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ModernUO/CLAUDE.md
Kamron Batman c39454137e
feat(network): pluggable connection filters; file blocklist + contribute-first CrowdSec (#2542) 
Reshapes IP banning around one idea: **core owns the question, content owns every answer.**

Core gains a single accept-path seam — `IConnectionFilter` — and loses everything that used to implement one. The firewall moves to UOContent, a new file-backed blocklist joins it there, and CrowdSec is repositioned from an in-app enforcer to a contribute-first reporter.

## The seam

```csharp
public interface IConnectionFilter
{
    string Name { get; }
    void Configure();
    void Start(CancellationToken token);
    void Stop();
    bool ShouldDeny(IPAddress address);
}
```

The accept path went from hardcoded branches to one question:

```csharp
else if (ConnectionFilters.ShouldDeny(remoteIP, out var deniedBy))
{
    logger.Debug("{Address} denied by connection filter '{Filter}'", remoteIP, deniedBy);
}
```

Filters register during the Configure sweep. The registry is a plain array walked by an indexed loop — no enumerator, no closure, no allocation — and the first denial short-circuits. An interface dispatch is noise next to the `accept()` syscall, so pluggability costs nothing measurable on the path that has to survive a DDoS.

Whatever a hit implies — persisting, promoting to an OS bouncer, contributing to the ban channel — is the filter's business, not the accept path's.

A filter that throws is **unregistered and the connection fails open**. A filter that faults once faults for every subsequent connection, so leaving it registered means an exception and a log line per accept — exactly the amplification an attacker wants — and a broken filter must not be able to deny everyone either.

This deliberately does **not** reuse `EventSink.InvokeSocketConnect`: that fires later and allocates a `SocketConnectEventArgs` per connection, which is what the accept path avoids for rejected traffic.

## What ships behind it

**`firewall`** (UOContent) — the existing admin-curated set. Collapsed from `Firewall` + `AdminFirewall` + a threaded enforcer into one single-threaded store with **zero concurrency primitives**: the accept path, admin gump, TTL expiry and boot load all run on the game loop. Persists to `Configuration/firewall.json` with automatic migration from the legacy `firewall.cfg`. No behavior change for operators — same namespace, same gump, same commands.

**`blocklist`** (UOContent) — new. Holds a millions-strong list in-app and **demand-pages** hits up to CrowdSec, which promotes them to the OS firewall.

The motivation is concrete: CrowdSec's Windows bouncer cannot load the ~3.9M IPs that 91 community feeds produce, but it handles ~100k fine. So the millions live in-process behind a binary search, and only addresses that *actually connect* get promoted. A `PromotedGuard` suppresses re-reporting an address until the bouncer picks it up.

The list is parsed straight from UTF-8 file bytes with no per-line string allocation, off the game loop, and published as an immutable snapshot swapped through a single `volatile` reference. Reloads yield to world saves.

**`tools/Export-IpBlocklist.ps1`** — the producer. Requires PowerShell 7 and runs on Windows, Linux and macOS; Windows PowerShell 5.1 is refused up front via `#requires`. Merges a thin, non-overlapping feed set into one de-duplicated, bogon-filtered file. Parsing runs in a compiled `Add-Type` hot loop (~1s for ~4M lines instead of minutes). Written to a `.tmp` sibling and swapped with `File.Replace`, so the shard never reads a half-written list, and a total feed outage refuses to overwrite a good list with an empty one. Re-running is idempotent — it exits without downloading anything while the list on disk is younger than `-MinInterval` (default 2h, the anchor feed's own refresh period), so a misconfigured scheduler can't hammer upstream.

## CrowdSec: contribute-first

`IBanReporter` + `BanChannel` fan locally-decided bans out to external systems. `CrowdSecReporter` (UOContent) posts to LAPI `POST /v1/alerts` and retracts via `DELETE /v1/decisions`.

Reporting is **enqueue-only** on the accept path: a bounded, coalescing channel drained off-loop with bounded retry, counted drops on overflow, and a flush on shutdown. Under a DDoS the accept path never does synchronous or lock-contending per-IP work.

### Why not pull decisions from CrowdSec?

The original design streamed decisions into an in-app snapshot and enforced them at the accept gate. That's the wrong layer: by the time the shard sees the connection, the TCP handshake and socket setup are already paid for. `cs-firewall-bouncer` drops the same traffic **at the kernel**, and it's what CrowdSec is built to do. So the shard now contributes what it uniquely knows (rate-limit trips, blocklist hits from real connection attempts) and lets the OS enforce.

