DHCP动态主机管理协议

DHCP通讯模式

通讯的4个步骤

(1) Client：dhcp discover
(2) Server: dhcp offer(IP/Netmask, GW, DNS Server, NTP Server, Wins Server, ...)
(3) Client：dhcp request
(4) Server：dhcp ack

相关名词

lease time: 租约期限
arp: IP ---> Mac
rarp: Mac ---> IP

DHCP动态主机管理协议(Dynamic Host Configuration Protocol)是一种基于UDP协议且仅限用于局域网的网络协议，主要用途是为局域网内部设备或网络供应商自动分配IP地址，通常会应用在大型的局域网环境中或局域网内存在比较多的移动办公设备，DHCP协议能够实现集中的管理、分配IP地址。
DHCP服务程序能够使局域网内的主机自动且动态的获取IP地址、子网掩码、网关地址以及DNS服务器地址等信息，且能够有效的提升地址使用率，提高配置效率，减少管理和维护成本。
DHCP协议能够保证任何IP地址在同一时刻只能由一台DHCP客户机使用，且能够为指定主机分配固定的IP地址。

DHCP服务器的配置

• 实现
  dhcp (isc.org) - The dhcp package provides the ISC DHCP service and relay agent.
  dnsmasq (dns, dhcp) - Dnsmasq is lightweight, easy to configure DNS forwarder and DHCP server. It is designed to provide DNS and, optionally, DHCP, to a small network.

• 安装:
  yum -y install dhcp

• 程序环境
  dhcp server：
  程序文件: /usr/sbin/dhcpd
  IPV4网络：
  配置文件: /etc/dhcp/dhcpd.conf # 默认为空,可从/usr/share/doc/dhcp/-xxx/dhcpd.conf.example拷贝
  UnitFile: /usr/lib/systemd/system/dhcpd.service
  IPV6网络：
  /etc/dhcp/dhcpd6.conf
/usr/lib/systemd/system/dhcpd6.service
  dhcprelay：中继,必须先对本地局域网中的主机提供服务
  /usr/sbin/dhcrelay
/usr/lib/systemd/system/dhcrelay.service

• 配置文件说明
  配置指令
  options --> 配置给客户端的相关信息
  指令 ---> 定义dhcp server的工作特性

# dhcpd.conf
# Sample configuration file for ISC dhcpd
# option definitions common to all supported networks...
option domain-name "example.org";    # 搜索的顶级域
option domain-name-servers ns1.example.org, ns2.example.org;  # 指定的DNS域名服务器
default-lease-time 600;       # 默认的租期时长
max-lease-time 7200;          # 最大的租期
# 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;
    }
}