DHCP protocol

• brief introduction

DHCP：（Dynamic Host Configuration Protocol） Dynamic Host Configuration Protocol , Mainly to let a computer plug in the network cable to achieve the effect of surfing the Internet , And a computer needs to get online and configure some information , and DDHCP The definition of message is to get these configuration information . Include ：

1. IP Address
2. Subnet mask
3. The default route is IP Address
4. Domain name server IP Address

• DHCP The interactive process on the client server

• client--->server：

need IP Address of the host A, Broadcast transmission DHCPDISCOVER message （ The source address is 0.0.0.0【 Because at this time client Not yet ip Address 】, The destination address is 255.255.255.255【 radio broadcast 】）, To discover DHCP Server's IP Address . 

• server--->client：

1. All hosts in the local network can receive DHCPDISCOVER message , But only DHCP The server will reply to this message
2. DHCP After the server receives the message , Find the configuration information of this computer , It can be found and returned to the host A.【 notes ： once DHCP The first successful client from DHCP Server side lease to IP After the address , You can use the address permanently . The place is irrevocable This distribution method is called ： Static allocation 】
3. The host cannot be found A Configuration information , from IP Address pool （address pool） One of them IP The address is assigned to the host A, It's called Dynamic allocation .
4. Assigned to IP The address is time sensitive , host A You can use this IP The time of the address is called Rental period . When When the lease expires , The client can request to extend the lease period and renew the lease , This operation is called The lease period is extended
5. Host computer A reply ： This message is called DHCPOFFER message , Indicates that IP Address and other information

• client：

host A Get IP Address and other information

• DHCP Relay agent

stay DHCP Forwarding between server and client DHCP Data packets . We don't want to provide too much in the subnet DHCP The server , So there is at least one network DHCP Relay agent （ It's usually a router ）.

• host A issue DHCPDISCOVER The broadcast message of ----> Router received -----> Routers send unicast messages to DHCP The server forwards this message ---> Router received DHCP Server feedback message ----> The router sends it to the host A

•  DHCP Detailed workflow of

 

1.  DHCP The server is opened passively UDP Of 67 port
2. DHCP The client from 68 Of UDP Port send DHCPDISCOVER message
3. DHCP The client receives... From the server DHCPOFFER message （ There may be a few ）
4. DHCP The client selects one DHCPOFFER message , And issue DHCPREQUEST message 【 Renew lease term 】
5. DHCP Server reply DHCPACK message , perhaps DHCPNACK message
6. DHCP Client issue DHCPRELEASE message 【 The client stops the lease period provided by the server , Send this message 】