New permission system

The release has three big stories — a new core permission system with optional client-cert principals on NRPE, a
PDH overhaul that fixes long-standing counter-collection crashes and adds counter functions, and a WEB hardening
option that lets monitoring-only deployments expose the WEB UI without seeding a privileged admin account. Everything
else is bug fixes, small features, and follow-ups around those three threads.

Highlights

• Core permission system — opt-in policy layer that gates which caller can run which command. Configured under
  /settings/permissions. Disabled by default; existing installs keep working.
  See https://nsclient.org/docs/concepts/permissions/ for the model, identity table, and rollout recipe.
• NRPE client identity from cert CN — when client identity source = cn is set on NRPEServer and the listener
  verifies the client cert, the CN is stamped as the policy principal so rules can be written per-cert (
  NRPEServer:icinga-master = ...). Hard guardrail at module start refuses to load the module if the TLS verify mode
  would let the CN be attacker-supplied.
• Global allow exec toggle — exec is now gated by a single on/off switch under /settings/permissions. The
  per-command rule table applies to queries only. Default true so enabling the policy system does
  not break exec callers.
• PDH (performance counter) overhaul — fixes for service crashes when PDH misbehaves (#592, #547), counter retry
  when temporarily unavailable (#634), reliable English counter lookup (#652, #906), a resource leak in the
  counter-lookup path, and a refactor to smart-buffer-based PDH enumeration. Most users running CheckSystem on Windows
  should see meaningfully better reliability.
• check_pdh counter scaling and functions (#281) — details-syntax and related rendering paths can now apply
  scaling and other functions, e.g. '${counter}'=${value:scale(/1024)}MB.
• check_network — human-readable strings, scaling, speed, and percentages (#329); team-network statistics (#625).
  See https://nsclient.org/docs/reference/check/CheckNet.
• Nagios range syntax in performance data (#748) — 1:10, ~:5, @10:20 etc. work in perfdata thresholds,
  matching the Nagios plugin spec.
• disable admin user on WEBServer — monitoring-only deployments can expose the WEB UI without ever seeding the
  built-in admin (and previously seeded admin entries are ignored). Pairs naturally with the new permission system to
  lock down reconfiguration surfaces.
• Path overrides moved to boot.ini + new --path-override CLI flag — path tokens (module-path,
  certificate-path, etc.) are now declared early in boot.ini so they take effect before the main config is loaded.
  Per-invocation overrides via --path-override KEY=VALUE. See https://nsclient.org/docs/concepts/settings.
• NRPE startup is no longer fatal on listener failure — bad bind address / port already in use logs a clear error
  and leaves the module loaded so settings and commands stay usable for diagnostics.
• Dual-stack listening fixed (#312) — v4 and v6 acceptors no longer trample each other's pending connection slot.
• disable admin user, client identity source, allow exec, and the policy table are all documented in
  https://nsclient.org/docs/concepts/permissions/ and https://nsclient.org/docs/setup/securing. Treat those two as the
  starting point for any new
  install.

Detailed changes

Security and permissions

Core permission system
A policy layer in the core decides whether a given caller may run a given command. Disabled by default; when enabled,
rules form a strict allow-list.

[/settings/permissions]
enabled = true
log denials = true
log allows = false      ; noisy, only flip on while rolling out
allow exec = true       ; queries-only rule table; exec is a global toggle

[/settings/permissions/policies]
NRPEServer = CheckHelpers.*, CheckSystem.check_cpu
WEBServer:admin   = *
WEBServer:viewer  = CheckSystem.check_cpu, CheckSystem.check_drivesize
Scheduler = CheckHelpers.*, CheckSystem.*

Subject is module[:principal]; object is module.command. Wildcards (*, ?) supported. Rules combine additively.
See https://nsclient.org/docs/concepts/permissions/ for the full identity model, the
CheckHelpers identity-forwarding behaviour, and a step-by-step rollout recipe.

NRPE client cert CN as principal
When two-way TLS is configured and verifying client certs against your CA, the Common Name is stamped as the policy
principal:

[/settings/NRPE/server]
client identity source = cn        ; default: none
verify mode = peer-cert
ca = /etc/nsclient/ca.pem

[/settings/permissions/policies]
NRPEServer:icinga-master   = CheckHelpers.*, CheckSystem.*
NRPEServer:metrics-shipper = CheckSystem.check_cpu, CheckSystem.check_drivesize

Guardrails: the module refuses to start if client identity source = cn is configured without SSL, without
verify_mode containing peer and fail-if-no-peer-cert (or the peer-cert alias), or without a non-empty
ca path. The CN is logged at debug level on every accepted handshake for diagnostics. CN-only (not full DN) because
INI key syntax uses = as the key/value separator and would corrupt DN-shaped policy keys; see the "Why CN-only"
section of the permissions doc. See https://nsclient.org/docs/reference/client/NRPEServer.

