vault backup: 2026-05-17 22:27:07
This commit is contained in:
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---
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tags: [计算机网络, ping, traceroute, ss, telnet, nc, netstat]
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create time: 2026-05-18 04:45
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---
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# 连通性与状态探测工具
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## 概述
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排查网络问题时，正确的第一步是判断"到底通不通"。本章覆盖从物理连通性到应用端口层的基础探测工具。
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## ping —— ICMP Echo Request/Reply
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### 基本用法速查
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```bash
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$ ping -c 4 example.com # 发 4 个包后自动停止
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$ ping -i 0.2 example.com # 每 0.2s 发一个（需 root）
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$ ping -s 1472 example.com # 载荷 1472 bytes (接近 MTU 1500)
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$ ping -t 64 example.com # 指定初始 TTL
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$ ping -q example.com # 静默模式，仅显示统计摘要
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$ ping -W 2 example.com # 每个包的超时等待 2 秒
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```
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### 解读输出指标
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```
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PING example.com (93.184.216.34) 56(84) bytes of data.
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64 bytes from 93.184.216.34: icmp_seq=1 ttl=56 time=14.2 ms
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64 bytes from 93.184.216.34: icmp_seq=2 ttl=56 time=13.8 ms
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--- example.com ping statistics ---
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packets transmitted: 2 # 发送包数
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packet loss: 0% # 丢包率
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rtt min/avg/max/mdev = 13.8/14.0/14.2/0.2 ms # 往返时间统计
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```
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| 指标 | 含义 | 合格阈值 |
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|------|------|---------|
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| packet loss | 丢包率 | 0% ~ 0.1%（局域网不应有丢包）|
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| rtt avg | 平均 RTT | LAN < 1ms / 同城 < 20ms / 跨省 30-80ms / 跨洋 80-150ms |
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| mdev | RTT 标准差 | < 5ms 正常；抖动大说明拥塞或不稳定 |
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> [!warning] ping 不通 ≠ 网络故障
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>
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> 很多服务器禁用了 ICMP Echo Reply（防火墙策略），此时 ping 不通不代表服务不可用。需要结合 `telnet port` 或 `curl` 综合判断。
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```bash
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# Linux 内置的 ICMP 防御（防 DDoS flood）
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$ sysctl net.ipv4.icmp_ratelimit=1000 # 每秒最多处理 1000 个 ICMP 包
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$ sysctl net.ipv4.icmp_ratemask=65535 # 允许的 ICMP 类型掩码
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$ sysctl net.ipv4.icmp_echo_ignore_all=1 # 完全忽略所有 ping（极端场景）
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```
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## telnet / nc —— 端口连通测试
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### telnet —— 最原始的端口探测
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```bash
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# 纯测试端口是否开放
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$ telnet example.com 80
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Trying 93.184.216.34...
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Connected to example.com. # ← Connected = 端口开放 ✅
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Escape character is '^]'.
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# 手动发 HTTP 请求测试
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GET / HTTP/1.1
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Host: example.com
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User-Agent: Mozilla/5.0
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Accept: */*
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(blank line 回车)
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HTTP/1.1 200 OK # ← 收到响应 = 链路完整 ✅
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Content-Type: text/html
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...
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```
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### nc (Netcat) —— 更强大的瑞士军刀
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```bash
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# 仅测试端口连通 (-z = zero-I/O 模式)
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$ nc -zv example.com 80
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Connection to example.com port 80 [http] succeeded! # ✅ TCP 3次握手成功
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$ nc -zv example.com 443
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Connection to example.com port 443 [https] succeeded! # ✅
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$ nc -zv example.com 8443
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nc: connect to example.com port 8443 (tcp): Connection refused # ❌
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# 发送数据并等待响应
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$ echo "GET / HTTP/1.1\r\nHost: example.com\r\n\r\n" | nc -w 2 example.com 80 | head -5
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# DNS 反向查询
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$ nc -dvl 0.0.0.0 12345 # 监听模式
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$ nc -u -z localhost 53 # UDP 端口测试 (DNS 常用)
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```
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## ss —— 连接状态快照（取代 netstat）
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### ss 基础用法
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```bash
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# 查看所有 TCP 连接（数字格式，不解析域名）
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$ ss -tan
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State Recv-Q Send-Q Local Address:Port Peer Address:Port Process
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ESTAB 0 0 192.168.1.10:45678 93.184.216.34:443 users:(("chrome",pid=1234,fd=42))
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SYN-SENT 0 1 192.168.1.10:54321 10.0.0.1:8080 # 正在尝试握手
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LISTEN 0 128 0.0.0.0:80 0.0.0.0:* users:(("nginx",pid=5678,fd=6))
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TIME-WAIT 0 0 192.168.1.10:443 93.184.216.34:52341 # 等待 2MSL
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# 查看特定端口的监听
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$ ss -tlnp sport = :80
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LISTEN 0 128 0.0.0.0:80 0.0.0.0:* users:(("nginx",pid=5678,fd=6))
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# 查看所有 TIME_WAIT 连接数（高并发服务器常见问题）
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$ ss -tan state time-wait | wc -l
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1247
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# 查看某进程的所有连接
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$ ss -tnp | grep nginx | wc -l
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# 诊断 CLOSE_WAIT 堆积（⚠️ 代码 Bug！）
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$ ss -tan state close-wait
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CLOSE-WAIT 0 0 server:8080 client:54321 # ← 对端已关闭，本地未 close()!
