dhcp option82包解析小记(之一)

已经忙了将近两个星期的dhcp 包解析，整个过程中option 82包的解析费掉的时间最多，因为我花了很久才抓到这个包，之前的开发过程没有写过日志，option82包的解析，我决定写点东西，一为备忘，二为纪念我的08年阳历年。

option82包是dhcp包option部分的一项，具体内容可以参考rfc3046。
这个包里面包括了交换机、dhcp客户端所在vlan以及端口号，这些信息对于某些应用是非常重要的。
本文的重点是如何让交换机启动填充option82。
1、交换机配置(2950)

configure terminal
ip dhcp snooping
ip dhcp snooping vlan 1
ip dhcp snooping information option

以上为交换机全局配置，接着配置dhcp server所连接的接口，这里是2

interface f0/2

设置该接口为信任接口

p dhcp snooping trust
ip dhcp snooping limit rate 1000

end
show running-config
copy running-config startup-config

show ip dhcp snooping

确保 dhcp客户端所连接接口是un trust的，否则交换机不会填充option82.

交换机上配置结束
2、配置dhcpd.

vim /etc/dhcpd.conf

看到内容如下：

#
# DHCP Server Configuration file.
# see /usr/share/doc/dhcp*/dhcpd.conf.sample
#

ok,我们来打开/usr/share/doc/dhcp*/dhcpd.conf.sample，内容是：

ddns-update-style interim;
ignore client-updates;

subnet 192.168.0.0 netmask 255.255.255.0 {

# — default gateway
option routers 192.168.0.1;
option subnet-mask 255.255.255.0;

option nis-domain “domain.org”;
option domain-name “domain.org”;
option domain-name-servers 192.168.1.1;

option time-offset -18000; # Eastern Standard Time
# option ntp-servers 192.168.1.1;
# option netbios-name-servers 192.168.1.1;
# — Selects point-to-point node (default is hybrid). Don’t change this unless
# — you understand Netbios very well
# option netbios-node-type 2;

range dynamic-bootp 192.168.0.128 192.168.0.254;
default-lease-time 21600;
max-lease-time 43200;

# we want the nameserver to appear at a fixed address
host ns {
next-server marvin.redhat.com;
hardware ethernet 12:34:56:78:AB:CD;
fixed-address 207.175.42.254;
}
}

然后我们直接复制这个文件覆盖/etc/dhcpd.conf
然后修改为：

ddns-update-style interim;
ignore client-updates;

subnet 192.168.1.0 netmask 255.255.255.0 {

# — default gateway
option routers 192.168.1.1;
option subnet-mask 255.255.255.0;

option nis-domain “domain.org”;
option domain-name “domain.org”;
option domain-name-servers 192.168.1.1;

option time-offset -18000; # Eastern Standard Time
# option ntp-servers 192.168.1.1;
# option netbios-name-servers 192.168.1.1;
# — Selects point-to-point node (default is hybrid). Don’t change this unless
# — you understand Netbios very well
# option netbios-node-type 2;

range dynamic-bootp 192.168.1.128 192.168.1.254;
default-lease-time 21600;
max-lease-time 43200;

# we want the nameserver to appear at a fixed address
host ns {
next-server marvin.redhat.com;
hardware ethernet 00:16:D3:B5:C4:9A;
fixed-address 192.168.1.101;
}
}

简单说明一下：
# — default gateway 是设置网关相关信息。
range dynamic-bootp 地址池
# we want the nameserver to appear at a fixed address 依据MAC地址固定绑定ip。

3. 打开wireshark, 设置filter: udp and (port 67 or port 68)，开启监听
让dhcp客户端来一把dhcp请求，这时候wireshark就会看到已经被填充了option 82的包。

Option: (t=82,l=18) Agent Information Option

且怒且放