Quick HOWTO : Ch08 : Configuring the DHCP Server/zh

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http://www.linuxhomenetworking.com/wiki/index.php/Quick_HOWTO_:_Ch08_:_Configuring_the_DHCP_Server

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简介

一般来讲，如果你有一个有线调制解调器或者数字用户线路，你就可以使用家里的电脑获取由你的服务提供商动态分配的IP地址。如果在调制解调器和本地网络之间安装了有线/数字用户线路的路由器，你的电脑很有可能是在启动过程中从路由器获取了IP地址。你也可以选择禁用本地路由器中DHCP服务器的功能，而设置一个Linux机器作为DHCP服务器。

这一章仅包含对一个可以提供IP地址的DHCP服务器的配置指南。关于Linux系统中，DHCP客户端从DHCP服务器端如何获取IP地址的配置可以参考第三章，“Linux Networking”中的Linux网络互联部分。

下载与安装 DHCP 包

许多 RedHat 以及 Fedora Linux 软件都是RPM格式的. 下载安装这些RPM软件很容易. 如果你想再回顾下如何安装, Chapter 6, "Installing Linux Software", 详细讲解了这部分内容.

找该软件包的时候，请注意DHCP服务器的RPM包的命名往往以dhcp开始，后跟一个版本号，比如这样：dhcp-3.0.1rc14-1.i386.rpm

Debian Note: 对 Debian / Ubuntu 来说软件包名称可能也包含版本号. 用 dpkg --list | grep dhcp 来的到一个所有dhcp软件包的列表，从而找到dhcp服务区的软件包名称. 这里我们不妨认为这个软件包是 dhcp3-server.如果想回顾一下DEB包的安装请参见 Chapter 6, "Installing Linux Software".

root@u-bigboy:/tmp# dpkg --list | grep dhcp
ii  dhcp3-client   3.0.3-6ubuntu7  DHCP Client
ii  dhcp3-common   3.0.3-6ubuntu7  Files used by all the dhcp3* packages
root@u-bigboy:/tmp#

/etc/dhcpd.conf 文件

DHCP 服务器启动时,它读取 /etc/dhcpd.conf文件，并使用里面的命令来配置你的网络。标准的DHCP RPM软件包不会安装这个文件，但是你可以在下面的目录里找到一个配置文件示例作为参照。

/usr/share/doc/dhcp-<version-number>/dhcpd.conf.sample

你需要拷贝这个文件 dhcpd.conf 到 /etc 目录下面然后进行编辑。对于3.0p11版本的RPM包，拷贝文件命令如下：

[root@bigboy tmp]# cp /usr/share/doc/dhcp-3.0pl1/dhcpd.conf.sample /etc/dhcpd.conf

Debian Note: 对 Debian / Ubuntu 来说配置文件是 /etc/dhcp*/dhcpd.conf ，语法跟 Redhat / Fedora一样.

下面是对dhcpd.conf文件的简单解释：最重要的是，你的Linux主机的每个网络接口都必须有一个子网段

ddns-update-style interim
ignore client-updates
 
subnet 192.168.1.0 netmask 255.255.255.0 {
 
   # 当DHCP客户端主机启动网络时
   # 服务器能够分配给他的IP地址
   # 的范围
 
   range 192.168.1.201 192.168.1.220;
 
   # 客户端能使用该IP地址的时间
   # 以秒计算

  default-lease-time 86400;
  max-lease-time 86400;
 
   # 客户端默认网关
 
   option routers 192.168.1.1;

   # 不从一个网口向另一个网口转发
   # DHCP请求
 
   option ip-forwarding off;
 
   # 设置客户端广播地址和子网掩码

  option broadcast-address 192.168.1.255;
  option subnet-mask 255.255.255.0;
 
   # 设置客户端DNS服务器

  option domain-name-servers 192.168.1.100;
 
   # 设置客户端NTP服务器
 
   option nntp-server 192.168.1.100;
 
   # 如果你为Windows客户端指定了一个WINS服务器，
   # 你必须在 dhcpd.conf中加入以下选项 

  option netbios-name-servers 192.168.1.100;
 
