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+# Homelab Infrastructure Exporter
+
+## Overview
+
+Build a Prometheus exporter for metrics specific to our homelab infrastructure. Unlike the generic nixos-exporter, this covers services and patterns unique to our environment.
+
+## Current State
+
+### Existing Exporters
+- **node-exporter** (all hosts): System metrics
+- **systemd-exporter** (all hosts): Service restart counts, IP accounting
+- **labmon** (monitoring01): TLS certificate monitoring, step-ca health
+- **Service-specific**: unbound, postgres, nats, jellyfin, home-assistant, caddy, step-ca
+
+### Gaps
+- No visibility into Vault/OpenBao lease expiry
+- No ACME certificate expiry from internal CA
+- No Proxmox guest agent metrics from inside VMs
+
+## Metrics
+
+### Vault/OpenBao Metrics
+
+| Metric | Description | Source |
+|--------|-------------|--------|
+| `homelab_vault_token_expiry_seconds` | Seconds until AppRole token expires | Token metadata or lease file |
+| `homelab_vault_token_renewable` | 1 if token is renewable | Token metadata |
+
+Labels: `role` (AppRole name)
+
+### ACME Certificate Metrics
+
+| Metric | Description | Source |
+|--------|-------------|--------|
+| `homelab_acme_cert_expiry_seconds` | Seconds until certificate expires | Parse cert from `/var/lib/acme/*/cert.pem` |
+| `homelab_acme_cert_not_after` | Unix timestamp of cert expiry | Certificate NotAfter field |
+
+Labels: `domain`, `issuer`
+
+Note: labmon already monitors external TLS endpoints. This covers local ACME-managed certs.
+
+### Proxmox Guest Metrics (future)
+
+| Metric | Description | Source |
+|--------|-------------|--------|
+| `homelab_proxmox_guest_info` | Info gauge with VM ID, name | QEMU guest agent |
+| `homelab_proxmox_guest_agent_running` | 1 if guest agent is responsive | Agent ping |
+
+### DNS Zone Metrics (future)
+
+| Metric | Description | Source |
+|--------|-------------|--------|
+| `homelab_dns_zone_serial` | Current zone serial number | DNS AXFR or zone file |
+
+Labels: `zone`
+
+## Architecture
+
+Single binary with collectors enabled via config. Runs on hosts that need specific collectors.
+
+```
+homelab-exporter
+├── main.go
+├── collector/
+│   ├── vault.go     # Vault/OpenBao token metrics
+│   ├── acme.go      # ACME certificate metrics
+│   └── proxmox.go   # Proxmox guest agent (future)
+└── config/
+    └── config.go
+```
+
+## Configuration
+
+```yaml
+listen_addr: ":9970"
+collectors:
+  vault:
+    enabled: true
+    token_path: "/var/lib/vault/token"
+  acme:
+    enabled: true
+    cert_dirs:
+      - "/var/lib/acme"
+  proxmox:
+    enabled: false
+```
+
+## NixOS Module
+
+```nix
+services.homelab-exporter = {
+  enable = true;
+  port = 9970;
+  collectors = {
+    vault = {
+      enable = true;
+      tokenPath = "/var/lib/vault/token";
+    };
+    acme = {
+      enable = true;
+      certDirs = [ "/var/lib/acme" ];
+    };
+  };
+};
+
+# Auto-register scrape target
+homelab.monitoring.scrapeTargets = [{
+  job_name = "homelab-exporter";
+  port = 9970;
+}];
+```
+
+## Integration
+
+### Deployment
+
+Deploy on hosts that have relevant data:
+- **All hosts with ACME certs**: acme collector
+- **All hosts with Vault**: vault collector
+- **Proxmox VMs**: proxmox collector (when implemented)
+
+### Relationship with nixos-exporter
+
+These are complementary:
+- **nixos-exporter** (port 9971): Generic NixOS metrics, deploy everywhere
+- **homelab-exporter** (port 9970): Infrastructure-specific, deploy selectively
+
+Both can run on the same host if needed.
