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 . 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 |
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> printFlags: 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> printFlags: 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> printFlags: 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> printFlags: 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>