DHCP是Dynamic Host Configuration Protocol的缩写，它是TCP/IP协议簇中的一种，主要是用来给网络客户机分配动态的IP地址，它的前身是BOOTP，BOOTP 原本是用于无磁盘主机连接的网络上面的。
一、DHCP的工作流程
(1) DHCP服务器发现阶段。DHCP客户机向广播地址255.255.255.255发送DHCPDiscover消息。
(2) DHCP服务器响应阶段。DHCP服务器从尚未出租的IP地址池中择一个分配给DHCP客户机，向DHCP客户机发送一个包含出租的IP地址和其他设置的DHCPoffer提供信息。
(3) IP地址择阶段。DHCP客户机择某个DHCP服务器提供的IP地址，以广播方式回答一个DHCPrequest请求信息.
(4) IP地址确认阶段。服务器回应包含它所提供的IP地址和其他设置的DHCPack确认信息。然后客户机将其与TCP/IP协议与网卡绑定。
(5) 客户机重新登录。再次发送前次分配的IP地址的DHCPRequest请求请求信息，DHCP服务器检查是否可以继续让客户机使用，然后发回DHCPack确认信息或者DHCPnck否认信息。如果IP已不能用，客户机需从(1)开始重新申请。
(6) 更新租约。IP地址租借期限到了，如果DHCP客户机要延长其租约，必须更新。DHCP客户机启动时和IP租约期限过一半时，都会向服务器发送更新期IP租约的信息。
二、DHCP服务器配置文件
路径在/etc/dhcp/dhcpd.conf。下面是一个配置文件的例子：
# dhcpd.conf
#
# Sample configuration file for ISC dhcpd
#
# option definitions common to all supported networks...
option domain-name "example.org";
option domain-name-servers ns1.example.org, ns2.example.org;
default-lease-time 600;
max-lease-time 7200;
# Use this to enble / disable dynamic dns updates globally.
#ddns-update-style none;
# If this DHCP server is the official DHCP server for the local
# network, the authoritative directive should be uncommented.
#authoritative;
# Use this to send dhcp log messages to a different log file (you also
# have to hack syslog.conf to complete the redirection).
log-facility local7;
# No service will be given on this subnet, but declaring it helps the
# DHCP server to understand the network topology.
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.
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.
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.
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.
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.
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;
}
}
DHCP配置文件通常包括3部分：declarations, parameters, options。
declarations：描述网络的布局；描述客户；提供客户的地址。
parameters：表明如何执行任务，是否要执行任务，或将哪些网络配置项发送给客户。
option：配置DHCP可参数。
常用声明：
shared-network 告知DHCP服务器是否为一些子网络分享相同网络
subnet 描述一个IP是否属于子网
range 提供动态分配IP地址的起始和结束范围
host 为特定的主机提供网络参数
group 为一组参数提供声明
常用参数：
ddns-update-style 配置DHCP-DNS互动更新模式
default-lease-time 指定默认租赁时间的长度，单位为秒
max-lease-time 指定最大租赁时间长度，单位为秒
hardware 指定网卡接口类型和MAC地址
server-name 告知DHCP客户服务器名称
fixed-address 为客户端指定一个固定的IP地址
常用项：
domain-name 为客户端指定域名
domain-name-servers为客户端指明DNS服务器IP地址
host-name 为客户端指明主机名称
routers 为客户端指明默认网关
broadcast-address 为客户端设定广播地址
subnet-mask 为客户端设定子网掩码
ntp-server 为客户端设定格林威治时间的偏移时间，单位为秒
下面是一个DHCPv6的配置文件的例子：
# Server configuration file example for DHCPv6
# From the file used for TAHI tests.
# IPv6 address valid lifetime
# (at the end the address is no longer usable by the client)
# (set to 30 days, the usual IPv6 default)
default-lease-time 2592000;
# IPv6 address preferred lifetime
# (at the end the address is deprecated, i.e., the client should use
# other addresses for new connections)
# (set to 7 days, the usual IPv6 default)
preferred-lifetime 604800;
# T1, the delay before Renew
# (default is 1/2 preferred lifetime)
# (set to 1 hour)
option dhcp-renewal-time 3600;
# T2, the delay before Rebind (if Renews failed)
# (default is 3/4 preferred lifetime)
# (set to 2 hours)
option dhcp-rebinding-time 7200;
# Enable RFC 5007 support (same than for DHCPv4)
allow leasequery;
# Global definitions for name server address(es) and domain search list
option dhcp6.name-servers 3ffe:501:ffff:100:200:ff:fe00:3f3e;
option dhcp6.domain-search "test.example.com","example.com";
# Set preference to 255 (maximum) in order to avoid waiting for
# additional servers when there is only one
##option dhcp6.preference 255;
# Server side command to enable rapid-commit (2 packet exchange)
##option dhcp6.rapid-commit;
# The delay before information-request refresh
# (minimum is 10 minutes, maximum one day, default is to not refresh)
# (set to 6 hours)
option dhcp6.info-refresh-time 21600;
# The path of the lease file
dhcpv6-lease-file-name "/usr/local/var/db/dhcpd6.leases";
# Static definition (must be global)
host myclient {
# The entry is looked up by this
host-identifier option
dhcp6.client-id 00:01:00:01:00:04:93:e0:00:00:00:00:a2:a2;
# A fixed address
fixed-address6 3ffe:501:ffff:100::1234;
# A fixed prefix
fixed-prefix6 3ffe:501:ffff:101::/64;
# Override of the global definitions,
# works only when a resource (address or prefix) is assigned
option dhcp6.name-servers 3ffe:501:ffff:100:200:ff:fe00:4f4e;
# For debug (to see when the entry statements are executed)
# (log "sol" when a matching Solicitation is received)
##if packet(0,1) = 1 { log(debug,"sol"); }
}
# The subnet where the server is attached
# (i.e., the server has an address in this subnet)
subnet6 3ffe:501:ffff:100::/64 {
# Two addresses available to clients
# (the third client should get NoAddrsAvail)
range6 3ffe:501:ffff:100::10 3ffe:501:ffff:100::11;
# Use the whole /64 prefix for temporary addresses
# (i.e., direct application of RFC 4941)
range6 3ffe:501:ffff:100:: temporary;
# Some /64 prefixes available for Prefix Delegation (RFC 3633)
prefix6 3ffe:501:ffff:100:: 3ffe:501:ffff:111:: /64;
}
# A second subnet behind a relay agent
subnet6 3ffe:501:ffff:101::/64 {
range6 3ffe:501:ffff:101::10 3ffe:501:ffff:101::11;
# Override of the global definitions,
# works only when a resource (address or prefix) is assigned
option dhcp6.name-servers 3ffe:501:ffff:101:200:ff:fe00:3f3e;
}
# A third subnet behind a relay agent chain
subnet6 3ffe:501:ffff:102::/64 {
range6 3ffe:501:ffff:102::10 3ffe:501:ffff:102::11;
}