The one thing the OS can't do — hold millions of entries on Windows — is exactly what the in-app blocklist covers, and it feeds the same pipeline.

## Threading policy

`CLAUDE.md` rule #3 is rewritten as an explicit three-part policy, with rule #10 restated in tandem:

- Anything touching game state runs **only** on the main loop.
- Heavy work that *needs* game state must be **chunked** across ticks, never threaded.
- Heavy work that does *not* need game state (large-file parse, external I/O) **must** run off-loop **and must yield to world saves**.

Results come back via an immutable snapshot swapped through a single `volatile` reference, or `Core.LoopContext.Post` — never by letting the scheduler decide where heavy work runs. Both new subsystems follow it.

## Shared primitives

`SortedRangeIndex<T> where T : IBinaryInteger<T>` — coalesced disjoint interval arrays plus a binary search. The firewall, the blocklist, and (as of this PR) core's reserved-network tables all use it.

Coalescing is a correctness requirement, not an optimization: multi-feed lists nest CIDRs (`/24` containing a `/32`), and a search that inspects only the rightmost run whose minimum is ≤ the value is sound **only** over disjoint runs. That bug was caught in review and is covered by regression tests.

`IPAddressUtility` collects the allocation-free `IPAddress` ↔ `UInt128` conversions and CIDR parsing that were previously scattered or duplicated.

## Config

| File | Owner | Keys |
|---|---|---|
| `Configuration/bans.json` | core | `reportRateLimitTrips`, `autoBanDuration` |
| `Configuration/blocklist.json` | content | `file`, `reloadInterval`, `reportHits`, `banDuration`, `promoteSuppression` |
| `Configuration/crowdsec.json` | content | `lapiUrl`, `machineId`, `password`, `origin`, `manualBanDuration`, `flushInterval`, `maxQueue` |
| `Configuration/firewall.json` | content | persisted firewall entries (migrated from `firewall.cfg`) |

Everything is inert by default. CrowdSec self-disables without credentials; the blocklist self-disables until its file exists. A shard that changes nothing sees no behavior change.

## Notes for review

- **Core no longer references `Firewall` or `IFirewallEntry` anywhere.** `NetworkUtilities` used to build its reserved-network tables out of `CidrFirewallEntry`, which coupled core to the firewall for something unrelated to banning; those are now a `SortedRangeIndex<UInt128>`, same semantics and public API.
- **`BanChannel.Stop()` no longer persists the firewall** — a contribution coordinator has no business saving an enforcement store. That's the firewall filter's `Stop()`.
- **A dead `whitelisted` parameter was dropped** from the blocklist gate: it was hardcoded `false` at its only call site, and no whitelist concept exists in core.
- **The blocklist filter is an instance, not a static.** The static version forced its tests onto the sequential collection with a reset hook; they now run in parallel.
- `dev-docs/networking-packets.md` documents the seam for content authors, plus a known wart in the `IPAddress` ↔ `UInt128` normalization flagged for a follow-up PR.
- The generator was verified on Linux, macOS and Windows under a temporary CI matrix (since removed). It caught two portability bugs — a Windows-only path separator, and a culture-sensitive duration parse that read `2.5` as `25` on comma-decimal locales and *silently* turned a 2.5h cooldown into 25h — plus a third that made the script unparseable on Windows PowerShell 5.1. The source is ASCII-only for that last reason: `#requires` is only honored once a file parses, so non-ASCII in a BOM-less script produces parse errors instead of the version message.

## Tests

**1344 pass** (782 `Server.Tests`, 562 `UOContent.Tests`). New coverage: filter registry (registration, short-circuit, fault-disable), blocklist parsing/CIDR/coalescing, snapshot reload markers, promote-guard TTL, ban-channel fan-out, CrowdSec alert building/dedup/flush-on-stop, and the generator's output-format contract pinned against the reader.
2026-07-25 11:59:37 -07:00

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ModernUO

.NET 10 Ultima Online server emulator. Single-threaded game loop. All game logic runs on one thread.

