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+---
+tags: [计算机网络, DNS, DHCP, WebSocket]
+create time: 2026-05-18 03:10
+---
+
+# DNS / DHCP / WebSocket
+
+## 一、DNS 原理与优化
+
+### DNS 查询流程（递归 vs 迭代）
+
+```mermaid
+sequenceDiagram
+    participant U as 用户浏览器
+    participant LRD as 本地 DNS Resolver<br/>ISP 或 1.1.1.1/9.9.9.9
+    participant TLD as TLD Server<br/>.com
+    participant AUTH as Authoritative NS<br/>example.com
+    
+    U->>LRD: 递归查询 "example.com?"
+    
+    Note over LRD: 检查本地缓存...
+    LRD->>TLD: 迭代查询 ".com?"
+    TLD-->>LRD: "去问 example.com 的 NS"
+    
+    LRD->>AUTH: 迭代查询 "example.com A 记录?"
+    AUTH-->>LRD: 93.184.216.34
+    
+    Note over LRD: TTL=300 → 缓存 5 分钟
+    LRD-->>U: ✅ 93.184.216.34
+```
+
+### 关键概念
+
+| 术语 | 说明 |
+|------|------|
+| **Recursion Desired (RD)** | 客户端要求 DNS 服务器递归查找 |
+| **Authority Section** | 指向下一级的授权 NS |
+| **CNAME Chain** | 别名链，最多 63 层嵌套 |
+| **TTL** | Time To Live，缓存有效期（秒） |
+| **ANAME/ALIAS** | 根域名的 CNAME 替代方案 |
+| **Any 查询** | `query type = ANY`，RFC 8753 建议禁用 |
+
+### 常见记录类型
+
+| 类型 | 名称 | 用途 | 示例 |
+|------|------|------|------|
+| A | Address | IPv4 地址 | `@ A 93.184.216.34` |
+| AAAA | Address V6 | IPv6 地址 | `@ AAAA 2606:2800:220:1:248:1893:25c8:1946` |
+| CNAME | Canonical Name | 别名 | `www CNAME example.com` |
+| MX | Mail Exchange | 邮件服务器优先级 | `@ MX 10 mail.example.com` |
+| TXT | Text | SPF/DKIM/域名验证 | `v=spf1 include:_spf.google.com ~all` |
+| SRV | Service | 服务定位 | `_sip._tcp SVC 1 0 5060 sip.example.com` |
+| NS | Name Server | 授权 NS 记录 | `@ NS ns1.provider.com` |
+| PTR | Pointer | 反向解析 (IP → 域名) | `34.216.184.93.in-addr.arpa PTR example.com` |
+| SOA | Start of Authority | 区域文件起始权威记录 | `ns1 admin email serial refresh retry expire minimum` |
+| DS / DNSKEY | — | DNSSEC 签名验证 | — |
+
+### DNS 缓存层次
+
+```
+┌─────────────────────────────────────┐
+│ Tier 1: Application/CPU Cache      │  ← Go sync.Map / HTTP cache header
+│   TTL controlled by Content-TTL    │     Browser cache varies 0~30min
+├─────────────────────────────────────┤
+│ Tier 2: OS Resolver Cache          │  ← nscd / systemd-resolved / DNSMasq
+│   Linux: nscd (default off!)       │     macOS: mDNSResponder
+│   macOS: mDNSResponder (~120s)      │     Windows: Dnscache
+├─────────────────────────────────────┤
+│ Tier 3: ISP / Recursive Resolver   │  ← Cloudflare 1.1.1.1, Google 8.8.8.8
+│   TTL from authoritative server    │     通常 60~300s 最小缓存
+├─────────────────────────────────────┤
+│ Tier 4: TLD + Authoritative        │  ← .com NS → example.com NS
+│   No user-controlable caching      │
+└─────────────────────────────────────┘
+```
+
+### DNS-over-HTTPS / DNS-over-TLS
+
+| 协议 | RFC | 端口 | 特点 |
+|------|-----|------|------|
+| DoH (DNS over HTTPS) | 8484 | 443 (TCP) | 伪装成 HTTPS 流量，CDN 友好 |
+| DoT (DNS over TLS) | 7858 | 853 (TCP) | 专用加密通道 |
+| DoQ (DNS over QUIC) | 9230 | 853 (UDP via QUIC) | HTTP/3 风格 |
+
+```bash
+# Linux 启用 DoH
+$ resolvectl dns eth0 1.1.1.1
+$ resolvectl domain eth0 "~."