   # 你也可以根据客户端MAC地址分配给他静态IP
   # (主机名是 "laser-printer"):

  host laser-printer {
      hardware ethernet 08:00:2b:4c:59:23;
     fixed-address 192.168.1.222;
   }
}
#
# 未使用网络接口
#
subnet 192.168.2.0 netmask 255.255.255.0 {
}

还有许多其他选项来配置 DHCP， 包括告诉 DHCP客户端哪里提供 finger 以及 IRC服务。在安装后请查看 dhcp-options man page :

[root@bigboy tmp]# man dhcp-options

Note: 示例文件dhcpd.conf文件里面的host指令非常有用. 像网络上打印机这样一些设备默认是通过DHCP获得IP地址的， 但是用户可以用固定IP地址区访问. 这个指令可以用来为一个已经预先知道网卡MAC地址的DHCP客户端提供静态IP地址。这可以降低系统管理开销

启动DHCP:

1) DHCP的一些旧版本的Fedora/RedHat将无法使用除非已有dhcpd.leases文件。如果没有的话可以用命令 touch /var/lib/dhcp/dhcpd.leases 去创建一个。 

[root@bigboy tmp]# touch /var/lib/dhcp/dhcpd.leases
2) Use the chkconfig command to get DHCP configured to start at boot:

[root@bigboy tmp]# chkconfig dhcpd on

With Debian / Ubuntu the equivalent command for the dhcp3-server package would be:

root@u-bigboy:/tmp# sysv-rc-conf dhcp3-server on

3) Use the service command to instruct the /etc/init.d/dhcpd script to start/stop/restart DHCP after

booting

[root@bigboy tmp]# service dhcpd start

[root@bigboy tmp]# service dhcpd stop [root@bigboy tmp]# service dhcpd restart

With Debian / Ubuntu the equivalent commands would be:

root@u-bigboy:/tmp# /etc/init.d/dhcp*-server start

root@u-bigboy:/tmp# /etc/init.d/dhcp*-server stop root@u-bigboy:/tmp# /etc/init.d/dhcp*-server restart

4) Remember to restart the DHCP process every time you make a change to the conf file for the changes to take effect on the running process. You also can test whether the DHCP process is running with the following command; you should get a response of plain old process ID numbers:

[root@bigboy tmp]# pgrep dhcpd

5) 最后，一定要记得设置你的电脑以使其可以通过DHCP来获得它的IP地址。

address via DHCP.

DHCP Servers with Multiple NICs

When a DHCP configured PC boots, it requests its IP address from the DHCP server. It does this by sending a standardized DHCP broadcast request packet to the DHCP server with a source IP address of 255.255.255.255.

If your DHCP server has more than one interface, you have to add a route for this 255.255.255.255 address so that it knows the interface on which to send the reply; if not, it sends it to the default gateway. (In both of the next two examples, we assume that DHCP requests will be coming in on interface eth0).

Note: More information on adding Linux routes and routing may be found in Chapter 3, "Linux Networking".

Note: You can't run your DHCP sever on multiple interfaces because you can only have one route to network 255.255.255.255. If you try to do it, you'll discover that DHCP serving working on only one interface.

Temporary Solution

You can temporarily add a route to 255.255.255.255 using the route add command as seen below.

[root@bigboy tmp]# route add -host 255.255.255.255 dev eth0

If you want this routing state to be maintained after a reboot, then use the permanent solution that's discussed next.

Permanent Solution

The new Fedora Linux method of adding static routes doesn't seem to support sending traffic out an interface that's not destined for a specific gateway IP address. The DHCP packet destined for address 255.255.255.255 isn't intended to be relayed to a gateway, but it should be sent using the MAC address of the DHCP client in the Ethernet frame.

You have one of two choices. Add the route add command to your /etc/rc.local script, or add an entry like this to your /etc/sysconfig/static-routes file.

#
# File /etc/sysconfig/static-routes
#
eth0 host 255.255.255.255

Note: The /etc/sysconfig/static-routes file is a deprecated feature and Fedora support for it will eventually be removed.

Now that you have configured your server, it's time to take a look at the DHCP clients.