Global allow exec toggle
Per-command rules apply to queries only. The exec surface (WEB scripts UI, lua/python core:simple_exec(...), CLI
exec) is gated by a single boolean:

[/settings/permissions]
allow exec = false   ; hard lockdown; default is true

When false and enabled = true, every exec call returns
Permission denied: exec is globally disabled (/settings/permissions/allow exec = false).
See "Why exec is a single toggle" in https://nsclient.org/docs/concepts/permissions/.

disable admin user on WEBServer
For installations that expose the WEB UI for status/visualisation only and never want a remote-reconfiguration surface:

[/settings/WEB/server]
disable admin user = true

With this set, the built-in admin is not seeded on first boot, and any existing admin entry in the user settings is
ignored at load time.

Security guide updates
https://nsclient.org/docs/setup/securing was rewritten with concrete configurations for NRPE (with
and without mTLS) and the WEB server. Read it before exposing either to a network you don't fully control.

Performance counters / PDH

The PDH subsystem (the Windows performance-counter collection backbone behind CheckSystem, check_cpu, check_pdh,
check_network, etc.) got a substantial reliability pass. Most users running NSClient++ as a long-running service on
Windows should see fewer crashes and more consistent results.

• Service crashes when PDH misbehaves on a particular machine (#592, #547) — root-caused and fixed. Misbehaving
  counter registrations no longer take the service down.
• Counter not retried if unavailable (#634) — counters that fail to bind at first sight now get retried on
  subsequent collection cycles, instead of being permanently unhealthy for the lifetime of the process.
• English counter lookup improved (#652, #906) — addresses reading of localised counters by their canonical English
  names on non- English Windows installs.
• Resource leak in PDH counter lookup fixed.
• PDH enumeration refactored to smart buffers — clearer memory ownership across the enumeration path, fewer footguns
  for future changes.
• check_pdh counter scaling and functions (#281) — all the details-syntax / rendering paths can now apply functions.
  Examples:

  check_pdh "counter=\Processor(_Total)\% Processor Time" \
            "details-syntax=${counter} = ${value:round(2)}%"
  

  See https://nsclient.org/docs/reference/check/CheckSystem for the function reference.

check_network

• Human-readable strings, scaling, speed, and percentages (#329) — perfdata and message output now render numbers in
  a way operators actually want to read:

  check_network 'filter=interface=Ethernet' \
                'top-syntax=${list}' \
                'detail-syntax=${interface}: ${total_rx_human}/s in, ${total_tx_human}/s out'
  

• Team network statistics (#625) — aggregate stats across Windows NIC teams.

See https://nsclient.org/docs/check/CheckNet.

Performance data formatting

• Nagios range syntax in performance data (#748) — the perfdata threshold fields now accept the standard Nagios
  range syntax: 5:10, ~:5, @10:20, etc. Brings NSClient++ into line with what Nagios consumers already expect.

Settings, paths, and CLI

• Path overrides moved to boot.ini — path tokens (module-path, certificate-path, data-path, log-path, …)
  now live under [paths] in boot.ini (next to nscp.exe), not in nsclient.ini. Overrides take effect before the
  main config is loaded — including the bootstrap step that decides where the main config itself lives.

  ; boot.ini
  [paths]
  module-path = D:\monitoring\modules
  certificate-path = D:\monitoring\certs

• --path-override CLI flag — per-invocation override, repeatable. (Renamed from --path to avoid colliding with
  the nscp settings --path subcommand option.)

  nscp client --path-override module-path=/build/modules --path-override log-path=. ...
  

• See https://nsclient.org/docs/concepts/settings for the precedence rules and the migration note for installs that had
  a [/paths] section in nsclient.ini.

Aliases and command registration

• CheckHelpers alias — aliases can now be defined under [/settings/check helpers/alias] and are registered by
  CheckHelpers directly, without requiring CheckExternalScripts to be loaded. This is the preferred place going
  forward; the legacy [/settings/external scripts/alias] is still honoured for backward compatibility.
• API to list registered query aliases (#506) — programmatic introspection of the alias table, useful for tooling.
• simple_command / simple_command_map — internal refactor that streamlines how modules register aliases. No
  user-visible behaviour change, but module authors may want to look at the new pattern.
• Icinga client alias (7c49a3d3) — minor module-specific addition.