+
+## Implementation
+
+### Language
+
+Go - consistent with labmon and nixos-exporter.
+
+### Phase 1: Core + ACME
+1. Create git repository (git.t-juice.club/torjus/homelab-exporter)
+2. Implement ACME certificate collector
+3. HTTP server with `/metrics`
+4. NixOS module
+
+### Phase 2: Vault Collector
+1. Implement token expiry detection
+2. Handle missing/expired tokens gracefully
+
+### Phase 3: Dashboard
+1. Create Grafana dashboard for infrastructure health
+2. Add to existing monitoring service module
+
+## Alert Examples
+
+```yaml
+- alert: VaultTokenExpiringSoon
+  expr: homelab_vault_token_expiry_seconds < 3600
+  for: 5m
+  labels:
+    severity: warning
+  annotations:
+    summary: "Vault token on {{ $labels.instance }} expires in < 1 hour"
+
+- alert: ACMECertExpiringSoon
+  expr: homelab_acme_cert_expiry_seconds < 7 * 24 * 3600
+  for: 1h
+  labels:
+    severity: warning
+  annotations:
+    summary: "ACME cert {{ $labels.domain }} on {{ $labels.instance }} expires in < 7 days"
+```
+
+## Open Questions
+
+- [ ] How to read Vault token expiry without re-authenticating?
+- [ ] Should ACME collector also check key/cert match?
+
+## Notes
+
+- Port 9970 (labmon uses 9969, nixos-exporter will use 9971)
+- Keep infrastructure-specific logic here, generic NixOS stuff in nixos-exporter
+- Consider merging Proxmox metrics with pve-exporter if overlap is significant
diff --git a/docs/plans/nixos-exporter.md b/docs/plans/nixos-exporter.md
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+# NixOS Prometheus Exporter
+
+## Overview
+
+Build a generic Prometheus exporter for NixOS-specific metrics. This exporter should be useful for any NixOS deployment, not just our homelab.
+
+## Goal
+
+Provide visibility into NixOS system state that standard exporters don't cover:
+- Generation management (count, age, current vs booted)
+- Flake input freshness
+- Upgrade status
+
+## Metrics
+
+### Core Metrics
+
+| Metric | Description | Source |
+|--------|-------------|--------|
+| `nixos_generation_count` | Number of system generations | Count entries in `/nix/var/nix/profiles/system-*` |
+| `nixos_current_generation` | Active generation number | Parse `readlink /run/current-system` |
+| `nixos_booted_generation` | Generation that was booted | Parse `/run/booted-system` |
+| `nixos_generation_age_seconds` | Age of current generation | File mtime of current system profile |
+| `nixos_config_mismatch` | 1 if booted != current, 0 otherwise | Compare symlink targets |
+
+### Flake Metrics (optional collector)
+
+| Metric | Description | Source |
+|--------|-------------|--------|
+| `nixos_flake_input_age_seconds` | Age of each flake.lock input | Parse `lastModified` from flake.lock |
+| `nixos_flake_input_info` | Info gauge with rev label | Parse `rev` from flake.lock |
+
+Labels: `input` (e.g., "nixpkgs", "home-manager")
+
+### Future Metrics
+
+| Metric | Description | Source |
+|--------|-------------|--------|
+| `nixos_upgrade_pending` | 1 if remote differs from local | Compare flake refs (expensive) |
+| `nixos_store_size_bytes` | Size of /nix/store | `du` or filesystem stats |
+| `nixos_store_path_count` | Number of store paths | Count entries |
+
+## Architecture
+
+Single binary with optional collectors enabled via config or flags.