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```
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### ss 状态速查表
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| ss 状态 | 含义 | 严重程度 |
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|---------|------|---------|
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| ESTABLISHED | 正常通信中 | ✅ 正常 |
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| LISTEN | 等待接受连接 | ✅ 正常 |
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| SYN-SENT | 客户端发出的 SYN 未收到回复 | ⚠️ 检查对端是否存活 |
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| SYN-RECV | 收到 SYN 但未完成三次握手 | ⚠️ 可能 SYN Flood |
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| TIME-WAIT | 己方主动关闭后等待 2MSL | ⚠️ 可调 tcp_tw_reuse |
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| CLOSE-WAIT | 对端关闭但己方未 close() | 🔴 **Bug！** 检查代码 |
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| LAST-ACK | 等待最后一个 ACK | ⚠️ 短暂状态，持续则异常 |
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| CLOSING | 同时关闭，互等对方 ACK | ⚠️ 罕见 |
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```bash
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# 按状态过滤
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$ ss -tan state established # 仅已建立
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$ ss -tan state syn-sent # 仅半连接中
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$ ss -tan state closing # 仅 CLOSING
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# 按源/目标 IP 过滤
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$ ss -tn daddr 10.0.0.0/8 # 只看内网连接
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$ ss -tn src :80 # 只看来自 80 端的
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```
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> [!tip] ss vs netstat
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> `ss` 使用 netlink socket 替代 `/proc/net/tcp`，速度更快、信息更全。Linux 5.x+ 系统上 netstat 已被标记为 deprecated。
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## traceroute —— 逐跳路径探测
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### 三种实现方式
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```bash
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# UDP 模式（默认）—— 向高端口发 UDP 包递增 TTL
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$ traceroute -n example.com
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1 192.168.1.1 0.5ms 0.3ms 0.4ms
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2 10.0.0.1 10.2ms 9.8ms 10.1ms
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12 93.184.216.34 45.1ms 44.8ms 45.0ms
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# ICMP 模式（更可靠，不容易被防火墙拦截）
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$ traceroute -I example.com
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# 现代替代：tracepath（不需要 root，自动 PMTUD）
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$ tracepath example.com
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1: 192.168.1.1 0ms reached
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2: 10.0.0.1 10ms
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...
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12: 93.184.216.34 45ms Ascent peak pmtu 1452
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```
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### traceroute 原理
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```mermaid
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flowchart LR
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P1["TTL=1 → 第1跳路由器<br/>回 ICMP Time Exceeded"] -->|"RTT₁"| R1["第1跳: 192.168.1.1"]
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P2["TTL=2 → 第2跳路由器<br/>回 ICMP Time Exceeded"] -->|"RTT₂"| R2["第2跳: 10.0.0.1"]
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P3["TTL=N → 到达目标<br/>回 ICMP Port Unreachable"] -->|"RTTN"| Rn["目的地: 93.184.216.34"]
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style R1 fill:#DDA0DD,color:#000
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style R2 fill:#FFD700,color:#000
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style Rn fill:#98FB98,color:#000
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```
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```bash
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# 识别路由问题
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$ traceroute -n 8.8.8.8
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...
