十分详细的DHCP服务工作原理剖析 推荐

以下内容摘自笔者最新出版的《深入理解计算机网络》一书，目前该书已在当当网、京东网和卓越网上可以正式预订了：

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在与读者的交流中，发现许多读者对DHCP服务的工作原理不是很清楚，如为什么DHCP服务报文不能穿越网关，在网络中有多台DHCP服务器时客户端到底该如何选择？各阶段的DHCP报文格式有哪些主要变化？DHCP客户端与DHCP服务器又是如何识别并建立通信的等。在此本文给出了具体介绍。

11.4.4 DHCP服务IP地址自动分配原理

DHCP在提供服务时，DHCP客户端是以UDP 68号端口进行数据传输的，而DHCP服务器是以UDP 67号端口进行数据传输的。DHCP服务不仅体现在为DHCP客户端提供IP地址自动分配过程中，还体现在后面的IP地址续约和释放过程中。本节仅介绍DHCP客户端初次分配IP地址的过程。

在整个DHCP服务器为DHCP客户端初次提供IP地址自动分配过程中，一共经过了以下四个阶段，利用了表11-10中的前四个报文：发现阶段（DHCP客户端在网络中广播发送DHCP DISCOVER请求报文，发现DHCP服务器，请求IP地址租约）、提供阶段（DHCP服务器通过DHCP OFFER报文向DHCP客户端提供IP地址预分配）、选择阶段（DHCP客户端通过DHCP REQUEST报文确认选择第一个DHCP服务器为它提供IP地址自动分配服务）和确认阶段（被选择的DHCP服务器通过DHCP ACK报文把在DHCP OFFER报文中准备的IP地址租约给对应DHCP客户端）。
在DHCP客户端在获得了一个IP地址以后，就可以发送一个免费ARP请求探测网络中是否还有其它主机使用IP地址，来避免由于DHCP服务器地址池重叠而引发的IP冲突。以上四个阶段如图11-19所示，相当于DHCP客户端与DHCP服务器的四次握手过程。具体描述如下：



图11-19 DHCP客户端从DHCP服务器获取IP地址的四个阶段
（1）发现阶段：即DHCP客户端获取网络中DHCP服务器信息的阶段。在客户端配置了DHCP客户端程序（如在Windows系统中进行了如图11-16所示的配置）并启动后，以广播方式发送DHCP DISCOVER报文寻找网络中的DHCP服务器。示例如下。
此广播报文采用传输层的UDP 68号端口发送（封装的目的端口为UDP 68号端口），经过网络层IP协议封装后，源IP地址为0.0.0.0（因为此时还没有分配IP地址），目的IP地址为255.255.255.255（有限广播IP地址）。如下是一个DHCP DISCOVER报文封装的IP报头示例，可以看到 Destination Address（目的地址）是 255.255.255.255，而 Source Address（源地址）是 0.0.0.0。IP报头中的其它字段含义大家可以参见本书第7章7.3.4节相关内容。

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  !---使用UDP传输层协议

IP:Checksum = 0x39A6

IP:Source Address = 0.0.0.0    !---源IP地址为0.0.0.0

IP:Destination Address = 255.255.255.255   !----目的IP地址为255.255.255.255

IP:Data:Number of data bytes remaining = 308 (0x0134)

【经验之谈】在以上DHCP DISCOVER报文中IP报头中的目的地址（Destination Address）是255.255.255.255这个有限广播地址。这个有限广播地址就是代表任意一个IPv4子网的广播地址，当然是发送报文的主机所在的子网和DHCP服务器所在子网的广播地址，但因为此时DHCP客户端并不知道DHCP服务器所在的是哪个子网。下面所有其它DHCP报文中的255.255.255.255地址的含义也是一样的。
至于IP报头中的源地址（Source Address），由于当前DHCP客户端主机并未分配具体的IP地址，所以只能用具有任意代表功能的0.0.0.0地址来表示了。下面所有其它DHCP报文中指定的0.0.0.0地址的含义也是一样的。
因为此时，DHCP客户端没有分配到IP地址，也不知道DHCP服务器或DHCP中继的IP地址，所以在DHCP DISCOVER报文中Ciaddr（客户端IP地址）、Yiaddr（被分配的DHCP客户端IP地址）、Siaddr（下一个为DHCP客户端分配IP地址的DHCP服务器地址）、Giaddr（DHCP中继IP地址）这四个字段均为0.0.0.0，如下所示。另外，从中可以看到，在CHADDR字段和DHCP选项中Client Identifier字段中都标识了DHCP客户端网卡MAC地址。

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)  = 1 (0x1)   !---标志位置1，代表以广播方式发送

DHCP:1............... = 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

DHCP:Server Host Name  (sname)  = <Blank>

DHCP:Boot File Name    (file)   = <Blank>

DHCP:Magic Cookie = [OK]