+
+```
+nixos-exporter
+├── main.go
+├── collector/
+│   ├── generation.go    # Core generation metrics
+│   └── flake.go         # Flake input metrics
+└── config/
+    └── config.go
+```
+
+## Configuration
+
+```yaml
+listen_addr: ":9971"
+collectors:
+  generation:
+    enabled: true
+  flake:
+    enabled: false
+    lock_path: "/etc/nixos/flake.lock"  # or auto-detect from /run/current-system
+```
+
+Command-line alternative:
+```bash
+nixos-exporter --listen=:9971 --collector.flake --flake.lock-path=/etc/nixos/flake.lock
+```
+
+## NixOS Module
+
+```nix
+services.prometheus.exporters.nixos = {
+  enable = true;
+  port = 9971;
+  collectors = [ "generation" "flake" ];
+  flake.lockPath = "/etc/nixos/flake.lock";
+};
+```
+
+The module should integrate with nixpkgs' existing `services.prometheus.exporters.*` pattern.
+
+## Implementation
+
+### Language
+
+Go - mature prometheus client library, single static binary, easy cross-compilation.
+
+### Phase 1: Core
+1. Create git repository
+2. Implement generation collector (count, current, booted, age, mismatch)
+3. Basic HTTP server with `/metrics` endpoint
+4. NixOS module
+
+### Phase 2: Flake Collector
+1. Parse flake.lock JSON format
+2. Extract lastModified timestamps per input
+3. Add input labels
+
+### Phase 3: Packaging
+1. Add to nixpkgs or publish as flake
+2. Documentation
+3. Example Grafana dashboard
+
+## Example Output
+
+```
+# HELP nixos_generation_count Total number of system generations
+# TYPE nixos_generation_count gauge
+nixos_generation_count 47
+
+# HELP nixos_current_generation Currently active generation number
+# TYPE nixos_current_generation gauge
+nixos_current_generation 47
+
+# HELP nixos_booted_generation Generation that was booted
+# TYPE nixos_booted_generation gauge
+nixos_booted_generation 46
+
+# HELP nixos_generation_age_seconds Age of current generation in seconds
+# TYPE nixos_generation_age_seconds gauge
+nixos_generation_age_seconds 3600
+
+# HELP nixos_config_mismatch 1 if booted generation differs from current
+# TYPE nixos_config_mismatch gauge
+nixos_config_mismatch 1
+
+# HELP nixos_flake_input_age_seconds Age of flake input in seconds
+# TYPE nixos_flake_input_age_seconds gauge
+nixos_flake_input_age_seconds{input="nixpkgs"} 259200
+nixos_flake_input_age_seconds{input="home-manager"} 86400
+```
+
+## Alert Examples
+
+```yaml
+- alert: NixOSConfigStale
+  expr: nixos_generation_age_seconds > 7 * 24 * 3600
+  for: 1h
+  labels:
+    severity: warning
+  annotations:
+    summary: "NixOS config on {{ $labels.instance }} is over 7 days old"
+
+- alert: NixOSRebootRequired
+  expr: nixos_config_mismatch == 1
+  for: 24h
+  labels:
+    severity: info
+  annotations:
+    summary: "{{ $labels.instance }} needs reboot to apply config"
+
+- alert: NixpkgsInputStale
+  expr: nixos_flake_input_age_seconds{input="nixpkgs"} > 30 * 24 * 3600
+  for: 1d
+  labels:
+    severity: info
+  annotations:
+    summary: "nixpkgs input on {{ $labels.instance }} is over 30 days old"
+```
+
+## Open Questions
+
+- [ ] How to detect flake.lock path automatically? (check /run/current-system for flake info)
+- [ ] Should generation collector need root? (probably not, just reading symlinks)
+- [ ] Include in nixpkgs or distribute as standalone flake?
+
+## Notes
+
+- Port 9971 suggested (9970 reserved for homelab-exporter)
+- Keep scope focused on NixOS-specific metrics - don't duplicate node-exporter
+- Consider submitting to prometheus exporter registry once stable