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5 * * * # ← 这里丢了？→ 路由器禁了 ICMP
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6 * * * # ← 可能是运营商之间互联点
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7 108.170.xxx.xxx 50ms 48ms 51ms # ← 回到 Google 边缘
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# Windows 等价命令
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tracert 8.8.8.8
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# macOS 等价命令
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traceroute 8.8.8.8
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```
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## 排错黄金思路回顾
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```
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问题: 无法访问服务
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↓
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ping 通吗？ → 否 → traceroute 定位断在哪一跳
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↓ 是
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telnet port 通吗？ → 否 → 检查防火墙/selinux → ss 看是否监听
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↓ 是
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curl 正常吗？ → 否 → 应用层问题 → 读日志
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↓ 是
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浏览器缓存/Cookie/Header 问题
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```
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## 关联笔记
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- [[hhs/NETWORK/ICMP与Ping-Traceroute]] — ping/traceroute 的协议底层原理
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- [[hhs/NETWORK/TCP三次握手与四次挥手]] — ss 中看到的状态都对应 TCP 状态机中的某个节点
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- [[hhs/NETWORK/DNS与DHCP与WebSocket]] — DNS 解析慢会影响 ping/curl 的表现
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- [[hhs/NETWORK/抓包HTTPDNS诊断工具]] — 进一步分析可用 tcpdump/curl/dig
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@@ -0,0 +1,249 @@
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---
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tags: [计算机网络, tcpdump, tshark, curl, dig, iptables, Wireshark]
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create time: 2026-05-18 04:50
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---
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# 抓包、HTTP 调试与 DNS 诊断工具
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## 概述
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在前一章完成了"通不通"的判断之后，本章深入"报文到了没"和"内容对不对"。涵盖 tcpdump 抓包分析、curl HTTP 调试、dig DNS 诊断以及 iptables 规则排查。
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## tcpdump —— 命令行抓包神器
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### 基础语法与 BPF 过滤器精选
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```bash
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sudo tcpdump -i eth0 # 抓取 eth0 所有流量
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sudo tcpdump -nn # -n: 不解析域名, -n: 不解析端口名
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sudo tcpdump -c 100 # 抓 100 个包后停止
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sudo tcpdump -w capture.pcap # 写入 pcap 文件（Wireshark 打开）
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sudo tcpdump -A 'port 80' # ASCII 模式打印 HTTP 内容
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sudo tcpdump -X 'port 80' # HEX + ASCII 双列输出
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```
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### BPF (Berkeley Packet Filter) 表达式
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```bash
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# 按 IP 过滤
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sudo tcpdump 'host 93.184.216.34' # 只看这个 IP
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# 按端口过滤
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sudo tcpdump 'port 80' # HTTP
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sudo tcpdump 'port 443' # HTTPS
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sudo tcpdump 'port range 8000-9000' # 端口范围
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# 组合过滤
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sudo tcpdump 'src host 10.0.0.1 and dst port 443' # 源 → 目标端口
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sudo tcpdump '(src net 192.168.1.0/24) or (dst net 10.0.0.0/8)' # 多网段
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# 匹配 TCP 标志位（非常强大！）
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sudo tcpdump 'tcp[tcpflags] & (tcp-syn|tcp-ack) != 0' # SYN+ACK
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sudo tcpdump 'tcp[tcpflags] & tcp-syn != 0' # SYN 包（新连接）
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sudo tcpdump 'tcp[13] & 0x04 != 0' # 同样匹配 SYN
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sudo tcpdump 'tcp[13] & 0x10 != 0' # ACK 标志位
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sudo tcpdump 'tcp[tcpflags] & tcp-rst != 0' # RST 包（异常断开）
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sudo tcpdump 'tcp[tcpflags] & tcp-fin != 0' # FIN 包（正常关闭）
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# 排除法
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sudo tcpdump 'not host 192.168.1.1 and not port 22' # 排除网关和 SSH
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```
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### SYN 包实时观察
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```bash
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# 实时监控新连接建立
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sudo tcpdump -nn 'tcp[tcpflags] & tcp-syn != 0'
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08:23:45.123456 IP 192.168.1.100:54321 > 10.0.0.1:8080: S 12345678:12345678(...)
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08:23:45.123789 IP 10.0.0.1:8080 > 192.168.1.100:54321: S 87654321:87654321(...)
