docs: document the connectivity probe config for hbc_mini.py and hbc_mini.c

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Uc9V1v7iXruubpoLiSE24M

README.md

@@ -659,7 +659,28 @@ Config: `~/.hbc.json` (JSON format, same keys as `~/.hbc.yaml`).
 | `cpu_monitor` | Linux (`/proc/stat`; no per-core, no frequency) |
 | `memory_monitor` | Linux (`/proc/meminfo`) |
 | `disk_monitor` | Linux, macOS, BSD (`df -P`) |
-| `network_monitor` | Linux (`/proc/net/dev`) |
+| `network_monitor` | Linux (`/proc/net/dev`); per-protocol connectivity probe (all platforms) |
+
+`network_monitor` also reports per-protocol (IPv4/IPv6) internet presence — whether the host
+has a working route and reply to a well-known address, plus its own address, the local
+gateway, and RTT — as `p4`/`p6` flags on every heartbeat and a `connectivity` object in the
+plugin data. Configurable, defaults shown:
+
+```json
+{
+  "plugins": {
+    "network_monitor": {
+      "probe_enabled": true,
+      "probe_v4": "1.1.1.1",
+      "probe_v6": "2606:4700:4700::1111",
+      "probe_interval": 60
+    }
+  }
+}
+```
+
+`probe_v4`/`probe_v6` must be IP literals (no hostnames). Gateway lookup works on Linux,
+FreeBSD/NetBSD/DragonFly/macOS (`route -n get`), and Windows (`route print`).
 
 Not available vs full `hbc`: no YAML config, no `filesystem_info`, no `zfs_monitor`, no IPv6 early-fail protection.
 
@@ -712,6 +733,7 @@ SIGHUP causes the process to restart itself via `execv()`. SIGTERM/SIGINT trigge
 | `disk_monitor` | All | `df -P` subprocess |
 | `network_monitor` | Linux | `/proc/net/dev` |
 | `network_monitor` | FreeBSD, NetBSD, DragonFly | `getifaddrs()` + `AF_LINK` |
+| `network_monitor` (connectivity probe) | Linux, FreeBSD, NetBSD, DragonFly | UDP `connect()` route check + one ICMP echo per protocol |
 | `ping_monitor` | All | `ping` subprocess |
 | `nagios_runner` | All | `popen()` subprocess |
 
@@ -743,12 +765,33 @@ SIGHUP causes the process to restart itself via `execv()`. SIGTERM/SIGINT trigge
 }
 ```
 
+`network_monitor` also runs the per-protocol connectivity probe described above, in a
+dedicated thread (its ICMP echo can block for a few seconds). Same config keys and defaults
+as `hbc_mini.py`:
+
+```json
+{
+  "plugins": {
+    "network_monitor": {
+      "probe_enabled": true,
+      "probe_v4": "1.1.1.1",
+      "probe_v6": "2606:4700:4700::1111",
+      "probe_interval": 60
+    }
+  }
+}
+```
+
+Gateway lookup on the C client covers Linux and FreeBSD/NetBSD/DragonFly only (no macOS or
+Windows build of `hbc_mini.c` exists).
+
 ### Differences from `hbc_mini.py`
 
 - No `filesystem_info` or `zfs_monitor` plugins
 - `UPD` (self-update) messages are logged but not acted on
 - No IPv6 early-fail protection
 - Config is JSON only (`~/.hbc.json`), no YAML
+- No macOS or Windows build (Linux and the BSDs only)
 
 ---