feat: DNS checker，自研 codec/transport，支持 system/server 双模式，UDP/TCP + TC fallback

probes.example.yaml

@@ -207,6 +207,67 @@ targets:
       durationMs:
         lte: 5000
 
+  # ========== DNS targets ==========
+
+  # 本机 DNS 解析检查（system 模式）
+  - id: "dns-system-localhost"
+    name: "本机 DNS 解析"
+    type: dns
+    group: "DNS"
+    dns:
+      resolver: system
+      name: "localhost"
+      family: ipv4
+    expect:
+      values:
+        exact:
+          - "127.0.0.1"
+      durationMs:
+        lte: 200
+
+  # DNS server 拨测（server 模式，A 记录）
+  - id: "dns-server-cf"
+    name: "Cloudflare DNS A 记录"
+    type: dns
+    group: "DNS"
+    dns:
+      resolver: server
+      server: "1.1.1.1"
+      name: "example.com"
+      recordType: A
+    expect:
+      rcode: ["NOERROR"]
+      ttlMin:
+        gte: 60
+      durationMs:
+        lte: 500
+
+  # 负向 DNS 检查（NXDOMAIN）
+  - id: "dns-nxdomain-check"
+    name: "负向 DNS 检查"
+    type: dns
+    group: "DNS"
+    dns:
+      resolver: server
+      server: "1.1.1.1"
+      name: "this-domain-should-not-exist.example.com"
+      recordType: A
+    expect:
+      rcode: ["NXDOMAIN"]
+
+  # MX 记录检查
+  - id: "dns-mx-check"
+    name: "MX 记录检查"
+    type: dns
+    group: "DNS"
+    dns:
+      resolver: server
+      server: "1.1.1.1"
+      name: "gmail.com"
+      recordType: MX
+    expect:
+      rcode: ["NOERROR"]
+
   # ========== UDP targets ==========
 
   - id: "udp-heartbeat"