# Security

> **v2 status (2026-05-26).** The four security concerns below are unchanged in v2.
> Paths + project names moved: `OtOpcUa.Server/Security/` → `OtOpcUa.Security/`
> (`Ldap/`, `Jwt/`, `Endpoints/AuthEndpoints.cs`), `OtOpcUa.Admin` is gone (its
> auth + role-grant pages live in `OtOpcUa.AdminUI`), and Admin auth policies
> register from `OtOpcUa.Host/Program.cs` via `AddOtOpcUaAuth`
> (`src/Server/ZB.MOM.WW.OtOpcUa.Security/ServiceCollectionExtensions.cs`) rather
> than in a separate Admin process. The Admin UI uses a **single Cookie
> authentication scheme** — there is no `AddJwtBearer` pipeline. The
> `Security:Jwt` section configures `JwtTokenService`, which mints a JWT at the
> `/auth/token` endpoint for **external** consumers (OPC UA clients / automation
> scripts); the cookie itself stores the `ClaimsPrincipal` directly. DataProtection
> keys persist to the shared Config DB (`PersistKeysToDbContext<OtOpcUaConfigDbContext>`)
> so cookies survive failover between admin-role nodes.
>
> See `docs/plans/2026-05-26-akka-hosting-alignment-design.md` §5 for the v2
> auth + DataProtection rationale.

OtOpcUa has four independent security concerns. This document covers all four:

1. **Transport security** — OPC UA secure channel (signing, encryption, X.509 trust).
2. **OPC UA authentication** — Anonymous / UserName / X.509 session identities; UserName tokens authenticated by LDAP bind.
3. **Data-plane authorization** — who can browse, read, subscribe, write, acknowledge alarms on which nodes. Evaluated by `TriePermissionEvaluator` over a `PermissionTrie` built from the Config DB `NodeAcl` tree.
4. **Control-plane authorization** — who can view or edit fleet configuration in the Admin UI. Gated by the `AdminRole` (`Viewer` / `Designer` / `Administrator`) claim resolved from `LdapGroupRoleMapping`.

Transport security and OPC UA authentication are per-node concerns configured in the Server's bootstrap `appsettings.json`. Data-plane ACLs and Admin role grants live in the Config DB.

---

## Transport Security

### Overview

The OtOpcUa Server supports configurable OPC UA transport security profiles that control how data is protected on the wire between OPC UA clients and the server.

There are two distinct layers of security in OPC UA:

- **Transport security** -- secures the communication channel itself using TLS-style certificate exchange, message signing, and encryption. This is what the `OpcUa:EnabledSecurityProfiles` setting controls.
- **UserName token encryption** -- protects user credentials (username/password) sent during session activation. The OPC UA stack encrypts UserName tokens using the server's application certificate regardless of the transport security mode. UserName authentication therefore works on `None` endpoints too — the credentials themselves are always encrypted. A secure transport profile adds protection against message-level tampering and eavesdropping of data payloads.

### Supported security profiles

The profiles are the members of the `OpcUaSecurityProfile` enum (`src/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer/OpcUaApplicationHost.cs`). The server ships **three** baseline profiles; the config value is the bare enum-member name (no hyphens, no underscores):

| Enum member                     | Security Policy  | Message Security Mode | Description                                      |
|---------------------------------|------------------|-----------------------|--------------------------------------------------|
| `None`                          | None             | None                  | No signing or encryption. Suitable for development and isolated networks only. |
| `Basic256Sha256Sign`            | Basic256Sha256   | Sign                  | Messages are signed but not encrypted. Protects against tampering but data is visible on the wire. |
| `Basic256Sha256SignAndEncrypt`  | Basic256Sha256   | SignAndEncrypt        | Messages are both signed and encrypted. Full protection against tampering and eavesdropping. |

`BuildSecurityPolicies` (`OpcUaApplicationHost.cs`) maps each configured profile to an SDK `ServerSecurityPolicy`. The server exposes a separate endpoint per configured profile and clients select the one they prefer at session open. The enum's XML doc notes that Aes128/Aes256 variants can be added later by extending the enum + `BuildSecurityPolicies` — the wiring is profile-agnostic — but they are **not implemented today**. There is no `SecurityProfileResolver` class.