NRPEServer

• Listener failure no longer kills the module — a bad bind to address that the resolver can't look up, or a port
  already in use, used to make the whole module fail to load. Now the failure is logged clearly, the listener stays
  down, and the module's settings and commands remain accessible for diagnostics and reconfiguration. Fix the config and
  reload — no service restart needed.
• Dual-stack fixed (#312) — the v4 and v6 acceptors used to share a single pending-connection slot, which caused
  intermittent Already open errors on v6 once v4 accepted a client. Each family now owns its own slot.
• Insecure mode produces an error-level log line — flipping insecure = true (for legacy check_nrpe interop) now
  surfaces as an ERROR so it shows up in monitoring dashboards, instead of silently disabling cert-based peer auth.

Plugin lifecycle

• prepare_shutdown hook — modules can opt in to a first-phase shutdown pass before any plugin is unloaded. Used
  by the Scheduler and similar long-running submitters to finish in-flight work cleanly. Operators see fewer "submission
  failed during shutdown" lines during service stop.

Settings store

• simpleini buffer NUL-termination fix — fixes a buffer allocation issue in the INI parser that could
  affect non-UTF-8 data paths.
• cache allowed host is now a real boolean — previously parsed as a string with surprising truthiness; matches
  what the docs always claimed.

Modules and clean-ups

• WMI module refactor — target handling and settings management cleaned up.
• IcingaClient cleanup — removed unused command-handling code paths.
• CheckLogFile config and descriptions — fixed misleading defaults and improved the help text.
• Web UI improvements — more settings elements exposed under modules, simpler module configuration. Web dependencies
  refreshed.
• Installer: UninstallString is now correct (#495) — removal via Windows "Apps & Features" works again.
• Rust dependencies bumped.

Upgrade notes

Most installs can upgrade in place — defaults are preserved. Read the specific items below if any of them apply.

Permission system

The new policy layer is disabled by default. Existing installs continue to behave exactly as before until an
operator opts in via /settings/permissions/enabled = true.

If you do opt in:

• Per-command rules under /settings/permissions/policies apply to queries only. Any rules you might have written
  for exec command patterns will be silently ignored for the exec dispatch path — exec is gated by the single global
  allow exec boolean.
• The default for allow exec is true, so enabling the policy will not silently break the WEB scripts UI, lua/python
  core:simple_exec(...), or CLI exec. Flip to false only if you want a hard exec lockdown.
• Roll out with log allows = true first so you can inventory what your actual traffic looks like before tightening to
  a real allow-list. See the step-by-step recipe in https://nsclient.org/docs/concepts/permissions/.

NRPEServer

• The new client identity source setting defaults to none, which matches the previous behaviour (subject is bare
  NRPEServer). Set to cn only when you want per-cert principals — and only after you've configured
  verify_mode = peer-cert and a ca path. The module will refuse to start with a clear error if you set cn without
  those.
• Pin the ca path to your private monitoring CA. The system trust store (Windows root store / Linux distro bundle)
  accepts certs from every public CA on the planet and would let an attacker with a public cert choose their own CN.
  See "Pin to a private CA" in the permissions doc.

Path overrides

• If you had a [/paths] section in nsclient.ini from an older NSClient++ install, those overrides moved to [paths]
  in boot.ini (note: same section name, different file). There is no automatic migration. Copy each key = value to a
  [paths] section in boot.ini (next to nscp.exe) and delete the old section from nsclient.ini.

WEB server

• The new disable admin user = true setting is opt-in. Existing installs keep their admin and continue to work
  unchanged. Use this when you want to expose the WEB UI for status-only viewing and have no need to reconfigure the
  agent through the web.

NRPEServer startup robustness

• A failed listener (bad bind address, port in use) used to make the whole NRPEServer module fail to load. It now logs
  an ERROR and leaves the module loaded with no active listener — so you can reconfigure via
  nscp settings --path /settings/NRPE/server --key ... --set ... and reload, without restarting the service. If you
  had monitoring on "module load failed" specifically, you may want to add "NRPE listener failed" as a separate signal.

insecure = true on NRPEServer

• This option (for legacy check_nrpe interop) now logs at ERROR rather than DEBUG/INFO. Behaviour is unchanged; the
  message is louder so it shows up in dashboards. If your monitoring filters by severity, you may want to whitelist this
  specific message on agents that intentionally run in insecure mode.

cache allowed host

• Previously parsed as a string with surprising truthiness; now a real boolean. If you had cache allowed host = yes or
  = on, switch to true. Numeric 1 / 0 still work.

Nagios range syntax in performance data

• This is additive — existing perfdata that doesn't use range syntax continues to work. Plain numbers still parse as
  before. Only consumers that previously had to special-case NSClient++'s output may need adjusting, but most
  Nagios-ecosystem tools handle both forms.

Full Changelog: 0.12.5...0.12.6