十分详细的DHCP服务工作原理剖析
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
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
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，Destination Address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。
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