Initial commit: LAN Checker

Network health monitoring script with MQTT reporting for Home Assistant.
- Ping, HTTP, and SNMP checkers
- MQTT Discovery for automatic entity creation
- Configurable check intervals

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

checkers/snmp.py

@@ -0,0 +1,62 @@
+import time
+
+from pysnmp.hlapi import (
+    SnmpEngine,
+    CommunityData,
+    UdpTransportTarget,
+    ContextData,
+    ObjectType,
+    ObjectIdentity,
+    getCmd,
+)
+
+from .base import BaseChecker, CheckResult
+
+
+class SnmpChecker(BaseChecker):
+    def check(self) -> CheckResult:
+        host = self.config["host"]
+        port = self.config.get("port", 161)
+        community = self.config.get("community", "public")
+        oid = self.config.get("oid", "1.3.6.1.2.1.1.1.0")  # sysDescr
+        timeout_val = self.config.get("timeout", 5)
+
+        start = time.time()
+        try:
+            iterator = getCmd(
+                SnmpEngine(),
+                CommunityData(community),
+                UdpTransportTarget((host, port), timeout=timeout_val, retries=1),
+                ContextData(),
+                ObjectType(ObjectIdentity(oid))
+            )
+
+            error_indication, error_status, error_index, var_binds = next(iterator)
+            response_time = (time.time() - start) * 1000  # ms
+
+            if error_indication:
+                return CheckResult(
+                    success=False,
+                    message=f"SNMP error: {error_indication}",
+                    response_time=None
+                )
+            elif error_status:
+                return CheckResult(
+                    success=False,
+                    message=f"SNMP error: {error_status.prettyPrint()}",
+                    response_time=None
+                )
+            else:
+                values = {str(oid): str(val) for oid, val in var_binds}
+                return CheckResult(
+                    success=True,
+                    message="SNMP response OK",
+                    response_time=response_time,
+                    details=values
+                )
+        except Exception as e:
+            return CheckResult(
+                success=False,
+                message=f"SNMP error: {e}",
+                response_time=None
+            )