> **Config value form.** The enum binds by member name, so a profile string with hyphens (e.g. `Basic256Sha256-Sign`) does **not** bind — use the exact enum-member spelling above. If `EnabledSecurityProfiles` is empty, the server falls back to a single `None` endpoint (logged, very visible) so it still has a listening endpoint.

### Configuration

Transport security is configured in the `OpcUa` section of the Host process's bootstrap `appsettings.json` (bound to `OpcUaApplicationHostOptions`):

```json
{
  "OpcUa": {
    "ApplicationName": "OtOpcUa",
    "ApplicationUri": "urn:node-a:OtOpcUa",
    "PublicHostname": "0.0.0.0",
    "OpcUaPort": 4840,
    "PkiStoreRoot": "C:/ProgramData/OtOpcUa/pki",
    "AutoAcceptUntrustedClientCertificates": false,
    "EnabledSecurityProfiles": [ "Basic256Sha256Sign", "Basic256Sha256SignAndEncrypt" ]
  }
}
```

`EnabledSecurityProfiles` is a **list** — the server publishes one endpoint per entry. The default (when the key is omitted) is all three baseline profiles (`None`, `Basic256Sha256Sign`, `Basic256Sha256SignAndEncrypt`); production deployments typically drop `None`. The list must contain at least one entry (`OpcUaApplicationHostOptionsValidator` enforces `MinCount(…, 1)`).

The server certificate is auto-generated on first start if none exists in `PkiStoreRoot/own/`. Always generated even for `None`-only deployments because UserName token encryption depends on it.

### PKI directory layout

```
{PkiStoreRoot}/
  own/        Server's own application certificate and private key
  issuer/     CA certificates that issued trusted client certificates
  trusted/    Explicitly trusted client (peer) certificates
  rejected/   Certificates that were presented but not trusted
```

### Certificate trust flow

When a client connects using a secure profile (`Sign` or `SignAndEncrypt`), the following trust evaluation occurs:

1. The client presents its application certificate during the secure channel handshake.
2. The server checks whether the certificate exists in the `trusted/` store.
3. If found, the connection proceeds.
4. If not found and `AutoAcceptUntrustedClientCertificates` is `true`, the certificate is automatically copied to `trusted/` and the connection proceeds.
5. If not found and `AutoAcceptUntrustedClientCertificates` is `false`, the certificate is copied to `rejected/` and the connection is refused.

The Admin UI `Certificates.razor` page (`src/Server/ZB.MOM.WW.OtOpcUa.AdminUI/Components/Pages/Certificates.razor`) lists the contents of each PKI sub-store (own / trusted / issuer / rejected) by reading the `OpcUa:PkiStoreRoot` path from configuration. It is currently a **read-only viewer** — promoting a rejected cert to trusted is still a file move (copy the `.der` from `rejected/` to `trusted/certs/`); the SDK trust list reloads on the next handshake.

### Production hardening

- Set `AutoAcceptUntrustedClientCertificates = false`.
- Drop `None` from `EnabledSecurityProfiles`.
- Promote trusted client certs by moving the `.der` from `rejected/` to `trusted/certs/` rather than relying on the auto-accept fallback. (The Admin UI Certificates page shows what is in each store.)
- Periodically audit the `rejected/` directory; an unexpected entry is often a misconfigured client or a probe attempt.

---

## OPC UA Authentication

The Server accepts three OPC UA identity-token types:

| Token | Handler | Notes |
|---|---|---|
| Anonymous | No `IOpcUaUserAuthenticator` call — the SDK admits anonymous sessions at the channel. | Data-plane authorization (below) still default-denies any node a session has no ACL grant for. |
| UserName/Password | `LdapOpcUaUserAuthenticator.AuthenticateUserNameAsync` (`src/Server/ZB.MOM.WW.OtOpcUa.Host/OpcUa/LdapOpcUaUserAuthenticator.cs`, implements `IOpcUaUserAuthenticator`), backed by the app `ILdapAuthService` — `OtOpcUaLdapAuthService` (`src/Server/ZB.MOM.WW.OtOpcUa.Security/Ldap/OtOpcUaLdapAuthService.cs`). | LDAP bind + group lookup. The returned LDAP groups are mapped to roles via `IGroupRoleMapper<string>` (`OtOpcUaGroupRoleMapper`) and attached to the OPC UA session identity for the downstream ACL evaluator. |
| X.509 Certificate | Stack-level acceptance during the secure-channel handshake. | The certificate must be trusted (see PKI trust flow); finer-grain authorization happens through the data-plane ACLs. |

When no authenticator is supplied, `OpcUaApplicationHost` falls back to `NullOpcUaUserAuthenticator`; the Host wires the real `LdapOpcUaUserAuthenticator` as a singleton in `Program.cs`.