DHCP:Option Field      (options)

DHCP:DHCP Message Type      = DHCP Discover   !---DHCP报文类型为DHCP Discover

DHCP:Client-identifier      = (Type:1) 08 00 2b 2e d8 5e

DHCP:Host Name              = JUMBO-WS    !---DHCP服务器主机名

DHCP:Parameter Request List = (Length:7) 01 0f 03 2c 2e 2f 06

DHCP:End of this option field

（2）提供阶段：即DHCP服务器向DHCP客户端提供预分配IP地址的阶段。网络中的所有DHCP服务器接收到客户端的DHCP DISCOVER报文后，都会根据自己地址池中IP地址分配的优先次序选出一个IP地址，然后与其它参数一起通过传输层的UDP 67号端口，在DHCP OFFER报文中以广播方式发送给客户端（目的端口是DHCP客户端的UDP 68号端口）。客户端通过封装在帧中的目的MAC地址（也就在DHCP DISCOVER报文中的CHADDR字段值）的比对来确定是否接收该帧。但这样一来，理论上DHCP客户端可能会收到多个DHCP OFFER报文（当网络中存在多个DHCP服务器时），但DHCP客户端只接受第一个到来的DHCP OFFER报文。
DHCP OFFER报文经过IP协议封装后的源IP地址DHCP服务器自己的IP地址，目的地址仍是255.255.255.255广播地址，使用的协议仍为UDP。下面是一个DHCP OFFER报文的IP报头示例。

IP:ID = 0x3C30; 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 = 15408 (0x3C30)

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 = 0x2FA8

IP:Source Address = 157.54.48.151

IP:Destination Address = 255.255.255.255

IP:Data:Number of data bytes remaining = 308 (0x0134)

在DHCP OFFER报文中，Ciaddr字段值仍为0.0.0.0，因为客户端仍没有分配到IP地址；Yiaddr字段已有值了，这是DHCP服务器为该客户端预分配的IP地址；因为此时仍没有得到客户端确认，所以Siaddr字段值仍为0.0.0.0；因为没有经过DHCP中继服务器，所以Giaddr字段值仍为0.0.0.0。另外，在 DHCP可选项部分，可以看到由服务器随 IP 地址一起发送的各种选项。在这种情况下，服务器发送的是子网掩码、默认网关（路由器）、租约时间、WINS 服务器地址（NetBIOS 名称服务）和 NetBIOS 节点类型。下面是一个DHCP OFFER报文示例。

DHCP:Offer              (xid=21274A1D)

DHCP:Op Code           (op)     = 2 (0x2)

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)  = 1 (0x1)

DHCP:1............... =  Broadcast

DHCP:Client IP Address (ciaddr) = 0.0.0.0

DHCP:Your   IP Address (yiaddr) = 157.54.50.5

DHCP:Server IP Address (siaddr) = 0.0.0.0

DHCP:Relay  IP Address (giaddr) = 0.0.0.0

DHCP:Client Ethernet Address (chaddr) = 08002B2ED85E

DHCP:Server Host Name  (sname)  = <Blank>

DHCP:Boot File Name    (file)   = <Blank>

DHCP:Magic Cookie = [OK]

DHCP:Option Field      (options)

DHCP:DHCP Message Type      = DHCP Offer    !---DHCP报文类型为DHCP Offer

DHCP:Subnet Mask            = 255.255.240.0    !---所分配IP地址的子网掩码为255.255.240.0

DHCP:Renewal Time Value (T1) = 8 Days,  0:00:00   !---想要继续租约原来分配的IP地址，则提出续约申请的期限为8天

DHCP:Rebinding Time Value (T2) = 14 Days,  0:00:00  !---如果上次申请续约失败，再次申请绑定原来分配到的IP地址的期限为14天

DHCP:IP Address Lease Time  = 16 Days,  0:00:00   !---租约期限为16天，也就是DHCP客户端可使用此IP地址的最长时间为16天

DHCP:Server Identifier      = 157.54.48.151  !---DHCP服务器的IP地址为157.54.48.151

DHCP:Router                 = 157.54.48.1  !---默认网关IP地址为157.54.48.1

DHCP:NetBIOS Name Service   = 157.54.16.154  !---DNS服务器IP地址为157.54.16.154

DHCP:NetBIOS Node Type      = (Length: 1) 04

DHCP:End of this option field

（3）选择阶段：即DHCP客户端选择IP地址的阶段。如果有多台DHCP服务器向该客户端发来DHCP OFFER报文，客户端只接受第一个收到的DHCP OFFER报文，然后以广播方式发送DHCP REQUEST报文。在该报文的“Requested Address”选项中包含DHCP服务器在DHCP OFFER报文中预分配的IP地址，对应的DHCP服务器IP地址等。这样也就相当于同时告诉得其他DHCP服务器，它们可以释放已提供的地址，并将这些地址返回到可用地址池中。

