The shard owner, on a Starlink CGNAT address, was blocked by the imported
reputation blocklist. The cause was not CrowdSec: the address was a literal line
in ip-blocklist.txt, so BlocklistFilter denied it at accept and then promoted it.
Clearing the CrowdSec decision could not fix it either, because the file entry
re-reports within promoteSuppression of every reconnect.
Reputation feeds list shared consumer address space constantly. On CGNAT one
public address fronts many subscribers at the same time, so a single abusive
customer gets the address listed and everyone else behind it is blocked with
them; where leases rotate, a listing says little about whoever holds the address
now. So exemptions go where they cost nothing, and escalation is driven by what a
connection actually does.
GENERATOR (tools/Export-IpBlocklist.ps1)
- -AllowlistFile takes multiple paths, subtracted from the merged set before the
output is written. Defaults to the operator's own ip-allowlist.txt (created
once, never rewritten) plus one generated file per network carve-out.
- Subtraction is range-correct: an allowlisted address inside a blocked CIDR
splits that CIDR around the hole instead of being ignored. This also fixes
-ExcludeAnonymizers, which parsed CIDR entries and then only subtracted
singles.
- Carve-outs are a table (name, ASN, reason, offline seed) rather than a
hardcoded network, so covering another CGNAT provider is one row.
-RefreshCarveouts re-fetches from the ASN's current routing announcements and
collapses them; -Carveout '' subtracts none. Announcements rather than
ownership records, because registry data disagrees with what is routed and caps
its result sets.
- Editing an allowlist bypasses -MinInterval, so a just-added exemption is not
indistinguishable from the allowlist not working.
- The shipped starlink carve-out costs ~0.1% of the list.
ALLOWLISTS
- FileAllowlist reads the same files the generator subtracts, so an operator
entry means "leave this address alone" for real. Subtraction alone only covers
being BLOCKED; behavioural detections never consult the blocklist, so without
this a carve-out was quietly routed around and one scanner behind a shared
address was enough to get everyone behind it firewalled. Reading the files also
means an entry applies on the next reload rather than the next regeneration.
- LoginAllowlist is earned by authenticating, with a 90 day TTL because an
address that logged in years ago is a stranger. Its own store rather than
Account.LoginIPs, which has no timestamps and cannot be backfilled. An entry is
evidence rather than a licence: 10 suppressed contributions in an hour revokes
it, and a fresh login forgives the tally.
- Both are consulted only AFTER the blocklist has already matched, so a normal
accept pays nothing for them and the accept gate stays allowlist-free.
- BanExemptions combines them behind BanChannel.IsExempt. Suppresses escalation
only; every local defence still applies.
BEHAVIOURAL DETECTION
- silent-connect (reaped having sent zero bytes) and invalid-seed (a zero seed)
are contributed. Both were already disconnected.
- ForeignProtocol positively identifies HTTP, TLS and SSH. Asking "is this a good
UO client?" cannot work: LoginEncryption.ClientDecrypt is a byte-for-byte
stream XOR, so a client with encryption on when the shard expects none sends a
structurally perfect connection whose payload is noise. Nothing assumes arrival
framing, since TCP has no message boundaries and a rule of the form "these
bytes must arrive together" drops real players on poor links.
- Keyed on bytes-received rather than elapsed time throughout. A connection that
sent something and ran out of time is far more likely a slow link, and banning
those makes the player retry, trip the rate limiter, and compound it.
- AutoDenylist holds behavioural detections locally for 15 minutes, as an
IConnectionFilter plus IBanReporter over one store so engine detection sites
never reach into content. Closes the gap where a flood pays for a socket and a
NetState per connection while waiting for an OS bouncer, and is the whole
defence on a shard running none. Not persisted: a holding pen that survives
restarts is a ban without a ban's review.
- BanReasons centralises the slugs. IsBehavioral is an opt-in set, not
"everything except manual", so a future reason escalates normally instead of
silently inheriting an exemption. The first cut of the exemption swallowed
manual admin bans; this is why.
FIXES
- BanConfiguration.Settings was null until Configure() ran while the reap path
dereferences it every Slice(), so a harness driving NetState.Slice() directly
hit an NRE that looked flaky because it depended on test ordering.
- -AllowlistFile was typed [string] while documented and used as a list.
LAYOUT AND DOCS
Content network code moves out of Misc into UOContent/Network, one concern per
folder. Namespaces are untouched, so these are pure file moves.
dev-docs/ip-bans-and-allowlists.md documents the subsystem, leading with the
operator process for unblocking a player -- including the three things that look
sufficient and are not: deleting the CrowdSec decision alone, editing
ip-blocklist.txt by hand, and cscli allowlists alone.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
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.
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