某局点DHCP中继问题经验案例

 某客户反馈我司无线控制器加瘦AP组网下，DHCP服务器Windows Server，无线控制器做DHCP中继，为终端分配地址，终端连接到SSID下后出现部分终端获取不到地址的问题。

通过一线反馈现场无线控制器配置，以及现场情况，发现问题现象主要出现在移动终端（苹果、安卓手机、平板）上，笔记本终端获取地址均正常。

由于DHCP协议较为简单，且设备上运行一直正常，首先检查相关配置。

#

 dhcp relay server-group 1 ip 10.154.73.11

 dhcp relay server-group 1 ip 10.154.73.12

#

#

wlan vlan-pool vp1

 vlan-id 405 to 406

#

interface Vlan-interface405

 ip address 10.154.97.253 255.255.255.0

 dhcp select relay

 dhcp relay server-select 1

#

interface Vlan-interface406

 ip address 10.154.98.253 255.255.255.0

 dhcp select relay

 dhcp relay server-select 1

#

interface WLAN-ESS1

 port link-type hybrid

 undo port hybrid vlan 1

 port hybrid vlan 200 405 to 406 untagged

 port hybrid pvid vlan 200

 mac-vlan enable

#

#

wlan service-template 1 clear

 ssid zjzyhx

 bind WLAN-ESS 1

 service-template enable

#

#

wlan ap zhwgcs model WA2620i-AGN id 1

 serial-id 219801A0CMC156000206

 radio 1

  service-template 1

 radio 2

  service-template 1 vlan-pool vp1

  radio enable

#

检查发现配置无异常，皆为基本配置，且PC机获取正常，说明基本的数据链路是没有问题的，于是搜集相关的debug信息。

debug  dhcp relay  all

发现手机终端（MAC地址38f8-8937-bfd1）

From client to server (Server-group 1):       

    Message type: request       

    Hardware type: 1, Hardware address length: 6       

    Hops: 1, Transaction ID: 1128803385       

    Seconds: 29, Broadcast flag: 0       

    Client IP address: 0.0.0.0   Your IP address: 0.0.0.0       

    Server IP address: 0.0.0.0   Relay agent IP address: 10.154.98.253       

    Client hardware address: 38f8-8937-bfd1       

    Server host name: Not Configured, Boot file name: Not Configured       

    DHCP message type: DHCP Discover

*Aug 17 14:27:41:162 2015 WX3024E_AC DHCPR/7/DHCPR_DEBUG_EVENT:

Pkt Sent: send request interface Vlan-interface406, dest IP: 10.154.73.11,

         chardAddr: 38f8.8937.bfd1, server-group: 1.

*Aug 17 14:27:41:162 2015 WX3024E_AC DHCPR/7/DHCPR_DEBUG_EVENT:

Pkt Sent: send request interface Vlan-interface406, dest IP: 10.154.73.12,

         chardAddr: 38f8.8937.bfd1, server-group: 1.

*Aug 17 14:27:41:164 2015 WX3024E_AC DHCPR/7/DHCPR_DEBUG_EVENT:

 Begin to deal with DHCP Offer packet.

*Aug 17 14:27:41:164 2015 WX3024E_AC DHCPR/7/DHCPR_DEBUG_PACKET:

Rx, DHCP reply packet, interface Vlan-interface406.

*Aug 17 14:27:41:165 2015 WX3024E_AC DHCPR/7/DHCPR_DEBUG_PACKET:

From server to client (Server-group 1):       

    Message type: reply       

    Hardware type: 1, Hardware address length: 6       

    Hops: 0, Transaction ID: 1128803385       

    Seconds: 0, Broadcast flag: 0       

    Client IP address: 0.0.0.0   Your IP address: 10.154.99.132       

    Server IP address: 0.0.0.0   Relay agent IP address: 10.154.98.253       

    Client hardware address: 38f8-8937-bfd1       

    Server host name: Not Configured, Boot file name: Not Configured       

    DHCP message type: DHCP Offer

*Aug 17 14:27:41:165 2015 WX3024E_AC DHCPR/7/DHCPR_DEBUG_EVENT:

 First VLAN ID of reply packets to client is 406.

*Aug 17 14:27:41:165 2015 WX3024E_AC DHCPR/7/DHCPR_DEBUG_EVENT:

Pkt Sent: send reply interface Vlan-interface406, dest IP: 10.154.99.132,

         chardAddr: 38f8.8937.bfd1, server-group: 1.

Debug信息中可以看出，无线控制器上中继功能完整的完成了中继流程，并且在最后一步debug信息中显示

Pkt Sent: send reply interface Vlan-interface406, dest IP: 10.154.99.132,

         chardAddr: 38f8.8937.bfd1, server-group: 1.

即无线控制器已经向该地址发送DHCP reply报文，并且走的vlan端口也是vlan406，为该终端所在vlan中。

查看PC的地址获取过程中的DEBUG信息：

*Aug 17 14:28:59:719 2015 WX3024E_AC DHCPR/7/DHCPR_DEBUG_EVENT:

 Begin to deal with DHCP Request packet.

*Aug 17 14:28:59:719 2015 WX3024E_AC DHCPR/7/DHCPR_DEBUG_PACKET:

Rx, DHCP request packet, interface Vlan-interface405.

