Add DNS checker

Checks DNS server availability via ping then performs configurable
DNS query (record_type: A, AAAA, TXT, MX, etc.).

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

checkers/dns.py

@@ -0,0 +1,68 @@
+import time
+
+import dns.resolver
+
+from .base import BaseChecker, CheckResult
+from .ping import PingChecker
+
+
+class DnsChecker(BaseChecker):
+    def check(self) -> CheckResult:
+        host = self.config["host"]
+        query_name = self.config["query"]
+        query_type = self.config.get("record_type", "A")
+        timeout = self.config.get("timeout", 5)
+
+        # First check if host is reachable via ping
+        ping_checker = PingChecker(self.name, {"host": host, "timeout": timeout})
+        ping_result = ping_checker.check()
+
+        if not ping_result.success:
+            return CheckResult(
+                success=False,
+                message=f"DNS server unreachable: {ping_result.message}",
+                response_time=None
+            )
+
+        # Now perform DNS query
+        resolver = dns.resolver.Resolver()
+        resolver.nameservers = [host]
+        resolver.timeout = timeout
+        resolver.lifetime = timeout
+
+        start = time.time()
+        try:
+            answers = resolver.resolve(query_name, query_type)
+            response_time = (time.time() - start) * 1000  # ms
+
+            records = [str(rdata) for rdata in answers]
+            return CheckResult(
+                success=True,
+                message=f"DNS {query_type} query OK ({len(records)} record(s))",
+                response_time=response_time,
+                details={"records": records, "query": query_name, "type": query_type}
+            )
+        except dns.resolver.NXDOMAIN:
+            return CheckResult(
+                success=False,
+                message=f"DNS query failed: {query_name} does not exist",
+                response_time=None
+            )
+        except dns.resolver.NoAnswer:
+            return CheckResult(
+                success=False,
+                message=f"DNS query failed: no {query_type} record for {query_name}",
+                response_time=None
+            )
+        except dns.resolver.Timeout:
+            return CheckResult(
+                success=False,
+                message="DNS query timeout",
+                response_time=None
+            )
+        except Exception as e:
+            return CheckResult(
+                success=False,
+                message=f"DNS error: {e}",
+                response_time=None
+            )