  • Server engine: Projects/Server/ — do NOT modify without explicit request
  • Game content: Projects/UOContent/ — primary editing target
  • Build: dotnet build from repo root

Code Audit Rules

Apply these when writing or reviewing .cs files under Projects/.

  1. LINQ — Tier 1 (zero-cost patterns) free on hot paths; Tier 2 (low overhead) OK on warm paths; Tier 3 (allocating) forbidden on hot paths → dev-docs/code-standards.md
  2. No Console.WriteLine — use LogFactory.GetLogger(typeof(MyClass)) → logger.Information(...) (requires using Server.Logging;)
  3. Threading policy — game logic runs only on the main loop; never touch game state (World, mobiles, items, maps, timers) from a background thread. Heavy work that needs game state must be chunked across ticks, not threaded. Heavy work that does not need game state (large-file parse, external I/O) must run on a background thread and must yield to world saves (defer while World.Saving/WorldState.PendingSave). Publish results back to the loop as an immutable snapshot swapped via a single volatile reference — the only sanctioned volatile. No lock/Mutex/ConcurrentDictionary in game logic. Rule #10 covers how background work hands results back to the loop → dev-docs/threading-model.md
  4. No World.Mobiles/World.Items iteration — use spatial queries: map.GetMobilesInRange<T>(), map.GetItemsInRange<T>()
  5. Clean up refs in OnDelete()/OnAfterDelete() — null out Item/Mobile references
  6. Cancel timers in OnDelete()/OnAfterDelete() — call _token.Cancel() or _timer?.Stop()
  7. STArrayPool<T>.Shared not ArrayPool<T>.Shared — single-threaded optimized, no locks
  8. PooledRefList<T> not new List<T>() on hot paths — zero GC pressure, stack-allocated ref struct
  9. Serialization — class must be partial, constructor needs [Constructible], TimerExecutionToken must NOT have [SerializableField]. New classes: use [SerializationGenerator(version)] (omit encoded). When bumping versions, add MigrateFrom(VXContent) (X = previous version). Never modify Deserialize(reader, version) for version bumps — that method is only for pre-codegen legacy saves. When migrating from pre-codegen Serialize/Deserialize: pass false if old code used reader.ReadInt(), bump version +1, and keep old logic as private void Deserialize(IGenericReader reader, int version) → dev-docs/runuo-migration-docs/02-serialization.md
  10. No Task.Run/new Thread() for game logic (tandem with rule #3) — game logic is the single-threaded event loop. Backgrounding is allowed only for work that does not itself touch game state (external service calls, large-file parse). When such work must feed game logic: run the heavy/I/O part off-loop and ConfigureAwait(false) its awaits so a continuation never resumes on the loop and silently foregrounds heavy work; then hand the result back explicitly — publish an immutable snapshot swapped via a volatile reference (the loop reads it lock-free), or marshal the apply step with Core.LoopContext.Post(() => …). Never touch game state off-thread; never let the scheduler decide where the heavy work runs → dev-docs/threading-model.md
  11. Never assume era — if code uses Core.AOS/Core.SE/etc., ask which expansion to target
  12. Naming — _camelCase private fields, PascalCase properties/methods/classes; don't flag legacy m_ but use _ for new code
  13. No empty gumps — every gump must produce visual elements. An empty gump leaks on client+server (no way to close it). Use static DisplayTo() to validate before constructing → dev-docs/gump-system.md
  14. PropertyList string literals must be holes — $"{"Map"}\t{value}" not $"Map\t{value}". The handler treats bare text as delimiters, {} holes as arguments. Only \t should be a bare literal → dev-docs/property-lists.md
  15. Braces required on all control flow — if, else, for, foreach, while, do, switch must always have braces, even for single-line bodies → dev-docs/code-standards.md
  16. Prefer switch expressions and switch-when — use switch expressions for value mapping and switch-when for pattern matching where they improve readability. Exception: skip if unreadable or cold path → dev-docs/code-standards.md
  17. No System.Text.StringBuilder — use ValueStringBuilder with stackalloc (bounded output) or ValueStringBuilder.Create() (unbounded). Supports $"..." interpolation directly. Always use using var for disposal. Use Reset() instead of reassigning → dev-docs/string-handling.md
  18. Interpolation anti-patterns on handler-aware APIs — Send*/Say/Emote/PublicOverhead*/IPropertyList.Add/gump AddLabel/AddHtml/Html.Center/SpanWriter.Write* all have ref RawInterpolatedStringHandler overloads that allocate zero strings, but only when the call-site argument is a $"..." literal directly. Avoid: ternaries with interpolated branches (Send(c ? $"a" : $"b")), switch expressions with interpolated arms, pre-built var s = $"..." locals (single-use), .ToString() / .String() / string.Format inside holes, string concat ({a + b}), LINQ string ops in holes. Use :L format spec for lowercase ({rank:L} not rank.ToString().ToLowerInvariant()) → dev-docs/string-handling.md § Interpolation Anti-Patterns