### LDAP bind flow (`OtOpcUaLdapAuthService`)

LDAP is configured under the `Security:Ldap` section (bound to `LdapOptions`, `src/Server/ZB.MOM.WW.OtOpcUa.Security/Ldap/LdapOptions.cs`, `SectionName = "Security:Ldap"`). The app authenticator is `OtOpcUaLdapAuthService` — a thin wrapper around the shared `ZB.MOM.WW.Auth.Ldap` directory client that adds two app-only concerns the shared library deliberately does not model: the `Enabled` master switch and `DevStubMode`. The same `ILdapAuthService` instance serves **both** the Admin UI cookie login (`/auth/login`) and the OPC UA UserName path (via `LdapOpcUaUserAuthenticator`), so operators use one credential across both planes.

`OtOpcUaLdapAuthService.AuthenticateAsync`:

1. If `Enabled = false`, denies outright — no bind, no DevStub bypass (the master switch wins).
2. If `DevStubMode = true`, accepts any non-empty credentials and grants the `Administrator` role **without any network bind** (dev only — must be `false` in production).
3. Refuses to bind over a plaintext transport (`Transport = None`) unless `AllowInsecure = true` (dev/test only). This is enforced at login, not at startup.
4. Delegates the real path to the shared `ZB.MOM.WW.Auth.Ldap` client: it binds (search-then-bind via `ServiceAccountDn`, or direct-bind `cn={user},{SearchBase}` when no service account is set), verifies the password, and reads the user's group memberships.
5. Returns an `LdapAuthResult` carrying the validated username + the **groups** (never roles). Failure codes are folded into opaque user-facing error strings so a probe cannot distinguish "unknown user" from "wrong password".

**Group → role mapping happens downstream**, not in the auth service: `LdapOpcUaUserAuthenticator` resolves `IGroupRoleMapper<string>` (`OtOpcUaGroupRoleMapper`) per call and unions its output with any pre-resolved roles (the DevStub `Administrator` grant). The roles are attached to the OPC UA session identity for the ACL evaluator. A mapper fault (e.g. a Config DB outage) falls back to the pre-resolved baseline rather than denying an otherwise-authenticated session.

`Transport` replaces the former `UseTls` bool: `Ldaps` (implicit TLS), `StartTls` (upgrade), or `None` (plaintext, requires `AllowInsecure`). Configuration example (Active Directory production):

```json
{
  "Security": {
    "Ldap": {
      "Enabled": true,
      "DevStubMode": false,
      "Server": "dc01.corp.example.com",
      "Port": 636,
      "Transport": "Ldaps",
      "AllowInsecure": false,
      "SearchBase": "DC=corp,DC=example,DC=com",
      "ServiceAccountDn": "CN=OtOpcUaSvc,OU=Service Accounts,DC=corp,DC=example,DC=com",
      "ServiceAccountPassword": "<from your secret store>",
      "GroupAttribute": "memberOf",
      "DisplayNameAttribute": "cn",
      "UserNameAttribute": "sAMAccountName",
      "GroupToRole": {
        "OPCUA-Designers": "Designer",
        "OPCUA-Admins": "Administrator",
        "OPCUA-Operators": "Operator"
      }
    }
  }
}
```

`GroupToRole` maps LDAP group names → Admin roles (case-insensitive); a user gets every role whose source group is in their membership. The values are the canonical control-plane role strings (`Viewer` / `Designer` / `Administrator`, plus the appsettings-only `Operator` for the `DriverOperator` policy). `UserNameAttribute: "sAMAccountName"` is the critical AD override — the GLAuth dev default is `cn`, which is not how AD users are looked up; use `userPrincipalName` instead if operators log in with `user@corp.example.com` form. `LdapOptionsValidator` (`src/Server/ZB.MOM.WW.OtOpcUa.Host/Configuration/LdapOptionsValidator.cs`) fails startup when `Transport = None` and `AllowInsecure = false` on a real-LDAP (non-DevStub) config.