Configuring Linux Clients to Use DHCP

A Linux NIC interface can be configured to obtain its IP address using DHCP with the examples outlined in , "Linux Networking". Please refer to this chapter if you need a quick refresher on how to configure a Linux DHCP client.

Configuring Windows Clients to Use DHCP

Fortunately Windows defaults to using DHCP for all its NIC cards so you don't have to worry about doing any reconfiguration.

Using a Single DHCP Server to Serve Multiple Networks

As stated before, DHCP clients send their requests for IP addresses to a broadcast address which is limited to the local LAN. This would imply that a DHCP server is required on each subnet. Not so. It is possible to configure routers to forward DHCP requests to a DHCP server many hops away. This is done by inserting the IP address of the router's interface on the DHCP client's network into the forwarded packet. To the DHCP server, the non-blank router IP address field takes precedence over the broadcast address and it uses this value to provide a DHCP address that is meaningful to the client. The DHCP server replies with a broadcast packet, and the router, which has kept track of the initial forwarded request, forwards it back towards the client. You can configure this feature on Cisco devices by using the ip helper-address command on all the interfaces on which DHCP clients reside. Here is a configuration sample that points to a DHCP server with the IP address 192.168.36.25:

interface FastEthernet 2/1
  ip address 192.168.1.30 255.255.255.0
  ip helper-address 192.168.36.25

Simple DHCP Troubleshooting

The most common problems with DHCP usually aren't related to the server; after the server is configured correctly there is no need to change any settings and it therefore runs reliably. The problems usually occur at the DHCP client's end for a variety of reasons. The following sections present simple troubleshooting steps that you can go through to ensure that DHCP is working correctly on your network.

DHCP Clients Obtaining 169.254.0.0 Addresses

Whenever Microsoft DHCP clients are unable to contact their DHCP server they default to selecting their own IP address from the 169.254.0.0 network until the DHCP server becomes available again. This is frequently referred to as Automatic Private IP Addressing (APIPA). Here are some steps you can go through to resolve the problem:

• Ensure that your DHCP server is configured correctly and use the pgrep command discussed earlier to make sure the DHCP process is running. Pay special attention to your 255.255.255.255 route, especially if your DHCP server has multiple interfaces.
• Give your DHCP client a static IP address from the same range that the DHCP server is supposed to provide. See whether you can ping the DHCP server. If you cannot, double-check your cabling and your NIC cards.
• DHCP uses the BOOTP protocol for its communication between the client and server. Make sure there are no firewalls blocking this traffic. DHCP servers expect requests on UDP port 67 and the DHCP clients expect responses on UDP port 68. Use tcpdump on the server's NIC to verify the correct traffic flows.

== == ==

DHCP的其它错误

如果你的DHCP服务器无法启动，可以使用第四章所描述的查错纠错技术，“常见网络错误及其解决方法”，来帮助纠正错误。 Most problems with an initial setup are often due to:

• Incorrect settings in the /etc/dhcpd.conf file such as not defining the networks for which the DHCP server is responsible;
• Firewall rules that block the DHCP bootp protocol on UDP ports 67 and 68;
• Routers failing to forward the bootp packets to the DHCP server when the clients reside on a separate network.

经常地查看/var/logs/messages文件以确定是否有DHCP错误发生，同时记得当你升级操作系统的时候寄存在配置文件里的关键字可能改变。经常检阅版本发布通知以确保关键字没有改变。

总结

大多数的家庭网络，一个CHCP server 是没有必要的，因为DSL router / firewall一般都有DHCP的功能，但是出于兴趣也可以尝试一下。务必确保一个网络的所有DHCP servers 发行的IP地址不要重复，否则将有可能导致不可预料的错误。 You might want to disable the router/firewall's DHCP server capabilities to experiment with your new Linux server.

A DHCP server may be invaluable

in an office environment where the time and cost of getting a network engineer to get the work done may make it simpler for Linux systems administrators to do it by themselves.

Creating a Linux DHCP server is straightforward and touches all the major themes in the previous chapters. 现在是时候尝试一些比较有难度的操作了，不过在开始之前，我们将快速地复习一下怎样创建将使用本书馀下部分介绍的许多功能的用户的方法。