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DHCP
Dynamic Host Configuration Protocol，动态主机配置协议
理解DHCP服务 使用DHCP时必须在网络上有一台DHCP服务器，而其他机器执行DHCP客户端。当DHCP客户端程序发出一个信息，要求一个动态的IP地址时，DHCP服务器会根据目前已经配置的地址，提供一个可供使用的IP地址和子网掩码给客户端。
1. 使用DHCP的优点 DHCP使服务器能够动态地为网络中的其他服务器提供IP地址，通过使用DHCP，就可以不给Intranet网中除DHCP、DNS和WINS服务器外的任何服务器设置和维护静态IP地址。使用DHCP可以大大简化配置客户机的TCP／IP的工作，尤其是当某些TCP／IP参数改变时，如网络的大规模重建而引起的IP地址和子网掩码的更改。
软件包dhcp yum install -y dhcp
监听端口 67/udp
配置文件 /etc/dhcp/dhcp.conf 里面的样例
cp usr/share/doc/dhcp*/dhcp.conf.sample /etc/dhcp/dhcp.conf
（2）相关配置文件 /etc/dhcpd.conf
主配置文件 /etc/sysconfig/dhcpd 指定开放dhcp服务的端口
/var/lib/dhcpd/dhcpd.lease 服务器租约文件 主配置说明，默认为空，需要拷贝
/usr/share/doc/dhcp*/dhcpd.conf.sample模版文件
主要文件/etc/dhcpd.conf选项说明：
==================================
192.168.0.0 netmask 255.255.255.0 { 定义一个可提供IP的网端，该网段必须存在于本机的某块网卡上。
option routers 192.168.0.1; 分配给客户端的默认路由，即网关。
option subnet-mask 255.255.255.0; 网关掩码
option nis-domain "domain.org"; NIS域信息，一般不使用
option domain-name "domain.org"; 域名信息
option domain-name-servers 192.168.1.1; 分配给主机的DNS
option time-offset -18000;
# Eastern Standard Time 同步时间，与格林威治的时间差
# option ntp-servers 192.168.1.1;
# option netbios-name-servers 192.168.1.1;
range dynamic-bootp 192.168.0.128 192.168.0.254; 定义可分配给客户的IP范围
# we want the nameserver to appear at a fixed address
host ns { 保留地址的主机，该主机对应指定Ip
next-server marvin.redhat.com; 用于指定tftp服务器，网络引导时才需使用
hardware ethernet 12:34:56:78:AB:CD; 指定主机的mac地址
fixed-address 207.175.42.254; 指定分配的IP地址
} }
cat /etc/dhcp/dhcpd.conf
#
# DHCP Server Configuration file.
# dhcpd.conf
#
# Sample configuration file for ISC dhcpd
#
# option definitions common to all supported networks...在所有支持的网络中常见的选项定义
option domain-name "example.org";
option domain-name-servers ns1.example.org, ns2.example.org;
#default-lease-time 600;
default-lease-time 21600; 默认租约时间
#max-lease-time 7200;
max-lease-time 43200; 最大租约时间
# Use this to enable / disable dynamic dns updates globally.
用这个来启用/禁用动态DNS更新全局。
#ddns-update-style none;
#ddns-update-style interim; 全局配置，定义所支持的DNS动态更新类型，默认临时
ignore client-updates; 全局配置，是否忽略客户端DNS更新，使用默认 subnet
# If this DHCP server is the official DHCP server for the local
# network, the authoritative directive should be uncommented.
如果DHCP服务器为本地网络官方的DHCP服务器，权威的指令应当取消注释。
#authoritative;
权威；
# Use this to send dhcp log messages to a different log file (you also
# have to hack syslog.conf to complete the redirection).
用它来发送DHCP消息记录到不同的日志文件（你也可以更改文件syslog.conf完成重定向）
log-facility local7; 日志设备 local7
# No service will be given on this subnet, but declaring it helps the
# DHCP server to understand the network topology.
没有服务将给这个子网，但定义它有助于DHCP服务器了解网络的拓扑结构
subnet 10.152.187.0 netmask 255.255.255.0 {
}
# This is a very basic subnet declaration.
subnet 10.254.239.0 netmask 255.255.255.224 {
range 10.254.239.10 10.254.239.20;
option routers rtr-239-0-1.example.org, rtr-239-0-2.example.org;