---

## Data-Plane Authorization

Data-plane authorization is the check run on every OPC UA operation against an OtOpcUa endpoint: *can this authenticated user Browse / Read / Subscribe / Write / HistoryRead / AckAlarm / Call on this specific node?*

Per decision #129 the model is **additive-only — no explicit Deny**. Grants at each hierarchy level union; absence of a grant is the default-deny.

### Hierarchy

ACLs are evaluated against the node's scope path. `NodeScope` (`src/Core/ZB.MOM.WW.OtOpcUa.Core/Authorization/NodeScope.cs`) carries a `Kind` that selects between two hierarchy shapes:

```
Equipment (UNS) kind:        Cluster → Namespace → UnsArea → UnsLine → Equipment → Tag
SystemPlatform (Galaxy) kind: Cluster → Namespace → FolderSegment(s) → Tag
```

On the Galaxy/SystemPlatform path each folder segment takes one trie level, so a deeply-nested Galaxy folder reaches the same depth as a full UNS path. Unset mid-path levels leave the corresponding id `null` and the evaluator walks only as far as the scope goes.

Each level can carry `NodeAcl` rows (`src/Core/ZB.MOM.WW.OtOpcUa.Configuration/Entities/NodeAcl.cs`) that grant a permission bundle to a set of `LdapGroups`.

### Permission flags

`NodePermissions` (`src/Core/ZB.MOM.WW.OtOpcUa.Configuration/Enums/NodePermissions.cs`), stored as an `int` bitmask in `NodeAcl.PermissionFlags`:

```csharp
[Flags]
public enum NodePermissions : int
{
    None = 0,

    Browse            = 1 << 0,
    Read              = 1 << 1,
    Subscribe         = 1 << 2,
    HistoryRead       = 1 << 3,
    WriteOperate      = 1 << 4,
    WriteTune         = 1 << 5,
    WriteConfigure    = 1 << 6,
    AlarmRead         = 1 << 7,
    AlarmAcknowledge  = 1 << 8,
    AlarmConfirm      = 1 << 9,
    AlarmShelve       = 1 << 10,
    MethodCall        = 1 << 11,

    ReadOnly  = Browse | Read | Subscribe | HistoryRead | AlarmRead,
    Operator  = ReadOnly | WriteOperate | AlarmAcknowledge | AlarmConfirm,
    Engineer  = Operator | WriteTune | AlarmShelve,
    Admin     = Engineer | WriteConfigure | MethodCall,
}
```

The three Write tiers map to Galaxy's v1 `SecurityClassification` — `FreeAccess`/`Operate` → `WriteOperate`, `Tune` → `WriteTune`, `Configure` → `WriteConfigure`. `SecuredWrite` / `VerifiedWrite` / `ViewOnly` classifications remain read-only from OPC UA regardless of grant.

### Evaluator — `PermissionTrie`

`src/Core/ZB.MOM.WW.OtOpcUa.Core/Authorization/`:

| Class | Role |
|---|---|
| `PermissionTrie` | Cluster-scoped trie; each node carries `(GroupId → NodePermissions)` grants. |
| `PermissionTrieBuilder` | Builds a trie from the current `NodeAcl` rows in one pass and installs it into the cache. |
| `PermissionTrieCache` | Process-singleton cache keyed on `(ClusterId, GenerationId)`. Generation-sealed: `Install(trie)` adds a new generation + advances the "current" pointer; older generations are retained (in-flight requests still resolve) and GC'd by `Prune`. `Invalidate(clusterId)` drops every cached trie for a cluster. There is **no** `AclChangeNotifier` — a publish installs a new generation rather than signalling an invalidation. |
| `TriePermissionEvaluator` | Implements `IPermissionEvaluator.Authorize(session, operation, scope)`. Walks the cluster trie for the supplied `NodeScope`, unions grants along the path, and returns an `AuthorizationDecision`. Evaluates against the **session's bound generation** (`session.AuthGenerationId`), not just "current", so a grant added/removed in a newer generation cannot take effect mid-session. |

`NodeScope` is described above (Equipment-kind vs SystemPlatform-kind). The evaluator unions the matched grants along the path — a tag-level ACL and an area-level ACL both contribute.

