1 当dhcp client 启动并进行dhcp 初始化时，它会在本地网络广播配置请求报文。 2 如果本地网络存在dhcp server，则可以直接进行dhcp 配置，不需要dhcp relay。 3 如果本地网络没有dhcp server，则与本地网络相连的具有dhcprelay 功能的网络设
备收到该广播报文后，将进行适当处理并转发给指定的其它网络上的dhcp server。 4 dhcp server 根据dhcp client 提供的信息进行相应的配置，并通过dhcp relay 将配置信息发送给dhcp client，完成对dhcp client 的动态配置。 事实上，从开始到最终完成配置，需要多个这样的交互过程。 报文交互过程如下图： 
dhcp relay设备修改dhcp消息中的相应字段，把dhcp的广播包改成单播包，并负责在服务器与客户机之间转换。
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http://wiki.mikrotik.com/wiki/Manual:IP/DHCP_Relay
Manual:IP/DHCP Relay
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Summary
DHCP Relay is just a proxy that is able to receive a DHCP request and resend it to the real DHCP server.
Properties
Sub-menu: /ip dhcp-relay
| Property | Description |
|---|---|
| add-relay-info (yes | no; Default: no) | Adds DHCP relay agent information if enabled according to RFC 3046. Agent Circuit ID Sub-option contains mac address of an interface, Agent Remote ID Sub-option contains MAC address of the client from which request was received. |
| delay-threshold (time | none; Default: none) | If secs field in DHCP packet is smaller than delay-threshold, then this packet is ignored |
| dhcp-server (string; Default: ) | List of DHCP servers' IP addresses which should the DHCP requests be forwarded to |
| interface (string; Default: ) | Interface name the DHCP relay will be working on. |
| local-address (IP; Default: 0.0.0.0) | The unique IP address of this DHCP relay needed for DHCP server to distinguish relays. If set to 0.0.0.0 - the IP address will be chosen automatically |
| name (string; Default: ) | Descriptive name for the relay |
DHCP relay does not choose the particular DHCP server in the dhcp-server
list, it just send the incoming request to all the listed servers.
Example setup
Let us consider that you have several IP networks 'behind' other routers, but you want to keep all DHCP servers on a single router. To do this, you need a DHCP relay on your network which relies DHCP requests from clients to DHCP server.
This example will show you how to configure a DHCP server and a DHCP relay which serve 2 IP networks - 192.168.1.0/24 and 192.168.2.0/24 that are behind a router DHCP-Relay.
IP Address Configuration
IP addresses of DHCP-Server:
[admin@DHCP-Server] ip address> print Flags: X - disabled, I - invalid, D - dynamic # ADDRESS NETWORK BROADCAST INTERFACE 0 192.168.0.1/24 192.168.0.0 192.168.0.255 To-DHCP-Relay 1 10.1.0.2/24 10.1.0.0 10.1.0.255 Public [admin@DHCP-Server] ip address>
IP addresses of DHCP-Relay:
[admin@DHCP-Relay] ip address> print Flags: X - disabled, I - invalid, D - dynamic # ADDRESS NETWORK BROADCAST INTERFACE 0 192.168.0.2/24 192.168.0.0 192.168.0.255 To-DHCP-Server 1 192.168.1.1/24 192.168.1.0 192.168.1.255 Local1 2 192.168.2.1/24 192.168.2.0 192.168.2.255 Local2 [admin@DHCP-Relay] ip address>
DHCP Server Setup
To setup 2 DHCP Servers on DHCP-Server router add 2 pools. For networks 192.168.1.0/24 and 192.168.2.0:
/ip pool add name=Local1-Pool ranges=192.168.1.11-192.168.1.100 /ip pool add name=Local1-Pool ranges=192.168.2.11-192.168.2.100 [admin@DHCP-Server] ip pool> print # NAME RANGES 0 Local1-Pool 192.168.1.11-192.168.1.100 1 Local2-Pool 192.168.2.11-192.168.2.100 [admin@DHCP-Server] ip pool>
Create DHCP Servers:
/ip dhcp-server add interface=To-DHCP-Relay relay=192.168.1.1 \ address-pool=Local1-Pool name=DHCP-1 disabled=no /ip dhcp-server add interface=To-DHCP-Relay relay=192.168.2.1 \ address-pool=Local2-Pool name=DHCP-2 disabled=no [admin@DHCP-Server] ip dhcp-server> print Flags: X - disabled, I - invalid # NAME INTERFACE RELAY ADDRESS-POOL LEASE-TIME ADD-ARP 0 DHCP-1 To-DHCP-Relay 192.168.1.1 Local1-Pool 3d00:00:00 1 DHCP-2 To-DHCP-Relay 192.168.2.1 Local2-Pool 3d00:00:00 [admin@DHCP-Server] ip dhcp-server>
Configure respective networks:
/ip dhcp-server network add address=192.168.1.0/24 gateway=192.168.1.1 \ dns-server=159.148.60.20 /ip dhcp-server network add address=192.168.2.0/24 gateway=192.168.2.1 \ dns-server 159.148.60.20 [admin@DHCP-Server] ip dhcp-server network> print # ADDRESS GATEWAY DNS-SERVER WINS-SERVER DOMAIN 0 192.168.1.0/24 192.168.1.1 159.148.60.20 1 192.168.2.0/24 192.168.2.1 159.148.60.20 [admin@DHCP-Server] ip dhcp-server network>
DHCP Relay Config
Configuration of DHCP-Server is done. Now let's configure DHCP-Relay:
/ip dhcp-relay add name=Local1-Relay interface=Local1 \ dhcp-server=192.168.0.1 local-address=192.168.1.1 disabled=no /ip dhcp-relay add name=Local2-Relay interface=Local2 \ dhcp-server=192.168.0.1 local-address=192.168.2.1 disabled=no [admin@DHCP-Relay] ip dhcp-relay> print Flags: X - disabled, I - invalid # NAME INTERFACE DHCP-SERVER LOCAL-ADDRESS 0 Local1-Relay Local1 192.168.0.1 192.168.1.1 1 Local2-Relay Local2 192.168.0.1 192.168.2.1 [admin@DHCP-Relay] ip dhcp-relay>