167 lines
5.0 KiB
Markdown
167 lines
5.0 KiB
Markdown
---
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tags: [计算机网络, 静态路由, 默认网关, 策略路由]
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create time: 2026-05-18 01:00
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---
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# 静态路由与默认网关
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## 概述
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路由是数据包从源到目的地所经历的路径选择过程。Linux 内核维护着一张**路由表**，每收到一个包就查表决定下一跳。最基础也是最常用的就是静态路由和默认网关。
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## Linux 路由表结构
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```bash
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$ ip route show
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default via 192.168.1.1 dev eth0 proto dhcp src 192.168.1.100 metric 100
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192.168.1.0/24 dev eth0 proto kernel scope link src 192.168.1.100 metric 100
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10.0.0.0/8 via 10.0.0.1 dev eth1 proto static metric 200
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172.16.0.0/12 dev eth2 scope link
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```
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### 关键字段含义
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| 字段 | 说明 |
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|------|------|
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| `default` | 匹配所有目标（等价于 `0.0.0.0/0`），即**默认网关** |
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| `via` | 下一跳的 IP 地址 |
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| `dev` | 出口网卡 |
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| `proto` | 路由来源：kernel/auto/static/dhcp |
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| `scope` | 范围：link（直连）/ host（单主机）/ global（全局可路由）|
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| `metric` | 优先级，值越小越优先 |
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## 最长前缀匹配原则
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当多条路由能匹配目标 IP 时，选择**前缀最长**的那条：
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```
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目标 IP: 10.0.0.5
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路由表:
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default via 192.168.1.1 → /0 (0 bits)
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10.0.0.0/8 via 10.0.0.1 → /8 (8 bits) ✅ 选中！
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10.0.0.0/24 via 10.0.0.2 → /24 (24 bits) ← 实际上这条更长，应该选它！
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10.0.0.5/32 via 10.0.0.3 → /32 (32 bits) ← 最佳匹配！
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结论: 最长前缀优先 → 选 /32 那条
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```
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```mermaid
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flowchart TD
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S["目标 IP: 10.0.0.5"] --> C{"/32 match?"}
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C -->|"是"| R3["10.0.0.5/32<br/>via 10.0.0.3 ✅"]
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C -->|"否"--> C2{"/24 match?"}
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C2 -->|"是"| R2["10.0.0.0/24<br/>via 10.0.0.2 ✅"]
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C2 -->|"否"| C8{"/8 match?"}
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C8 -->|"是"| R1["10.0.0.0/8<br/>via 10.0.0.1 ✅"]
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C8 -->|"否"| D["default route<br/>via 192.168.1.1 ✅"]
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style R3 fill:#98FB98,color:#000
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style R2 fill:#DDA0DD
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style R1 fill:#FFD700
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style D fill:#B0C4DE
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```
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## 默认网关
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默认网关是所有未知目的地的兜底路径：
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```bash
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# 查看当前默认网关
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$ ip route show default
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default via 192.168.1.1 dev eth0 proto dhcp
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# 添加默认网关
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$ ip route add default via 192.168.1.1 dev eth0 metric 100
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# 删除默认网关
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$ ip route del default via 192.168.1.1
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# 修改默认网关（先删后加）
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$ ip route replace default via 192.168.1.254 dev eth0 metric 50
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```
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### 多默认网关（负载均衡与故障转移）
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```bash
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# ECMP（Equal-Cost Multi-Path）— 同开销的多条路径负载均衡
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$ ip route add default via 10.0.0.1 dev eth0
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$ ip route add default via 10.0.1.1 dev eth0
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# 流量会自动在两条链路间负载均衡（基于哈希）
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$ ip -d route show
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default nexthop via 10.0.0.1 dev eth0 weight 1
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nexthop via 10.0.1.1 dev eth0 weight 1
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```
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## 静态路由配置
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### 基本语法
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```bash
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# 格式: ip route add <网络>/<掩码> via <下一跳IP> dev <网卡> metric <优先级>
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$ sudo ip route add 172.16.0.0/12 via 192.168.1.1 dev eth0 metric 100
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$ sudo ip route add 10.100.0.0/16 via 10.0.0.1 dev eth1 onlink
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# onlink: 强制使用，即使下一跳不在同一子网
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```
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### 不同发行版的持久化方式
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| 系统 | 配置文件 | 示例 |
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|------|---------|------|
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| Debian/Ubuntu | `/etc/network/interfaces` | `up ip route add ...` |
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| RHEL/CentOS 7+ | `/etc/sysconfig/network-scripts/route-eth0` | `10.0.0.0/8 via 192.168.1.1` |
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| Systemd-networkd | `.network` 文件 | `Route:` section |
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| NetworkManager | `nmcli connection modify` | `ipv4.routes "..."` |
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| Alpine | `/etc/network/interfaces` | `post-up ip route add ...` |
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### Netplan (Ubuntu 18.04+)
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```yaml
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# /etc/netplan/01-netcfg.yaml
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network:
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version: 2
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ethernets:
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eth0:
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dhcp4: true
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routes:
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- to: 10.100.0.0/16
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via: 192.168.1.1
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```
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## 策略路由（Policy-Based Routing）
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Linux 支持基于源 IP、端口、协议等条件选择不同路由表：
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```bash
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# 创建自定义路由表
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echo "100 web-table" >> /etc/iproute2/rt_tables
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echo "200 db-table" >> /etc/iproute2/rt_tables
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# 为 web-table 添加路由
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ip route add 10.0.0.0/8 via 10.0.0.1 dev eth1 table web-table
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# 策略：来自 192.168.10.0/24 的流量走 web-table
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ip rule add from 192.168.10.0/24 table web-table
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# 效果：即使是同一个目的地，不同源 IP 走不同路径
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```
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```mermaid
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flowchart LR
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SrcA["192.168.10.5"] -->|"Source IP match"| Rule["ip rule policy"]
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SrcB["192.168.20.5"] -->|"Source IP match"| Rule
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Rule -->|"web-table → eth1"| WebRouter["Web Router<br/>10.0.0.1"]
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Rule -->|"db-table → eth2"| DBRouter["DB Router<br/>10.1.0.1"]
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style WebRouter fill:#DDA0DD,color:#000
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style DBRouter fill:#98FB98,color:#000
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```
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## 关联笔记
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- [[hhs/NETWORK/CIDR与子网划分]] — CIDR 前缀长度决定路由表的精确度
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- [[hhs/NETWORK/NAT原理与应用]] — NAT 常作为默认网关的角色存在
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- [[hhs/NETWORK/动态路由协议]] — OSPF/BGP 自动生成路由项，无需手动配置
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