*Aug 17 14:28:59:720 2015 WX3024E_AC DHCPR/7/DHCPR_DEBUG_PACKET:

From client to server (Server-group 1):       

    Message type: request       

    Hardware type: 1, Hardware address length: 6       

    Hops: 1, Transaction ID: 1738251759       

    Seconds: 1024, Broadcast flag: 1       

    Client IP address: 10.154.99.9   Your IP address: 0.0.0.0       

    Server IP address: 0.0.0.0   Relay agent IP address: 10.154.97.253       

    Client hardware address: e006-e6ce-36b8       

    Server host name: Not Configured, Boot file name: Not Configured       

    DHCP message type: DHCP Request

*Aug 17 14:28:59:720 2015 WX3024E_AC DHCPR/7/DHCPR_DEBUG_EVENT:

Pkt Sent: send request interface Vlan-interface405, dest IP: 10.154.73.11,

         chardAddr: e006.e6ce.36b8, server-group: 1.

*Aug 17 14:28:59:720 2015 WX3024E_AC DHCPR/7/DHCPR_DEBUG_EVENT:

Pkt Sent: send request interface Vlan-interface405, dest IP: 10.154.73.12,

         chardAddr: e006.e6ce.36b8, server-group: 1.

*Aug 17 14:28:59:728 2015 WX3024E_AC DHCPR/7/DHCPR_DEBUG_EVENT:

 Begin to deal with DHCP Ack packet.

*Aug 17 14:28:59:728 2015 WX3024E_AC DHCPR/7/DHCPR_DEBUG_PACKET:

Rx, DHCP reply packet, interface Vlan-interface405.

*Aug 17 14:28:59:728 2015 WX3024E_AC DHCPR/7/DHCPR_DEBUG_PACKET:

From server to client (Server-group 1):       

    Message type: reply       

    Hardware type: 1, Hardware address length: 6       

    Hops: 0, Transaction ID: 1738251759       

    Seconds: 0, Broadcast flag: 1       

    Client IP address: 10.154.99.9   Your IP address: 10.154.99.9       

    Server IP address: 0.0.0.0   Relay agent IP address: 10.154.97.253       

    Client hardware address: e006-e6ce-36b8       

    Server host name: Not Configured, Boot file name: Not Configured       

    DHCP message type: DHCP Ack

*Aug 17 14:28:59:729 2015 WX3024E_AC DHCPR/7/DHCPR_DEBUG_EVENT:

 First VLAN ID of reply packets to client is 405.

*Aug 17 14:28:59:729 2015 WX3024E_AC DHCPR/7/DHCPR_DEBUG_EVENT:

Pkt Sent: send reply interface Vlan-interface405, dest IP: 255.255.255.255,

         chardAddr: e006.e6ce.36b8, server-group: 1.

发现交互流程的信息只有Broadcast flag: 1该位有区别。

又在debug信息中发现了

*Aug 17 14:29:09:833 2015 WX3024E_AC DHCPR/7/DHCPR_DEBUG_PACKET:

From server to client (Server-group 1):       

    Message type: reply       

    Hardware type: 1, Hardware address length: 6       

    Hops: 0, Transaction ID: 1077829468       

    Seconds: 0, Broadcast flag: 0       

    Client IP address: 10.154.99.9   Your IP address: 10.154.99.9       

    Server IP address: 0.0.0.0   Relay agent IP address: 10.154.97.253       

    Client hardware address: e006-e6ce-36b8       

    Server host name: Not Configured, Boot file name: Not Configured       

DHCP message type: DHCP Ack

即PC终端也发送过Broadcast flag为0的报文，且该种情况下也是获取异常。

现象明了，即Broadcast flag位置零的终端获取异常(单播)，Broadcast flag位置一的DHCP 中继流程正常（广播）。

查看RFC2131可知DHCP ACK报文“broadcast or unicast, based on bootp flags.”而debug信息中，无线控制器回应的ACK报文该位与终端要求的均一一对应，不存在问题。

由于debug信息中显示已经向问题终端发送ACK报文，于是协调AP AC间抓包。发现报文确实没有发送到AP。

继续排查，在无线控制器内联口抓包发现单播ACK报文出现在了，内联口且未做LWAPP封装，即该报文走向了有线口，进一步查看配置。

发现业务vlan 405 406三层接口配置：

#

interface Vlan-interface405

 ip address 10.154.97.253 255.255.255.0

 dhcp select relay

 dhcp relay server-select 1

#

interface Vlan-interface406

 ip address 10.154.98.253 255.255.255.0

 dhcp select relay

 dhcp relay server-select 1

分别为10.154.98.0/24 10.154.98.0/24网段，而debug中终端拿到的地址是Client IP address: 10.154.99.9   Your IP address: 10.154.99.9      

10.154.99.0/24网段，随即向客户询问配置缘由，经了解，客户在有线网关上有相应的portal认证，设备在没有过认证之前，获取的是10.154.99.0/24网段地址，现场确实有该需求。

    至此，原因查明，手机终端DHCP报文广播位置为0，即请求服务器单播回应。

PC机会分别尝试单播广播两种方式。当请求单播时，由于设备上没有相应的（10.154.99.0/24网段）三层接口，于是ACK的单播报文走默认路由，走无线内联口发出，所以终端没有收到报文。

根据问题发生的原因，在业务vlan 405 406下配置（10.154.99.0/24网段）子地址，问题解决。

在业务vlan 405 406下配置（10.154.99.0/24网段）子地址，问题解决。

在做DHCP中继的情况下，业务vlan最好有跟终端相同的网段地址。

在业务vlan 405 406下配置（10.154.99.0/24网段）子地址，问题解决。