resource existence. Publisher
authentication also completes before any tenant/resource operation, while the
pre-authentication source budget prevents invalid credentials from bypassing
load shedding.

## Bounded state and recovery

`Rendezvous:AbuseProtection:MaxTrackedKeys` is a hard combined ceiling for rate
and active-concurrency keys. General HTTP and UDP traffic cannot consume the
configured `CriticalTrackedKeyReserve`; lease operations, health probes, and
allowlisted operator controls may
use that reserve but never exceed the hard ceiling. A request that would exceed
its applicable ceiling fails closed without adding state. Fixed-window rate keys
are cleared at the next window boundary; concurrency keys are removed as their
request leases finish. HTTP and UDP trackers have separate locks and cardinality
partitions, so a UDP flood cannot block HTTP admission on a shared lock or
consume HTTP key capacity. This gives
deterministic burst recovery and prevents an attacker from growing a permanent
high-cardinality address, credential, or resource table.

The complete default profile is checked into
`src/FinalFactory.Rendezvous.Server/appsettings.json`. Operators may lower or
tune limits for a measured deployment profile, but must preserve all dimensions
and leave the tracker ceiling above the maximum simultaneous key set. A rolling
deployment should use the same profile on every instance. These per-process
limits are a final service boundary; an edge proxy may add stricter distributed
limits but is not a substitute for them.

When an HTTP reverse proxy is used, every immediate proxy address must be
allowlisted in `Rendezvous:AbuseProtection:TrustedProxyAddresses` (or indexed
environment variables such as
`Rendezvous__AbuseProtection__TrustedProxyAddresses__0`). Only one forwarded
hop is accepted. With an empty allowlist, forwarded headers are ignored and the
direct TCP peer is the source. Never add a broad network range or accept
untrusted `X-Forwarded-For` input: that would let a caller choose its own rate
partition.

## Reflection, disclosure, and logging rules

- UDP sends nothing for malformed, oversized, unauthenticated, stale,
  replayed, wrong-role, or rate-limited input.
- Introductions are emitted only after both role-scoped capabilities bind to
  their observed gameplay-socket sources. HTTP never supplies a public
  introduction target.
- Private candidates must be same-family private unicast addresses and are used
  only for peers observed behind the same public address.
- Abuse keys, exceptions, and responses never include bearer credentials,
  capabilities, tickets, raw endpoints, metadata values, or hostile markup.
- Endpoint and capability values are not used as metric labels or log fields.

## Verification

The deterministic test corpora use the recorded seeds `0x152026`, `0x154A50`,
and `0x1557A7E`. They exercise 10,000 arbitrary UDP envelopes through the
production decoder, 5,000 arbitrary HTTP/credential parser inputs, and 1,000
mutated state transitions, including the oversized and configured-capacity
boundaries.
Focused tests cover IPv4 and IPv6 prefix
partitioning, tenant/principal/resource concurrency, tracker exhaustion,
window recovery, wire-operation isolation, a steady-state allocation ceiling,
typed `429`/`413` responses, secret fingerprint redaction, and silent
authenticated UDP shedding. The existing state, contract, HTTP, client,
and mediator suites continue to cover cross-tenant access, replay, role swaps,
credential rotation, bounded metadata, endpoint validation, and one-shot
amplification behavior.