### Dispatch gate — `IPermissionEvaluator`

`IPermissionEvaluator.Authorize(UserAuthorizationState session, OpcUaOperation operation, NodeScope scope)` (default impl `TriePermissionEvaluator` at `src/Core/ZB.MOM.WW.OtOpcUa.Core/Authorization/TriePermissionEvaluator.cs`) returns an `AuthorizationDecision`. The dispatch path calls it on every Read, Write, HistoryRead, Browse, Subscribe, AckAlarm, Call; a `NotGranted` decision denies the operation.

Key properties:

- **Driver-agnostic.** No driver-level code participates in authorization decisions. Drivers report `SecurityClassification` as metadata on tag discovery; everything else flows through the evaluator.
- **Strictly fail-closed (default-deny).** Every guard path returns `NotGranted` — a stale session (past the staleness ceiling, decision #152), a cluster mismatch between session and scope, a missing trie, a pruned bound generation, or simply no matching grant. There is no `StrictMode` / fail-open mode; absence of a grant is always a deny.
- **Evaluator stays pure.** `TriePermissionEvaluator` has no OPC UA stack dependency — it's tested directly from xUnit.

### Full model

See [`docs/v2/acl-design.md`](v2/acl-design.md) for the complete design: trie invalidation, flag semantics, per-path override rules, and the reasoning behind additive-only (no Deny).

---

## Control-Plane Authorization

Control-plane authorization governs **the Admin UI** — who can view fleet config, edit drafts, publish generations, manage cluster nodes + credentials.

Per decision #150 control-plane roles are **deliberately independent of data-plane ACLs**. An operator who can read every OPC UA tag in production may not be allowed to edit cluster config; conversely a `Designer` may not have any data-plane grants at all.

### Roles

The `AdminRole` enum (`src/Core/ZB.MOM.WW.OtOpcUa.Configuration/Enums/AdminRole.cs`) defines three roles. Task 1.7 standardized the member names on the canonical `ZB.MOM.WW.Auth` `CanonicalRole` vocabulary (`ConfigViewer → Viewer`, `ConfigEditor → Designer`, `FleetAdmin → Administrator`); a data migration (`CanonicalizeAdminRoles`) rewrote existing rows. This was a rename, not a permission change.

| Role | Capabilities |
|---|---|
| `Viewer` | Read-only access to drafts, generations, audit log, fleet status. (Was `ConfigViewer`.) |
| `Designer` | Viewer plus draft authoring (UNS, equipment, tags, ACLs, driver instances, reservations, CSV imports). Cannot publish. (Was `ConfigEditor`.) |
| `Administrator` | Designer plus publish, cluster/node CRUD, credential management, role-grant management. Satisfies both the `FleetAdmin` and `DriverOperator` authorization policies. (Was `FleetAdmin`.) |

`DriverOperator` is an **authorization policy name** (kept stable), not an `AdminRole` member. It gates **Reconnect** / **Restart** commands against live driver instances from the Admin UI `DriverStatusPanel` and requires the canonical role `Operator` or `Administrator` (`policy.RequireRole("Operator", "Administrator")` in `AddAuthorization`, `src/Server/ZB.MOM.WW.OtOpcUa.Security/ServiceCollectionExtensions.cs`). `Operator` is an appsettings-only string role (not an `AdminRole` member); map an LDAP group to it via `GroupToRole`, e.g. `"ot-driver-operator": "Operator"`. The `FleetAdmin` policy requires the `Administrator` role.

In v2 the authentication + authorization stack is wired centrally by `AddOtOpcUaAuth` (`src/Server/ZB.MOM.WW.OtOpcUa.Security/ServiceCollectionExtensions.cs`), which also installs a `FallbackPolicy` that requires an authenticated user. Razor pages gate inline with the canonical role names, e.g. `@attribute [Authorize(Roles = "Administrator,Designer")]`. Nav-menu sections hide via `<AuthorizeView>`.

### Role grant source

Admin reads `LdapGroupRoleMapping` rows from the Config DB (`src/Core/ZB.MOM.WW.OtOpcUa.Configuration/Entities/LdapGroupRoleMapping.cs`) — the same pattern as the data-plane `NodeAcl` but scoped to Admin roles + (optionally) one cluster for multi-site fleets (a system-wide row, `IsSystemWide = true`, stacks additively with cluster-scoped rows). The `RoleGrants.razor` page lets `Administrator`s edit these mappings without leaving the UI.