在DHCP OFFER报文封装的IP协议头部中，客户端的 Source Address仍然是 0.0.0.0，数据包的 Destination 仍然是 255.255.255.255。但在DHCP OFFER报文中Ciaddr、Yiaddr、Siaddr、Giaddr 字段的地址均0.0.0.0，大家自己分析一下为什么，很简单的。下面是一个DHCP OFEER报文头部和DHCP OFFER报文示例。

IP:ID = 0x100; 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 = 256 (0x100)

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 = 0x38A6

IP:Source Address = 0.0.0.0

IP:Destination Address = 255.255.255.255

IP:Data:Number of data bytes remaining = 308 (0x0134)

DHCP:Request            (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)  = 1 (0x1)

DHCP:1............... = 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

DHCP:Server Host Name  (sname)  = <Blank>

DHCP:Boot File Name    (file)   = <Blank>

DHCP:Magic Cookie = [OK]

DHCP:Option Field      (options)

DHCP:DHCP Message Type      = DHCP Request

DHCP:Client-identifier      = (Type:1) 08 00 2b 2e d8 5e

DHCP:Requested Address      = 157.54.50.5

DHCP:Server Identifier      = 157.54.48.151

DHCP:Host Name              = JUMBO-WS

DHCP:Parameter Request List = (Length:7) 01 0f 03 2c 2e 2f 06

DHCP:End of this option field

（4）确认阶段：即DHCP服务器确认分配级DHCP客户端IP地址的阶段。某个DHCP服务器在收到DHCP客户端发来的DHCP REQUEST报文后，只有DHCP客户端选择的服务器会进行如下操作：如果确认将地址分配给该客户端，则以广播方式返回DHCP ACK报文；否则返回DHCP NAK报文，表明地址不能分配给该客户端。

在DHCP 服务器发送的DHCP ACK报文的IP协议头部，Source Address是 DHCP 服务器IP地址，DestinationAddress仍然是广播地址255.255.255.255。在DHCP ACK报文中的Yiaddr字段包含要分配给客户端的IP地址，而Chaddr和DHCP:Client Identifier字段是发出请求的客户端中网卡的MAC地址。同时在选项部分也会在DHCP OFFER报文中把所分配的IP地址的子网掩码、默认网关、DNS服务器、租约期、续约时间等信息加上。

IP:ID = 0x3D30; 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 = 15664 (0x3D30)

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 = 0x2EA8

IP:Source Address = 157.54.48.151

IP:Destination Address = 255.255.255.255

IP:Data:Number of data bytes remaining = 308 (0x0134)

DHCP:ACK                (xid=21274A1D)

DHCP:Op Code           (op)     = 2 (0x2)

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)  = 1 (0x1)

DHCP:1............... = Broadcast

DHCP:Client IP Address (ciaddr) = 0.0.0.0

DHCP:Your   IP Address (yiaddr) = 157.54.50.5

DHCP:Server IP Address (siaddr) = 0.0.0.0

DHCP:Relay  IP Address (giaddr) = 0.0.0.0

DHCP:Client Ethernet Address (chaddr) = 08002B2ED85E

DHCP:Server Host Name  (sname)  = <Blank>

DHCP:Boot File Name    (file)   = <Blank>

DHCP:Magic Cookie = [OK]

DHCP:Option Field      (options)

DHCP:DHCP Message Type      = DHCP ACK

DHCP:Renewal Time Value (T1) = 8 Days,  0:00:00

DHCP:Rebinding Time Value (T2) = 14 Days,  0:00:00

DHCP:IP Address Lease Time  = 16 Days,  0:00:00

DHCP:Server Identifier      = 157.54.48.151

DHCP:Subnet Mask            = 255.255.240.0

DHCP:Router                 = 157.54.48.1

DHCP:NetBIOS Name Service   = 157.54.16.154

DHCP:NetBIOS Node Type      = (Length: 1) 04

DHCP:End of this option field

【说明】客户端在收到服务器返回的DHCP-ACK确认报文后，会以广播的方式发送免费ARP报文（该报文中，源IP地址和目标IP地址都是本机IP地址，源MAC地址是本机MAC地址，目的MAC地址是广播MAC地址），探测是否有主机使用服务器分配的IP地址，如果在规定的时间内没有收到回应，客户端才使用此地址。否则，客户端会发送DHCP DECLINE报文给DHCP服务器，并重新申请IP地址。
如果网络中存在多个DHCP服务器，除DHCP客户端选中的服务器外，其它DHCP服务器中本次未分配出的IP地址仍可分配给其他客户端。