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# ↑ 这就是三次握手的前两个包（SYN 和 SYN-ACK）
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# 监控 RST（异常断开）
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sudo tcpdump -nn 'tcp[tcpflags] & (tcp-rst) != 0'
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```
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> [!tip] tcpdump → Wireshark 工作流
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> ```bash
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> # 1. tcpdump 后台抓包
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> sudo tcpdump -w /tmp/app.pcap 'host 10.0.0.1 and port 8080' &
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> # 2. 复现问题
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> # 3. Ctrl+C 停止抓包
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> # 4. Wireshark 打开分析
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> wireshark /tmp/app.pcap
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> # 5. Wireshark 支持深度解码 HTTP/gRPC/TLS 等协议
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> ```
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## tshark —— CLI 版 Wireshark
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```bash
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# 实时过滤 HTTP 请求
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$ tshark -i eth0 -Y 'http.request'
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No. Time Source Destination Protocol Length Info
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12 0.123456 10.0.0.1 93.184.xxx HTTP 450 GET /api/users HTTP/1.1
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# 导出字段（CSV 风格）
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tshark -r capture.pcap -Y 'http' \
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-T fields -e ip.src -e http.host -e http.request.uri
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# 只追踪 TLS 握手的 ClientHello
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tshark -Y 'tls.handshake.type == 1'
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# 1 = ClientHello, 11 = ServerHello, 12 = Certificate, etc.
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```
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## curl —— HTTP 调试利器
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### 详细输出与计时分解
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```bash
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# -v: verbose 模式，看到完整的握手过程和头部交换
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curl -v https://example.com
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* Trying 93.184.216.34:443...
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* Connected to example.com (93.184.216.34) port 443
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* ALPN, offering h2
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* ALPN, offering http/1.1
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* TLSv1.3, TLS handshake, Client hello (1):
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* TLSv1.3, TLS handshake, Server hello (2):
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* TLSv1.3, Encrypted Extensions (8):
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* TLSv1.3, Certificate (11):
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* TLSv1.3, TLS handshake, Finished (5):
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* TLSv1.3, Encrypted Change Cipher (1):
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* ... SSL connection using TLSv1.3 / TLS_AES_256_GCM_SHA384
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# -w: custom output 精确测量各阶段耗时 💡
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curl -w '\nDNS Resolve: %{time_namelookup}s\nTCP Connect: %{time_connect}s\nTLS Handshake: %{time_appconnect}s\nFirst Byte: %{time_starttransfer}s\nTotal: %{time_total}s\n' \
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-o /dev/null -s https://example.com
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# 输出示例:
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# DNS Resolve: 0.01234s
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# TCP Connect: 0.01567s
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# TLS Handshake: 0.04567s
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# First Byte: 0.15678s
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# Total: 0.23456s
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```
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```
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耗时瓶颈分析:
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├─ time_namelookup > 0.1s → DNS 慢 → 考虑 DNS 缓存
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├─ time_connect - time_namelookup > 0.1s → TCP 连接慢 → 距离远/NAT 问题
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├─ time_appconnect - time_connect > 0.2s → TLS 握手慢 → 证书链长/无 session resumption
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└─ time_starttransfer - time_appconnect > 0.5s → 服务端处理慢 → 查应用日志
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||||
```
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||||
### curl 实战用法
|
||||
|
||||
```bash
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||||
# 自定义 Header 和认证
|
||||
curl -H 'Authorization: Bearer token' \
|
||||
-H 'Content-Type: application/json' \
|
||||
-X POST -d '{"name":"alice"}' \
|
||||
https://api.example.com/users
|
||||
|
||||
# 跟踪重定向（最多 5 层）
|
||||
curl -L --max-redirs 5 https://short.link
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||||
|
||||
# 查看响应头（快速判断 CDN/缓存状态）
|
||||
curl -I https://cdn.example.com/style.css
|
||||
HTTP/2 200
|
||||
cache-control: public, max-age=86400
|
||||
age: 1234
|
||||
x-cache: HIT # ← CDN 命中
|
||||
cf-cache-status: HIT # ← Cloudflare 标记
|
||||
content-encoding: br # ← Brotli 压缩
|
||||
|
||||
# SSL 证书检查
|
||||
openssl s_client -connect example.com:443 -servername example.com
|
||||
# 关注:
|
||||
# Verify return code: 0 (ok) ✅
|
||||
# Verify return code: 20 (unable to get local issuer certificate) ❌
|
||||
|
||||
# JSON 美化输出
|
||||
curl https://jsonplaceholder.typicode.com/users/1 | python3 -m json.tool
|
||||
```
|
||||
|
||||
## dig —— DNS 诊断最强工具
|
||||
|
||||
### 常用查询模式
|
||||
|
||||
```bash
|
||||
# A 记录简洁输出
|
||||
$ dig example.com +short
|
||||
93.184.216.34
|
||||
|
||||
# AAAA 记录（IPv6）
|
||||
$ dig example.com AAAA +short
|
||||
2606:2800:220:1:248:1893:25c8:1946
|
||||
|
||||
# 权威追踪（从根域名开始逐级查询）
|
||||
$ dig example.com +trace
|
||||
;; ->>HEADER<<- opcode: QUERY, status: NOERROR
|
||||
;; QUESTION SECTION: ;example.com. IN A
|
||||
;; ANSWER SECTION: example.com. 300 IN A 93.184.216.34
|
||||
|
||||
# 指定 DNS 服务器对比
|
||||
$ dig @8.8.8.8 example.com # Google DNS
|
||||
$ dig @1.1.1.1 example.com # Cloudflare DNS
|
||||
$ dig @local_dns_server example.com # 本地 DNS（可能被污染）
|
||||
|
||||
# TXT 记录（SPF / DKIM 验证邮件配置）
|
||||
$ dig example.com TXT +short
|
||||
"v=spf1 include:_spf.google.com ~all"
|
||||
|
||||
# MX 记录（邮件服务器）
|
||||
$ dig example.com MX +short
|
||||
10 mail.example.com.
