DHCP（动态主机配置协议）基本信息

概要

动态主机配置协议 (DHCP) 是 RFC 1541（已被 RFC 2131 取代）定义的标准协议，该协议答应服务器向客户端动态分配 IP 地址和配置信息。通常，DHCP 服务器至少向客户端提供以下基本信息： • IP 地址

• 子网掩码

• 默认网关它还可以提供其他信息，如域名服务 (DNS) 服务器的地址和 Windows Internet 名称服务 (WINS) 服务器的地址。系统治理员配置 DHCP 服务器分配给客户端的选项。

当客户端被配置为接收 DHCP 信息后首次初始化时，它将启动与服务器的对话。

以下是客户端和服务器之间对话的摘要表，最后一列是数据包级的过程说明：

Source     Dest        Source     Dest              Packet
MAC addr   MAC addr    IP addr    IP addr           Description
-----------------------------------------------------------------
Client     Broadcast   0.0.0.0    255.255.255.255   DHCP Discover
DHCPsrvr   Broadcast   DHCPsrvr   255.255.255.255   DHCP Offer
Client     Broadcast   0.0.0.0    255.255.255.255   DHCP Request
DHCPsrvr   Broadcast   DHCPsrvr   255.255.255.255   DHCP ACK

DHCP 客户端和 DHCP 服务器之间的具体对话如下：

DHCPDISCOVER

客户端发送 DHCPDISCOVER 数据包。以下内容摘自网络监视器的捕捉信息，显示了 DHCPDISCOVER 数据包中的 IP 和 DHCP 部分。在 IP 部分，可以看到 Destination 地址是 255.255.255.255，而 Source 地址是 0.0.0.0。DHCP 部分将数据包标识为 Discover 数据包，并使用网卡的物理地址在两处标识客户端。注重 CHADDR 字段和 DHCP 中的值：Client Identifier 字段是相同的。

IP:ID = 0x0; Proto = UDP; Len: 328
IP:Version = 4 (0x4)
IP:Header Length = 20 (0x14)
IP:Service Type = 0 (0x0)
IP:Precedence = Routine
IP:...0.... = Normal Delay
IP:....0... = Normal Throughput
IP:.....0.. = Normal Reliability
IP:Total Length = 328 (0x148)
IP:Identification = 0 (0x0)
IP:Flags Summary = 0 (0x0)
IP:.......0 = Last fragment in datagram
IP:......0. = May fragment datagram if necessary
IP:Fragment Offset = 0 (0x0) bytes
IP:Time to Live = 128 (0x80)
IP:Protocol = UDP - User Datagram
IP:Checksum = 0x39A6
IP:Source Address = 0.0.0.0
IP:Destination Address = 255.255.255.255
IP:Data:Number of data bytes remaining = 308 (0x0134)

DHCP:Discover           (xid=21274A1D)
DHCP:Op Code           (op)     = 1 (0x1)
DHCP:Hardware Type     (htype)  = 1 (0x1) 10Mb Ethernet
DHCP:Hardware Address Length (hlen) = 6 (0x6)
DHCP:Hops              (hops)   = 0 (0x0)
DHCP:Transaction ID    (xid)    = 556223005 (0x21274A1D)
DHCP:Seconds           (secs)   = 0 (0x0)
DHCP:Flags             (flags)  = 0 (0x0)
DHCP:0............... = No Broadcast
DHCP:Client IP Address (ciaddr) = 0.0.0.0
DHCP:Your   IP Address (yiaddr) = 0.0.0.0
DHCP:Server IP Address (siaddr) = 0.0.0.0
DHCP:Relay  IP Address (giaddr) = 0.0.0.0
DHCP:Client Ethernet Address (chaddr) = 08002B2ED85E