Dev-Docs Reference

Topic File
Code standards & LINQ tiers dev-docs/code-standards.md
Serialization system dev-docs/serialization.md
Content patterns (Items, Mobiles, Creatures) dev-docs/content-patterns.md
Era & expansion handling dev-docs/era-expansion.md
Timer system dev-docs/timers.md
Event scheduler (wall-clock/calendar) dev-docs/event-scheduler.md
Object property lists (tooltips) dev-docs/property-lists.md
Gump (UI dialog) system dev-docs/gump-system.md
Commands & targeting dev-docs/commands-targeting.md
Event system dev-docs/events.md
Threading model dev-docs/threading-model.md
Server lifecycle & bootstrap phases (Configure/ConfigurePrompts/Initialize) dev-docs/server-lifecycle.md
Configuration system dev-docs/configuration.md
Networking & packets dev-docs/networking-packets.md
Region system dev-docs/regions.md
String handling & ValueStringBuilder dev-docs/string-handling.md
RunUO migration (overview) dev-docs/runuo-migration-docs/00-overview.md
RunUO migration (all docs) dev-docs/runuo-migration-docs/

Claude Skills (Opt-In)

Detailed Claude Code skills live in dev-docs/claude-skills/. They are not auto-loaded — they must be copied to .claude/skills/ to activate.

When to offer: If the user is building complex content (new items, creatures, spells, gumps, quests, packets, serialization work, etc.), ask:

I have detailed Claude Code skills for this kind of work. Want me to enable them? I'll copy the relevant files from dev-docs/claude-skills/ to .claude/skills/.

Then copy only the relevant skill files based on the task:

Task Skills to enable
New Item or Mobile modernuo-content-patterns, modernuo-serialization, modernuo-property-lists
Creature / spawn modernuo-content-patterns, modernuo-serialization, modernuo-timers
Spell or ability modernuo-content-patterns, modernuo-serialization, modernuo-timers, modernuo-era-expansion
Gump / UI dialog modernuo-gump-system, modernuo-commands-targeting
Quest or event system modernuo-events, modernuo-content-patterns, modernuo-configuration
Scheduled / seasonal / holiday events modernuo-event-scheduler, modernuo-timers
Custom regions / dynamic areas modernuo-regions, modernuo-content-patterns
Packet / networking modernuo-networking, modernuo-threading
Commands modernuo-commands-targeting
Timer work modernuo-timers, modernuo-serialization
Config system modernuo-configuration
Era-conditional code modernuo-era-expansion
String building / formatting modernuo-string-handling
Code review / audit modernuo-code-audit
Any .cs file edit modernuo-code-audit (always offer for code changes)
RunUO Migration
Migrate any RunUO script migrate-from-runuo/migrate-foundation (always), plus system-specific skills below
Migrate Item/Mobile/Creature migrate-from-runuo/migrate-foundation, migrate-from-runuo/migrate-serialization, migrate-from-runuo/migrate-items-mobiles
Migrate serialization migrate-from-runuo/migrate-serialization
Migrate timers migrate-from-runuo/migrate-timers
Migrate gumps migrate-from-runuo/migrate-gumps
Migrate packets migrate-from-runuo/migrate-packets
Migrate property lists migrate-from-runuo/migrate-property-lists
Migrate events/commands migrate-from-runuo/migrate-commands-events
Migrate persistence (WorldSave) migrate-from-runuo/migrate-persistence
Migrate multi-file system migrate-from-runuo/migrate-systems

To enable a skill: cp dev-docs/claude-skills/<name>.md .claude/skills/

Migration skills reference the deep docs in dev-docs/runuo-migration-docs/ and point to existing ModernUO skills for best practices.

The modernuo-code-audit skill auto-triggers on .cs file edits and flags convention violations (warnings only, asks before fixing).