|
||||
20 mail2.example.com.
|
||||
|
||||
# SRV 记录（服务发现，gRPC/Kafka 常用）
|
||||
$ dig _grpc._tcp.service.consul SRV +short
|
||||
0 100 8080 api.service.consul.
|
||||
|
||||
# 查看 CNAME 链全貌
|
||||
$ dig www.example.com +multiline
|
||||
```
|
||||
|
||||
### dig 高级用法
|
||||
|
||||
```bash
|
||||
# 区域传输（AXFR，通常被禁止）
|
||||
$ dig @ns1.example.com example.com axfr
|
||||
|
||||
# EDNS Client Subnet（CDN 优化用户体验的关键）
|
||||
$ dig +ecs=203.0.113.0/24 example.com @1.1.1.1
|
||||
# DNS resolver 把自己的子网发给 authoritative server
|
||||
# → authoritative server 返回离用户最近的 CDN 节点 IP
|
||||
|
||||
# DoH（DNS over HTTPS）检测
|
||||
$ curl https://dns.google/resolve?name=example.com&type=A
|
||||
{"Status":0,"Answer":[{"name":"example.com","type":5,"data":"93.184.216.34",...}]}
|
||||
```
|
||||
|
||||
## iptables / nftables 规则排查
|
||||
|
||||
```bash
|
||||
# iptables 规则排查步骤
|
||||
sudo iptables -L -n -v # 列出 FILTER 表所有规则及计数器
|
||||
sudo iptables -t nat -L -n -v # 查看 NAT 表（SNAT/DNAT/MASQUERADE）
|
||||
sudo iptables -t mangle -L -n -v # 查看 MANGLE 表（TOS/TTL 修改）
|
||||
|
||||
# 关键排查命令
|
||||
sudo iptables-save | grep -A 5 INPUT # 只看 INPUT chain 的上下文
|
||||
sudo iptables -L FORWARD -n -v --line-numbers # 转发规则（容器/代理场景必查）
|
||||
|
||||
# 常见规则导致的"假故障"
|
||||
# ❌ 忘记放行容器的桥接流量
|
||||
iptables -I FORWARD -i docker0 -j ACCEPT
|
||||
|
||||
# ❌ 没有启用 IP forwarding
|
||||
sysctl net.ipv4.ip_forward=1
|
||||
|
||||
# nftables（iptables 的现代替代品）
|
||||
sudo nft list ruleset # 列出所有 nft 规则
|
||||
sudo nft add rule inet filter input tcp dport 22 accept
|
||||
```
|
||||
|
||||
## 关联笔记
|
||||
|
||||
- [[hhs/NETWORK/连通性与状态探测工具]] — 先看 ping/ss/telnet 再决定要不要抓包
|
||||
- [[hhs/NETWORK/ICMP与Ping-Traceroute]] — ping/traceroute 协议原理
|
||||
- [[hhs/NETWORK/DNS与DHCP与WebSocket]] — DNS 递归查询流程补充 dig 实操
|
||||
- [[hhs/NETWORK/TCP三次握手与四次挥手]] — tcpdump 中观察 SYN/ACK 交互细节
|
||||
Reference in New Issue
Block a user