}
# This declaration allows BOOTP clients to get dynamic addresses,
# which we don't really recommend.
这一声明允许BOOTP客户获取动态地址，我们真的不建议。
（BOOTP（Bootstrap Protocol，引导程序协议）是一种引导协议，基于IP/UDP协议，也称自举协议，是DHCP协议的前身。BOOTP用于无盘工作站的局域网中，可以让无盘工作站从一个中心服务器上获得IP地址。通过BOOTP协议可以为局域网中的无盘工作站分配动态IP地址，这样就不需要管理员去为每个用户去设置静态IP地址。）
（BOOTP客户端会根据该回应帧来获得自己的IP地址并通过专用文件服务器（如TFTP服务器）下载启动镜像文件，模拟成磁盘来完成启动。）
subnet 10.254.239.32 netmask 255.255.255.224 {
range dynamic-bootp 10.254.239.40 10.254.239.60;
option broadcast-address 10.254.239.31;
option routers rtr-239-32-1.example.org;
}
# A slightly different configuration for an internal subnet.
一个稍微为内部子网的不同的配置
subnet 10.5.5.0 netmask 255.255.255.224 {
range 10.5.5.26 10.5.5.30;
option domain-name-servers ns1.internal.example.org;
option domain-name "internal.example.org";
option routers 10.5.5.1;
option broadcast-address 10.5.5.31;
default-lease-time 600;
max-lease-time 7200;
}
# Hosts which require special configuration options can be listed in
# host statements. If no address is specified, the address will be
# allocated dynamically (if possible), but the host-specific information
# will still come from the host declaration.
需要特殊配置选项的主机可以在主机语句中列出.。如果没有指定地址，地址将动态分配（如果可能的话），但主机特定的信息仍然来自主机声明.。
host passacaglia {
hardware ethernet 0:0:c0:5d:bd:95;
filename "vmunix.passacaglia";
server-name "toccata.fugue.com";
}
# Fixed IP addresses can also be specified for hosts. These addresses
# should not also be listed as being available for dynamic assignment.
# Hosts for which fixed IP addresses have been specified can boot using
# BOOTP or DHCP. Hosts for which no fixed address is specified can only
# be booted with DHCP, unless there is an address range on the subnet
# to which a BOOTP client is connected which has the dynamic-bootp flag
# set.
固定IP地址也可以为主机指定。这些地址不应该被列为动态分配.。主机的固定IP地址指定了可以使用BOOTP或者DHCP引导。它没有固定的地址是指定只能启动DHCP主机，除非有一个地址范围的子网，BOOTP客户端连接具有动态BOOTP旗标。
host fantasia {
hardware ethernet 08:00:07:26:c0:a5;
fixed-address fantasia.fugue.com;
}
# You can declare a class of clients and then do address allocation
# based on that. The example below shows a case where all clients
# in a certain class get addresses on the 10.17.224/24 subnet, and all
# other clients get addresses on the 10.0.29/24 subnet.
您可以声明一个类的客户，然后做地址分配的基础上。下面的例子显示的情况下，在某一类的所有客户获得在10.17.224/24子网地址，和所有其他的客户得到的10.0.29/24子网地址。
class "foo" {
match if substring (option vendor-class-identifier, 0, 4) = "SUNW";
}
shared-network 224-29 {
subnet 10.17.224.0 netmask 255.255.255.0 {
option routers rtr-224.example.org;
}
subnet 10.0.29.0 netmask 255.255.255.0 {
option routers rtr-29.example.org;
}
pool {
allow members of "foo";
range 10.17.224.10 10.17.224.250;
}
pool {
deny members of "foo";
range 10.0.29.10 10.0.29.230;
}
}
# see /usr/share/doc/dhcp*/dhcpd.conf.sample
# see 'man 5 dhcpd.conf'
#
完成配置文件开启服务 service dhcpd start
DHCP语法检查工具：dhcpd
错误1: Not configured to listen on any interfaces!
网卡网段必须和DHCP分配的网段匹配
DHCP客户端获取IP的四个过程：
1. DHCPDISCOVER client ----广播-----> ALL
2. DHCPOFFERserver--------------> client
3. DHCPREQUESTclient ----广播-----> ALL
#第二次依然使用广播，目的是为告诉其他DHCP服务器，自己已经得到DHCP回应。
4. DHCPACK server---------> client