---

## OTOPCUA0001 Analyzer — Compile-Time Guard

Per-capability resilience (retry, timeout, circuit-breaker, bulkhead) is applied by `CapabilityInvoker` in `src/Core/ZB.MOM.WW.OtOpcUa.Core/Resilience/`. A driver-capability call made **outside** the invoker bypasses resilience entirely — which in production looks like inconsistent timeouts, un-wrapped retries, and unbounded blocking.

`OTOPCUA0001` (Roslyn analyzer at `src/Tooling/ZB.MOM.WW.OtOpcUa.Analyzers/UnwrappedCapabilityCallAnalyzer.cs`) fires with category `OtOpcUa.Resilience` and default severity **Warning** (per `AnalyzerReleases.Shipped.md`) when a method on one of the seven guarded capability interfaces (`IReadable`, `IWritable`, `ITagDiscovery`, `ISubscribable`, `IHostConnectivityProbe`, `IAlarmSource`, `IHistoryProvider` — all in `ZB.MOM.WW.OtOpcUa.Core.Abstractions`) is invoked **outside** a lambda passed to `CapabilityInvoker.ExecuteAsync` / `ExecuteWriteAsync`. `AlarmSurfaceInvoker` is **not** a wrapper home — its own implementation is covered transitively because it routes through the inner `CapabilityInvoker.ExecuteAsync`. The analyzer walks up the syntax tree from the call site, finds any enclosing invoker invocation, and verifies the call lives transitively inside that invocation's anonymous-function argument — a sibling pattern (do the call, then invoke `ExecuteAsync` on something unrelated nearby) does not satisfy the rule.

The xunit.v3 + Shouldly suite at `tests/Tooling/ZB.MOM.WW.OtOpcUa.Analyzers.Tests/UnwrappedCapabilityCallAnalyzerTests.cs` covers the common fail/pass shapes + the sibling-pattern regression guard.

The rule is intentionally scoped to async surfaces — pure in-memory accessors like `IHostConnectivityProbe.GetHostStatuses()` return synchronously and do not require the invoker wrap.

---

## Audit Logging

- **Server**: authentication, certificate-validation, and write-denial events are logged through the regular Serilog rolling file sink.
- **Admin**: `AuditWriterActor` (`src/Server/ZB.MOM.WW.OtOpcUa.ControlPlane/Audit/AuditWriterActor.cs`) writes `ConfigAuditLog` rows (`src/Core/ZB.MOM.WW.OtOpcUa.Configuration/Entities/ConfigAuditLog.cs`) to the Config DB for publish, rollback, cluster-node CRUD, and credential rotation. Visible on the cluster Audit page (`ClusterAudit.razor`) for operators with `Viewer` or above.

---

## Troubleshooting

### Certificate trust failure

Check `{PkiStoreRoot}/rejected/` for the client's cert. Copy the `.der` file to `trusted/certs/`; the SDK trust list reloads on the next handshake. The Admin UI Certificates page shows what is in each store but does not move certs.

### LDAP users can connect but fail authorization

Verify (a) `Security:Ldap:GroupAttribute` (default `memberOf`) returns the user's groups, (b) `Security:Ldap:GroupToRole` maps those groups to the expected roles, and (c) a `NodeAcl` grant exists at some level of the node's scope path that unions to the required permission. The data-plane evaluator is strictly default-deny — there is no fail-open mode to fall back on.

### LDAP bind rejected as "insecure"

Set `Security:Ldap:Transport = "Ldaps"` (or `"StartTls"`) with the matching port (636 for AD `Ldaps`), or temporarily set `Security:Ldap:AllowInsecure = true` in dev. Production Active Directory increasingly refuses plain-LDAP bind under LDAP-signing enforcement.

### Stale ACL trie after a publish

A publish installs a **new generation** into `PermissionTrieCache` via `PermissionTrieBuilder` rather than signalling an invalidation; the evaluator binds each session to a generation. If grants appear stale, confirm the new generation was installed (publish completed) and that sessions re-resolved their auth state — a session past its staleness ceiling fails closed and must re-authenticate. As a last resort `PermissionTrieCache.Invalidate(clusterId)` drops a cluster's cached tries.
