DHCP service

DHCP(Dynamic Host Configuration Protocol, Dynamic Host Configuration Protocol ) It's a LAN Of Network protocol , Use UDP Agreement work , There are two main uses : To the internal network or Network services Suppliers are automatically assigned IP Address , To users or internal network administrators as for all Computer As a means of central management , stay RFC 2131 Detailed description in .DHCP Yes 3 Ports , among UDP67 and UDP68 For the normal DHCP Service port , Respectively as DHCP Server and DHCP Client Service port for ;546 Port number is used for DHCPv6 Client, Not for DHCPv4, Is for DHCP failover service , This is a service that needs to be opened specially ,DHCP failover Is used to do " hot standby " Of .

The form of distribution

First ﹐ There must be at least one DHCP Work on the Internet ﹐ It will monitor the Internet DHCP request ﹐ And negotiate with the client TCP/IP Setting environment of . It offers two kinds of IP Positioning mode ﹕

AutomaticAllocation

Automatically assigned ﹕ once DHCP For the first time, the client succeeded from DHCP The server is rented to IP After address ﹐ Always use this address .

DynamicAllocation

Dynamic allocation : When DHCP First time from HDCP The server is rented to IP After address ﹐ The address is not used permanently ﹐ As long as the lease expires ﹐ The client has to release (release) This IP Address ﹐ For other workstations . Of course ﹐ The client can continue ahead of other hosts (renew) lease ﹐ Or rent something else IP Address .

working principle

for the first time Sign in When ﹕

1. seek Server. When DHCP The first time a client logs into the network ﹐ That is to say, the customer finds that there is no IP Data setting ﹐ It will send a message to the network DHCPDISCOVER Packet . Because the client does not know which network it belongs to ﹐ So the source address of the packet will be 0.0.0.0﹐ The destination address is 255.255.255.255﹐ Then attach Dhcpdiscover Information about ﹐ Broadcast to the Internet .

stay Windows By default ,Dhcpdiscover The waiting time of is preset to 1 second ﹐ That is, when the client will be the first Dhcpdiscover After the package is delivered ﹐ stay 1 If you don't get a response in seconds ﹐ It's going to be a second time Dhcpdiscover radio broadcast . If there is no response ﹐ There will be four times for the client Dhcpdiscover radio broadcast ( Including the first time )﹐ Except for waiting for the first time 1 Seconds away ﹐ The remaining three waiting times are 9﹑13﹑16 second . If you don't get it DHCP Server response ﹐ The client will display an error message ﹐ Declare Dhcpdiscover The failure of the . after ﹐ Based on the user's choice ﹐ The system will continue to 5 Repeat in minutes Dhcpdiscover The process of .

2. Provide IP Rental address . When DHCP The server listens to Dhcpdiscover After the broadcast ﹐ It'll be in the range of addresses that haven't been leased yet ﹐ Choose the empty one in the front IP, Together with others TCP/IP Set up , Respond to the client with a DHCPOFFER Packet .

Because the client didn't at the beginning IP Address ﹐ So in its Dhcpdiscover It will be included in the package MAC Address information ﹐ And there's a XID Number to identify the packet ﹐DHCP The server responded Dhcpoffer The packet is then passed to the client requesting the lease based on this information . According to the server settings ﹐Dhcpoffer The packet will contain information about the term of the lease .

3. Accept IP lease . If the client receives more than one on the network DHCP Server response ﹐ Only one of them will be selected Dhcpoffer nothing more ( Usually the first one to arrive )﹐ And will send a Dhcprequest Broadcast packets ﹐ Tell all DHCP Server it will specify which server to accept IP Address .

meanwhile ﹐ The client will also send a ARP Packet ﹐ Check if there are any other machines on the Internet using this IP Address ﹔ If you find this IP Already occupied ﹐ The client will send out a DHCPDECLINE Packet to DHCP Server ﹐ Refuse to accept it Dhcpoffer﹐ And resend Dhcpdiscover Information .

in fact ﹐ Not all DHCP Clients will unconditionally accept DHCP Server's offer﹐ In particular, these hosts are installed with other TCP/IP Related customer software . The client can also use Dhcprequest Propose to the server DHCP choice ﹐ And these choices will be filled in with different numbers in DHCPOptionField Inside ﹕

In other words ﹐ stay DHCP The settings on the server ﹐ Not all clients accept ﹐ The client can keep some of its own TCP/IP Set up . And the initiative is always on the client side .

4. Lease confirmation . When DHCP The server received from the client Dhcprequest after ﹐ Will send a message to the client DHCPACK Respond ﹐ To confirm IP The formal entry into force of the lease ﹐ It ends a complete DHCP working process .