{% hint style="info" %}
DHCP 中的 单播/广播
* Client 不知道 Server 的地址，因此**广播**
* Server 收到广播包后，知道了 Client 的MAC地址，因此**单播**
{% endhint %}
1. 发现阶段：Client 不知道 Server 的地址，因此**广播** `DHCP Discover` 包
2. 响应阶段：Server 收到 `DHCP Discover` 包后，向Client**单播** `DHCP Offer` 包，包含 IP 地址、子网掩码、租约时间等信息
3. 选择阶段：Client 收到 `DHCP Offer` 包后，选择一个 Server，广播 `DHCP Request` 包
4. 租约确认：Server 收到 `DHCP Request` 包后，返回 `DHCP Ack`（或 `DHCP Nak`）包，确认或拒绝租约
5. 租期续约：Client 在租期中，向提供 IP 地址的 Server `DHCP Request` 包，收到 Server 的 `DHCP Ack` 包后更新配置
6. 租期释放：Client 在租期中，向提供 IP 地址的 Server发送 `DHCP Release` 包，Server 收到后释放 IP 地址
## 报文结构
```plaintext
0 7 15 23 31
+------------------------------------------+
| Op (1) | Htype (1) | Hlen (1)| Hops (1)|
+------------------------------------------|
| Xid (4) |
+------------------------------------------|
| Secs (2) | Flags (2) |
+------------------------------------------|
| Ciaddr (4) |
+------------------------------------------|
| Yiaddr (4) |
+------------------------------------------|
| Siaddr (4) |
+------------------------------------------|
| Giaddr (4) |
+------------------------------------------+
| Chaddr (16) |
+------------------------------------------+
| Sname (64) |
+------------------------------------------|
| File (128) |
+------------------------------------------+
| Options(variable) |
+------------------------------------------+
```
## DHCP 欺骗
* 欺骗：伪造 DHCP Server，抢先回答，向 Client 发送虚假 IP 地址，进而导致数据窃听
* 实际应用的网络中采用 DHCP 中继，使得本网络的非授权 DHCP Server 快于其他网络的授权 DHCP Server，导致 DHCP 欺骗更容易完成
* 防范措施：DHCP Snooping，使用 DHCP Snooping 绑定表过滤不可信的信息
### 配置
```bash
switch(config)# ip dhcp snooping
switch(config)# ip dhcp snooping vlan 1
switch(config)# interface GigabitEthernet 0/1
switch(config-if)# ip dhcp snooping trust # 默认是非信任端口，需要改为确认
```
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