DHCP relay 原理

DHCP relay 原理

    1 當dhcp client 啟動並進行dhcp 初始化時，它會在本網廣播配置請求報文。     2 如果本網存在dhcp server，則可以直接進行dhcp 配置，不需要dhcp relay。     3 如果本網沒有dhcp server，則與本網相連的具有dhcprelay 功能的網路設
備收到該廣播報文後，將進行適當處理並轉寄給指定的其它網路上的dhcp server。     4 dhcp server 根據dhcp client 提供的資訊進行相應的配置，並通過dhcp relay 將配置資訊發送給dhcp client，完成對dhcp client 的動態配置。     事實上，從開始到最終完成配置，需要多個這樣的互動過程。     報文互動過程如：    

     dhcp relay裝置修改dhcp訊息中的相應欄位，把dhcp的廣播包改成單播包，並負責在伺服器與客戶機之間轉換。   

 

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 http://wiki.mikrotik.com/wiki/Manual:IP/DHCP_Relay

Manual:IP/DHCP Relay

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> 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   10.1.0.2/2410.1.0.010.1.0.255Public[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>