+
+# dig 指定 DNS 服务器
+$ dig @1.1.1.1 example.com +short
+93.184.216.34
+```
+
+## 二、DHCP 自动分配
+
+### DHCP 四步交互（DORA）
+
+```mermaid
+sequenceDiagram
+    participant Client as 新设备<br/>(无 IP)
+    participant Server as DHCP Server
+    
+    Note over Client: BOOTING state, src_ip=0.0.0.0
+    Client->>Server: DHCPDISCOVER (broadcast ff:ff:ff:ff:ff:ff)
+    Note over Server: 收到后从可用池选择一个 IP
+    
+    Server-->>Client: DHCPOFFER (ip=x.x.x.x, lease_time=T, gw=y.y.y.y)
+    Note over Client: SELECTING state
+    
+    Client->>Server: DHCPREQUEST (requested ip=x.x.x.x)
+    Note over Server: 确认分配
+    
+    Server-->>Client: DHCPACK (confirmed allocation)
+    Note over Client: BOUND state ✅ IP = x.x.x.x
+```
+
+### DHCP 续租机制
+
+```
+租约时间 = Lease Time（默认通常 24h）
+
+续约时机:
+• T1 = 50% 租约时 → RENEW (单播到原服务器)
+• T2 = 87.5% 租约时 → REBIND (广播到新服务器)
+• 到期前必须续约成功，否则释放 IP
+```
+
+```bash
+# Linux DHCP 客户端配置
+$ cat /etc/dhcp/dhclient.conf
+timeout 300;           # 超时重试间隔
+retry 60;              # 初始重试间隔
+reboot 10;             # 重启时重尝试获取 IP
+request subnet-mask, broadcast-address, time-offset, routers;
+
+# 手动刷新 IP
+$ sudo dhclient -r eth0   # release
+$ sudo dhclient eth0      # request new
+```
+
+## 三、WebSocket 全双工通信
+
+### 为什么需要 WebSocket？
+
+```
+传统轮询 vs WebSocket:
+═══════════════════════
+Polling:   Client ─→ GET /status ─→ empty
+           Client ─→ GET /status ─→ {"msg": "hi"}
+           Client ─→ GET /status ─→ empty
+           ... 浪费带宽，延迟高
+
+Long Polling:
+           Client ─→ GET /status (挂起) ─→ ...等待... ─→ {"msg": "hi"}
+           Client ─→ GET /status (再挂起) ─→ ...
+
+WebSocket ✨:
+           Client ─── 握手升级 ───→ 持久双向通道
+           Server ─── 推送消息 ───→ Client
+           Client ─── 推送消息 ───→ Server
+           ← 全双工、低开销! -->
+```
+
+### WebSocket 握手升级过程
+
+```
+HTTP 请求 → WebSocket → HTTP 响应
+
+Client:                                   Server:
+GET /ws/chat HTTP/1.1                     101 Switching Protocols
+Host: chat.example.com                    Upgrade: websocket
+Upgrade: websocket                        Connection: Upgrade
+Connection: Upgrade                       Sec-WebSocket-Accept: <base64-hash>
+Sec-WebSocket-Key: dGhlIHNhbXBsZSBub25jZQ==
+Sec-WebSocket-Version: 13
+```
+
+**关键：** 这不是 TCP 层面的升级，而是 HTTP 协议的升级——服务器返回 `101` 状态码告诉客户端："好的，我们从这里开始用 WebSocket 协议"。
+
+### Sec-WebSocket-Accept 计算
+
+```go
+import (
+    "crypto/sha1"
+    "encoding/base64"
+    "strings"
+)
+
+const magicGUID = "258EAFA5-E914-47DA-95CA-C5AB0DC85B11"
+
+func computeAccept(key string) string {
+    h := sha1.New()
+    h.Write([]byte(key + magicGUID))
+    return base64.StdEncoding.EncodeToString(h.Sum(nil))
+}
+
+// 例: key = "dGhlIHNhbXBsZSBub25jZQ=="
+// accept = "s3pPLMBiTxaQ9kYGzzhZRbK+xOo="
+```
+
+### WebSocket Frame 格式
+
+```
+┌───────────┬───────────┬─────────────┬───────────────┬────────────────┐
+│ FIN(1 bit)│ RSV(3bit) │ Opcode(4bit)│ Mask(1 bit)   │ Payload Len    │
+│           │           │             │               │ + Ext Len      │
+├───────────┴───────────┴─────────────┴───────────────┼────────────────┤
+│                   Extended Payload Length            │   Mask Key   │
+│              (0, 126, or 127 bytes)                  │    (4 bytes) │
+├─────────────────────────────────────────────────────┴────────────────┤
+│                   Masked Payload Data (variable)                    │
+└─────────────────────────────────────────────────────────────────────┘
+```
+
+| Opcode | 含义 |
+|--------|------|
+| 0x0 | Continuation frame |
+| 0x1 | Text frame |
+| 0x2 | Binary frame |
+| 0x8 | Connection Close |
+| 0x9 | Ping |
+| 0xA | Pong |
+
+### Go 中的 WebSocket
+
+```go
+package main
+
+import (
+    "log"
+    "net/http"
+    "github.com/gorilla/websocket"
+)
+
+var upgrader = websocket.Upgrader{
+    CheckOrigin: func(r *http.Request) bool {
+        return true // 生产环境应严格校验 Origin
+    },
+}
+
+func wsHandler(w http.ResponseWriter, r *http.Request) {
+    conn, err := upgrader.Upgrade(w, r, nil)
+    if err != nil {
+        log.Println("upgrade error:", err)
+        return
+    }
+    defer conn.Close()
+    
+    for {
+        mt, message, err := conn.ReadMessage()
+        if err != nil {
+            break
+        }
+        log.Printf("recv: %s", message)
+        conn.WriteMessage(mt, message)
+    }
+}
+
+func main() {
+    http.HandleFunc("/ws", wsHandler)
+    log.Fatal(http.ListenAndServe(":8080", nil))
+}
+```
+
+## 关联笔记
+
+- [[hhs/NETWORK/HTTPS与TLS握手]] — SNI 扩展在 DNS 解析中的应用
+- [[hhs/NETWORK/WebSocket全双工通信]] — 心跳机制防止 NAT/代理超时断开
+- [[hhs/NETWORK/NAT原理与应用]] — WebSocket 也受 NAT 影响