DHCP技术(整理版)

www.net130.com     日期:2006-12-26    浏览次数:
作者:病书生    出处:bbs.net130.com

 

DHCP基本知识点

1 DHCP协议在RFC2131种定义,使用udp协议进行数据报传递,使用的端口是67以及68。

2 DHCP最常见的应用是,自动给终端设备分配ip地址,掩码,默认网关,但是DHCP也同样可以给终端设备自动配置其他options,比如DNS server, 域名(比如 net130.com),time zones, NTP servers 以及其他的配置内容,更有些厂家,利用自己开发的第3放软件,把自己的一些配置信息,利用dhcp协议来实现对终端设备的自动配置。

3 DHCP服务的系统最基本的构架是 client/server模式,并且如果client 和server不再同一个2层网络内(即广播可以到达的网络范围),则必须要有能够透过广播报文的中继设备,或者能把广播报文转化成单播报文的设备(cisco的ios就引经了这种功能)

There are three distinct element types in a DHCP network. There must be a client and a server. If these two elements are not on the same Layer 2 network, there also must be a proxy, which usually runs on the router. The proxy is needed because the client device initially doesn't know its own IP address, so it must send out a Layer 2 broadcast to find a server that has this information. The router must relay these broadcasts to the DHCP server, then forward the responses back to the correct Layer 2 address so that the right end device gets the right configuration information.

4 CISCO的路由器(IOS12.0 T1以后),可以配置为dhcp的中继设备,DHCP的客户端设备,也可以配置为DHCP的服务器。

5 同一个网段DHCP服务器可以有多个,这不会影响终端设备从服务器获取配置信息,终端设备以接受到的第一组配置信息为准。以后又服务器段返回的DHCP配置信息被抛弃。

. Most DHCP networks of any size include two or more DHCP servers for redundancy. The end devices typically just need to talk to a DHCP server at startup time, but they will not work at all without it. So redundancy is important. This also means that it is not unusual for an end device to see several responses to a DHCP request. It will generally just use the first response. However, this also underscores the importance of ensuring that all of the DHCP servers distribute the same information. Their databases of end device configuration parameters must be synchronized.

6 DHCP 服务器往往遵守先来先服务的规则(first-come, first-served),或者说他能够建立一个IP地址和终端设备MAC地址之间的映射表(或者叫做database),由此可以保证特定的终端(也就是特定的MAC)每次开机后都能够获得此相同的ip地址。

.The server can allocate IP addresses from a pool on a first-come, first-served basis, or it can associate IP addresses with end device MAC addresses to ensure that a particular client always receives the same address.

用 IP Helper Addresses 命令配置DHCP中继服务

1典型配置命令

The ip helper-address configuration command allows the router to forward local DHCP requests to one or more centralized DHCP servers:

Router1#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Router1(config)#interface Ethernet0
Router1(config-if)#ip helper-address 172.25.1.1 /*指定dhcp服务器的地址,表示通过Ethernet0向该服务器发送DHCP请求包*/
Router1(config-if)#ip helper-address 172.25.10.7 /*作用同上*/
Router1(config-if)#end
Router1#


关于以上配置的讨论
1 在客户端设备和DHCP服务器不再同一广播域内的时候,中间设备即路有器(路有功能的设备)必须要能够转发这种广播包,具体到cisco的设备上,则启用ip helper-address命令,来实现这种中继。
2 DHCP服务器要给终端设备分配地址则需要掌握两个重要的信息,第一,该客户端设备所在网络的子网掩码,DHCP服务器依据子网掩码的信息来判断,服务器该分配哪个IP地址,以使得该ip地址在那个子网内,第二,DHCP服务器必须知道客户端的MAC地址,以维护DHCP服务器的ip 地址和MAC之间的映射关系,由此保证同样一台客户机,每次启动后能获得和前一次相同的ip地址。
3 配置了ip helper-address命令之后的路由器在中继DHCP请求时的工作过程如下

a,DHCP客户端发送请求,由于没有ip地址,所以自己的源IP地址为0.0.0.0,而且也不知道目的DHCP服务器的地址,所以为广播255.255.255.255。该数据报中当然还包含其他信息,比如二层的信息,源mac地址,和目的mac地址FFFFFFFFFFFF。

b,当路由器接收到该数据报的时候,他就用自己的接口地址(接收到数据报的接口)来取代源地址0.0.0.0,并且用ip help-address 命令中指定的地址(上例中为172.25.1.1以及172.25.10.7)来取代目的地址255.255.255.255

The router must replace the source address with its own IP address, for the interface that received the request. And it replaces the destination address with the address specified in the ip helper-address command. The client device's MAC address is included in the payload of the original DHCP request packet, so the router doesn't need to do anything to ensure that the server receives this information.

c 当DHCP服务器接收到路有器转发过来的DHCP请求包时,他有了足够的信息,(由源IP地址中的地址,确定客户机所在的子网掩马,由此分配相应地址池中的空闲地址,并且知道了客户记得MAC地址,把它写入自己的数据库,建立IP地址和MAC的映射关系)然后DHCP服务器做出响应,并且由路有器把数据报转发会客户端。(整个过程应该在客户机和服务器之间还有一次会话,由于这不是路由器DHCP配置的讨论重点,这里不谈)

4 例子中配置了两个DHCP服务器,我们必须分别用ip helper-address 命令指明,路有器会转发DHCP请求包到所有的DHCP服务器上。很多企业的做法都是至少有两台DHCP服务器,有提高冗余和可靠性的作用。此时,如果客户端受到几个来自不同DHCP服务器的应答,则只选择最先接收到的应答数据报。

5 必须要注意的是;ip helper-address 命令不仅仅是只转发DHCP请求包,事实上,在默认情况下,他还转发其他的UDP报(比如DNS请求)到ip helper-address命令所指定的服务器上,所以这种额外的数据流量可能会增加DHCP服务器链路的负担以及服务器CPU负担,可能会引起问题,关于解决办法,将在后面讨论。

最后 用show ip interface 显示相关的ip help-address配置信息:

Router1#show ip interface Ethernet0
Ethernet0 is up, line protocol is up
Internet address is 192.168.30.1/24
Broadcast address is 255.255.255.255
Address determined by setup command
MTU is 1500 bytes
Helper addresses are 172.25.1.3
172.25.1.1

Directed broadcast forwarding is disabled

Router1#

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